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Synchrotron X-ray diffraction analysis of epitaxial GaN layer laterally overgrown
The GaN layer grown by epitaxial lateral overgrowth on sapphire (0001) has been investigated by synchrotron X-ray diffraction. The results show that ELO GaN stripes bend towards the SiN sub x mask in directions perpendicular to the stripe direction. This lead to the GaN (0001) crystal planes in the 'wings' (overgrown GaN) exhibit crystallographic tilts away from those in the 'window' (seed) regions. The GaN (0002) diffraction was used to determine the grain sizes in the wing region and window region, respectively. It is found that the grain size in the wing region increases about three times comparing to those in window region
2000-07-01
A buried tungsten (W) mask structure with GaN is successfully obtained by epitaxial lateral overgrowth (ELO) technique via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The selectivity of GaN growth on the window region vs. the mask region is good. An underlying GaN with a striped W metal mask is easily decomposed above 500 C by the W catalytic effect, by which radical hydrogen is reacted with GaN. It is difficult to bury the W mask because severe damage occurs in the GaN epilayer under the mask. It is found that an underlying AlGaN/GaN layer with a narrow W stripe mask width (mask/window - 2/2 {micro}m) leads the ELO GaN layer to be free from damage, resulting in an excellent W-buried structure.
Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template
2005-01-01
We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110 nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned ...
Growth and properties of semi-polar GaN on a patterned silicon substrate
2009-01-01
Adopting anisotropy etching method, a (111) facet of Si is obtained on a Si substrate and selective area growth (SAG) of GaN is performed with metal-organic vapor phase epitaxy on the facet. The epitaxial lateral overgrowth of (1101), (1122) GaN is investigated on (001) and (113) Si substrate, respectively, and the incorporation properties of Si, C, and Mg elements are discussed in relation to the atomic configuration on the surface. Analyzing the optical and electrical properties of C-doped (1101) GaN layer, it is shown that carbon creates a shallow acceptor level. On the thus prepared (1101) GaN layer, a light emitting diode (LED) with a C-doped p-type layer is fabricated.
Abnormal photoluminescence properties of GaN nanorods grown on Si(111) by molecular-beam epitaxy
2008-01-01
We have studied the photoluminescence properties of GaN nanorods grown on Si(111) substrates by radio-frequency plasma-assisted molecular-beam epitaxy. The hexagonal shaped nanorods with lateral average diameters from 30 to 150 nm are obtained by controlling the Ga flux with a fixed amount of nitrogen. As the diameters decrease, the main emission lines assigned as donor bound excitons are blueshifted, causing a spectral overlap of this emission line with that of the free exciton at 10 K due to the quantum size effect in the GaN nanorods. The temperature-dependent photoluminescence spectra show an abnormal behaviour with an 'S-like' shape for higher diameter nanorods. The activation energy of the free exciton for GaN nanorods with different diameters was also evaluated
GaN and AlN layers grown by nano epitaxial lateral overgrowth technique on porous substrates
2000-07-01
Defect density and stress reduction in heteroepitaxial GaN and AlN materials is one of the main issues in group III nitride technology. Recently, significant progress in defect density reduction in GaN layers has been achieved using lateral overgrowth technique. In this paper, the authors describe a novel technique based on nano-scale epitaxial lateral overgrowth. GaN layers were overgrown by hydride vapor phase epitaxy (HVPE) on porous GaN. Porous GaN was formed by anodization of GaN layers grown previously on SiC substrates. Pore's size was in nano-scale range. Thickness of overgrown layers ranged from 2 to 120 microns. It was shown that GaN layers overgrown on porous GaN have good surface morphology and high crystalline quality. The surface of overgrown GaN material was uniform and flat without any traces of porous structure. Raman spectroscopy measurements indicated that the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1.3 GPa. Preliminary experiments were done on HVPE growth of AlN layer on porous substrates. Improvement of surface morphology and crack density reduction has been observed.
Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth
1999-01-11
Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1{micro}m nucleation zones. Although emission is comprised of both UV ({approximately}365nm) and yellow ({approximately}550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.
Maskless Lateral Epitaxial Growth of Gallium Nitride Using Dimethylhydrazine as a Nitrogen Precursor
2007-01-01
Lateral epitaxial growth (LEG) is a key technology to improve the lifetime of III-V nitride-based laser diodes (LDs) by reducing the dislocation density in the materials. To increase the area of low dislocation density, the lateral growth rate needs to be increased. In addition, suppression of the vertical growth is strongly desired to avoid unnecessarily thick growth, which would result in cracks in the epitaxial film. This paper reports the maskless LEG of GaN with extremely high lateral-to-vertical growth rate ratio using dimethylhydrazine as a nitrogen precursor. The lateral growth only occurs from the sidewalls of the etched mesa stripes without any dielectric masks. The lateral growth rate toward the Formula Not Shown direction is extremely high, as high as 10m/h, while no vertic...
Growth of GaN on porous SiC and GaN substrates
2003-11-01
GaN films were grown on porous SiC and GaN templates using both plasma-assisted molecular beam epitaxy (PAMBE) and metal-organic chemical vapor deposition (MOCVD) to evaluate possible advantage of epitaxy on a porous substrate. For the growth of GaN on porous SiC by PAMBE, transmission electron microscopy (TEM) observations indicate that the exposed SiC suface pores tend to extend into the GaN film as open tubes and to trap Ga droplets. The GaN layers grown on porous templates have fewer threading dislocations originating at the interface, but they have additional defects in the form of half-loop dislocations which act to relieve the strain in the films. For PAMBE of GaN on porous GaN, dislocations existing in the porous seed layer are seen to propagate through the porous layer into the overgrown GaN, resulting in no dislocation reduction. For MOCVD of GaN on porous GaN, the initial regrowth tend to bend laterally the dislocations and enhance their annihilation, resulting in 5-10 x fewer dislocations in the overgrown film. (copyright 2003 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Study of wing tilt in asymmetrically and laterally overgrowth of thick GaN films
2003-01-01
The evolution of wing tilt in laterally and epitaxially overgrown (LEO) GaN thick film was studied using cross-sectional transmission electron microscope (TEM) observation and high-resolution X-ray diffraction (XRD). The LEO GaN film was grown by hydride vapor epitaxy (HVPE) on Si(1 1 1) substrate. Horizontally propagating dislocations (HDs) and low density of threading dislocation (TD) in LEO wings were observed in TEM micrographs. Tilt of LEO wing to the GaN seed region was measured as 3.3 deg. at interface of HVPE-GaN/metal organic vapor phase epitaxy (MOVPE)-GaN by selective area electron diffraction, which is much more than that in the surface region estimated as 0.4 deg. by XRD measurements. With phi-scan XRD technique, we found that there exist many differences of tilts and twists of c-axis between the opposite wings on each mask. Asymmetry growth of the opposite ...
Study of wing tilt in asymmetrically and laterally overgrowth of thick GaN films
2003-03-01
The evolution of wing tilt in laterally and epitaxially overgrown (LEO) GaN thick film was studied using cross-sectional transmission electron microscope (TEM) observation and high-resolution X-ray diffraction (XRD). The LEO GaN film was grown by hydride vapor epitaxy (HVPE) on Si(1 1 1) substrate. Horizontally propagating dislocations (HDs) and low density of threading dislocation (TD) in LEO wings were observed in TEM micrographs. Tilt of LEO wing to the GaN seed region was measured as 3.3 deg. at interface of HVPE-GaN/metal organic vapor phase epitaxy (MOVPE)-GaN by selective area electron diffraction, which is much more than that in the surface region estimated as 0.4 deg. by XRD measurements. With phi-scan XRD technique, we found that there exist many differences of tilts and twists of c-axis between the opposite wings on each mask. Asymmetry growth of the opposite wings on SiO{sub 2} mask may be due to the flow asymmetry of precursor in vertical reactor. The tilting generation and diminishing increasing with film thickness was discussed too.
Aqueous lateral epitaxy overgrowth of ZnO on (0001) GaN at 90^oC
2010-01-01
In Part I, it was shown that the critical thickness required for mechanical failure of lateral epitaxial overgrown films of ZnO on GaN buffered Al2O3 substrates could be significantly increased by masking and patterning the substrate to produce periodic, hexagonal arrays of circular windows. It was also observed that the critical thickness could be increased by increasing the distance between the circular windows. In Part II, it is shown, using Raman piezo-spectroscopy, that a stress gradient exists within the hexagonal prisms of ZnO that grow from the circular windows. The compressive stress, due to lattice mismatch, was highest in the center of the prisms and lowest near the perimeter where they overgrow the masked region to form 'wings'. For one orientation of the hexagonal array of cir...
Growth behavior of GaN nanoneedles with changing HCl/NH3 flow ratio
2009-01-01
We grew one-dimensional GaN nanoneedles on AlN/Si(111) substrates at HCl/NH3 gas-flow ratios of 1/20, 1/30, and 1/50 using the hydride vapor-phase epitaxy (HVPE) method. Field emission-scanning electron microscopy (FE-SEM) images of GaN nanoneedles show that the vertical growth rate of GaN nanoneedles increases with increasing gas-flow ratio, but there is little growth in the lateral direction. X-ray diffraction patterns indicate that GaN nanoneedles grew with c-axes oriented perpendicular to the substrate. The room-temperature PL spectrum of GaN nanoneedles was detected at 3.237eV of near-band-edge transitions.
2010-01-01
Thick, epitaxial ZnO thin films have been grown on (0001) GaN buffered Al2O3 substrates using an aqueous solution at 90^oC. Films with improved structural, optical and electrical characteristics, were grown using a lateral epitaxial overgrowth (LEO) method. Different photoresist masks were used to enable LEO. The masks included linear windows and two different hexagonal arrays of circular windows. Films that exceeded a critical thickness mechanically failed through buckling, consistent with the large compressive stresses expected due to the mismatch of the ZnO lattice with the underlying GaN substrate. It was shown that improved mechanical stability could be achieved using the LEO method. Without LEO, a film thickness no greater than 4 m could be grown without buckling. The critical thickn...
Mechanism of molecular beam epitaxy growth of GaN nanowires on Si(111)
GaN nanowires have been grown without external catalyst on Si(111) substrates by plasma-assisted molecular beam epitaxy. Nanowire aspect ratios (length/diameter) of about 250 have been achieved. During the initial stage of the growth, there is a nucleation process in which the number of wires increases and the most probable nucleation diameter of about 10 nm has been observed, which slowly increases with deposition time. For deposition time longer than the nucleation stage, the nanowire length as a function of diameter monotonically decreases. This phenomenon can be explained by adatom diffusion on the nanowire lateral surface towards the tip.
2010-01-01
Phosphor-free apple-white light emitting diodes (LEDs) have been fabricated using dual stacked InGaN/GaN multiple quantum wells (MQWs) comprising a lower set of long wavelength emitting indium rich nanostructures incorporated in MQWs with an upper set of cyan-green emitting MQWs. The LEDs were grown on nano-epitaxial lateral overgrown (ELO) GaN template formed by regrowth of GaN over SiO2 film patterned using an anodic alumina oxide mask with holes of 125 nm diameter and a period of 250 nm. The MQWs grown on the nano-ELO GaN templates show stronger photoluminescence intensity and a higher activation energy for their peak emission. A minimal shift in the electroluminescence (EL) spectra with higher injection current applied for LEDs grown on ELO-GaN compared to conventional GaN template, su...
A New Method for Lift-off of III-Nitride Semiconductors for Heterogeneous Integration
2010-01-01
The release and transfer of GaN epilayers to other substrates is of interest for a variety of applications, including heterogeneous integration of silicon logic devices, IIIV power devices and optical devices. We have developed a simple wet chemical etching method to release high-quality epitaxial III-nitride films from their substrates. This method builds on a nanoepitaxial lateral overgrowth (NELO) process that provides III-Nitride films with low dislocation densities. NELO is accomplished using a nanoporous mask layer patterned on GaN substrates. Chemical removal of the SiO2 layer after growth of III-Nitride overlayers causes fracture at the interface between the GaN film and the original GaN substrate, resulting in free-standing GaN films with nanostructured surfaces on one side. Th...
Microstructure of laterally overgrown GaN layers
2001-04-03
Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.
Microstructure of laterally overgrown GaN layers
Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.
Microstructure of laterally overgrown GaN layers
Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.
Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD
2008-08-07
We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 {mu}m. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson-Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8 K show that a strong band edge photoluminescence of GaN on Si (1 1 1) emits light at an energy of 3.449 eV with a full width at half maximum (FWHM) of approximately 16 meV. At room temperature, the peak position and FWHM of this emission become 3.390 eV and 58 meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1 1 1) epitaxial layers.
Nanoscale lateral epitaxial overgrowth of GaN on Si (111)
We demonstrate that GaN can selectively grow by metalorganic chemical vapor deposition into the pores and laterally over the nanoscale patterned SiO{sub 2} mask on a template of GaN/AlN/Si. The nanoporous SiO{sub 2} on GaN surface with pore diameter of approximately 65 nm and pore spacing of 110 nm was created by inductively coupled plasma etching using anodic aluminum oxide template as a mask. Cross-section transmission electron microscopy shows that the threading-dislocation density was largely reduced in this nanoepitaxial lateral overgrowth region. Dislocations parallel to the interface are the dominant type of dislocations in the overgrown layer of GaN. A large number of the threading dislocations were filtered by the nanoscale mask, which leads to the dramatic reduction of the threading dislocations during the growth within the nano-openings. More importantly, due to the nanoscale size of the mask area, the very fast coalescence and subsequent lateral overgrowth of GaN force the threading dislocations to bend to the basal plane within the first 50 nm of the film thickness. The structure of overgrown GaN is a truncated hexagonal pyramid which is covered with six {l_brace}1101{r_brace} side facets and (0001) top surface depending on the growth conditions.
Fabrication of GaN epitaxial thin film on InGaZnO4 single-crystalline buffer layer
2010-01-01
Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO4 (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH3 source. The epitaxial relationships are (0001)GaN//(0001)IGZO//(111)YSZ in out-of-plane and [112 0]GaN//[112 0]IGZO//[11 0]YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30^o with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.
Defect reduction in (112_O) a-plane GaN by two-stage epitaxiallateral overgrowth
2006-10-20
In the epitaxial lateral overgrowth (ELO) of (11{bar 2}0) a-plane GaN, the uneven growth rates of two opposing wings, Ga- and N-wings, makes the coalescence of two neighboring wings more difficult than that in c-plane GaN. We report a two-stage growth method to get uniformly coalesced epitaxial lateral overgrown a-plane GaN using metalorganic chemical vapor deposition (MOCVD) by employing relatively lower growth temperature in the first step followed by enhanced lateral growth in the second. Using this method, the height differences between Ga-polar and N-polar wings at the coalescence front could be reduced, thereby making the coalescence of two wings much easier. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing areas was 1.0x10{sup 8}cm{sup -2}, more than two orders of magnitude lower than that in the window areas (4.2x10{sup 10}cm{sup -2}). However, high density of basal stacking faults of 1.2x104 cm-1 was still observed in the wing areas as compared to c-plane GaN. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-GaN sample also indicated improved material quality.
Recent Progress in GaN-Based Light-Emitting Diodes
2009-01-01
In the last few years the GaN-based white light-emitting diode (LED) has been remarkable as a commercially available solid-state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V-grooved c-plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light-emitting efficiency (because of the enhanced reflection from the V-grooved plane). Furthermore, anomalous tunneling-assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to 72%. Finally, a...
In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN
2000-06-26
By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO{sub 2} mask. This was done by repeatedly performing line scans through the 101(bar sign)3 peak in the direction perpendicular to the [101(bar sign)0]{sub GaN} stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of {approx}1.19 degree sign after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to {approx}1.36 degree sign , indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt. (c) 2000 American Institute of Physics.
Lateral epitaxial overgrowth of GaN on Si(111). Progress report
1998-09-01
The lateral epitaxial overgrowth of GaN on Si(111) substrates was achieved using an extension of the standard LEO process on GaN/Al{sub 2}O{sub 3} substrates, and the reduction of the dislocation density was demonstrated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth on the Si(111) substrate was initiated with the deposition of a thin AlN buffer layer to avoid the formation of potentially detrimental silicon nitride at the interface. The wafers were then patterned with a SiO{sub 2} layer in which 5 micron wide opening separated by 35 microns were etched using buffered HF. After reloading the samples in the MOCVD chamber, the LEO growth was performed using the authors` standard parameters. There are a few unresolved issues concerning the effect of the AlN buffer thickness and its chemical compatibility with the SiO{sub 2} mask layer, but after a basic optimization they were able to obtain 5 microns of lateral overgrowth with smooth sidewalls in a reproducible manner. They are currently investigating the use of mask materials other than SiO{sub 2} to achieve LEO on Si(111) over a wider range of process parameters.
Lateral epitaxial overgrowth of GaN on Si(111). Progress report
The lateral epitaxial overgrowth of GaN on Si(111) substrates was achieved using an extension of the standard LEO process on GaN/Al{sub 2}O{sub 3} substrates, and the reduction of the dislocation density was demonstrated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth on the Si(111) substrate was initiated with the deposition of a thin AlN buffer layer to avoid the formation of potentially detrimental silicon nitride at the interface. The wafers were then patterned with a SiO{sub 2} layer in which 5 micron wide opening separated by 35 microns were etched using buffered HF. After reloading the samples in the MOCVD chamber, the LEO growth was performed using the authors` standard parameters. There are a few unresolved issues concerning the effect of the AlN buffer thickness and its chemical compatibility with the SiO{sub 2} mask layer, but after a basic optimization they were able to obtain 5 microns of lateral overgrowth with smooth sidewalls in a reproducible manner. They are currently investigating the use of mask materials other than SiO{sub 2} to achieve LEO on Si(111) over a wider range of process parameters.
Realization of high-crystalline-quality and thick GaInN films
2009-01-01
We report on the growth of a completely relaxed, high- crystalline-quality, and thick GaInN layer on a grooved m-plane GaN template using sidewall epitaxial lateral overgrowth (SELO) technology. We grew GaInN-based multiple quantum wells (MQWs) on this GaInN layer, and the PL intensity was compared with that for the same GaInN/GaN MQWs on m-plane GaN grown by SELO. The PL peak intensity obtained from the GaInN/GaN MQW on the high-crystalline-quality thick GaInN on grooved GaN was approximately 1.7 times higher than that on the SELO m-plane GaN template. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
The polarity of laterally epitaxially overgrown (LEO) GaN on Si(111) with an AlN buffer layer grown by MOCVD has been studied by convergent beam electron diffraction (CBED). The LEO GaN was studied by cross-section and plan-view transmission electron microscopy (TEM). The threading dislocation density is less than 10{sup 8} cm{sup {minus}2} and no inversion domains were observed. CBED patterns were obtained at 200 kV for the <1 {bar 1} 00> zone. Simulation was done by many-beam solution with 33 zero-order beams. The comparison of experimental CBED patterns and simulated patterns indicates that the polarity of GaN on Si(111) is Ga face.
2000-07-01
The polarity of laterally epitaxially overgrown (LEO) GaN on Si(111) with an AlN buffer layer grown by MOCVD has been studied by convergent beam electron diffraction (CBED). The LEO GaN was studied by cross-section and plan-view transmission electron microscopy (TEM). The threading dislocation density is less than 10{sup 8} cm{sup {minus}2} and no inversion domains were observed. CBED patterns were obtained at 200 kV for the <1 {bar 1} 00> zone. Simulation was done by many-beam solution with 33 zero-order beams. The comparison of experimental CBED patterns and simulated patterns indicates that the polarity of GaN on Si(111) is Ga face.
2007-01-01
We have used spatially and temporally resolved cathodoluminescence (CL) to study the carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and (1101)-oriented facets of GaN triangular prisms prepared by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system. The effects of In migration during growth on the resulting QW thickness and composition were examined. We employed a variable temperature time-resolved CL imaging approach that enables a spatial correlation between regions of enhanced exciton localization, luminescence efficiency, and radiative lifetime with the aim of distinguishing between excitons localized in In-rich quantum dots and those in the surrounding Ga-rich QW regions
2008-07-01
GaN nanocolumns are one-dimensional high-quality dislocation-free nano-crystals. The effect of Be-doping on GaN nanocolumns was investigated using InGaN/GaN nanocolumn LED structure grown by RF-plasma-assisted molecular-beam epitaxy. Be was doped into the upper cladding region of the nanocolumn light-emitting diodes (LEDs). Be is expected to be a p-type dopant of GaN with a shallow accepter level, but p-type conductivity was not obtained in these experiments. However, we observed a remarkable enhancement of the lateral growth of the GaN nanocolumn by Be doping; that is, the surface of the Be-doped region became smooth and continuous with increasing Be doping concentration. The root mean square (RMS) surface roughness was reduced from 35 to 10 nm with increasing Be cell temperature from 710 to 975 C. Cross-sectional TEM observation revealed that no threading dislocations were generated in the observed area at the conjunction boundary between neighboring nanocolumns during lateral growth. This suggests that a high-quality overgrowth layer with a low threading dislocation density can be realized on Be-doped GaN nanocolumns. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
A New Method for Lift-off of III-Nitride Semiconductors for Heterogeneous Integration
Full Text Available.The release and transfer of GaN epilayers to other substrates is of interest for a variety of applications, including heterogeneous integration of silicon logic devices, III–V power devices and optical devices. We have developed a simple wet chemical etching method to release high-quality epitaxial III-nitride films from their substrates. This method builds on a nanoepitaxial lateral overgrowth (NELO) process that provides III-Nitride films with low dislocation densities. NELO is accomplished using a nanoporous mask layer patterned on GaN substrates. Chemical removal of the SiO
In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN
2000-01-01
By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 101(bar sign)3 peak in the direction perpendicular to the [101(bar sign)0]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of approx1.19 degree sign after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to approx1.36 degree sign , indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and ...
Structural Defects in Laterally Overgrown GaN Layers Grown onNon-polar Substrates
Transmission electron microscopy was used to study defects in lateral epitaxial layers of GaN which were overgrown on a template of a-plane (11{und 2}0) GaN grown on (1{und 1}02) r-plane Al2O3. A high density of basal stacking faults is formed in these layers because the c-planes of wurtzite structure are arranged along the growth direction. Density of these faults is decreasing at least by two orders of magnitude lower in the wings compared to the seed areas. Prismatic stacking faults and threading dislocations are also observed, but their densities drastically decrease in the wings. The wings grow with opposite polarities and the Ga-wing width is at least 6 times larger than N-wing and coalescence is rather difficult. Some tilt and twist was detected using Large Angle Convergent Beam Electron Diffraction.
Structural Defects in Laterally Overgrown GaN Layers Grown onNon-polar Substrates
2007-02-14
Transmission electron microscopy was used to study defects in lateral epitaxial layers of GaN which were overgrown on a template of a-plane (11{und 2}0) GaN grown on (1{und 1}02) r-plane Al2O3. A high density of basal stacking faults is formed in these layers because the c-planes of wurtzite structure are arranged along the growth direction. Density of these faults is decreasing at least by two orders of magnitude lower in the wings compared to the seed areas. Prismatic stacking faults and threading dislocations are also observed, but their densities drastically decrease in the wings. The wings grow with opposite polarities and the Ga-wing width is at least 6 times larger than N-wing and coalescence is rather difficult. Some tilt and twist was detected using Large Angle Convergent Beam Electron Diffraction.
2006-01-01
Exciton dynamics in nonpolar (11 anti 20)InxGa1-xN/GaN multiple quantum wells (QWs) grown on GaN templates prepared by lateral epitaxial overgrowth were studied. Due to the absence of the polarization fields normal to the QW plane, the photoluminescence (PL) peak energy moderately shifted to the higher energy and the radiative lifetime did not change remarkably with the decrease in the well thickness. Similar to the case for polar InGaN QWs, time-resolved PL signals exhibited the nonexponential decay shape, which can be explained by thermalization and subsequent localization of excitons. Although the growth conditions were not fully optimized, values of the PL intensity at 300 K divided by that at 8 K were 25 and 17% for the peaks at 2.92 and 2.60 eV, respectively. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
1999-12-27
GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked anisotropic structures in the form of hexagonal pyramids and ridges. Spatially resolved optical emission from these structures, using both cathodoluminescence and collection-mode near-field scanning optical microscopy, of radiation centered near 550 nm, the so-called yellow band, indicates that the emission arises predominantly from the apex regions of the pyramids and ridges. In contrast, transmission electron microscopy shows that the apex region is nearly dislocation free and that dislocations cluster at the vertical growth core region. The spatial separation of the dislocations and optical emission indicates that the yellow-band emission has no direct relationship to dislocations. The observation of yellow-band emission strongly localized in the apical regions of both types of structures and the tendency of impurity species to concentrate in these areas argues that it arises instead from impurity states, the most likely candidate of which is a complex formed between a gallium vacancy, V{sub Ga}, and Si or O. (c) 1999 American Institute of Physics.
The results of comparative studies of the growth kinetics of the GaN layers of different polarity during ammonia molecular beam epitaxy and plasma-assisted molecular beam epitaxy (PA MBE) of nitrogen with the use of sapphire substrates and GaN(0001-bar)/c-Al{sub 2}O{sub 3} templates grown by gas-phase epitaxy from metalorganic compounds are presented. The possibility is shown of obtaining the GaN layers with an atomically smooth surface during molecular beam epitaxy with plasma activation of nitrogen. For this purpose, it is suggested to carry out the growth in conditions enriched with metal near the mode of formation of the Ga drops at a temperature close to the decomposition temperature of GaN (TS {approx} 760 deg. C). The conclusion is made that an increase in the growth temperature positively affects the structural, optical, and electrical properties of the GaN (0001-bar) layers. A high quality of the GaN (0001) films grown by the PA MBE method at a low temperature of {approx}700 deg. C on the GaN/c-Al{sub 2}O{sub 3} templates is shown.
Epitaxially-Grown GaN Junction Field Effect Transistors
1999-05-19
Junction field effect transistors (JFET) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The DC and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (g
2006-06-15
The growth of GaN by plasma assisted molecular beam epitaxy on GaN template substrates (GaN on sapphire) is investigated with in-situ multi-channel spectroscopic ellipsometry. Growth is performed under various Ga/N flux ratios at growth temperatures in the range 710-780 C. The thermal roughening of the GaN template caused by decomposition of the surface is investigated through the temporal variation of the GaN pseudodielectric function over the temperature range of 650 C to 850 C. The structural, morphological, and optical properties are also discussed. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Alpha particle detection with GaN Schottky diodes
2009-01-01
Ni/GaN Schottky diode radiation detectors were fabricated on 3-mum-thick unintentionally doped n-GaN films grown by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) and on 12-mum-thick undoped n-GaN layers prepared by epitaxial lateral overgrowth (ELOG). The reverse current of all detector structures was -9 A for bias voltages necessary for detector operation, with the level of background donor doping of 15 cm-3. With this doping level the space charge region of the Schottky diode could be extended to the entire thickness of the films. The charge collection efficiency of the detectors was close to 100% for MOCVD and ELOG detectors for alpha-particles with range comparable to the thickness of the layer. Electrical properties and deep trap spectra were also ...
2008-01-01
Diffuse x-ray scattering from threading dislocations in epitaxial structures is simulated numerically by a Monte Carlo method. The method allows one to simulate diffraction curves for dislocation types, where macroscopic approaches fail. That includes dislocation types for which analytical ensemble averaging is not feasible as well as microdiffraction curves from small sample volumes. In the latter case, the degree of statistic fluctuation of characteristic features is determined. The Monte Carlo method makes it possible to correlate quantitatively the widths of the microdiffraction curves to the densities of various dislocation types. The potential of the method has been demonstrated by a quantitative estimation of the density distribution of edge and screw threading dislocations in laterally overgrown epitaxial GaN structures, which is investigated by a ...
Effects of surface treatment for sapphire substrate on gallium nitride films
2009-01-01
High quality GaN films with low dislocation density have been successfully grown on the c-plane sapphire substrate, which the sapphire is etched by molted KOH solution. The solid-phase epitaxy starts from the regions with no etched pits where there are some micro-sidesteps to help to nucleate and then spreads laterally by increasing V/III ratio to form continuous GaN films. The crystal quality of epilayer is improved with the lengthening of etching time, but when the etching time is prolonged to some extent, the size of etching pit is too large to coalesce of two wings, the epilayer crystal quality is decreased, so the etching time for sapphire substrate has crucial effect on the epilayer crystal quality. The full widths at half maximum of the X-ray diffraction curves for the GaN films grown on the sapphire substrate, which has been etched for 50 min, in (0 0 0 2) plane and (101-bar2) ...
2010-06-15
Phosphor-free apple-white light emitting diodes (LEDs) have been fabricated using dual stacked InGaN/GaN multiple quantum wells (MQWs) comprising a lower set of long wavelength emitting indium rich nanostructures incorporated in MQWs with an upper set of cyan-green emitting MQWs. The LEDs were grown on nano-epitaxial lateral overgrown (ELO) GaN template formed by regrowth of GaN over SiO{sub 2} film patterned using an anodic alumina oxide mask with holes of {proportional_to}125 nm diameter and a period of 250 nm. The MQWs grown on the nano-ELO GaN templates show stronger photoluminescence intensity and a higher activation energy for their peak emission. A minimal shift in the electroluminescence (EL) spectra with higher injection current applied for LEDs grown on ELO-GaN compared to conventional GaN template, suggests a reduction in strain of the quantum well layers on the nano-ELO GaN template. An enhancement in the light extraction efficiency is also achieved with multiple scattering via the embedded SiO{sub 2} mask. (Abstract Copyright [2010], Wiley Periodicals, Inc.)
Molecular beam epitaxy and structural anisotropy of m-plane InN grown on free-standing GaN
This study reports on the growth of high-quality nonpolar m-plane [1100] InN films on free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized growth conditions (In/N ratio {approx}1 and T=390-430 deg. C) yielded very smooth InN films with undulated features elongated along the [1120] orientation. This directionality is associated with the underlying defect structure shown by the anisotropy of x-ray rocking curve widths parallel to the [1120] (i.e., 0.24 deg. - 0.34 deg.) and [0001] (i.e., 1.2 deg. - 2.7 deg.) orientations. Williamson-Hall analysis and transmission electron microscopy identified the mosaic tilt and lateral coherence length and their associations with different densities of dislocations and basal-plane stacking faults. Ultimately, very low band gap energies of {approx}0.67 eV were measured by optical absorption similar to the best c-plane InN.
Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces
2008-01-01
Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O2 system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbOx films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (001) GaN surface leads to the growth of epitaxial (111) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50nm lateral dimensions. Rocking curves in and have full-width half-maxim...
GaN exists in both wurtzite and zinc-blende phases and the growths of the two on its (0001) or (111) surfaces are achieved by choosing proper deposition conditions of molecular-beam epitaxy (MBE). At low substrate temperatures but high gallium fluxes, metastable zinc-blende GaN films are obtained, whereas at high temperatures and/or using high nitrogen fluxes, equilibrium wurtzite phase GaN epilayers resulted. This dependence of crystal structure on substrate temperature and source flux is not affected by deposition rate. Rather, the initial stage nucleation kinetics plays a primary role in determining the crystallographic structures of epitaxial GaN by MBE.
2009-01-01
The nucleation of HVPE GaN on misoriented sapphire substrates and the transition from the nucleation layer to an epitaxial film were investigated. After a KOH/NaOH eutectic etch of the approximately Formula Not Shown thick GaN layer, grown on sapphire using a low temperature nucleation, high temperature epitaxy process, the cross-sections revealed columnar structures, up to roughly Formula Not Shown above the sapphire substrate. Photoetching of the thick GaN layers revealed inhomogeneous defect distributions along the cross-sections, which appeared to be related to the numerous pinholes originating at the GaN/sapphire interface. We present a model explaining the formation of pinholes by the coalescence of the GaN nuclei during the epitaxial overgrowth.
GaN nucleation on (0 0 0 1)-sapphire via ion-induced nitridation of gallium
2006-08-31
The growth of epitaxial GaN films on (0 0 0 1)-sapphire has been investigated using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). In order to investigate the mechanism of the growth in detail, we have focused on the nitridation of pre-deposited Ga layers (droplets) using ion beam-assisted molecular beam epitaxy (IBA-MBE). Comparative analysis of XPS core-level spectra and LEED patterns reveals, that nitride films nucleate as epitaxial GaN islands. The wetting of the surface by GaN proceeds via reactive spreading of metallic Ga, supplied from the droplets. The discussed growth model confirms, that excess of metallic Ga is beneficial for GaN nucleation.
Approach for dislocation free GaN epitaxy
2010-01-01
The characteristics of confined epitaxial growth are investigated with the goal of determining the contributing effects of mask attributes (spacing, feature size) and growth conditions (V/III ratio, pressure, temperature) on the efficiency of the approach for dislocation density reduction of GaN. In addition to standard (secondary electron and atomic force) microscopy, electron channeling contrast imaging (ECCI) is employed to identify extended defects over large (tens of microns) areas. Using this method, it is illustrated that by confining the epitaxial growth, high quality GaN can be grown with dislocation densities approaching zero.
Growth of free-standing GaN layer on Si(111) substrate
2009-01-01
This investigation demonstrates the epitaxial growth of a free-standing GaN layer on Si(111) using a funnel-like GaN nano-rod buffer structure. The funnel-like GaN nano-rods were directly grown on Si substrates by RF-plasma molecular beam epitaxy. Free-standing GaN layers were achieved through the coalescence of funnel-like GaN nano-rods by the metalorganic chemical vapor deposition. This study examines the structure, optical characteristics and stress of GaN nano-rods and free-standing GaN layers. The c-axis lattice constant of the strain-free Ga-face GaN layer on Si is 5.1844A, as determined by high-resolution X-ray diffraction. The fully relaxed band edge at 3.468eV without deep-level emission around 2.3eV, was revealed in a free-standing GaN layer on Si, using photoluminescence.
GaN epitaxial layers have been grown by plasma-assisted molecular beam epitaxy on Si-face 4H- and 6H-SiC(0001){sub Si} substrates. The impact of the SiC surface preparation and oxide removal achieved via a Ga flash-off process followed by nitridation on the structure and properties of GaN epitaxial layers is articulated. A correlation among the SiC surface nitridation conditions, the Ga wetting layer development, the nucleation layer, and GaN crystalline properties is revealed.
2006-01-01
A technique was developed to deposit GaN on a Si(1 1 1) substrate by a four-step process in a single reactor: formation of ultra-thin oxide, conversion to an oxynitride via NH3 exposure at the onset of growth, low-temperature MOCVD of GaN, followed by HVPE of GaN. It was found that this oxynitride compliant layer served to relieve stress at the GaN/Si interface as well as protect the Si substrate from reaction with free Ga atoms present during the initial stages of growth and interfacial reactions with the adjoining GaN film. Electron microscopy revealed that an ultra-thin (less than 2 nm) oxynitride could maintain the hexagonal epitaxial relationship of the Si substrate allowing growth of single crystal GaN. It was shown experimentally that formation of a thicker oxynitride layer was detrimental to GaN epitaxy due to loss of the epitaxial relationship. In the final ...
2009-01-01
The results of comparative studies of the growth kinetics of the GaN layers of different polarity during ammonia molecular beam epitaxy and plasma-assisted molecular beam epitaxy (PA MBE) of nitrogen with the use of sapphire substrates and GaN(0001-bar)/c-Al2O3 templates grown by gas-phase epitaxy from metalorganic compounds are presented. The possibility is shown of obtaining the GaN layers with an atomically smooth surface during molecular beam epitaxy with plasma activation of nitrogen. For this purpose, it is suggested to carry out the growth in conditions enriched with metal near the mode of formation of the Ga drops at a temperature close to the decomposition temperature of GaN (TS approx 760 deg. C). The conclusion is made that an increase in the growth temperature positively affects the structural, optical, and electrical ...
2009-01-01
The results of comparative studies of the growth kinetics of the GaN layers of different polarity during ammonia molecular beam epitaxy and plasma-assisted molecular beam epitaxy (PA MBE) of nitrogen with the use of sapphire substrates and GaN(000 Formula Not Shown )/c-Al2O3 templates grown by gas-phase epitaxy from metalorganic compounds are presented. The possibility is shown of obtaining the GaN layers with an atomically smooth surface during molecular beam epitaxy with plasma activation of nitrogen. For this purpose, it is suggested to carry out the growth in conditions enriched with metal near the mode of formation of the Ga drops at a temperature close to the decomposition temperature of GaN (TS 760C). The conclusion is made that an increase in the growth temperature positively...
Mechanical Deformation Behavior of Nonpolar GaN Thick Films by Berkovich Nanoindentation
Full Text Available.In this study, the deformation mechanisms of nonpolar GaN thick films grown on m-sapphire by hydride vapor phase epitaxy (HVPE) are investigated using nanoindentation with a Berkovich indenter, cathodoluminescence (CL), and Raman microscopy. Results show that nonpolar GaN is more susceptible to plastic deformation and has lower hardness than c-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 11\\bar{2}0\\rangle$$\\end{document} orientation due to anisotropic defect-related stresses. Moreover, the quenching of CL luminescence can be observed to extend exclusively out from the center of the indentations along the \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 11\\bar{2}0\\rangle$$\\end{document} orientation, a trend which is consistent with the evolution of cracks. The recrystallization process happens in the indented regions for the load of 500 mN. Raman area mapping indicates that the distribution of strain field coincides well with the profile of defect-expanded dark regions, while the enhanced compressive stress mainly concentrates in the facets of the indentation.
Recombination of free and bound excitons in GaN
2008-01-01
We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to
A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors, GaN heterostructure and metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.
2000-07-17
A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors, GaN heterostructure and metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.
GaN Electronics For High Power, High Temperature Applications
2000-06-12
A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers. GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.
In this paper, we present the study of the structural, optical and electrical of n-type GaN grown on silicon (111) by RF plasma-assisted molecular beam epitaxy (RF-MBE). X-ray diffraction (XRD) measurement reveals that the GaN was epitaxially grown on silicon. For the photoluminescence (PL) measurement, a sharp and intense peak at 364.5 nm indicates that the sample is of high optical quality. Hall effect measurement shows that the film has a carrier concentration of 3.28x10{sup 19} cm{sup -3}. The surface of the n-type GaN was smooth and no any cracks and pits.
Low defect-mediated reverse-bias leakage in (0001) GaN via high-temperature molecular beam epitaxy
2010-01-01
Conductive atomic force microscopy, scanning electron microscopy, and x-ray diffraction were used to determine the effects of Ga/N flux ratio on the conductivity of current leakage paths in GaN grown by molecular beam epitaxy. Our data reveal a band of fluxes near Ga/Napprox =1 for which these pathways ceased to be observable. We conclude that changes in surface defects surrounding or impurities along screw-component threading dislocations are responsible for their conductive nature. These observations suggest a method for controlling the primary source of reverse-bias Schottky contact leakage in n-type GaN grown by molecular beam epitaxy
Optical properties of GaN quantum dots grown on nonpolar (11-20) SiC by molecular-beam epitaxy
We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H-SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.
High-quality GaN grown by molecular beam epitaxy on Ge(001)
1999-07-01
The authors report on growth of GaN on Germanium as an alternative substrate material. The GaN films were deposited on Ge(001) substrates by plasma-assisted molecular beam epitaxy. Atomic force microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy were used to characterize the structural and optical properties of the films. They observed that the Ga/N ratio plays a crucial role in determining the phase purity and crystal quality. Under N-rich conditions the films were phase-mixed, containing cubic and hexagonal GaN, while in the Ga-rich regime they were purity hexagonal. The latter samples show bandedge luminescence with linewidths as small as 31 meV at low temperatures.
High electron mobility GaN grown under N-rich conditions by plasma-assisted molecular beam epitaxy
An alternative approach is presented for the plasma-assisted molecular beam epitaxy of high-quality GaN. Under N-rich growth conditions, an unexpected layer-by-layer growth mode was found for a wide range of growth temperatures in the GaN thermal decomposition regime (>750 deg. C). Consequently, superior surface morphologies with roughness of less than 1 nm (rms) have been achieved. For lightly Si-doped GaN films, room-temperature electron mobilities exceeding 1100 cm{sup 2}/V s were measured, surpassing the commonly insulating nature of GaN grown under N-rich conditions at low temperature.
Epitaxial GaN Nanorods via Catalytic Capillary Condensation
2005-03-09
Intrinsic catalytic process by capillary condensation of Ga-atoms into nanotrenches, formed among impinging islands during the wurzite-GaN thin film deposition, is shown to be an effective path to growing GaN nanorods without metal catalysts. The nano-capillary brings within it a huge imbalance in equilibrium partial pressure of Ga relative to the growth ambient. GaN nanorods thus always grow out of a holding nanotrench and conform to the boundaries of surrounding islands. The nanorods are epitaxially orientated with GaN // Si and GaN // Si similar to the matrix. Concaved geometry is essential and is a condition that limits the axial dimension of the nanorods protruding above the base (matrix) material region. Revelation of the growth mechanism in the current context suggests that fabrication of nano quantum structures with controlled patterns is enabling for any attainable dimensions
Atomistic View of the Autosurfactant Effect during GaN Epitaxy
The Ga-N site exchange critical to the autosurfactant effect during GaN epitaxy is studied. On the GaN(0001) pseudo (1x1), the first site exchange results in N incorporation at the subsurface T{sub 1} site, forming ghost islands. The second exchange that converts these islands to that of bilayer height can be triggered by continued scanning tunneling microscopy imaging, which involves electrons tunneling to or from localized states associated with the second layer Ga. The resulting electrostatic force sets off a chain reaction which frees these Ga atoms, allowing N to form covalent Ga-N-Ga bonds of a new GaN bilayer.
Atomistic View of the Autosurfactant Effect during GaN Epitaxy
2007-01-01
The Ga-N site exchange critical to the autosurfactant effect during GaN epitaxy is studied. On the GaN(0001) pseudo (1x1), the first site exchange results in N incorporation at the subsurface T1 site, forming ghost islands. The second exchange that converts these islands to that of bilayer height can be triggered by continued scanning tunneling microscopy imaging, which involves electrons tunneling to or from localized states associated with the second layer Ga. The resulting electrostatic force sets off a chain reaction which frees these Ga atoms, allowing N to form covalent Ga-N-Ga bonds of a new GaN bilayer
A growth diagram for plasma-assisted molecular beam epitaxy of GaN nanocolumns on Si(111)
2009-01-01
The morphology of GaN samples grown by plasma-assisted molecular beam epitaxy on Si(111) was systematically studied as a function of impinging Ga/N flux ratio and growth temperature (730-850 deg. C). Two different growth regimes were identified: compact and nanocolumnar. A growth diagram was established as a function of growth parameters, exhibiting the transition between growth regimes, and showing under which growth conditions GaN cannot be grown due to thermal decomposition and Ga desorption. Present results indicate that adatoms diffusion length and the actual Ga/N ratio on the growing surface are key factors to achieve nanocolumnar growth
Low-temperature GaN growth on silicon substrates by single gas-source epitaxy and photo-excitation
2005-01-01
We report a unique low-temperature growth method for epitaxial GaN on Si(111) substrates via a ZrB2(0001) buffer layer. The method utilizes the decomposition of a single gas-source precursor (D2GaN3)3 on the substrate surface to form GaN. The film growth process is further promoted by irradiation of ultraviolet light to enhance the growth rate and ordering of the film. The best epitaxial film quality is achieved at a growth temperature of 550 deg. C with a growth rate of 3 nm/min. The films exhibit intense photoluminescence emission at 10 K with a single peak at 3.48 eV, indicative of band-edge emission for a single-phase hexagonal GaN film. The growth process achieved in this study is compatible with low Si processing temperatures and also enables direct epitaxy of GaN on ZrB2 in contrast to conventional ...
Improved molecular beam epitaxy for fabricating AlGaN/GaN heterojunction devices
2002-03-16
The polarity-control technology for Ga-polar GaN with rf plasma nitrogen-source molecular beam epitaxy (RF-MBE) was established. Inserting high-temperature grown AlN multiple intermediate layers (HT-AlN-MIL) into MBE-grown GaN at 750 C suppressed the propagation of dislocations into upper GaN, and the electrical and optical properties were improved. The HT-AlN-MIL was deposited on GaN templates grown by metal organic chemical vapor deposition (MOCVD), followed by GaN grown by MBE. The dislocations with screw character in MBE-grown GaN were reduced by about two orders of magnitude compared to those of the bottom GaN template, which produced the step-like surface morphology for MBE-grown GaN. To demonstrate the capability of MBE in heterojunction control, GaN/AlN double barrier resonant tunneling diode (RTD) structures were fabricated to show the negative differential resistance with a peak-valley ratio of 3.1. (orig.)
Atomic-Level Study of Melting Behavior of GaN Nanotubes
2006-09-20
Molecular dynamics simulations with a Stillinger-Weber potential have been used to investigate the melting behavior of wurtzite-type single crystalline GaN nanotubes. The simulations show that the melting temperature of GaN nanotubes is much lower than that of bulk GaN, which may be associated with the large surface-to-volume ratio of the nanotubes. The melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a GaN slab. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. The melting temperature of a single-crystalline GaN nanotube with [100]-oriented lateral facets is higher than that with [110]-oriented lateral facets for the same thickness.
Mg doping and its effect on the semipolar GaN(1122) growth kinetics
2009-01-01
We report the effect of Mg doping on the growth kinetics of semipolar GaN(1122) synthesized by plasma-assisted molecular-beam epitaxy. Mg tends to segregate on the surface, inhibiting the formation of the self-regulated Ga film which is used as a surfactant for the growth of undoped and Si-doped GaN(1122). We observe an enhancement of Mg incorporation in GaN(1122) compared to GaN(0001). Typical structural defects or polarity inversion domains found in Mg-doped GaN(0001) were not observed for the semipolar films investigated in the present study
Dislocation Clustering and Luminescence Nonuniformity in Bulk GaN and Its Homoepitaxial Film
2010-01-01
Cathodoluminescence (CL) spectroscopy and mapping techniques were used to study defect and impurity distributions in free-standing bulk GaN substrates prepared by hydride vapor-phase epitaxy. It was found that, in the bulk GaN substrates investigated, dislocation clusters appearing as dark cores in the CL map were surrounded by bright disk-like regions with higher luminescence efficiency than that of the outside areas. This large-area luminescence nonuniformity disappeared in homoepitaxial GaN grown on top of the GaN substrate. Schottky barrier diodes fabricated on the homo-epilayer exhibited low average reverse leakage current, while dislocation clusters duplicated from the original bulk GaN substrate still limited device yield.
2009-01-01
Epitaxial SrTiO3 films were fabricated by laser molecular beam epitaxy on bare and TiO2 buffered GaN(0002), respectively. The whole deposition processes were in situ monitored by reflection high energy electron diffraction (RHEED). X-ray diffraction (XRD) was carried out to study the growth orientation and crystalline quality of STO films. The interfacial characters and epitaxial relationships were also investigated by high revolution transition electron microscope and selected area electron diffraction (SAED). According to the RHEED observation, the lowest epitaxy temperature of STO on TiO2 buffered GaN was decreased compared with the direct deposited one. The epitaxial relationship was (111)[110]STO//(0002)[1120]GaN in both cases as confirmed by RHEED, XRD, and SAED. The full width at half maximum of ...
Growth of GaN films on nitrided GaAs substrates using hot-wire CVD
2003-04-22
Epitaxial growth of cubic-type gallium nitride (c-GaN) by hot-wire CVD on GaAs(100) substrates was investigated. Prior to the epitaxial growth, a nitridation layer was formed using ammonia plasma generated by electron cyclotron resonance (ECR). It was found that the crystal phase of the epitaxial layer was predominantly determined by that of the nitrided layer. The best nitridation condition using ECR plasma for the growth of the GaN films with preponderant cubic-type structure was obtained.
Long wavelength GaN blue laser (400-490nm) development
2000-10-26
Room temperature (RT) pulsed operation of blue nitride based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates was achieved. Atmospheric pressure MOCVD was used to grow the active region of the device which consisted of a 10 pair In{sub 0.21}Ga{sub 0.79}N (2.5nm)/In{sub 0.07}Ga{sub 0.93}N (5nm) InGaN MQW. The threshold current density was reduced by a factor of 2 from 10 kA/cm{sup 2} for laser diodes grown on sapphire substrates to 4.8 kA/cm{sub 2} for laser diodes grown on lateral epitaxial overgrowth (LEO) GaN on sapphire. Lasing wavelengths as long as 425nm were obtained. LEDs with emission wavelengths as long as 500nm were obtained by increasing the Indium content. These results show that a reduction in nonradiative recombination from a reduced dislocation density leads to a higher internal quantum efficiency. Further research on GaN based laser diodes is needed to extend the wavelength to 490nm which is required for numerous bio-detection applications. The GaN blue lasers will be used to stimulate fluorescence in special dye molecules when the dyes are attached to specific molecules or microorganisms. Fluorescein is one commonly used dye molecule for chemical and biological warfare agent detection, and its optimal excitation wavelength is 490 nm. InGaN alloys can be used to reach this wavelength.
TEM studies of laterally overgrown GaN layers grown on non-polarsubstrates
2006-01-05
Transmission electron microscopy (TEM) was used to study pendeo-epitaxial GaN layers grown on polar and non-polar 4H SiC substrates. The structural quality of the overgrown layers was evaluated using a number of TEM methods. Growth of pendeo-epitaxial layers on polar substrates leads to better structural quality of the overgrown areas, however edge-on dislocations are found at the meeting fronts of two wings. Some misorientation between the 'seed' area and wing area was detected by Convergent Beam Electron Diffraction. Growth of pendeo-epitaxial layers on non-polar substrates is more difficult. Two wings on the opposite site of the seed area grow in two different polar directions with different growth rates. Most dislocations in a wing grown with Ga polarity are 10 times wider than wings grown with N-polarity making coalescence of these layers difficult. Most dislocations in a wing grown with Ga polarity bend in a direction parallel to the substrate, but some of them also propagate to the sample surface. Stacking faults formed on the c-plane and prismatic plane occasionally were found. Some misorientation between the wings and seed was detected using Large Angle Convergent Beam Diffraction.
We report on the InGaN multiquantum laser diodes (LDs) made by rf plasma-assisted molecular beam epitaxy (PAMBE). The laser operation at 408 nm is demonstrated at room temperature with pulsed current injections using 50 ns pulses at 0.25% duty cycle. The threshold current density and voltage for the LDs with cleaved uncoated mirrors are 12 kA/cm{sup 2} (900 mA) and 9 V, respectively. High output power of 0.83 W is obtained during pulse operation at 3.6 A and 9.6 V bias with the slope efficiency of 0.35 W/A. The laser structures are deposited on the high-pressure-grown low dislocation bulk GaN substrates taking full advantage of the adlayer enhanced lateral diffusion channel for adatoms below the dynamic metallic cover. Our devices compare very favorably to the early laser diodes fabricated using the metalorganic vapor phase epitaxy technique, providing evidence that the relatively low growth temperatures used in this process pose no intrinsic limitations on the quality of the blue optoelectronic components that can be fabricated using PAMBE.
New buffer layers for GaN on sapphire by atomic layer and molecular stream epitaxy
1996-11-01
The current approach of depositing a low temperature then annealed AlN or GaN buffer for the growth of GaN on sapphire results in a high dislocation density. These dislocations thread through the GaN layer to the surface. Reducing their density either by growing thicker films or using a strained layer superlattice is ineffective. Two new approaches for AlN/GaN buffer layer growth for GaN on sapphire have been employed: Atomic Layer Epitaxy (ALE) and molecular Stream Epitaxy (MSE). ALE is distinguished by organo-metallic/ammonia separation while MSE is distinguished by cyclic annealing of the growing film. Both ALE and MSE enhance two dimensional growth of single crystal GaN on sapphire. The structural quality of epitaxial GaN grown on these buffer layers was studied by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The initial result for the ALE buffer shows an improved quality GaN film with lower defect densities. The MSE grown buffer layer closely resembles that of conventionally grown MOCVD buffer layers observed by others, with dislocations threading through the GaN epilayer. The effects of these buffer layers on the structural and optical properties of GaN grown on sapphire will be presented.
Low dislocation density GaN growth on high-temperature AlN buffer layers on (0001) sapphire
2010-01-01
Epitaxial films of GaN were grown on c-plane sapphire using a high-temperature AlN buffer layer by metalorganic vapour phase epitaxy. The GaN crystal quality was characterized by electron microscopy, atomic force microscopy, cathodoluminescence and high-resolution X-ray diffraction. The best GaN film had a threading dislocation (TD) density of (4.6±0.2)×107cm−2 with 006 and 302 ω-scan full widths at half maximum both less than 180arcsec. The GaN growth method involved the deposition of facetted GaN islands under conditions of reduced temperature and increased reactor pressure, followed by island coalescence at higher temperature and reduced pressure. The deposition of an in-situ SiNx micromask on the AlN buffer controlled the GaN island density during the in...
GaN directly deposited on m-sapphire by plasma-assisted molecular-beam epitaxy settles into two main crystalline orientation domains: GaN(1122) and GaN(1013). The dominant phase is GaN(1122) with <1123>{sub GaN} parallel <0001>{sub sapphire} and <1100>{sub GaN} parallel <1120>{sub sapphire} in-plane epitaxial relationships. Deposition of GaN on top of an AlN(1122) buffer layer and growth under slightly Ga-rich conditions reduce GaN(1013) precipitates below the detection limits. Studies of Ga adsorption demonstrate that it is possible to stabilize up to one Ga monolayer on the GaN(1122) surface. The presence of this monolayer of Ga excess on the growth front reduces the (1122) surface energy and hence minimizes the surface roughness. Photoluminescence from two-dimensional GaN(1122) layers is dominated by a near-band-edge emission, which is assigned to excitons bound to stacking faults, present with a density around 3x10{sup 5} cm{sup -1}.
AlN buffer layer growth for GaN epitaxy on (111) Si: Al or N first?
2009-01-01
AlN is generally used as buffer layer for the epitaxial growth of GaN on Si(111) substrate. In this work, we specifically address the relationship between the way the AlN growth is initiated on the Si(111) surface and the overall properties of the final GaN epitaxial layer. The growth is performed by molecular beam epitaxy with ammonia (NH3) as nitrogen source. Two procedures have been compared: exposing the Si surface first to NH3 or Al. The AlN nucleation is followed in real-time by reflection high-energy electron diffraction and critical stages are also investigated in real space using scanning tunnelling microscopy and transmission electron microscopy. Atomic force microscopy, X-ray diffraction and photoluminescence are also used to assess the properties of the final GaN epitaxial laye...
Transient atomic behavior and surface kinetics of GaN
2009-01-01
An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination
Strain effects in GaN thin film growth
1996-12-31
Strain effects in GaN thin film crystals are analyzed by Photoluminescence (PL), X-ray and Raman scattering. All three methods can be consistently used to monitor strain that depends on the choice of substrate and other parameters. The high amount of stress incorporated in GaN hetero-epitaxial samples causes confusion as to assignment of the PL lines of donor bound excitons and donor-acceptor transitions. Homo-epitaxially grown GaN films provide a reference for unstrained films. The PL line positions of the donor acceptor pair transitions are found to exhibit a different stress coefficient than the excitonic line.
The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer. {copyright} 2001 American Institute of Physics.
2001-06-11
The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer. {copyright} 2001 American Institute of Physics.
To identify the species which contribute to GaN growth, the authors investigated the discharge parameter (0.3-4.8 SCCM (SCCM denotes cubic centimeter per minute at STP), 150-400 W) dependences of the atomic N flux by appearance mass spectrometry and of the incorporated nitrogen atoms into GaN layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using the rf-plasma source. Ion fluxes were also evaluated by ion current measurements. A good correlation between the supplied atomic N flux and the incorporated nitrogen flux was obtained under a wide range of plasma conditions. It was clarified that the atomic N plays a dominant role in the growth of GaN by PAMBE.
2003-01-01
The optical properties of thick GaN films grown by hydried vapor phase epitaxy (HVPE) using a low-temperature intermediate GaN buffer layer grown on a (111) Si substrate with a ZnO thin film were investigated by using photoluminescence (PL) measurement at 300 K and 77 K. The strong donor bound exciton (DBE) at 357 nm with a full width at half maximum (FWHM) of 15 meV was observed at 77 K. The value of 15 meV is extremely narrow for GaN grown on Si substrate by HVPE. An impurity-related peak was also observed at 367 nm. The origin of impurity was investigated using Auger spectroscopy
Metastable behavior of the UV luminescence in Mg-doped GaN layers grown on quasibulk GaN templates
2007-01-01
Metastability of near band gap UV emissions in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by cathodoluminescence (CL). The CL spectrum changes its initial shape within a few minutes under electron irradiation. The acceptor bound exciton line vanishes while the emissions related to the stacking faults (SFs) of different geometry rise significantly. The increase of the defect-related luminescence is likely caused by recombination enhanced SF formation under electron irradiation. The CL spectrum transformation is permanent at low temperatures. However, the metastable process is reversible if samples are heated to room temperature
Defect reduction in gallium nitride using cantilever epitaxy.
Cantilever epitaxy (CE) has been developed to produce GaN on sapphire with low dislocation densities as needed for improved devices. The basic mechanism of seeding growth on sapphire mesas and lateral growth of cantilevers until they coalesce has been modified with an initial growth step at 950 C. This step produces a gable with (11{bar 2}2) facets over the mesas, which turns threading dislocations from vertical to horizontal in order to reduce the local density above mesas. This technique has produced material with densities as low as 2-3x10{sup 7}/cm{sup 2} averaged across extended areas of GaN on sapphire, as determined with AFM, TEM and cathodoluminescence (CL). This density is about two orders of magnitude below that of conventional planar growths; these improvements suggest that locating wide-area devices across both cantilever and mesa regions is possible. However, the first implementation of this technique also produced a new defect: cracks at cantilever coalescences with associated arrays of lateral dislocations. These defects have been labeled 'dark-block defects' because they are non-radiative and appear as dark rectangles in CL images. Material has been grown that does not have dark-block defects. Examination of the evolution of the cantilever films for many growths, both partial and complete, indicates that producing a film without these defects requires careful control of growth conditions and crystal morphology at multiple steps. Their elimination enhances optical emission and uniformity over large (mm) size areas.
Defect reduction in gallium nitride using cantilever epitaxy.
2003-08-01
Cantilever epitaxy (CE) has been developed to produce GaN on sapphire with low dislocation densities as needed for improved devices. The basic mechanism of seeding growth on sapphire mesas and lateral growth of cantilevers until they coalesce has been modified with an initial growth step at 950 C. This step produces a gable with (11{bar 2}2) facets over the mesas, which turns threading dislocations from vertical to horizontal in order to reduce the local density above mesas. This technique has produced material with densities as low as 2-3x10{sup 7}/cm{sup 2} averaged across extended areas of GaN on sapphire, as determined with AFM, TEM and cathodoluminescence (CL). This density is about two orders of magnitude below that of conventional planar growths; these improvements suggest that locating wide-area devices across both cantilever and mesa regions is possible. However, the first implementation of this technique also produced a new defect: cracks at cantilever coalescences with associated arrays of lateral dislocations. These defects have been labeled 'dark-block defects' because they are non-radiative and appear as dark rectangles in CL images. Material has been grown that does not have dark-block defects. Examination of the evolution of the cantilever films for many growths, both partial and complete, indicates that producing a film without these defects requires careful control of growth conditions and crystal morphology at multiple steps. Their elimination enhances optical emission and uniformity over large (mm) size areas.
Polycrystalline GaN for light emitter and field electron emitter applications
2005-09-01
We have grown polycrystalline GaN on quartz, refractory metal (W, Mo, Ta and Nb) and Si substrates by using plasma-assisted molecular beam epitaxy. It has been found that polycrystalline GaN grown on quartz and refractory metal substrates shows a strong band-edge emission without yellow emission. GaN growth on Si with native oxide produces well c-orientated nanorods exhibiting a low field emission threshold and high emission current density. We will review the growth of polycrystalline GaN films and the evaluation of their optical properties and electron field emission characteristics.
Growth and characterization of free-standing zinc-blende GaN layers and substrates
2010-01-01
We have investigated the growth of bulk, free-standing zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). The PA-MBE technique was used for bulk crystal growth and GaN layers up to 100 m in thickness were produced. We have established that the best structural properties of free-standing zinc-blende GaN can be achieved with initiation under Ga-rich conditions, but without Ga droplet formation. The procedure to produce free-standing bulk zinc-blende substrates from thick GaN layers grown on GaAs substrates has been developed. We have demonstrated the scalability of the process by growing free-standing GaN layers up to 3 in. in diameter. Growth of free-standing bulk GaN layers has allowed us to refine the values for the basic parameters of zinc-blende GaN. We h...
Surfactant effect of gallium during molecular-beam epitaxy of GaN on AlN (0001)
2001-01-01
We study the adsorption of Ga on (0001) GaN surfaces by reflection high-energy electron diffraction. It is shown that a dynamically stable Ga bilayer can be formed on the GaN surface for appropriate Ga fluxes and substrate temperatures. The influence of the presence of this Ga film on the growth mode of GaN on AlN(0001) by plasma-assisted molecular-beam epitaxy is studied. It is demonstrated that under nearly stoichiometric and N-rich conditions, the GaN layer relaxes elastically during the first stages of epitaxy. At high temperatures the growth follows a Stranski-Krastanov mode, whereas at lower temperatures kinetically formed flat platelets are observed. Under Ga-rich conditions--where a Ga bilayer is rapidly formed due to excess Ga accumulating on the surface--the growth follows a Frank-van der Merwe layer-by-layer mode at any growth temperature and no initial ...
2000-07-01
GaN growth by electron-cyclotron-resonance plasma-excited molecular beam epitaxy using hydrogen-nitrogen mixed gas plasma were carried out on GaN templates with a different polar-surface. Structure and surface morphology of the GaN layers were characterized using transmission electron microscopy. The GaN layer grown with hydrogen on N-polar template showed a relatively flat morphology including hillocks. Columnar domain existed in the center of the hillock, which might be attributed to the existence of tiny inversion domain with Ga-polarity. On the other hand, columnar structure was formed in the GaN layer grown with hydrogen on Ga-polar template.
Improved MOCVD growth of GaN on Si-on-porous-silicon substrates
2010-01-01
The mechanism of the poor surface morphology of MOCVD-grown GaN on porous Si (PSi) substrates was explored. Transmission electron microscopy suggests that a vacancy growth of pores starts, which is accompanied by the migration of the top surface of the PSi layer during the initial growth stage of GaN on the AlN inter layer. This causes a lot of pits on top GaN and wavy interface between GaN of the PSi layer, deteriorating the poor surface morphology. The epitaxial Si layer on PSi is effective in suppressing the migration of the top surface of the PSi layer. The GaN film on the epitaxial-Si/PSi shows a mirror surface with pits-free and very few cracks. No voids at the interface between the AlN IL and Si/PSi substrate are observed. (Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Efficient p-type doping of GaN films by plasma-assisted molecular beam epitaxy
In this paper we report on the mechanism of efficient incorporation of Mg in GaN films during growth by plasma-assisted molecular beam epitaxy. It is found that Mg incorporates more efficiently during growth of GaN films at high temperatures (770 deg. C) under extreme Ga-rich conditions. We propose that this result is due to the dissolution of Mg in the excess Ga on the growth surface and its incorporation into the GaN film via liquid-phase processes. Transport measurements at 300 K together with secondary-ion-mass-spectroscopy indicate that the Mg-doping efficiency of GaN under these conditions of growth is 10%. Using this method of doping, p-type GaN films free of Ga droplets, with hole concentrations varying from 2x10{sup 17} to 3x10{sup 18} cm{sup -3} and corresponding mobilities varying from 30 to 2 cm{sup 2}/V s, are obtained. The lowest resistivity achieved is 0.3 {omega} cm.
Strongly localized donor level in oxygen doped gallium nitride
1996-08-01
A classification in terms of localization of donor defects in GaN is performed by Raman spectroscopy under large hydrostatic pressure. We observe a significant decrease of free carrier concentration in highly O doped GaN epitaxial films at 22 GPa, indicating the presence of a strongly localized donor defect at large pressure. Monitoring the phonon plasmon coupled mode, we find similarities with results on highly n-type bulk crystals. We refine the model of localized defects in GaN and transfer it to the AlGaN system.
Strong coupling of light with A and B excitons in GaN microcavities grown on silicon
2006-01-01
We present experimental results demonstrating strong-light matter coupling at low and room temperature in bulk GaN microcavities with epitaxial (Al,Ga)N Bragg mirrors grown on silicon (111). At low temperature, the strong coupling of both the A and B excitonic features of GaN with the cavity mode is clearly resolved in the microcavity. At room temperature a Rabi energy of 50 meV is observed and well reproduced using transfer-matrix reflectivity calculations describing the interaction of both the A and B excitonic states with the photonic mode
Self-confined GaN heterophased quantum wells
2010-01-01
Wurtzite/zinc-blende/wurtzite GaN heterophased quantum wells (QWs) were grown by plasma-assisted molecular beam epitaxy. A self-assembling mechanism was used to simulate the heterophased QW, in which a wurtzite/zinc-blende phase transition was created by rotating the threefold symmetric N-Ga vertical bond 60 deg. The GaN heterophased QW was attested by transmission electron microscopy, selective area electron diffraction and cathodoluminescence measurements
Self-assembled GaN hexagonal micropyramid and microdisk
The self-assembled GaN hexagonal micropyramid and microdisk were grown on LiAlO{sub 2} by plasma-assisted molecular-beam epitaxy. It was found that the (0001) disk was established with the capture of N atoms by most-outside Ga atoms as the (1x1) surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from M-plane GaN.
GaN monocrystals preparation for substrates of epitaxy MBE and MOCVD
1994-12-31
The possibility of growing of AlN, GaN and InN crystals at high pressure of N has been analysed. Thermodynamical properties and its consequences for crystals growth have been shortly discussed on the basis of the results of high pressure experiments up to 20 kbar. At this pressure range the best results have been obtained for GaN. AlN requires higher temperatures to increase the solubility of N in the liquid Al. For InN the pressure of 20 kbar is not sufficient for effective crystallization. 21 refs, 6 figs, 1 tab.
GaN monocrystals preparation for substrates of epitaxy MBE and MOCVD
1994-01-01
The possibility of growing of AlN, GaN and InN crystals at high pressure of N has been analysed. Thermodynamical properties and its consequences for crystals growth have been shortly discussed on the basis of the results of high pressure experiments up to 20 kbar. At this pressure range the best results have been obtained for GaN. AlN requires higher temperatures to increase the solubility of N in the liquid Al. For InN the pressure of 20 kbar is not sufficient for effective crystallization. 21 refs, 6 figs, 1 tab
Evolution of deep centers in GaN grown by hydride vapor phaseepitaxy
Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.
Evolution of deep centers in GaN grown by hydride vapor phaseepitaxy
2001-04-18
Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.
Elasto-optical properties of zinc-blende (cubic) GaN measured by picosecond acoustics
2009-01-01
Using the technique of picosecond acoustics, we measure the basic elastic and optical properties of a micrometre-thickness zinc-blende (cubic) GaN epitaxial film grown on GaAs. We provide low temperature values of the speed of sound, c11 elastic constant and refractive index. Our value of the elastic constant is in good agreement with the theoretical calculations for cubic GaN (Wright 1997 J. Appl. Phys. 82 2833)
2009-01-01
Reciprocal space mapping with a two-dimensional (2D) area detector in a grazing incidence geometry was applied to determine crystallographic orientations of GaN nanostructures epitaxially grown on a sapphire substrate. By using both unprojected and projected reciprocal space mapping with a proper coordinate transformation, the crystallographic orientations of GaN nanostructures with respect to that of a substrate were unambiguously determined. In particular, the legs of multipods in the wurtzite phase were found to preferentially nucleate on the sides of tetrahedral cores in the zinc blende phase
Improvement of crystal quality of GaN grown on AlN template by MOCVD using HT-AlN interlayer
2009-06-15
Two GaN samples, with and without high temperature (HT)-AlN interlayer (labelled as sample A and B, respectively) grown by MOCVD on AlN template, were investigated by double-crystal X-ray diffraction (DC-XRD), photoluminescence (PL), and atomic force microscope (AFM) measurements. It was found that the crystal quality of GaN could be greatly improved by the HT-AlN interlayer. The full width at half maximum (FWHM) of (102) reflection in XRD rocking curve was narrower for sample A than that for sample B. However, the FWHMs of (002) reflections were almost the same for the two samples. In addition, the tilt degree which reflected screw dislocation density was almost the same, while the twist degree which reflected edge dislocation density changed from 0.214 to 0.152 when the HT-AlN interlayer was used. Both the intensities of (102) reflection in XRD and band edge emission in PL for sample A were stronger too. In the AFM images, the atomic growth steps of sample A were clearer than those of sample B. According to the results of the in situ optical reflectivity spectra and the atomic force microscope (AFM) images, the above results were attributed to the three-dimensional (3D) growth mode of the HT-AlN interlayer. The HT-AlN interlayer may work as a kind of ''micro-area'' seed for epitaxial lateral overgrowth (ELOG) resulting in bending some dislocations. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
X-ray diffraction peak profiles from threading dislocations in GaN epitaxial films
2004-10-20
We analyze the lineshape of x-ray diffraction profiles of GaN epitaxial layers with large densities of randomly distributed threading dislocations. The peaks are Gaussian only in the central, most intense part of the peak, while the tails obey a power law. The $q^{-3}$ decay typical for random dislocations is observed in double-crystal rocking curves. The entire profile is well fitted by a restricted random dislocation distribution. The densities of both edge and screw threading dislocations and the ranges of dislocation correlations are obtained.
Observation of stimulated emission from an MBE grown GaN film on sapphire
1993-11-12
The authors report the first observation of optically pumped stimulated emission from an GaN epilayer at 77K and at room temperature grown by reactive ion-beam molecular beam epitaxy. The observed uv optical emission profile was a nonlinear function of the pump power density, with line narrowing at threshold power densities. The similarity in the emission profile as compared with those of films grown with low-pressure metal-organic chemical vapor deposition and metal-organic vapor phase epitaxy techniques will be noted.
High resolution x-ray diffraction from epitaxial gallium nitride films
1997-12-31
The width of double axis X-ray rocking curves of epitaxial GaN layers is shown to be critically dependent on the width of the detector aperture. The authors show that triple axis diffraction measurements using a crystal analyzer before the detector enables the instrument function to be defined and the tilt and dilation distributions separated. All GaN samples examined showed a mosaic blocks. In reciprocal space maps this was revealed as a wide distribution of intensity in a direction perpendicular to the reciprocal lattice vector.
Exciton diffusion in GaN epitaxial layers
2001-11-08
The photoluminescence (PL) and reflection excitonic spectra of GaN epitaxial layers grown on sapphire substrate by MOVPE were measured in the temperature region from 50 to 200 K. A three-layer model of the crystal was used for fitting the reflection spectrum. In this way the dead layer thickness, resonance energies and the broadening parameters of the free excitons were obtained. These parameters were then used for fitting the PL spectra assuming a thermal equilibrium for excitons. A decrease in the diffusion coefficient and increase in the radiative lifetime of excitons was found with increasing temperature. (orig.)
Polar and nonpolar GaN quantum dots
2008-01-01
Growth, structural and optical properties of GaN quantum dots are reviewed, with a special emphasis on plasma-assisted molecular beam epitaxy. The versatility of this technique makes it particularly adapted to growth of quantum dots, either polar (c-plane) or nonpolar (a-plane and m-plane). After describing in detail the growth process and analyzing the morphology of the dots, we review the optical properties of these nanostructures and discuss the properties of single dots. (topical review)
Initial growth behaviors of GaN layers overgrown by HVPE on one-dimensional nanostructures
2010-01-01
We developed a novel technique for obtaining a residual-strain-free GaN layer by the hydride vapor phase epitaxy (HVPE) method using one-dimensional nanostructures. The GaN layer was grown on a Si(1 1 1) substrate with a conventional AlN film and one-dimensional GaN nanostructures. The nanostructures were grown for 2 h with a HCl:NH3 gas flow ratio of 1:50. The growth rate of nanoneedles at 600 deg. C and nanorods at 650 deg. C were 2.553 and 2.193 mum/h, respectively. The overgrown GaN layer was grown at 1050 deg. C for 5 and 10 min. We obtained a GaN layer of 1.833 mum thickness and c = 5.1849 A. The morphology, crystalline structure, and optical characteristics of the GaN layer were examined by field emission scanning electron microscopy, X-ray diffraction, and photoluminescence
Initial growth behaviors of GaN layers overgrown by HVPE on one-dimensional nanostructures
2010-01-01
We developed a novel technique for obtaining a residual-strain-free GaN layer by the hydride vapor phase epitaxy (HVPE) method using one-dimensional nanostructures. The GaN layer was grown on a Si(111) substrate with a conventional AlN film and one-dimensional GaN nanostructures. The nanostructures were grown for 2h with a HCl:NH3 gas flow ratio of 1:50. The growth rate of nanoneedles at 600^oC and nanorods at 650^oC were 2.553 and 2.193mm/h, respectively. The overgrown GaN layer was grown at 1050^oC for 5 and 10min. We obtained a GaN layer of 1.833mm thickness and c=5.1849A. The morphology, crystalline structure, and optical characteristics of the GaN layer were examined by field emission scanning electron microscopy, X-ray diffraction, and photoluminescence.
Growth and electron microscopy study of GaN/MgAl2O4 heterostructures
2010-01-01
We successfully grew Gallium nitride (GaN) films on MgAl2O4 (111) substrates using low temperature pulsed laser deposition (PLD). X-ray diffraction rocking curves revealed the high quality of as-grown GaN with its full width at half maximum (FWHM) along [0002] as small as 0.08degree. Atomic force microscopy images showed very smooth surface of as-grown GaN and indicated a two-dimensional growth. High-resolution transmission electron microscopy showed high structural perfection of GaN films. The crystalline structure for MgAl2O4 substrate from its body to its top part adjacent to GaN epitaxy kept constant, and the interface between GaN and MgAl2O4 was atomically abrupt. We attribute the high quality of as-grown GaN films to the well-controlled Mg atom evaporation from MgAl2O4 substrate surf...
Enhanced optical properties of porous GaN by using UV-assisted electrochemical etching
In this paper we report the fabrication of porous GaN by UV-assisted electrochemical etching with direct current densities of 5, 10 and 20 mA/cm2 for 20 min in electrolytes consisting of aqueous HF and ethanol C2H5OH (1:4). Scanning electron microscopy (SEM) demonstrates that current density has significant effect on the size and shape of the pores. Raman spectra of both as-grown and porous GaN exhibit phonon mode E2 (high), A1 (LO), A1 (TO) and E2 (low). There was a red shift in E2 (low) and E2 (high) and blue shift in A1 (LO). The red shift in E2 (high) indicates a relaxation of compressive stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Peak intensity of photoluminescence (PL) spectra of the nanoporous GaN samples was observed to be enhanced and the crystal quality was improved with increase in etching current density as compared to as-grown GaN.
Supersonic jet epitaxy: An improved method for nitride deposition
1996-11-01
GaN was grown by supersonic jet epitaxy (SSJE), seeding triethylgallium in helium carrier gas. Activated nitrogen was supplied by a microwave plasma source. Single crystalline GaN films were deposited on the Si-face 6H-SiC and the c-plane sapphire substrates at 600--670 C. A cubic SiC buffer layer was grown on Si(111) at 800 C by SSJE using dichlorosilane, acetylene, and a high quality GaN crystal was grown on this template at 630 C. The materials high quality was proved by hard rectifying characteristics of a diode with an N-GaN/{beta}-SiC/P-Si(111) structure.
Strong light-matter coupling in ultrathin double dielectric mirror GaN microcavities
2008-01-01
Strong light-matter coupling is demonstrated at low temperature in an ultrathin GaN microcavity fabricated using two silica/zirconia Bragg mirrors, in addition to a three-period epitaxial (Al,Ga)N mirror serving as an etch stop and assuring good quality of the overgrown GaN. The lambda/2 cavity is grown by molecular beam epitaxy on a Si substrate. Analysis of angle-resolved data reveal key features of the strong coupling regime in both reflectivity and transmission spectra at 5 K: anticrossing with a normal mode splitting of 43+-2 meV and 56+-2 meV for reflectivity and transmission, respectively, and narrowing of the lower polariton linewidth near resonance
The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations.
Growth of M-plane GaN on (100) LiGaO2 by plasma-assisted molecular beam epitaxy
2010-01-01
Growth of M-plane GaN on (100) LiGaO2 was achieved using plasma-assisted molecular beam epitaxy. Thermal annealing of the LiGaO2 wafer was found to lead to a substrate surface suitable for growth. Structural and morphological analysis was performed using x-ray and reflective high energy electron diffraction, scanning electron and atomic force microscopy. X-ray diffraction results show very high phase purity and a relaxation state of the GaN film close to 80%. The surface morphology, showing characteristic M-plane streaks, is flat and smooth.
GaN doped with neodymium by plasma-assisted molecular beam epitaxy
We report in situ doping of GaN with the rare earth element Nd by plasma-assisted molecular beam epitaxy. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectroscopy data indicate {approx}5 at. % Nd in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation under nitrogen-rich conditions with up to {approx}1 at. % Nd, with the highest luminescence intensities corresponding to doping of {approx}0.5 at. %. Spectral correlation of the Nd emission multiplets for above (325 nm) and below (836 nm) GaN bandgap excitations implies enhanced substitutional doping at the Ga site.
2002-01-01
Light emitting diodes and blue laser diodes grown on GaN have been demonstrated despite six orders of magnitude higher dislocation density than that for III-V arsenide and phosphide diodes. Understanding and determination of dislocation cores in GaN is crucial since both theoretical and experimental work are somewhat contradictory. Transmission Electron Microscopy (TEM) has been applied to study the layers grown by hydride vapor-phase epitaxy (HVPE) and molecular beam epitaxy (MBE) (under Ga rich conditions) in plan-view and cross-section samples. This study suggests that despite the fact that voids are formed along the dislocation line in HVPE material, the dislocations have closed cores. Similar results of closed core are obtained for the screw dislocation in the MBE material, confirming earlier studies
1997-12-31
By monitoring RHEED reconstruction patterns during gas source molecular beam epitaxy growth, the optimization of the growth for cubic GaN was carried out successfully. Cubic GaN epilayer having a X-ray diffraction width of 16min and a low temperature photoluminescence emission width of 19 meV was obtained on a 3C-SiC substrate by adjusting the effective III/V ratio in-situ during the growth, which can be inferred from the surface reconstruction transitions. It was found that the surface reconstructions of cubic GaN surfaces are good indices for the optimization of growth parameters.
Stress and Defect Control in GaN Using Low Temperature Interlayers
1998-12-04
In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.
Quasi-horizontal GaN nanowire array network grown by sublimation sandwich technique
2008-08-15
Quasi-horizontal GaN nanowire array network has been grown on Au-film-coated MgO substrates via a sublimation sandwich technique. These GaN nanowire arrays principally grew along two directions which were perpendicular to each other and nearly parallel to the substrate, forming a regular network. The formation of the nanowire network was a hetero-epitaxial vapor-liquid-solid (VLS) process assisted by Au catalysts and was dependent on the substrates. Transmission electron microscopy revealed that the nanowires were single-crystalline wurtzite GaN. Raman scattering spectrum of the nanowire network presented some new features.
Quasi-horizontal GaN nanowire array network grown by sublimation sandwich technique
2008-01-01
Quasi-horizontal GaN nanowire array network has been grown on Au-film-coated MgO substrates via a sublimation sandwich technique. These GaN nanowire arrays principally grew along two directions which were perpendicular to each other and nearly parallel to the substrate, forming a regular network. The formation of the nanowire network was a hetero-epitaxial vapor-liquid-solid (VLS) process assisted by Au catalysts and was dependent on the substrates. Transmission electron microscopy revealed that the nanowires were single-crystalline wurtzite GaN. Raman scattering spectrum of the nanowire network presented some new features
Lattice-matched HfN buffer layers for epitaxy of GaN on Si
Gallium nitride is grown by plasma-assisted molecular-beam epitaxy on (111) and (001) silicon substrates using sputter-deposited hafnium nitride buffer layers. Wurtzite GaN epitaxial layers are obtained on both the (111) and (001) HfN/Si surfaces, with crack-free thickness up to 1.2 (mu)m. Initial results for GaN grown on the (111) surface show a photoluminescence peak width of 17 meV at 11 K, and an asymmetric x-ray rocking curve width of 20 arcmin. Wurtzite GaN on HfN/Si(001) shows reduced structural quality and peculiar low-temperature luminescence features. However, growth on the (001) surface results in nearly stress-free films, suggesting that much thicker crack-free layers could be obtained.
Lattice-matched HfN buffer layers for epitaxy of GaN on Si
Gallium nitride is grown by plasma-assisted molecular-beam epitaxy on (111) and (001) silicon substrates using sputter-deposited hafnium nitride buffer layers. Wurtzite GaN epitaxial layers are obtained on both the (111) and (001) HfN/Si surfaces, with crack-free thickness up to 1.2 (mu)m. Initial results for GaN grown on the (111) surface show a photoluminescence peak width of 17 meV at 11 K, and an asymmetric x-ray rocking curve width of 20 arcmin. Wurtzite GaN on HfN/Si(001) shows reduced structural quality and peculiar low-temperature luminescence features. However, growth on the (001) surface results in nearly stress-free films, suggesting that much thicker crack-free layers could be obtained.
Lattice-matched HfN buffer layers for epitaxy of GaN on Si
2002-05-08
Gallium nitride is grown by plasma-assisted molecular-beam epitaxy on (111) and (001) silicon substrates using sputter-deposited hafnium nitride buffer layers. Wurtzite GaN epitaxial layers are obtained on both the (111) and (001) HfN/Si surfaces, with crack-free thickness up to 1.2 (mu)m. Initial results for GaN grown on the (111) surface show a photoluminescence peak width of 17 meV at 11 K, and an asymmetric x-ray rocking curve width of 20 arcmin. Wurtzite GaN on HfN/Si(001) shows reduced structural quality and peculiar low-temperature luminescence features. However, growth on the (001) surface results in nearly stress-free films, suggesting that much thicker crack-free layers could be obtained.
Growth of group III nitride films by pulsed electron beam deposition
2009-01-01
We have grown group III nitride films on Al2O3 (0 0 0 1), 6H-SiC (0 0 0 1), and ZnO (0001-bar) substrates by pulsed electron beam deposition (PED) for the first time and investigated their characteristics. We found that c-plane AlN and GaN grow epitaxially on these substrates. It has been revealed that the growth of GaN on atomically flat 6H-SiC substrates starts with the three-dimensional mode and eventually changes into the two-dimensional mode. The GaN films exhibited strong near-band-edge emission in their room temperature photoluminescence spectra. We also found that the use of PED allows us to reduce the epitaxial growth temperature for GaN down to 200 deg. C. - Graphical abstract: We have grown group III nitride films by pulsed electron beam deposition (PED) and found that the films of group III nitrides grow epitaxially on 6H-SiC and ...
2003-01-01
A novel single-source precursor with composition D2GaN3 is used to grow GaN films on Si(1 1 1) substrates via AlN buffer layers by gas-source molecular beam epitaxy. The morphological and optical properties of the films are determined as a function of the reaction conditions including deposition temperature. Monocrystalline GaN nanopillars with faceted terminations are readily formed at unusually low temperatures of 275-300 deg. C from the thermodynamically driven decomposition of the precursor via elimination of D2 and N2. Highly oriented layers with columnar morphology and wurtzite microstructure are obtained between 450-750 deg. C via basal plane growth. The photoluminescence and cathodoluminescence spectra of these materials display strong band edge emission peaks at 360 nm, and in some cases they show broad and ...
Ti-mask selective-area growth of GaN nanorings by RF-plasma-assisted molecular-beam epitaxy
2009-06-15
GaN nanorings with InGaN/GaN multiple quantum wells (MQWs) were successfully fabricated on a GaN template by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) using a titanium (Ti) mask selective-area growth (SAG) technique. We employed focused ion beam (FIB) milling to make patterns on the Ti-mask (ring groove patterns). 3-period InGaN/GaN MQWs integrated into the GaN nanorings exhibited a clear room-temperature photoluminescence (RT-PL) spectrum with a peak wavelength of 462 nm and the full width at half maximum (FWHM) of the RT-PL spectrum was 145 meV. In addition, we fabricated GaN double nanorings and hexagonal nanorings with InGaN/GaN MQWs by controlling the amount of FIB etching, and the growth conditions. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Ti-mask selective-area growth of GaN nanorings by RF-plasma-assisted molecular-beam epitaxy
2009-01-01
GaN nanorings with InGaN/GaN multiple quantum wells (MQWs) were successfully fabricated on a GaN template by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) using a titanium (Ti) mask selective-area growth (SAG) technique. We employed focused ion beam (FIB) milling to make patterns on the Ti-mask (ring groove patterns). 3-period InGaN/GaN MQWs integrated into the GaN nanorings exhibited a clear room-temperature photoluminescence (RT-PL) spectrum with a peak wavelength of 462 nm and the full width at half maximum (FWHM) of the RT-PL spectrum was 145 meV. In addition, we fabricated GaN double nanorings and hexagonal nanorings with InGaN/GaN MQWs by controlling the amount of FIB etching, and the growth conditions. (Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
2009-01-01
GaN films were grown by hydride vapor phase epitaxy on sapphire substrates with orientations c-(0 0 0 1), a-(1 1 2-bar 0), m-(1 0 1-bar 0) and r-(1 0 1-bar 2) using N2 as a carrier gas. The crystalline perfection of the grown films was studied by X-ray diffraction, by optical microscopy and scanning electron microscopy and by microcathodoluminescence (MCL). It was found that, for c- and a-oriented sapphire substrates, the GaN films showed (0 0 0 1) orientation, for m-oriented sapphire the films showed semi-polar (1 0 1-bar 3) orientation, while for r-sapphire substrates GaN layers with non-polar a(1 1 2-bar 0) orientation could be grown. The surface morphology of the GaN films and their crystalline structure strongly depended on the substrate orientation. With increasing the layer thickness the halfwidth of the X-ray rocking curves monotonically decreased which ...
2007-01-01
GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to approx 2.8 x 109/cm2. The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission ...
2007-01-01
GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50nm thick high-temperature grown AlN buffer layer, 150nm thick AlGaN layer, 30nm low-temperature grown AlN layer, 300nm GaN layer, 50nm AlGaN superlattice layer, followed by 100nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to 2.8109/cm2. The optical quality was considerably improved. The photoluminescence emission at 3.423.45eV is attributed to the recombination of free hole-to-d...
RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers
Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates were irradiated with 150 MeV Ag ions at a fluence of 5x10{sup 12} ions/cm{sup 2}. Samples used in this study are 2 {mu}m thick GaN layers, with and without a thin AlN cap-layer. Energy dependent RBS/Channeling measurements have been carried out on both irradiated and unirradiated samples for defects characterization. Observed results are compared and correlated with previous HRXRD, AFM and optical studies. The {chi}{sub min} values for unirradiated samples show very high value and the calculated defect densities are of the order of 10{sup 10} cm{sup -2} as expected in these samples. Effects of irradiation on these samples are different as initial samples had different defect densities. Epitaxial reconstruction of GaN buffer layer has been attributed to the observed changes, which are generally grown to reduce the strain between GaN and Sapphire.
Influence of AlN buffer layer thickness and deposition methods on GaN epitaxial growth
2009-01-01
A gallium nitride (GaN) epitaxial layer was grown by metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates with aluminum nitride (AlN) buffer layers at various thicknesses. The AlN buffer layers were deposited by two methods: radio frequency (RF) magnetron sputtering and MOCVD. The effect of the AlN deposition method and layer thickness on the morphological, structural and optical properties of the GaN layers was investigated. Field emission scanning electron microscopy showed that GaN did not coalesce on the sputtered AlN buffer layer. On the other hand, it coalesced with a single domain on the MOCVD-grown AlN buffer layer. Structural and optical analyses indicated that GaN on the MOCVD-grown AlN buffer layer had fewer defects and a better aligned lattice to the a- and c-...
We investigated the effect of AlN nucleation layers (NLs) on the structural and electrical properties of N-face GaN grown on C-face 6H-SiC substrates by plasma-assisted molecular beam epitaxy. The GaN films were characterized by secondary ion mass spectroscopy, x-ray diffraction, and transistor electrical measurements. It was found that an AlN NL grown in the N-rich regime was essential for realizing highly resistive GaN buffers. The mosaic structure of the GaN epilayers was systematically correlated with the AlN nucleation conditions. N-face high electron mobility transistors fabricated on these low-leakage buffers demonstrated the highest output power density at 4 GHz to date of 8.1 W/mm with an associated power-added efficiency of 54%.
2009-01-01
We investigated the effect of AlN nucleation layers (NLs) on the structural and electrical properties of N-face GaN grown on C-face 6H-SiC substrates by plasma-assisted molecular beam epitaxy. The GaN films were characterized by secondary ion mass spectroscopy, x-ray diffraction, and transistor electrical measurements. It was found that an AlN NL grown in the N-rich regime was essential for realizing highly resistive GaN buffers. The mosaic structure of the GaN epilayers was systematically correlated with the AlN nucleation conditions. N-face high electron mobility transistors fabricated on these low-leakage buffers demonstrated the highest output power density at 4 GHz to date of 8.1 W/mm with an associated power-added efficiency of 54%
Growth of A-plane GaN on (010) LiGaO2 by plasma-assisted MBE
2010-01-01
The (010) surface of LiGaO2 is closely lattice matched to A-plane Formula Not Shown GaN making it an interesting candidate as a substrate for heteroepitaxy of non-polar GaN. We demonstrate successful, first-time growth of A-plane GaN on (010) LiGaO2 using plasma-assisted molecular beam epitaxy. Structural and morphological analysis is performed using X-ray and reflective high energy electron diffraction, scanning electron and atomic force microscopy. Very high phase purity of A-plane GaN is shown. Apart from defects of the epitaxial film originating from substrate scratches, the film is smooth and shows an rms roughness of 10nm over an area of Formula Not Shown .
Doping of GaN by ion implantation: Does It Work?
1998-01-01
Epitaxially grown GaN by metal organic chemical vapor deposition (MOCVD) on SiC were implanted with 100 keV Si+ (for n-type) and 80 keV Mg+ (for p-type) with various fluences from 1 x 1012 to 7 x 10 15 ions/cm2 at liquid nitrogen temperature (LT), room temperature (RT), and 700 C (HT). High temperature (1,200 C and 1,500 C) annealing was carried out after capping the GaN with epitaxial AlN by MOCVD to study damage recovery. Samples were capped by a layer of AlN in order to protect the GaN surface during annealing. Effects of implant temperature, damage and dopant activation are critically studied to evaluate a role of ion implantation in doping of GaN. The damage was studied by Rutherford Backscattering/Channeling, spectroscopic ellipsometry and photoluminescence. Results show dependence of radiation damage level on ...
2010-01-01
We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al x Ga1x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain bu...
Defect reduction in (11-20) a-plane GaN by two step epitaxiallateral overgrowth
2006-11-25
We report a two-step growth method to obtain uniformly coalesced epitaxial lateral overgrown a-plane GaN by metal-organic chemical vapor deposition (MOCVD). By obtaining a large wing height to width aspect ratio in the first step followed by enhanced lateral growth in the second step via controlling the growth temperature, we reduced the tilt angle between the advancing Ga-polar and N-polar wings for improved properties. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing area was 1.0 x 10{sup 8}cm{sup -2}, more than two orders of magnitude lower than that in the window area (4.2 x 10{sup 10} cm{sup -2}). However, a high density of basal stacking faults, 1.2 x 10{sup 4} cm{sup -1}, was still observed in the wing area. Near field scanning optical microscopy (NSOM) at room temperature revealed that the luminescence was mainly from the wing regions with very little contribution from the windows and meeting fronts. These observations suggest that due to significant reduction of threading dislocations radiative recombination is enhanced in the wings.
2010-01-01
We present true green InGaN ridge waveguide (RWG) laser diodes (LDs) at 520 nm on c-plane GaN substrates in pulse operation at room temperature. Defect reduction in the In-rich quantum wells by improving growth conditions of the epitaxial layers is the key parameter to demonstrate laser operation at this wavelength. Carrier lifetime measurements in combination with electroluminescence (EL) data and simulations of competing recombination processes below laser threshold confirm that the reduction of defects in the light emitting InGaN quantum wells is essential to realize true green nitride-based LDs with low laser threshold of 125 mA and good slope efficiency of 220 mW/A for optical output power level up to 50 mW. Investigation of the lateral far field of 520 nm RWG LDs shows a perfect beam...
HIGH-EFFICIENCY NITRIDE-BASED SOLID-STATE LIGHTING
In this second annual report we summarize the progress in the second-year period of Department of Energy contract DE-FC26-01NT41203, entitled ''High- Efficiency Nitride-Based Solid-State Lighting''. The two teams, from the University of California at Santa Barbara (Principle Investigator: Dr. Shuji Nakamura) and Rensselaer Polytechnic Institute (led by Dr. N. Narendran), are pursuing the goals of this contract from thin film growth, characterization, and packaging standpoints. The UCSB team has recently made significant progress in the development of light-emitting diodes (LEDs) with AlGaN active regions emitting in the ultraviolet (UV), resonant-cavity LEDs (RCLEDs), as well as lateral epitaxial overgrowth (LEO) techniques to obtain large-area non-polar GaN films with low average dislocation density. The Rensselaer team has benchmarked the performance of commercially available LED systems and has also conducted efforts to develop an optimized RCLED packaging scheme, including development of advanced epoxy encapsulant chemistries.
HIGH-EFFICIENCY NITRIDE-BASED SOLID-STATE LIGHTING
2003-10-30
In this second annual report we summarize the progress in the second-year period of Department of Energy contract DE-FC26-01NT41203, entitled ''High- Efficiency Nitride-Based Solid-State Lighting''. The two teams, from the University of California at Santa Barbara (Principle Investigator: Dr. Shuji Nakamura) and Rensselaer Polytechnic Institute (led by Dr. N. Narendran), are pursuing the goals of this contract from thin film growth, characterization, and packaging standpoints. The UCSB team has recently made significant progress in the development of light-emitting diodes (LEDs) with AlGaN active regions emitting in the ultraviolet (UV), resonant-cavity LEDs (RCLEDs), as well as lateral epitaxial overgrowth (LEO) techniques to obtain large-area non-polar GaN films with low average dislocation density. The Rensselaer team has benchmarked the performance of commercially available LED systems and has also conducted efforts to develop an optimized RCLED packaging scheme, including development of advanced epoxy encapsulant chemistries.
2009-01-01
Hf-doped Bi4Ti3O12 (BTH) ferroelectric films with excellent electrical properties were epitaxially integrated with GaN semiconductor using (111) SrTiO3 (STO)/rutile (200) TiO2 as buffer layer. The STO/TiO2 buffer layer was deposited by laser molecular beam epitaxy. The structural characteristics of the buffer layer were in situ and ex situ characterized by reflective high energy electron diffraction, x-ray diffraction (XRD), and high resolution transmission microscopy. The overlaying SrRuO3 (SRO) and BTH films were then deposited by pulsed laser deposition. XRD spectra, including theta-2theta and PHI scans, show that the (208) BTH films were epitaxially grown on GaN, and the BTH films inherit the in-plane twin-domain of STO buffer ...
2009-01-01
Epitaxial growth of semiconductor alloys onto a fixed substrate has become the method of choice to make high quality crystals. In the coherent epitaxial growth, the lattice mismatch between the alloy film and the substrate induces a particular form of strain, adding a strain energy term into the free energy of the alloy system. Such epitaxial strain energy can alter the thermodynamics of the alloy, leading to a different phase diagram and different atomic microstructures. In this paper, we present a general-purpose mixed-basis cluster expansion method to describe the thermodynamics of an epitaxial alloy, where the formation energy of a structure is expressed in terms of pair and many-body interactions. With a finite number of first-principles calculation inputs, our method can predict the energies of various atomic structures with an accuracy comparable to that of ...
TEM study of c-plane GaN layers grown on {gamma}-LiAlO{sub 2}(100)
2008-07-01
Freestanding c-plane GaN layers were grown by hydride vapor phase epitaxy on {gamma}-LiAlO{sub 2}(100) substrates. The layers appear transparent and colorless. However, they contain a large number of dodecagonal pinholes (V-pits), which are surrounded by brownish GaN pyramids. It is shown that the V-pit formation occurs at the top of N-polar inversion domains, which appear in the Ga-polar GaN matrix. The GaN layers contain threading dislocations, which are partly attracted to the V-pit surface in order to minimize the dislocation energy. The appearance of brown GaN pyramids around the V-shaped pinholes seems to be a direct consequence of impurity incorporation, which might stem from the LiAlO{sub 2} substrate. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
TEM study of c-plane GaN layers grown on g-LiAlO2(100)
2008-01-01
Freestanding c-plane GaN layers were grown by hydride vapor phase epitaxy on g-LiAlO2(100) substrates. The layers appear transparent and colorless. However, they contain a large number of dodecagonal pinholes (V-pits), which are surrounded by brownish GaN pyramids. It is shown that the V-pit formation occurs at the top of N-polar inversion domains, which appear in the Ga-polar GaN matrix. The GaN layers contain threading dislocations, which are partly attracted to the V-pit surface in order to minimize the dislocation energy. The appearance of brown GaN pyramids around the V-shaped pinholes seems to be a direct consequence of impurity incorporation, which might stem from the LiAlO2 substrate. (Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Selective area growth of GaN nanorods on patterned W/SiO2/Si substrates by RF-MBE
2009-01-01
We report on the selective area growth (SAG) of GaN nanorods on Si substrates masked with W or SiO2 and also on bare Si substrates by RF plasma-assisted molecular beam epitaxy (RF-MBE). The growth of GaN (i.e. irradiation of Ga and RF plasma-activated N2) on the W mask layer results in the appearance of a ring reflection high-energy electron diffraction (RHEED) pattern coming from a-W. In contrast, broken ring RHEED patterns from GaN nanorods are clearly observed on SiO2 and Si surfaces. Ex-situ scanning Auger microscopy analysis confirms that no growth of GaN takes place on W. Utilizing this phenomenon, we have demonstrated the SAG of GaN nanorods on Si substrates partly masked with W. We will discuss this phenomenon in terms of the difference in the desorption energy of Ga on W, SiO2, an...
Effect of stoichiometry on defect distribution in cubic GaN grown on GaAs by plasma-assisted MBE
High resolution electron microscopy was used to study the structure of {beta}-GaN epilayers grown on (001) GaAs substrates by plasma- assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N{sub 2} flow for fixed rf power). The best quality of GaN layers was achieved by ``stoichiometric`` growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the ``major flat`` of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN- GaAs interfaces.
Effect of stoichiometry on defect distribution in cubic GaN grown on GaAs by plasma-assisted MBE
1996-12-31
High resolution electron microscopy was used to study the structure of {beta}-GaN epilayers grown on (001) GaAs substrates by plasma- assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N{sub 2} flow for fixed rf power). The best quality of GaN layers was achieved by ``stoichiometric`` growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the ``major flat`` of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN- GaAs interfaces.
Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD
2008-01-01
We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 mum. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. ...
2010-01-01
The properties of Ga-face and N-face GaN surfaces were studied by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED) as well as high resolution electron energy loss spectroscopy (HREELS). The Ga-face GaN was grown on 6H-SiC(0001) and the N-face GaN directly on a-Al2O3(0001) by plasma assisted molecular beam epitaxy (PAMBE) and in situ analysed by photoelectron spectroscopy. The Ga-face GaN surfaces show a 2 x 2 reconstruction measured by RHEED. From the analysis of the XPS spectra a valence band maximum located 2.9 eV below the Fermi level results. Furthermore, the UPS spectra exhibit two surface states. The N-face GaN presented a 1 x 1 surface with one surface state and a valence band maximum loca...
Al composition dependence of breakdown voltage in Al{sub x}Ga{sub 1-x}N Schottky rectifiers
2000-03-27
Planar geometry, lateral Schottky rectifiers were fabricated on high resistivity Al{sub x}Ga{sub 1-x}N (x=0-0.25) epitaxial layers grown on sapphire substrates. The reverse breakdown voltages of unpassivated devices increased with Al composition, varying from 2.3 kV for GaN to 4.3 kV for Al{sub 0.25}Ga{sub 0.75}N. The reverse current-voltage (I-V) characteristics showed classical Shockley-Read-Hall recombination as the dominant mechanism, with I{proportional_to}V{sup 0.5}. The reverse current density in all diodes was in the range 5-10x10{sup -6} A cm{sup -2} at 2 kV. The use of p{sup +} guard rings was effective in preventing premature edge breakdown and with optimum ring width increased V{sub B} from 2.3 to 3.1 kV in GaN diodes. (c) 2000 American Institute of Physics.
RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers
2009-01-01
Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates were irradiated with 150 MeV Ag ions at a fluence of 5x1012 ions/cm2. Samples used in this study are 2 mum thick GaN layers, with and without a thin AlN cap-layer. Energy dependent RBS/Channeling measurements have been carried out on both irradiated and unirradiated samples for defects characterization. Observed results are compared and correlated with previous HRXRD, AFM and optical studies. The chimin values for unirradiated samples show very high value and the calculated defect densities are of the order of 1010 cm-2 as expected in these samples. Effects of irradiation on these samples are different as initial samples had different defect densities. Epitaxial reconstruction of GaN buffer layer has been attributed to ...
Observation of mid-gap states in GaN with optical-isothermal capacitance transient spectroscopy
1997-12-31
Optical-isothermal capacitance transient spectroscopy (O-ICTS) was used to distinguish the deep levels which occur in unintentionally doped n-type GaN by means of their characteristic optical cross section. GaN grown by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were /compared. Correspondence between optical and thermal emission characteristics of previously discovered levels, E2 ({approximately}E{sub c}-0.55 eV) and E4 ({approximately}E{sub c}-1.0 eV), were clearly determined by observing their sequential appearance in the ICTS spectra. Whether by thermal or optical simulation, the emission from E4 was found to be broad in nature; it is consequently believed to involve a defect. The total measured concentration of deep levels, including a prominent level which photoionizes in the range 2.5 to 3.0 eV below the conduction band, is greater in the GaN grown by MOVPE than by HVPE that was tested.
2004-01-01
Influence of growth conditions in plasma assisted molecular beam epitaxy on quality of GaN layers and GaN/AlGaN heterojunctions is studied. The growth diagram for step-flow growth mode and different nitrogen flux is presented. The low defect density of bulk GaN substrates together with very low impurity background concentrations resulted in high electron mobility for GaN/AlGaN heterojunctions: 109,000 cm{sup 2}/Vs at 1.5 K, and 2500 cm{sup 2}/Vs at 295 K. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Growth mechanism of catalyst-free [0001] GaN and AlN nanowires on Si by molecular beam epitaxy
2010-01-01
Real-time in-situ X-rays scattering experiments were performed to study the nucleation process of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). From the comparison between the case of nanowires and the case of GaN quantum dots, it is concluded that nanowire precursor islands are completely relaxed from the beginning. Next, based on high resolution electron microscopy analysis, it is demonstrated that relaxation of nanowire precursors is associated to formation of dislocations at the AlN/GaN interface, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth. Along the same lines, we demonstrate that three-dimensional islanding of AlN on SiO2 leads to formation of AlN nanowires w...
Epitaxy and strain in the growth of GaN on AlN: A polarized x-ray absorption spectroscopy study
2002-01-01
We present a study of the local structure of 7-8 nm thick GaN epilayers deposited on AlN by molecular beam epitaxy at temperatures ranging between 620 and 790 deg. C by x-ray absorption spectroscopy, exploiting the polarization dependence of the technique. Both the near-edge and extended spectra suggest that atomic intermixing between GaN and AlN takes place at most for one monolayer. An interpretation of the near-edge spectra of wurtzite GaN and of the variation with the sample orientation in the framework of multiple scattering theory is provided. Values of the in-plane and out-of-plane strain due to heteroepitaxial growth are obtained from the interatomic distances in the second coordination shell around Ga. We compare the obtained values to literature values of the elastic constants, finding good agreement with most of them
Epitaxially-Grown GaN Junction Field Effect Transistors
Junction field effect transistors (JFET) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The DC and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (gm) of 48 mS/mm and a maximum source-drain current of 270 mA/mm are achieved on a 0.8 µ m gate JFET device at VGS= 1 V and VDS=15 V. The intrinsic transconductance, calculated from the measured gm and the source series resistance, is 81 mS/mm. The fT and fmax for these devices are 6 GHz and 12 GHz, respectively. These JFETs exhibit a significant current reduction after a high drain bias is applied, which is attributed to a partially depleted channel caused by trapped hot-electrons in the semi-insulating GaN buffer layer. A theoretical model describing the current collapse is described, and an estimate for the length of the trapped electron region is given.
The microstructure of AlN nucleation layers grown on semi-insulating 4H-SiC substrates by plasma-assisted molecular beam epitaxy has a major effect on subsequent GaN and AlGaN/GaN film quality and electronic properties. Characterization by transmission electron microscopy shows that the morphology of the GaN films is critically dependent on the Al to active nitrogen flux ratio used during nucleation layer growth. Structures grown with Al/N flux ratios slightly above approximately 0.98 have many basal-plane stacking faults in the AlN nucleation layer, the GaN/AlN interfaces are wavy and ill defined, and regions of cubic GaN exist close to the GaN/AlN interfaces. In comparison, for structures grown with lower Al/N flux ratios, the AlN nucleation layers have no stacking faults and the GaN/AlN interfaces are sharp. The impact of these changes in microstructure on the electron Hall mobility and buffer leakage are discussed.
Droplet epitaxy of zinc-blende GaN quantum dots
2010-01-01
Zinc-blende GaN quantum dots were grown on 3C-AlN(001) by a vapor–liquid–solid process in a molecular beam epitaxy system. We were able to control the density of the quantum dots in a range of 5×108–5×1012cm−2. Photoluminescence spectroscopy confirmed the optical activity of the GaN quantum dots in a range of 1011–5×1012cm−2. The data obtained give an insight to the condensation mechanism of the vapor–liquid–solid process in general, because the GaN quantum dots condense in metastable zinc-blende crystal structure supplied by the substrate, and not in the wurtzite crystal structure expected from free condensation in the droplet.
In this study, we observed optically pumped stimulated emission at room temperature in quasi-bulk GaN prepared from thick-film GaN grown on a sapphire substrate by using hydride vapor-phase epitaxy and subsequent mechanical removal of the sapphire substrate. The stimulated emission from the surface and 1-mm-wide-cleaved cavity of the GaN was red-shifted compared to the spontaneous emission by increasing the optical pumping-power density, and the full width at half maximum (FWHM) of the peak decreased. The stimulated emission was demonstrated to have a highly TE-mode polarized nature, and the super-linear dependence of the integrated emission intensity on the excitation power indicated a threshold pump-power density of I sub t sub h = 2 MW/cm sup 2 for one set of stimulated emissions.
Analysis of the carbon-related 'blue' luminescence in GaN
2004-09-24
The properties of a broad 2.86 eV photoluminescence band in carbon-doped GaN were studied as a function of C-doping level, temperature, and excitation density. The results are consistent with a C{sub Ga}-C{sub N} deep donor-deep acceptor recombination mechanism as proposed by Seager et al. For GaN:C grown by molecular-beam epitaxy (MBE) the 2.86 eV band is observed in Si co-doped layers exhibiting high n-type conductivity as well as in semi-insulating material. For low excitation density (4 W/cm{sup 2}) the 2.86 eV band intensity decreases as a function of cw-laser exposure time over a period of many minutes. The transient behavior is consistent with a model based on carrier diffusion and charge trapping-induced Coulomb barriers. The temperature dependence of the blue luminescence below 150 K was different for carbon-contaminated GaN grown by metalorganic vapor phase epitaxy (MOVPE) compared to C-doped MBE GaN.
1991-07-01
We present the first comprehensive investigation of the bulk properties, both optical and structural, of cubic GaN as grown by plasma-assisted molecular-beam epitaxy on vicinal (100) GaAs substrates. X-ray measurements determined the crystal structure of GaN/GaAs to be cubic with a lattice constant of 4.5 A. High resolution transmission electron microscopy revealed a high density of planar defects propagating along the GaN {l brace}111{r brace} planes. The majority of the defects originated from disordered regions at the GaN/GaAs interface. The optical properties of the films were investigated by cathodoluminescence which revealed a broad midgap peak as well as several sharp emission peaks just below the expected band gap. The data imply that the room temperature band gap of cubic GaN is approximately 3.45 eV.
Cathodoluminescence studies of defects and piezoelectric fields in GaN
dislocations were observed to generate in-plane dislocation half-loops, which moved under the influence of the stress present. These dislocation half loops were shown to act as non-radiative recombination centres. observed in CL and photoluminescence (PL) experiments. It is suggested that separation across the well of charge carriers of opposite signs screened the piezoelectric field. The blue shift of the QW luminescence was observed to remain after high electron dose irradiation in the SEM. This effect was not observed in PL measurements after high laser power exposure. The CL blue shift could be removed by irradiation with a UV lamp. It was shown that the minority carrier diffusion length (MCDL) increased from 200nm at low electron beam irradiation doses to 330nm after higher dose irradiation in the SEM. It is proposed that the change in the MCDL and the prolonged nature of the blue shift of the QW CL peak can be explained by the filling of trap states in the vicinity of the QW. It is suggested that these trap states take the form of fluctuations in the In content of the QW. An epitaxial laterally overgrown GaN sample (ELOG) was studied using similar techniques employed on the InGaN/GaN SQW. ELOG films are often used commercially as substrates for blue and violet GaN based LDs. The ELOG sample was shown to have a significantly reduced threading dislocation density in the overgrown regions. It was shown that the reduction in dislocation density resulted in an improved luminescence efficiency. The technique was non-ideal however, as the coalescence boundary of the ELO regions was shown to be a tilt-boundary described by the generation of threading dislocations. Some of these High spatial resolution cathodoluminescence (CL) mapping studies were carried out in GaN based material systems similar to those used commercially in the manufacture of light emitting diodes (LED) and laser diodes (LD). The area of prime interest was the effect of defects in these materials, such as dislocations, on the luminescence production. For this purpose a field emission scanning electron microscope (SEM) was modified and calibrated so that cathodoluminescence studies could be carried out at temperatures down to 8K on thinned samples suitable for characterisation by transmission electron microscopy. An InGaN/GaN single quantum well (QW) similar to those used in the fabrication of short-wavelength GaN LEDs was studied. It was demonstrated that threading edge dislocations act as non-radiative recombination centres in this sample. It was estimated from CL maps and from computer simulations that these threading dislocations reduce the overall QW luminescence by at least 60% compared to that from a dislocation free sample. Is has also been shown that 'V-shaped' pits present reduce the QW luminescence production in this sample by a further 10% due to the missing QW material. The presence of a large piezoelectric field across the QW was shown to red shift the QW luminescence peak due to the quantum confined Stark effect (QCSE). It was possible to partially screen this field, under high intensity excitation, producing a blue shift of the QW peak. This blue shift was
Persistent photoconductivity in p-type GaN epilayers and n-type AlGaN/GaN heterostructures
1997-12-31
Persistent photoconductivity (PPC) effect has been observed in p-type GaN epilayers grown both by metal-organic chemical vapor deposition (MOCVD) and reactive molecular beam epitaxy (MBE) as well as in a two-dimensional electron gas (2DEG) system formed by an AlGaN/GaN heterostructure grown by MOCVD. Its properties have been investigated at different conditions.
Multicolored light emitters on silicon substrates
We demonstrate the operation of multicolored light emitting diodes (LEDs) on a silicon wafer using ultraviolet/violet GaN LEDs grown by molecular beam epitaxy on Si(111) in conjunction with organic dye based color converters. These organic converters, applied on top of the nitride diodes, absorb the nitride LED electroluminescence at ~360 nm, and fluoresce in the green (~530 nm), and orange (~600 nm). In this fashion, multicolored light emitters may be made to operate side by side and monolithically on the same Si wafer in an approach that may be attractive for miniature GaN based full color displays.
GaN and InN nanowires grown by MBE: A comparison
2007-06-15
Morphological, optical and transport properties of GaN and InN nanowires grown by molecular beam epitaxy (MBE) have been studied. The differences between the two materials in respect to growth parameters and optimization procedure was stressed. The nanowires crystalline quality has been investigated by means of their optical properties. A comparison of the transport characteristics was given. For each material a band schema was shown, which takes into account transport and optical features and is based on Fermi level pinning at the surface. (orig.)
Europium doping of zincblende GaN by ion implantation
2009-01-01
Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration ( direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal ...
Dislocation-enhanced spin relaxation in wurtzite GaN
2009-01-01
By performing non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial gallium nitride (GaN). Characteristic times of the different spin-relaxation channel (electron, heavy- and light- or split-off-hole spin flips) for different samples as a function of temperature are extracted. We show that the high dislocation density increases the spin relaxation of electrons and holes through the defect-assisted Elliot-Yafet mechanism
Crystalline growth of wurtzite GaN on (111) GaAs
Gallium Nitride films were grown on (111) gallium arsenide substrates using reactive rf magnetron sputtering. Despite a 20% lattice mismatch and different crystal structure, wurtzite. Heteroepitaxy was observed for growth temperatures between 550-600{degrees}C. X-ray diffraction patterns revealed (0002) GaN peak with a full-width-half-maximum (FWHM) as narrow as 0.17{degree}. Possible surface reconstructions to explain the epitaxial growth are presented.
Crystalline growth of wurtzite GaN on (111) GaAs
Gallium Nitride films were grown on (111) gallium arsenide substrates using reactive rf magnetron sputtering. Despite a 20% lattice mismatch and different crystal structure, wurtzite. Heteroepitaxy was observed for growth temperatures between 550-600{degrees}C. X-ray diffraction patterns revealed (0002) GaN peak with a full-width-half-maximum (FWHM) as narrow as 0.17{degree}. Possible surface reconstructions to explain the epitaxial growth are presented.
Crystalline growth of wurtzite GaN on (111) GaAs
1991-12-01
Gallium Nitride films were grown on (111) gallium arsenide substrates using reactive rf magnetron sputtering. Despite a 20% lattice mismatch and different crystal structure, wurtzite. Heteroepitaxy was observed for growth temperatures between 550-600{degrees}C. X-ray diffraction patterns revealed (0002) GaN peak with a full-width-half-maximum (FWHM) as narrow as 0.17{degree}. Possible surface reconstructions to explain the epitaxial growth are presented.
Crystalline growth of wurtzite GaN on (111) GaAs
1991-12-01
Gallium Nitride films were grown on (111) gallium arsenide substrates using reactive rf magnetron sputtering. Despite a 20% lattice mismatch and different crystal structure, wurtzite. Heteroepitaxy was observed for growth temperatures between 550-600{degrees}C. X-ray diffraction patterns revealed (0002) GaN peak with a full-width-half-maximum (FWHM) as narrow as 0.17{degree}. Possible surface reconstructions to explain the epitaxial growth are presented.
Admittance spectroscopy of Mg-doped GaN grown by molecular beam epitaxy using RF nitrogen sources
Thermal activation energies of Mg in GaN grown using RF nitrogen source with varying Mg flux were examined using an admittance spectroscopy technique. There was no noticeable difference or trend in the activation energy with varying Mg flux. The thermal activation energy for GaN:Mg was approx 115 meV under the investigated Mg flux range. Negligible persistent photo-conductivity and yellow luminescence peak in PL observed in the samples suggest possible reduction of the thermal activation energies compared to the values in the literature.
2010-01-01
Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be 28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.
We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-instilating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 365nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x-rays with energies ranging from 60kVp to 90kVp.
We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-instilating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 365nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x-rays with energies ranging from 60kVp to 90kVp.
1995-08-01
We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-instilating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 365nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x-rays with energies ranging from 60kVp to 90kVp.
2010-01-01
We report on the influence of different surface orientations of Si substrates on the properties of GaN-based layer structures grown by metalorganic vapor-phase epitaxy. By using a high-temperature AlN seed layer, a monocrystalline and c-axis-oriented GaN-layer can be obtained on all substrates with Si(110), Si(111), Si(115), Si(117), and Si(119) surface planes. In particular, the samples on Si(110) substrates exhibit a high quality of the GaN layer, which is comparable or even better than that of identically grown test structures on standard Si(111) substrates. This result can be explained by a more suited epitaxial relation between the c-plane of the high-temperature AlN seed layer and the Si(110) surface. The crystallographic structure of approximately 500-nm-thick GaN layers on Si(110) ...
Metallic impurities in gallium nitride grown by molecular beam epitaxy
1997-01-01
Transition metals are often encountered in trace amounts in semiconductors. They have been extensively studied in most elemental and compound systems, since they form deep donor and/or acceptor levels which usually degrade the electronic and optical material properties. Only very little is known about transition metals in recent III-V semiconducting materials, such as GaN, AlN and InN. These few studies have been done exclusively on Metal-Organic Chemical Vapor Deposition (MOCVD) or Hybrid Vapor Phase Epitaxy HVPE-grown GaN. Preliminary x-ray fluorescence studies at the Advanced Light Source, beamline 10.3.1, Lawrence Berkeley National Laboratory have revealed that GaN materials grown by Molecular Beam Epitaxy (MBE) have Fe, Ni and Cr as the dominant transition metal contaminants. This finding is commensurate with the extremely high concentrations of hydrogen, ...
Resistivity control of unintentionally doped GaN films
2005-05-01
GaN epilayers were grown on sapphire substrates via low temperature GaN and AlN nucleation layers (NL) by metalorganic chemical vapor phase epitaxy (MOCVD). The morphology of the individual NLs strongly depends on the carrier gas used during the growth and recrystallization and this is the key factor for control of the resistivity of the GaN layer grown on it. The GaN nucleation layer grown in presence of N{sub 2} has a higher density of islands with a statistically smaller diameter than the samples grown in H{sub 2} atmosphere. The NL grown in N{sub 2} enables the growth GaN with a sheet resistivity higher than 3 x 10{sup 4} {omega}.cm as opposed to a 0.5 {omega}.cm value obtained for the NL grown in H{sub 2}. Introduction of an additional intermediate (IL) low temperature (GaN or AlN) nucleation layer changes the GaN epilayer resistivity to about 50 {omega}.cm, regardless of the carrier gas used during the growth of the IL. Defect selective etching demonstrated that control of the type and density of the dislocations in GaN enables the growth of highly resistive layers without any intentional acceptor doping (Mg, Zn). It will be demonstrated that by changing the ratio of edge type to screw dislocations the resistivity of the layer can be changed by a few orders of magnitude. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Metallic impurities in gallium nitride grown by molecular beam epitaxy
1997-04-01
Transition metals are often encountered in trace amounts in semiconductors. They have been extensively studied in most elemental and compound systems, since they form deep donor and/or acceptor levels which usually degrade the electronic and optical material properties. Only very little is known about transition metals in recent III-V semiconducting materials, such as GaN, AlN and InN. These few studies have been done exclusively on Metal-Organic Chemical Vapor Deposition (MOCVD) or Hybrid Vapor Phase Epitaxy HVPE-grown GaN. Preliminary x-ray fluorescence studies at the Advanced Light Source, beamline 10.3.1, Lawrence Berkeley National Laboratory have revealed that GaN materials grown by Molecular Beam Epitaxy (MBE) have Fe, Ni and Cr as the dominant transition metal contaminants. This finding is commensurate with the extremely high concentrations of hydrogen, carbon and oxygen (up to 10{sup 20} cm{sup {minus}3}) measured by Secondary Ion Mass Spectroscopy (SIMS). Preliminary work using the mapping capabilities of the x-ray fluorescence microprobe revealed the metal impurities were inhomogeneously distributed over the film. Future work of this collaboration will be to find a correlation between the existence of transition metals in MBE films, as revealed by x-ray fluorescence, and Photoluminescence (PL) spectra taken in the infrared region. Also, the authors will make use of the 1 {mu}m spatial resolution of x-ray microprobe to locate the contaminants in relation to structural defects in the GaN films. Because of the large strain caused by the lattice mismatch between the GaN films and the substrates, the films grow in a columnar order with high densities of grain boundaries and dislocations. These structural defects offer preferential sites for metal precipitation or agglomeration which could degrade the optical properties of this material more so than if the impurities were left dissolved in the GaN.
Photoluminescence and Raman studies of GaN films grown by MOCVD
2009-09-01
The photoluminescence (PL) of high quality GaN epitaxial layer grown by MOCVD was investigated for various excitation power and temperatures from 8.3 to 300K. The PL at 8.3 K and with relatively low excitation power of GaN film grown on c-plane sapphire by using MOCVD shows clearly free-exciton A, B and exciton A bound to neutral donors (D{sup 0}X) at 3.502 eV, 3.509 eV, and 3.496 eV, respectively. The full width at half maximum (FWHM) and binding energy of exciton A of the high quality GaN film were evaluated as small as 3.7 meV and 27.9 {+-} 0.5 meV, respectively. In addition, PL and Raman scattering of GaN films grown on r-plane sapphire and (111) Si substrates by using MOCVD were examined. The residual stress effect was detected in all films.
Photoluminescence and Raman studies of GaN films grown by MOCVD
2009-01-01
The photoluminescence (PL) of high quality GaN epitaxial layer grown by MOCVD was investigated for various excitation power and temperatures from 8.3 to 300K. The PL at 8.3 K and with relatively low excitation power of GaN film grown on c-plane sapphire by using MOCVD shows clearly free-exciton A, B and exciton A bound to neutral donors (D0X) at 3.502 eV, 3.509 eV, and 3.496 eV, respectively. The full width at half maximum (FWHM) and binding energy of exciton A of the high quality GaN film were evaluated as small as 3.7 meV and 27.9 +- 0.5 meV, respectively. In addition, PL and Raman scattering of GaN films grown on r-plane sapphire and (111) Si substrates by using MOCVD were examined. The residual stress effect was detected in all films
Nucleation of GaN/AlN quantum dots
We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, bidimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, the height and the density of the islands increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimodal height distribution appears. The variation of island height and density as a function of substrate temperature is discussed in the framework of an equilibrium model for Stranski-Krastanov growth [R. E. Rudd et al., Phys. Rev. Lett. 90, 146101 (2003)].
Nucleation and Growth of GaN/AlN Quantum Dots
2003-11-03
We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, two-dimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, island height and density increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimodal height distribution appears. The variation of island height and density as a function of substrate temperature is discussed in the framework of an equilibrium model for Stranski-Krastanov growth.
2009-01-01
High-quality thick GaN films without cracks were achieved by using a new nozzle structure in the reactor grown by the hydride vapor phase epitaxy on sapphire substrates. Optical contrast microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray double diffraction (XRD) and cathodoluminescence (CL) were carried out to reveal the surface, crystal and optical properties of the GaN epilayer. It was found that the nozzle structure in the reactor has a large effect on the growth rate, surface flat, crystal quality, and the uniformity of the growth. Compared with the old one, the new nozzle structure (denoted as multi-layers nozzle) can improve dramatically the properties of thick GaN. Mirror, colorless and flat GaN thick film was obtained and its (0 0 0 2) FWHM results were reduced from 1000 to 300 arcsec when the new nozzle was used. AFM result revealed a ...
N-plasma assisted MBE grown GaN films on Si(111)
2006-06-15
GaN films were grown by rf-plasma assisted molecular beam epitaxy on nitrified Si(111) surfaces. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the structural and chemical properties of the GaN films. The XPS results show that GaN growth can be initiated only at temperatures below 650 C. LEED indicates an improvement of the crystalline quality after introduction of a crystalline Si{sub 3}N{sub 4} interface layer. This is confirmed by STM, where an atomically resolved 3 x 3 reconstruction for thin GaN films is observed as well as a smooth growth morphology. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Depth profile high-energy spectroscopic study of Mn-doped GaN prepared by thermal diffusion
2007-03-16
We have performed an $in$-$situ$ depth profile study of Mn-doped GaN prepared by a low temperature thermal diffusion method using photoemission and x-ray absorption spectroscopy. It was revealed from the core-level photoemission measurements that Mn ions are diffused into a deep ($\\sim$ 70 \\AA) region of the GaN substrates and that the line shapes of Mn 3$d$ partial density of states obtained by resonant photoemission measurements was close to that of Ga$_{1-x}$Mn$_x$N thin films grown by molecular-beam epitaxy. From x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Mn $L$-edge, it was revealed that the doped Mn ions were in the divalent Mn$^{2+}$ state and primarily paramagnetic. In magnetization measurements, weak hysteresis was detected in samples prepared using $p$-type GaN substrates while samples using $n$-type GaN substrates showed only paramagnetism.
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