Crystalline Quality Of GaN Layer Research Articles

Improvement in the crystalline quality of GaN and defects analysis using cathodoluminescence

The crystalline quality of GaN layer grown on 4H-SiC substrate using Metal Organic Chemical Vapor Deposition (MOCVD) has been improved by reducing the growth rate of GaN. Depth resolved CL (cathodoluminescence) study was also done to analyze the defects in the GaN layers. Two samples, one with fast growth rate (∼0.7 nm/s; Sample-A) of GaN and other with slower growth rate (∼0.2 nm/s; Sample-B) were grown. The surface morphology and crystalline quality of GaN epi-layers have been studied by Atomic Force Microscopy (AFM) and High Resolution X-Ray Diffraction (HRXRD), respectively. It was observed that growth rate of the GaN layer strongly influenced its crystalline quality and surface morphology. The GaN layer grown with slow growth rate exhibited improved surface morphology with lower rms roughness of ∼0.18 nm and lower surface pits density. X-ray rocking curve measurement for (0 0 2) and (1 0 2) planes showed that both screw and edge dislocation densities were reduced for the GaN grown with slow growth rate. Depth profiling by CL spectroscopy revealed intense Yellow (YL) and Blue luminescence (BL) band in Sample-A, while in Sample B, the YL and BL bands were much weaker. The higher yellow and blue luminescence in the GaN grown at higher growth rate may be correlated with nitrogen vacancies (VN) and two charged state of CN(−/0 and 0/+) acceptor.

Influence of the AlN nucleation layer on the properties of AlGaN/GaN heterostructure on Si (1 1 1) substrates

AlGaN/GaN heterostructures were grown on Si (1 1 1) substrates with different AlN nucleation layers (NL) by metal–organic chemical vapor deposition (MOCVD). The results indicate that the growth temperature of AlN NL has a noticeable influence on the structural, electronic and optical properties of the AlGaN/GaN heterostructures. Optimizing the growth temperature to 1040 °C led to quasi-2D smooth surface of the AlN NL with providing sufficient compressive stress to suppress cracking of the subsequent GaN layer during the cooling process, resulting in improved crystalline quality of GaN layer and superior two-dimensional electron gas (2DEG) performance of the AlGaN/GaN heterostructure.

Structural properties of GaN layers grown on Al2O3 (0001) and GaN/Al2O3 template by reactive radio-frequency magnetron sputter epitaxy

GaN single-crystalline layers were grown by reactive radio-frequency magnetron sputter epitaxy using N2/Ar ambient gas and Ga target. The GaN layers were grown directly on Al2O3 (0001) substrates and GaN/Al2O3 templates. The crystalline quality and the surface morphology of GaN layer depending on substrate temperature and N2 composition ratio, were measured. The crystalline quality of GaN layer was improved by increasing substrate temperature and also decreasing N2 composition ratio. The full-width at half-maximum (FWHM) value of X-ray rocking curve (XRC) of GaN layer grown on Al2O3 at 900 °C and 6% N2, was 27 and 338 arcsec for highly c-axis oriented columnar domains and disordered structure inside the GaN layer, respectively, for (0002) plane. The FWHM values of XRCs of the GaN layers on GaN/Al2O3 for both (0002) and (101̅2) planes, decreased with the increase of substrate temperature, and at above 890 °C they were similar to those of the metal-organic vapor phase epitaxy-grown GaN template, which were approximately 300 and 310 arcsec, respectively. The threading dislocation densities were also estimated using the FWHM values of XRCs, and those of GaN layers on GaN/Al2O3 were considerably decreased as compared with those on Al2O3 at substrate temperatures more than 890 °C.

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.

Low Temperature Growth of GaN Epitaxial Layers on Sapphire (0001) by Pulsed Laser Deposition Using Liquid Gallium Target

Smooth surface GaN epilayers have been grown on sapphire (0001) by pulsed laser ablation of liquid Ga target under radio frequency assisted atomic nitrogen ambient. Laser energy density, laser frequency and substrate-to-target distance were adjusted to optimize the Ga flux for a fixed nitrogen plasma condition. From in-situ reflection high energy electron diffraction and atomic force microscopy analyses, it is found that the Ga/N flux ratio plays a crucial role in determining the growth mode and that only the Ga-rich condition promotes a two dimensional GaN growth. Also, the increase of Ga flux by increasing the laser energy density above 8 J/cm(2) is observed to produce Ga droplets on the grown GaN surface even if the growth front is excess with nitrogen flux. Instead, the increase of laser frequency (up to 50 Hz, in this study) is preferable. The effect of growth temperature on the crystalline quality of GaN layers has also been investigated in the range from 300 to 700 degrees C. Highly c-axis oriented GaN layers, with a (0002) plane X-ray rocking curve full width at half maximum of 245 arcsec, have been obtained at 500-600 degrees C, which is about 150 degrees C lower than the conventional MBE growth.

High power AlGaN/GaN HFETs on 4 inch Si substrates

AbstractIn this paper, we successfully demonstrate an AlGaN/GaN HFET with a high breakdown voltage on 4‐inch Si substrates. In order to obtain the high breakdown voltage and to improve the crystalline quality of GaN layers, a thick GaN epitaxial layer including a buffer layer was grown. The breakdown voltage and the maximum drain current were achieved to be over 1.3 kV and 120 A, respectively. Furthermore, the suppression of a current collapse phenomenon was examined. The on‐resistance was not significantly increased up to a high drain off‐bias‐stress of 900 V. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy

Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al 2O 3 (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N 2 composition ratio in N 2/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N 2 composition ratios, and the GaN layer grown at 30% N 2 condition was transparent and colorless. Al x Ga 1− x N layers with x = 0.06–0.08 and In x Ga 1− x N layers with x = 0.45–0.5, were obtained at 30–40% and 30–50% N 2 composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys ( x = 0.15 in both Al x Ga 1− x and In x Ga 1− x alloys).

Nanoheteroepitaxial lateral overgrowth of GaN on nanoporous Si(111)

Nanoheteroepitaxial (NHE) lateral overgrowth of GaN on nanoporous Si(111) substrates has been demonstrated. Nanopore arrays in Si(111) surfaces were fabricated using anodized aluminum oxide templates as etch masks, resulting in an average pore diameter and depth of about 60 and 160–180nm, respectively. NHE growth of AlN and GaN was found to result in a significant reduction in the threading dislocation density (<108cm−2) compared to that on flat Si(111). Most dislocations that originate at the Si interface bent to lie in the GaN (0001) basal plane during lateral growth over the pore openings. E2 phonon blueshifts in the Raman spectra indicate a significant relaxation of the tensile stress in the coalesced GaN films, due to three-dimensional stress relaxation mechanisms on porous substrates. Our results show that a single step lateral overgrowth of GaN on nanopatterned Si(111) substrates without a dielectric mask is a simple way to improve the crystalline quality of GaN layers for microelectronic applications.

Effect of the Parameters of Sapphire Substrates on the Crystalline Quality of GaN Layers

The effect of certain types of sapphire substrate treatment on the properties of gallium nitride layers grown by metal-organic vapor-phase epitaxy is investigated.

Effects of 6H-SiC surface reconstruction on lattice relaxation of AlN buffer layers in molecular-beam epitaxial growth of GaN

Growth of GaN on on-axis 6H-SiC (0001)Si substrates with an AlN buffer layer was performed by molecular-beam epitaxy. The effects of SiC surface reconstruction on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers were studied. High-temperature HCl-gas etching followed by HF chemical treatment resulted in an atomically flat SiC surface with a 1×1 structure. The AlN layer grown on the surface showed slow lattice relaxation. GaN grown on the AlN buffer layer exhibited the narrowest (0002) x-ray rocking curve of 70 arcsec and 107 cm−2 screw-type dislocation density, which was two orders of magnitude smaller than that of GaN grown on as-received substrates.

Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN

ABSTRACTThe effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.

Effects of thermal treatment of low-temperature GaN buffer layers on the quality of subsequent GaN layers

The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are proposed and discussed.