Yu.V. Melnik

Properties of Free-Standing GaN Bulk Crystals Grown by HVPE

GaN wafers 200 μm thick and 30 mm diameter were fabricated. GaN was grown by hydride vapor phase epitaxy on SiC substrates and removed from the substrate by reactive ion etching. Lateral size of the GaN wafers was equal to the size of the initial SiC substrates. GaN wafers were cleaved in pieces and these pieces were characterised. It was found that after the fabrication, GaN crystals were slightly deformed and strained. An anneal at 830°C in nitrogen ambient eliminated the residual strains. The FWHM of ω-scan (0002) x-ray rocking curve for annealed crystals was less than 140 arcsec for both sides of the best GaN crystals. The lattice constants measured from both sides of the crystals were c =5.1853±0.0003 A and a = 3.1889±0.0001 A. The N d – N a concentration determined by a mercury probe was about 2×10 17 cm −3 for as-grown GaN surface and about 2×10 19 cm −3 for former interface surface. Photoluminescence spectrum taken at 17 K revealed an edge peak at 3.472 eV with the FWHM value of 2.3 meV. A ratio of the edge peak intensity to the intensity of yellow band was higher than 1000. Initial TEM experiments were performed.

Physical Properties of Bulk GaN Crystals Grown by HVPE

Free standing GaN platelets were fabricated by hydride vapor phase epitaxy (HVPE). The platelets having a current maximum size of 7×6×0.1 mm 3 were obtained by HVPE growth of ~100 μm thick GaN layers on SiC substrates and subsequent removal of the substrates by reactive ion etching (RIE). Surface of the GaN platelets was characterized by reflectance high energy election diffraction (RHEED), and Auger electron spectroscopy (AES). Crystal structure and optical properties of the platelets were studied by x-ray diffraction and photoluminescence (PL), respectively. Raman spectroscopy was also applied for material characterization. Residual strain was detected in the crystals. The stress was eliminated by high temperature anneal.

Structural properties of GaN grown on SiC substrates by hydride vapor phase epitaxy

Abstract Hydride vapor phase epitaxy (HVPE) was used for GaN deposition on SiC substrates without a buffer layer. The influence of substrate parameters (structural perfection, surface polarity, misorientation) on the structural properties of GaN layers was investigated. X-ray differential diffractometry (XRDD) and the triple crystal modification of the Bond method were used to study the structural properties. The X-ray mapping was done over a 30 mm diameter 6H-SiC substrate. It was shown, that the full width at half maximum (FWHM) of X-ray ω-scan rocking curves (RC) from GaN layers monotonically increases as the FWHM of the RC from the substrate increases.

Residual strains in GaN grown on 6H-SiC

Abstract The triple crystal modification of Bond method was used for lattice constant measurements and for the study of residual strains in GaN layers grown on 6H-SiC (0001) substrates. GaN layers grown by MOCVD employing AlN and AlGaN buffer layers and GaN layers grown by HVPE without buffer layer were investigated. It was found that the residual strains in GaN were considerably reduced by use of the AlGaN buffer layer. The dependence of residual strains on thickness and composition of buffer layer could be explained by the different degree of relaxation mismatch stresses and change of thermal stresses in GaN layers, grown on SiC with different buffer layers.

GaN PN-Structures Grown by Hydride Vapor Phase Epitaxy

For the first time, GaN pn-junctions were fabricated by hydride vapor phase epitaxy. GaN pn-structures were grown directly on 6H-SiC substrates without any buffer layer. Undoped GaN layers were n-type with N d -N a concentration ranged from 1×10 17 to 5×10 18 cm −3 . Magnesium was used as an acceptor to grow p-type GaN layers. Mg atomic concentration determined by secondary ion mass spectroscopy ranged from 5×10 19 to 5×10 20 cm −3 . As-grown GaN layers doped with Mg were p-type, and p-type conductivity was improved by post-growth anneal. Mesa diodes with a vertical current flow geometry were formed by reactive ion etching. The position of the GaN pn-junction was determined by the electron beam induced current method. The electrical characteristics of the pn diodes were studied. Electroluminescence from the pn diodes was measured.

Characterization of Thin GaN Layers Deposited by Hydride Vapour Phase Epitaxy (HVPE) on 6H- SiC Substrates

In this study, the near-surface regions of air-exposed thin GaN layers deposited by hydride vapour phase (HVPE) epitaxy on 6H-SiC substrates have been examined. Chemical-state identification and in-depth elemental distribution profiles are evaluated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and secondary ion mass spectroscopy (SIMS). The epilayers show a high degree of chemical purity as determined by XPS and SIMS. Low temperature photoluminescence (PL) were performed and is dominated by donor-acceptor pairs (DAP) emission. Layer thickness was measured to be ∼ 600-700 nm and an abrupt, well-defined heterointerface is observed using scanning electron microscopy (SEM) and SIMS.