C. H. Wei

The role of trimethylgallium flow during nucleation layer deposition in the optimization of epitaxial GaN films

Abstract The effects of the trimethylgallium flow (14–55 μmol/min) during the deposition of the GaN nucleation layer on the structure and electronic properties of GaN epilayers were examined. X-ray and mobility studies indicate that GaN epilayers, grown using non-optimal trimethylgallium (TMG) flow, result in wide FWHM peak and low electron mobility. On the contrary, an optimal TMG flow during the nucleation layer growth leads to films with superior structural and electronic properties. Atomic force microscopy (AFM) was used to systematically investigate the morphological evolution of as-grown nucleation layers, and the nucleation layers were heated to 1000°C under different TMG flows.

Effects of Surface Preparation on Epitaxial GaN on 6H-SIC Deposited Via Mocvd

A comparison was made of 6H-SiC surfaces etched with H 2, C2H4/H2, and HCl/H2, and the resulting crystal quality of epitaxial GaN films deposited on these substrates. To remove the many fine scratches and to smooth the rough surfaces typical of commercial SiC substrates, the Si-face 6H-SiC substrates were etched in H 2, C2H4/H2, and HCl/H2 at 1450 C. GaN was subsequently deposited on these etched surfaces after first depositing a low temperature GaN buffer layer via metalorganic chemical vapor deposition (MOCVD). The surface morphologies after etching and after GaN deposition were characterized by atomic force microscopy and Normaski differential interference contrast microscopy, while the crystal quality of the GaN films was assessed by double crystal x-ray rocking curves and x-ray topography. 6H-SiC substrate surfaces were improved in terms of the removal of scratches and the reduction of surface roughness, and both surface morphology and crystal quality of the subsequently deposited GaN films were enhanced. However, the dislocation density was not decreased by the surface etching. The best GaN film was produced by etching the substrate in pure H 2 fo 40 minutes before growth. Recommendations for the optimum substrate treatment are made.

Growth and Characterization of B x Ga l-x N on 6H-SiC (0001) by MOVPE

Boron was incorporated into GaN in order to determine its limits of solubility, its ability of reducing the lattice constant mismatch with 6H-SiC, as well as its effects on the structural and optical properties of GaN epilayers. B x Ga l-x N films were deposited on 6H-SiC (0001) substrates at 950 °C by low pressure MOVPE using diborane, trimethylgallium, and ammonia as precursors. A single phase alloy with x=0.015 was successfully produced at a gas reactant B/Ga ratio of 0.005. Phase separation into pure GaN and B x Ga l-x N alloy with x=0.30 was deposited for a B/Ga reactant ratio of 0.01. This is the highest B fraction of the wurtzite structure alloy ever reported. For B/Ga ratio ≥ 0.02, no B x Ga l-x N was formed, and the solid solution contained two phases: wurtzite GaN and BN based on the results of Auger and x-ray diffraction. The band edge emission of B x Ga l-x N varied from 3.451 eV for x=0 with FWHM of 39.2 meV to 3.465 eV for x=0.015 with FWHM of 35.1 meV. The narrower FWHM indicated that the quality of GaN epilayer was improved with small amount of boron incorporation.