Structural and morphological properties of GaN buffer layers grown by ammonia molecular beam epitaxy on SiC substrates for AlGaN/GaN high electron mobility transistors

The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH(3):Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH(3):Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of similar to 1.0 nm over 2x2 mu m(2) atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 mu m/h were achieved. TD densities in the buffers as low as 3x10(9) cm(-2) were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of >11 W/mm at 4 and 10 GHz. (C) 2008 American Institute of Physics.