Influence of the AlGaN buffer layer on the biaxial strain of GaN epilayers grown on 6H-SiC (0001) by molecular-beam epitaxy

The role of AlxGa1-xN buffers in the control of residual strain in GaN epitaxial layers grown on 6H-SiC (0001) substrates by molecular-beam epitaxy was investigated. The initial GaN layer on the AlxGa1-xN (x=0%-10%) buffer nucleates in two-dimensional form, which promotes a step-flow growth mode in subsequently grown GaN layers. As the Al content in the AlxGa1-xN (x∼20%) buffer was increased, GaN began to nucleate as incoherent polygonal islands, coalescing as the thickness increased to 15 nm. The overall residual strain induced in GaN is tensile (εxx) for GaN grown directly on SiC and compressive (-εxx) for GaN grown using an AlGaN buffer. The stiffness coefficient (-2C13/C33) of GaN on the AlxGa1-xN (x∼10%) buffer was estimated to be -0.53 assuming that the defect-induced hydrostatic stress of unstrained GaN (c0/a0) was constant