Influence of AlN nucleation layers on growth mode and strain relief of GaNgrown on 6H-SiC(0001)

We study the growth mode and strain state of GaN layers grown either direct ly on 6H- SiC(0001) or on thin (5 nm), coherently strained AlN nucleation l ayers. Using a combination of structural, optical, and vibrational characte rization methods, we show that the 3.4% compressive lattice mismatch strain is fully relieved in the former case, whereas in the latter case a signifi cant amount (0.3%) remains even after 1 mu m of growth. This finding is cla rified by in situ reflection high-energy electron diffraction and transmiss ion electron microscopy. We demonstrate that the strain state of the GaN la yer is determined by its growth mode, which in turn is governed by the degr ee of wetting of the underlayer rather than by lattice mismatch. (C) 1999 A merican Institute of Physics. [S0003-6951(99)00324-1].