Structural quality and ordering of MBE grown AlxGa1−xN-layers

Epitaxial AlxGa1−xN-layers (x=1–0) with the hexagonal wurzite structure were grown on 6H-SiC- and α-Al2O3-substrates for the development of semiconductor UV detectors. The experiments were performed in a plasma enhanced molecular beam epitaxy system (MBE) by varying substrate temperature, III/V ratio and growth rates. A detailed analysis of dislocations was performed for pure AlN-layers by RBS-channeling and was compared to the analysis of cross sectional and plan view TEM images. The annihilation of threading dislocations during the growth process was observed by RBS-channeling depending on the type of substrate and the growth mechanism. Lowest dislocation densities are obtained for 2D-growth on SiC substrate in the range of 2×108 cm−2. The decrease of the growth temperature from 1000 to 900°C lead to an decrease of the dislocation density by about an order of magnitude. Depending on the growth conditions chemical ordering was observed for ternary AlxGa1−xN-layers. The space group symmetry P63mc of the wurzite structure is reduced to P3m1 due to the ordering of the group III sublattice into alternating aluminium and gallium layers. The phenomenon was studied by XRD-measurements of the symmetric superlattice peaks.