Silicon on GaN(0001) and (000 ) Surfaces

Surface reconstructions and adatom kinetics of silicon on GaN(0001) and (000 ) surfaces are studied by scanning tunneling microscopy, electron diffraction and first-principles calculations. In the low silicon coverage regime a 2×2 structure is observed, and is interpreted in terms of a model consisting of a Ga adatom on a monolayer of 3Ga + 1 Si and a SiGa atom in the third layer. For high silicon coverage a 4×4 structure appears containing disordered, partially 2×2 and “1×1” domains. After annealing above 300 C the “1×1” region become dominant and the 4×4 region is seen only near step edges. It is concluded that the silicon adatoms tend to reside in subsurface sites on the Ga-polar surface. Surface morphology in the presence of Si is smooth for the (0001) surface but rough for the (000 ) surface. This difference is attributed to the presence of multiple Ga surface layers in the former case, which enhance surface diffusivities.