Indium segregation mechanism and V-defect formation at the [0001] InAlN surface: an ab-initio investigation

First-principle calculations were performed to investigate adsorption and diffusion of indium and aluminum atoms on (0001) and (0001) In (18%) AlN surfaces. First, it was shown that these surfaces are most stable when they contain complex defects. The presence of vacancies causes the In to be strongly bound to the surface with the adsorption energy increasing by 0.11 eV for metal-polar and by 0.78 eV N-polar. In contrast, the adsorption strength of Al to the surface with defects decreases; the corresponding energy goes from 3.96 eV-2.29 eV (metal-polar) and from 8.30 eV-5.05 eV (N-polar). Simultaneously, the diffusion of In is enhanced; its energy barrier decreases by 0.74 eV (0.06 eV) for the N-polar (metal-polar) InAlN surface, whereas that of the Al adatom increases by 0.32 eV for metal-polar (0.08 eV for N-polar), which should limit its diffusion on the surface. Therefore, the indium atoms will tend to migrate towards the complex defects. Eventually, during epitaxial growth, this aggregation of indium atoms around the defects and the low mobility of Al atoms could be the origin of the observed V defects, the phase separation and the crystallographic degradation of the InAlN epitaxial layers with increasing thickness.