Polarization properties of AlN (1010) and (1120) non-polar surfaces: Maximally localized Wannier functions study

AlN (1010) and (1120) surfaces are usually considered to be non-polar facets. However, polarization may be induced in the surface due to the surface dangling bonds. In this work, using the average local dipole evaluated by maximally localized Wannier functions, we investigated polarization properties of these two surfaces layer-by-layer. It is found that a remarkable polarization which is more than eight times that of the bulk, is induced on these two surfaces. Moreover, we found that the polarization oscillates around the bulk value in the inner surface layers, and quickly approaches to the bulk value at a depth of four layers. Such polarization variation was further analyzed from the viewpoints of atomic rearrangement and charge transfer. Thus (1010) and (1120) surfaces are actually not completely non-polar facets, but rather non-polar bulk core combined with polar surface layers which would potentially affect the surface properties. The present study may provide a deeper understanding of the non-polar AlN surfaces.