Reconstructions and origin of surface states on AlN polar and nonpolar surfaces

The AlN (0001), (000 (1) over bar), (10 (1) over bar0), and (11 (2) over bar0) surfaces and their electronic structures are studied based on density-functional theory using the generalized gradient approximation as well as the hybrid functional approach. The stable reconstructions generally satisfy the electron-counting rule, except for cases where Al adlayers are present. We find that the transitions between different reconstructions exhibit a distinct trend for group-III nitrides. For all surfaces, Al dangling-bond states tend to be close to the conduction-band minimum (CBM) and N dangling-bond states close to the valence-band maximum (VBM). Al-N bonding states also occur near the VBM, while Al-Al bonding states occur in the middle of the gap. We find that Al dangling-bond states on the Al-polar (0001) surface can pin the Fermi level of n-type AlN at 1.0 eV below the CBM at moderate Al/N growth ratios. At high Al/N ratios, metallic Al adlayers form which pin the Fermi level in the middle of the gap. The lack of a surface donor state in the upper part of the gap suggests that the surface states on clean AlGaN surfaces are unlikely to be the source of carriers in the two-dimensional electron gas in AlGaN/GaN high-electron-mobility transistors.