Electronic structure of nitride surfaces

Knowledge of surface reconstructions and the corresponding surface electronic structure is important to control growth, since Fermi-level pinning can affect defect creation and incorporation. In addition, surface states can play an important role in devices, for instance in high-electron mobility transistors where the surface acts as a source of electrons for the channel. In the case of InN a very high, and thus far unexplained, electron accumulation has been observed on all polar surfaces. We have addressed these issues by performing a systematic computational study of reconstructed GaN and InN surfaces in various orientations, including (11-20) ($a$ plane) and (10-10) ($m$ plane), as well as the polar (0001) (+$c)$ and (000-1) (-$c)$ planes. The calculations are based on density-functional theory, combined with an extensively tested approach for correcting the band-gap error through use of modified pseudopotentials. For GaN we identify the microscopic origins of the experimentally observed Fermi-level pinning. For InN we find that on polar surfaces occupied surface states occur above the conduction-band minimum, thus explaining the observed electron accumulation. We predict an absence of electron accumulation on \textit{nonpolar} surfaces grown at moderate In/N ratios.