Disorder-induced electron localization and electronic states coupling in nitride semiconductors

The electronic states of surface oxidized nitride semiconductors (GaN, AlGaN, and AlN) are investigated by valence electron energy-loss spectroscopy. It is shown that the loss of long-range atomic order in the surface oxide produces localized electronic states below the surface states in the bandgap. Using an off-axis electron probe, the coupling between local electronic states and localized states in the amorphous layers of GaN, AlGaN, and AlN is studied. Regarding the excitation of surface states under aloof conditions via long-range electrostatic forces, the surface and the localized states are independent of the electron scattering vector. Interestingly, though the surface sub-bandgap transitions are directionally forbidden, localized states show dispersion due to states coupling between bulk electronic states and localized states. These findings challenge the conventional model of built-in potential barriers at surface oxides and should be useful in understanding the local electrical properties of nano-scale structures.