Electronic surface states on clean and oxygen‐exposed GaAs surfaces

Electron‐energy‐loss measurements are reported for the polar {111} and (100) surfaces, as well as for the nonpolar (110) surface. The surface composition of the polar faces was changed by various annealing treatments. Surfaces with unsaturated Ga bonds exhibit empty surface states 0.9±0.2 eV above the bulk valence band edge. Their origin is attributed to the dangling Ga bonds. Dangling As bonds, if present, do not contribute to any surface states in the band‐gap region, but seem to be responsible for filled states 0.5–2 eV below the valence band edge—the value depending on the particular surface. An energy‐level scheme of surface states is proposed for the various surfaces. Room temperature oxidation saturates the dangling Ga bonds and eliminates the empty, intrinsic surface states for coverages as low as 0.3 of the saturation coverage. The oxygen is chemically bonded only to the Ga atoms—the As atoms do not seem to play a direct role in the oxidation process. The nature of the bonding seems to be identic...