Electronic states at aluminum nitride (0001)-1×1 surfaces

We investigate the electronic structure of aluminum nitride (0001)-1×1 surfaces via direct and inverse photoemission spectroscopy. Bulk and surface sensitive measurements on clean surfaces and surfaces exposed to oxygen or cesium demonstrate the existence of filled and empty surface states which extend more than 1 eV beyond the valence- and conduction-band edges. The filled states are tentatively associated with Al dangling or back bonds. The measurement of the top of the valence band upon removal of the filled states leads to a determination of an electron affinity equal to 1.9±0.2 eV. The empty surface states are presumed to play a role in the pinning of the Fermi level in the upper part of the gap and are consistent with the anticipated metallicity of the surface.