  We report on our investigation of the electrical properties of
metal/Al2O3/GaN metal-insulator-semiconductor (MIS) capacitors. We determined
the conduction band offset and interface charge density of the alumina/GaN
interface by analyzing capacitance-voltage characteristics of atomic layer
deposited Al2O3 films on GaN substrates. The conduction band offset at the
Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical
predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band
conditions in the GaN was inferred, which we attribute to a fixed net positive
charge density of magnitude 4.60x1012 cm-2 at the Al2O3/GaN interface. We
provide hypotheses to explain the origin of this charge by analyzing the energy
band line-up.
