Measurement of the variable surface charge concentration in gallium nitride and implications on device modeling and physics
We have evaluated the density of interface trap states (Dit) at the surface of a GaN/AlGaN/GaN heterojunction by the previously described gated van der Pauw experiments, as well as by a UV assisted gated van der Pauw method, described in this article. The obtained Dit values are about two orders of magnitude lower than assumed by the surface-donor theory and three orders of magnitude lower than required to compensate the polarization surface charge in GaN. Previous experimental studies using a variety of other techniques reported similarly low Dit values. We hence conclude that variable midgap surface-charge is not responsible for the formation of the two-dimensional electron gas, and cannot compensate for the large surface polarization charge in GaN. A yet unexplained polarization self-compensating (PSC) surface charge must be invoked to account for experiments. A few comments about the physical nature of the proposed PSC charge are provided.