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 (D-it) 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 paper. The obtained D-it values are about two orders of magnitude lower than the assumed by the surface-donor theory and three orders of magnitude lower than the required to compensate the polarization surface charge in GaN. Previous experimental studies using a variety of other techniques reported similarly low D-it values. We, hence, conclude that the variable midgap surface charge is not responsible for the formation of the 2-D 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.