Measurement of the variable surface charge concentration in gallium nitride and implications on device modeling and physics

TL;DR

This study measures the interface trap density on GaN surfaces using advanced gated van der Pauw techniques, revealing much lower trap densities than previously assumed, challenging existing theories on surface charge compensation in GaN devices.

Contribution

The paper introduces a novel UV-assisted gated van der Pauw method to accurately measure interface trap densities on GaN surfaces, providing new insights into surface charge phenomena.

Findings

Measured Dit values are two orders of magnitude lower than surface-donor theory predictions.

Variable midgap surface charge does not account for 2DEG formation in GaN.

A polarization self-compensating surface charge likely explains experimental observations.

Abstract

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.