Evidence of an impurity band at an n-GaN/sapphire interface

Abstract

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on a silicon-doped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that close to the interface, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Over the whole temperature range, the mobility does not change appreciably for the first 1.35 μm of removed doped material, but then falls rapidly over the remaining undoped region. Using a model based upon one donor, a deep acceptor and one conducting layer, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors were also found.