Temperature-dependent Hall-effect measurements in hydride vapor phase epitaxial GaN grown on sapphire can be well fitted over the temperature range 10–400 K by assuming a thin, degenerate n-type region at the GaN/sapphire interface. This degenerate interfacial region dominates the electrical properties below 30 K, but also significantly affects those properties even at 400 K, and can cause a second, deeper donor to falsely appear in the analysis. However, by using a two-layer Hall model, the bulk mobility and carrier concentration can be accurately ascertained.
Look, D. C.,
& Molnar, R. J.
(1997). Degenerate Layer at GaN/Sapphire Interface: Influence on Hall-Effect Measurements. Applied Physics Letters, 70 (25), 3377-3379.