Electron mobility in very low density GaN/AlGaN/GaN heterostructures

We report on the transport properties of a two-dimensional electron gas (2DEG) confined at the lower interface of a GaN/Al0. 06Ga0.94N/GaN heterostructure grown by molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy. Using an insulated gate Hall bar structure, the electron density is continuously tuned from ~ 2x1012cm- 2 down to 1.5x1011cm-2. At T=300mK, the 2DEG displays a maximum mobility of 80,000cm2/Vs at a sheet density of 1.75x1012cm- 2. At low densities, the mobility exhibits a power law dependence on density- m ~ nea, with a ~ 0.83, over the range of 2x1011cm- 2 to 1x1012cm-2. In this density regime, the mobility is no longer limited by alloy scattering and long-range Coulomb scattering dominates. The 2DEG remains metallic at a density of 1.5x1011cm- 2 at T=300mK. We discuss the dominant scattering mechanisms that presently limit low temperature mobility at electron densities below 1x1012cm-2.