Electron scattering mechanisms at polar GaN/AlGaN interfaces

We present recent results of calculations of charge transfer and electron mobilities in nominally undoped AlGaN/GaN heterostructures. It has previously been proposed that the two-dimensional electron gas (2-DEG) originates from donor-like defects on the surface of the AlGaN barrier. We have made detailed calculations of a model in which these defects are created under thermodynamic equilibrium at the growth temperature and show that the spontaneous and strain-induced piezoelectric fields in the AlGaN barrier enhance the formation of these defects. In calculating the low temperature electron mobility in these structures, we consider all the major scattering mechanisms including acoustic phonons, Coulomb scattering from charged centers, and alloy disorder scattering. The relative importance of the different scattering mechanisms depends strongly on the 2-DEG density. At densities smaller than about 2×10¹² cm^-2, the mobility is limited by Coulomb scattering. At higher densities, alloy disorder scattering becomes the dominant electron scattering process. Finally, we have calculated the ratio of the transport to quantum lifetimes τ_t/τ_q for various AlGaN/GaN heterostructures and find that the value of the ratio cannot be used to infer the nature of the dominant scattering mechanism, as is traditionally assumed.