Transport properties of two-dimensional electron gases induced by spontaneous and piezoelectric polarisation in AlGaN/GaN heterostructures

We have performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al_xGa_l-xN/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n_s = 1.05 × 10¹³ cm^-2. From SdH measurements of 2DEGs with sheet carrier concentrations of 2.1 × 10¹² and 4.6 × 10¹² cm^-2, effective electron masses were determined to be 0.24 and 0.207m₀, respectively. In addition, angle resolved SdH measurements were performed to evaluate the effective g-factor from the angle of zero oscillation amplitude. Including the measured electron masses, the g-factors were calculated to be 2.11 and 2.47, respectively. In order to identify the main electron scattering mechanism we determined the ratio between transport-and quantum scattering times as a function of sheet carrier concentration. We observe a significant decrease of the τ_t/τ_q ratio with increasing 2DEG carrier concentration, indicating a transition from a dominant small angle to large angle scattering mechanism when n_s exceeds 7 × 10¹² cm^-2.