Al0.15Ga0.85N/GaN heterostructure:: Effective mass and scattering times

We have observed well-reolved Shubnikov-de Haas oscillations in the two-dimensional electron gas in AlxGa1-xN/GaN heterojunctions, and determined the GaN electron effective mass (m*) and the quantum scattering time (tau(q)). We found m*=0.18m(0)+/-0.02m(0) in agreement with theoretical calculations, but slightly smaller that the values previously reported from optical experiments. The value of tau(q) was found to be 0.13 x 10(-12) sec, which is about a factor of 6 smaller than the classical scattering time (tau(c)=0.77 x 10(-12) sec). This difference between tau(q) and tau(c) is attributed to a significant amount of small angle scattering, most likely due to charged defects the epilayer/substrate interface.