Calculations were made of donor defect profiles in pseudomorphic AlGaN layers which had been grown onto GaN. Account was taken of the effects of the strain polarization field upon the defect formation energy. Under certain conditions, the defect concentration could be enhanced by more than an order of magnitude. These large concentrations, combined with the band-bending effects of the piezoelectric field, made charge transfer from the AlGaN barrier to the GaN well extremely efficient; thus resulting in a 2-dimensional electron gas of very high density and low mobility.

Effects of piezoelectric field upon defect formation, charge transfer and electron transport at GaN/AlxGa1-xN interfaces L.Hsu, W.Walukiewicz: Applied Physics Letters, 1998, 73[3], 339-41