Polarization-Induced Charges in Modulation-Doped Al_xGa_1-xN/GaN Heterostructures Through Capacitance-Voltage Profiling

Polarization-induced charges in modulation-doped Al_0.22Ga_0.78N/GaN heterostructures were investigated by the capacitance-voltage (C--V) method. The C--V profile of the Pt/Al_0.22Ga_0.78N/GaN Schottky diodes with various Al_0.22Ga_0.78N thickness shows significant differences due to change of the polarization field in the heterostructures. Numerical simulation based on the experimental results indicates that the sheet density of the polarization-induced charges at the heterointerface is 6.78 x 10¹²cm^-2 in the samples with the Al_0.22Ga_0.78N thickness of 30 nm or 45 nm. The charge density reduces to 1.30 x 10¹²cm^-2 in the sample with the Al_0.22Ga_0.78N thickness of 75 nm. It is thought that the reduction of the polarization-induced charges at the heterointerface is due to the partial relaxation of the Al_0.22Ga_0.78N layer on GaN. This work provides a technique for quantitative characterization of the polarization-induced charges in Al_xGa_1-xN/GaN heterostructures.