Origin of positive fixed charge at insulator/AlGaN interfaces and its control by AlGaN composition

The key feature for the precise tuning of V-th in GaN-based metal-insulator-semiconductor (MIS) high electron mobility transistors is the control of the positive fixed charge (Q(f)) at the insulator/III-N interfaces, whose amount is often comparable to the negative surface polarization charge (Q(pol)(-)). In order to clarify the origin of Q(f), we carried out a comprehensive capacitance-voltage (C-V) characterization of SiO2/AlxGa1-xN/GaN and SiN/AlxGa1-xN/GaN structures with Al composition (x) varying from 0.15 to 0.4. For both types of structures, we observed a significant V-th shift in C-V curves towards the positive gate voltage with increasing x. On the contrary, the Schottky gate structures exhibited Vth shift towards the more negative biases. From the numerical simulations of C-V curves using the Poisson's equation supported by the analytical calculations of V-th, we showed that the V-th shift in the examined MIS structures is due to a significant decrease in the positive Q(f) with rising x. Finally, we examined this result with respect to various hypotheses developed in the literature to explain the origin of the positive Q(f) at insulator/III-N interfaces