Limitations of threshold voltage engineering of AlGaN/GaN heterostructures by dielectric interface charge density and manipulation by oxygen plasma surface treatments

The interface charge density between the gate dielectric and an AlGaN/GaN heterostructure has a significant impact on the absolute value and stability of the threshold voltage V-th of metalinsulator- semiconductor (MIS) heterostructure field effect transistor. It is shown that a dryetching step (as typically necessary for normally off devices engineered by gate-recessing) before the Al2O3 gate dielectric deposition introduces a high positive interface charge density. Its origin is most likely donor-type trap states shifting V-th to large negative values, which is detrimental for normally off devices. We investigate the influence of oxygen plasma annealing techniques of the dry-etched AlGaN/GaN surface by capacitance-voltage measurements and demonstrate that the positive interface charge density can be effectively compensated. Furthermore, only a low V-th hysteresis is observable making this approach suitable for threshold voltage engineering. Analysis of the electrostatics in the investigated MIS structures reveals that the maximum V-th shift to positive voltages achievable is fundamentally limited by the onset of accumulation of holes at the dielectric/barrier interface. In the case of the Al2O3/Al0.26Ga0.74N/GaN material system, this maximum threshold voltage shift is limited to 2.3V. Published by AIP Publishing.