Adjustment of threshold voltage in AlN/AlGaN/GaN high-electron mobility transistors by plasma oxidation and Al₂O₃ atomic layer deposition overgrowth

We discuss possibilities of adjustment of a threshold voltage V_T in normally off GaN high-electron mobility transistors (HEMTs) without compromising a maximal drain current I_DSmax. Techniques of a low power plasma or thermal oxidation of 2-nm thick AlN cap over 3-nm thick AlGaN barrier are developed and calibrated for a thorough oxidation of the cap with a minimal density of surface donors at the inherent oxide-semiconductor interface. It has been shown that while a thermal oxidation technique leads to the channel and/or interface degradation, low density of surface donors and scalability of V_T with additionally overgrown Al₂O₃ may be obtained for plasma oxidized HEMTs. With 10-nm thick Al₂O₃ deposited at 100 °C by atomic-layer deposition, we obtained V_T of 1.6 V and I_DSmax of 0.48 A/mm at a gate voltage of V_GS = 8 V. Density of surface donors was estimated to be about 1.2 × 10¹³ cm⁻², leaving most of the negative polarization charge at the semiconductor surface uncompensated. Further reduction of surface donors may be needed for even higher V_T.