Surface passivation is critically important to improve the current collapse and the overall device performance in metal-oxide semiconductor high-electron mobility transistors (MOS-HEMTs) and, thus, their reliability. In this paper, we demonstrate the surface passivation effects in AlGaN/AlN/GaN-based MOS-HEMTs using ultraviolet-ozone (UV/O3) plasma treatment prior to SiO2 -gate dielectric deposition. X-ray photoelectron spectroscopy (XPS) was used to verify the improved passivation of the GaN surface. The threshold voltage (VTH) of the MOS-HEMT was shifted towards positive due to the band bending at the SiO2/GaN interface by UV/O3 surface treatment. In addition, the device performance, especially the current collapse, hysteresis, and 1/f characteristics, was further significantly improved with an additional 15 nm thick hafnium silicate (HfSiOX) passivation layer after the gate metallization. Due to combined effects of the UV/O3 plasma treatment and HfSiOX surface passivation, the magnitude of the interface trap density was effectively reduced, which further improved the current collapse significantly in SiO2-MOS-HEMT to 0.6% from 10%. The UV/O3-surface-modified, HfSiOX-passivated MOS-HEMT exhibited a decent performance, with IDMAX of 655 mA/mm, GMMAX of 116 mS/mm, higher ION/IOFF ratio of approximately 107, and subthreshold swing of 85 mV/dec with significantly reduced gate leakage current (IG) of 9.1 ×10−10 A/mm.

In this paper, we have demonstrated the optimized device performance in the Γ-shaped gate AlGaN/AlN/GaN metal oxide semiconductor high electron mobility transistor (MOS-HEMT) by incorporating aluminum into atomic layer deposited (ALD) HfO2 and comparing it with the commonly used HfO2 gate dielectric with the N2 surface plasma treatment. The inclusion of Al in the HfO2 increased the crystalline temperature (~1000 °C) of hafnium aluminate (HfAlOX) and kept the material in the amorphous stage even at very high annealing temperature (>800 °C), which subsequently improved the device performance. The gate leakage current (IG) was significantly reduced with the increasing post deposition annealing (PDA) temperature from 300 to 600 °C in HfAlOX-based MOS-HEMT, compared to the HfO2-based device. In comparison with HfO2 gate dielectric, the interface state density (Dit) can be reduced significantly using HfAlOX due to the effective passivation of the dangling bond. The greater band offset of the HfAlOX than HfO2 reduces the tunneling current through the gate dielectric at room temperature (RT), which resulted in the lower IG in Γ-gate HfAlOX MOS-HEMT. Moreover, IG was reduced more than one order of magnitude in HfAlOX MOS-HEMT by the N2 surface plasma treatment, due to reduction of N2 vacancies which were created by ICP dry etching. The N2 plasma treated Γ-shaped gate HfAlOX-based MOS-HEMT exhibited a decent performance with IDMAX of 870 mA/mm, GMMAX of 118 mS/mm, threshold voltage (VTH) of −3.55 V, higher ION/IOFF ratio of approximately 1.8 × 109, subthreshold slope (SS) of 90 mV/dec, and a high VBR of 195 V with reduced gate leakage current of 1.3 × 10−10 A/mm.