Epitaxial ZnO gate dielectrics deposited by RF sputter for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors

Radio frequency (RF)-sputtered ZnO gate dielectrics for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) were investigated with varying O₂/Ar ratios. The ZnO deposited with a low oxygen content of 4.5% showed a high dielectric constant and low interface trap density due to the compensation of oxygen vacancies during the sputtering process. The good capacitance-voltage characteristics of ZnO-on-AlGaN/GaN capacitors resulted from the high crystallinity of oxide at the interface, as investigated by x-ray diffraction and high-resolution transmission electron microscopy. The MOS-HEMTs demonstrated comparable output electrical characteristics with conventional Ni/Au HEMTs but a lower gate leakage current. At a gate voltage of -20 V, the typical gate leakage current for a MOS-HEMT with a gate length of 6 μm and width of 100 μm was found to be as low as 8.2 ×10^-7 mA mm^-1, which was three orders lower than that of the Ni/Au Schottky gate HEMT. The reduction of the gate leakage current improved the on/off current ratio by three orders of magnitude. These results indicate that RF-sputtered ZnO with a low O₂/Ar ratio is a good gate dielectric for high-performance AlGaN/GaN MOS-HEMTs.