Band Alignment and Performance Improvement Mechanisms of Chlorine-Treated ZnO-Gate AlGaN/GaN Metal–Oxide–Semiconductor High-Electron Mobility Transistors

The intrinsic ZnO (i-ZnO) film deposited by a vapor cooling condensation system was used as the gate dielectric layer of the AlGaN/GaN MOS-HEMTs. The chlorine surface treatment was utilized to obtain a high-quality i-ZnO/AlGaN interface due to the reduced surface state density. The chlorine-treated MOS-HEMTs showed the better direct current and pulsed output performances than those of the untreated MOS-HEMTs. The resulting unit gain cutoff frequency and the maximum frequency of oscillation were 9.5 and 19.4 GHz, respectively. The Hooge´s coefficient was 7.23 × 10^-6, when the chlorine-treated ZnO-gate MOS-HEMTs operated at 100 Hz and the gate-source voltage of -4 V. Compared with the untreated MOS-HEMTs, the chlorine-treated MOS-HEMTs revealed better performances. The valence-band offset of i-ZnO/AlGaN was measured by X-ray photoelectron spectroscopy. The valence-band offset of the i-ZnO film on the untreated and the chlorine-treated AlGaN was 1.53 and 2.05 eV, respectively. The conduction-band offset of the i-ZnO film on the untreated AlGaN and the chlorine-treated AlGaN was deduced to be 0.77 and 1.29 eV, respectively. The improved performances of the chlorine-treated MOS-HEMTs and the enhanced conduction-band offset of the i-ZnO/AlGaN interface were attributed to the decrease of Ga dangling bonds and the passivation of N vacancies on the AlGaN surface by using the chlorine surface treatment.