Achievement of normally-off AlGaN/GaN high-electron mobility transistor with p-NiO_x capping layer by sputtering and post-annealing

Highlights: • A technique to fabricate normally off GaN-based high-electron mobility transistor (HEMT) by sputtering and post-annealing p-NiO_x capping layer. • The V_th shifts from −3 V in the conventional transistor to 0.33 V, and on/off current ratio became 10⁷. • The reverse gate leakage current is 10⁻⁹ A/mm, and the off-state drain-leakage current is 10⁻⁸ A/mm. • The V_th hysteresis is extremely small at about 33 mV. - Abstract: In this paper, we present a technique to fabricate normally off GaN-based high-electron mobility transistor (HEMT) by sputtering and post-annealing p-NiO_x capping layer. The p-NiO_x layer is produced by sputtering at room temperature and post-annealing at 500 °C for 30 min in pure O₂ environment to achieve high hole concentration. The V_th shifts from −3 V in the conventional transistor to 0.33 V, and on/off current ratio became 10⁷. The forward and reverse gate breakdown increase from 3.5 V and −78 V to 10 V and −198 V, respectively. The reverse gate leakage current is 10⁻⁹ A/mm, and the off-state drain-leakage current is 10⁻⁸ A/mm. The V_th hysteresis is extremely small at about 33 mV. We also investigate the mechanism that increases hole concentration of p-NiO_x after annealing in oxygen environment resulted from the change of Ni²⁺ to Ni³⁺ and the surge of (111)-orientation.