Normally-off GaON/p-GaN gate HEMTs with using selective plasma oxidation: from structural characterization, performance improvement to physical mechanism

In this letter, a p-GaN gate high-electron-mobility transistor (HEMT) with a high threshold voltage and better gate reliability was demonstrated by using selective plasma oxidation (SPO) and an additional low-temperature annealing step before gate metal is evaporated. After the SPO, a gallium oxynitride (GaON) dielectric layer was formed on the surface of p-GaN under the gate metal, and was studied by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), high-resolution transmission electron microscopy (HR-TEM) and energy dispersion spectroscopy (EDS). In addition, the fabricated metal/GaON/p-GaN gate HEMT exhibited a large threshold voltage (VTH) improvement from 1.46 V to 2.47 V. Furthermore, the forward gate breakdown voltage (VGS,BD) increased from 7.55 V to 11.10 V, and the maximum forward gate operating voltage (VGS-max) significantly improved from 5.0 V to 7.80 V for a ten-year lifetime with a 63.2% failure rate. Kelvin Probe Force Microscopy (KPFM) reveals that the surface potential increased after SPO, and the shift of valence band maximum (VBM) obtained by XPS spectra was 0.7 eV lower than that of the p-GaN, which further improves the Schottky barrier height (SBH) at the gate metal/GaON interfaces to holes, thereby improving VTH and reducing IGS of the device. As a barrier layer, GaON suppressed the injection of carrier into the depletion region under a high electric field and enhanced the reliability of the gate.