Design principle for p-type oxide gate layer on AlGaN/GaN toward normally-off HEMTs: Li-doped NiO as model

Integration of p-type oxide gate layer on AlGaN/GaN is a promising approach to explore normally-off high electron mobility transistors (HEMTs). However, critical reason for ultralow threshold voltage in intrinsic p-type oxides gated HEMTs remains elusive. Herein, Li-doped NiO thin films with various doping contents were grown on AlGaN/GaN/Si substrates by pulsed laser deposition (PLD) to identify the most key physical property and design principle of p-type oxide for normally-off HEMTs. With increasing of Li doping content up to 25%, Ni0.75Li0.25O on AlGaN/GaN exhibits excellent epitaxial growth quality with good interfacial state, wide band gap of 3.65 eV and ultrahigh hole concentration of 6.81×1019 cm-3. Nevertheless, Ni0.75Li0.25O/AlGaN/GaN/Si heterostructure still suffers from low threshold voltage of merely -2.12 V. By resolving band alignment at Ni0.75Li0.25O/AlGaN interface and depletion mechanism for p-type Ni0.75Li0.25O on 2DEG, the band alignment matching is ascribed to be the most critical issue for intrinsic p-type oxide gated normally-off HEMTs with low threshold voltafge, that is, relatively small energy level difference value between conduction band of GaN and valence band of intrinsic p-type oxide. Based on the results, design principle of p-type oxide gate layer on AlGaN/GaN for normally-off HEMTs is proposed, and p-type oxides doped from intrinsic n-type oxides are suspected to be competitive candidates.