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

Integration of the p-type oxide gate layer on AlGaN/GaN is a promising approach to explore normally-off high electron mobility transistors (HEMTs). However, the critical reason for the ultralow threshold voltage in intrinsic p-type oxide 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 important physical properties and design principle of p-type oxide for normally-off HEMTs. With the increasing Li doping content up to 25%, Ni_0.75Li_0.25O on AlGaN/GaN exhibits excellent epitaxial growth quality with good interfacial state, a wide band gap of 3.65 eV and an ultrahigh hole concentration of 6.81 × 10¹⁹ cm^-3. Nevertheless, the Ni_0.75Li_0.25O/AlGaN/GaN/Si heterostructure still suffers from a low threshold voltage of merely -2.12 V. By resolving the band alignment at the Ni_0.75Li_0.25O/AlGaN interface and the depletion mechanism for p-type Ni_0.75Li_0.25O on 2DEG, the band alignment matching is ascribed to be the most critical issue for intrinsic p-type oxide gated normally-off HEMTs with a low threshold voltage, that is, a relatively small energy level difference value between the conduction band of GaN and the valence band of intrinsic p-type oxide. Based on the results, the design principle of the 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.