Reduction in Gap State Density near Valence Band Edge at Al₂O₃/p‐type GaN Interface by Photoelectrochemical Etching and Subsequent SiO₂ Cap Annealing

The process‐dependent properties of Al 2 O 3 /p‐type GaN (p‐GaN) interfaces formed by atomic layer deposition at 300 °C after photoelectrochemical (PEC) etching are reported. For investigating the gap states at the Al 2 O 3 /p‐GaN interface, metal‐oxide‐semiconductor (MOS) diodes are fabricated and examined by sub‐bandgap‐light‐assisted and temperature‐dependent capacitance–voltage ( C–V ) measurements. PEC etching prior to Al 2 O 3 /p‐GaN interface formation is conducted with the etching depth varied in the range between 12.5 and 32.1 nm. The C–V characteristics of the MOS diodes without PEC etching indicate Fermi‐level pinning due to the near‐surface defect level in p‐GaN at 0.7 eV above the valence band edge E V and a high density of gap states around the midgap. However, all samples with PEC etching exhibit C–V characteristics, indicating a reduction in the density of the defect states at E V + 0.7 eV and midgap states. Still, PEC etching after capless annealing at 800 °C for the activation of Mg acceptors cannot reduce the density of gap states near the valence band edge. On the other hand, annealing of a sample with a SiO 2 cap layer at 800 °C after PEC etching can reduce the gap state density near the valence band edge.