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 Al2O3/p-type GaN (p-GaN) interfaces formed by atomic layer deposition at 300 degrees C after photo electrochemical (PEC) etching are reported. For investigating the gap states at the Al2O3/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 Al2O3/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 EV 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 EV + 0.7 eV and midgap states. Still, PEC etching after capless annealing at 800 degrees 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 SiO2 cap layer at 800 degrees C after PEC etching can reduce the gap state density near the valence band edge.