Investigation of Al2O3/GaN interface properties by sub-bandgap photo-assisted capacitance-voltage technique

Interfaces in Al2O3/n-GaN capacitors fabricated on free-standing GaN substrates were investigated using sub-bandgap photo-assisted capacitance-voltage measurements. After post-metallization annealing (PMA) at 300 °C, the metal-oxide-semiconductor (MOS) devices exhibited excellent capacitance-voltage (C-V) characteristics without frequency dispersion under dark conditions and a quite low interface state density (Dit) of ∼7×1010 cm-2 eV-1 for energies less than ∼1.2 eV from the conduction band edge. Despite the outstanding characteristics under dark conditions, it was found that sub-bandgap irradiation also resulted in shifts of the C-V curves toward the negative bias direction, which indicates the existence of donor-type interface traps. As a result, the Al2O3/n-GaN Dit distribution does not show the generally reported U-shape: for energies less than ∼1.2 eV from the conduction band edge, a Dit of ∼7×1010 cm-2 eV-1 was observed, although Dit rapidly increased to ∼2-4×1012 cm-2 eV-1 near the valence band edge. These interface states near the valence band edge are positively charged due to trapped holes when the states are located above the Fermi level.