Detection of defect levels in vicinity of Al2O3/p-type GaN interface using sub-bandgap-light-assisted capacitance–voltage method

Abstract

Defect levels in the vicinity of the Al2O3/p-type GaN interface were characterized using a sub-bandgap-light-assisted capacitance–voltage ( C–V) method. For metal–oxide–semiconductor (MOS) diodes prepared using p-type GaN (p-GaN) and Al2O3 formed by atomic layer deposition, the C–V curves measured in the dark showed capacitance saturation at a negative bias and a large negative voltage shift compared with ideal curves, which implied the effects of donor-like gap states in the vicinity of the Al2O3/p-GaN interface. Upon illumination with monochromated sub-bandgap light with photon energies higher than 2.0 eV under a large positive bias, the subsequently measured C–V curves showed three plateaus. The plateau under the positive bias voltage due to the surface inversion appeared despite the sub-bandgap illumination, which did not appear at 1.8 eV light illumination, indicating the existence of midgap defect levels. Moreover, the other plateaus were attributed to defect levels at 0.60 and 0.7–0.8 eV above the valence band maximum. For a sample whose surface was prepared by photo-electrochemical (PEC) etching to a depth of 16.5 nm, the C–V curve measured in the dark showed a reduced voltage shift compared with the unetched sample. Furthermore, sub-bandgap-light-assisted C–V curves of the sample with PEC etching showed no plateau at a positive bias, which indicated the reduction in the density of the midgap defect states. Possible origins of the detected defect levels are discussed. The obtained results showed that the interface control can improve the properties of p-GaN MOS structures.