Study of SiNx and SiO2 passivation of GaN surfaces

The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence (PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the surface potential measured by SKPM increases as a function of distance from the Schottky contact much faster after SiNx passivation. We suggest that the passivation reduces the total density of surface states and therefore reduces surface recombination. (c) 2007 American Institute of Physics.