Interface gap states and Schottky barrier inhomogeneity at metal/n-type GaN Schottky contacts

The barrier heights (BH) of various metals including Pd, Pt and Ni on n-type GaN (M/n-GaN) have been measured in the temperature range 80–400 K with using a current–voltage (I–V) technique. The temperature dependence of the I–V characteristics of M/n-GaN have shown non-ideal behaviors and indicate the presence of a non-uniform distribution of surface gap states, resulting from the residual defects in the as grown GaN. The surface gap states density Nss, as well as its temperature dependence were obtained from the bias and temperature dependence of the ideality factor n(V,T) and the barrier height ΦBn(V,T). Further, a dependence of zero-bias BH Φ0Bn on the metal work function (Φm) with an interface parameter coefficient of proportionality of 0.47 is found. This result indicates that the Fermi level at the M/n-GaN interface is unpinned. Additionally, the presence of lateral inhomogeneities of the BH, with two Gaussian distributions of the BH values is seen. However, the non-homogeneous SBH is found to be correlated to the surface gap states density, in that Φ0Bn becomes smaller with increasing Nss. These findings suggest that the lateral inhomogeneity of the SBH is connected to the non-uniform distribution of the density of surface gap states at metal/GaN which is attributed to the presence of native defects in the as grown GaN. Deep level transient spectroscopy confirms the presence of native defects with discrete energy levels at GaN and provides support to this interpretation.