Surface potential measurements on GaN and AlGaN/GaN heterostructures by scanning Kelvin probe microscopy

Surface potentials on GaN epilayers and Al0.35Ga0.65N/GaN heterostructures have been studied by scanning Kelvin probe microscopy (SKPM) in conjunction with noncontact atomic force microscopy. The dependence of the surface pot ential on doping in GaN films, as well as the variation of surface potentia l with Al0.35Ga0.65N barrier layer thickness has been investigated. The bar e surface barrier height (BSBH), as measured by SKPM, is observed to decrea se from similar to1. 40 +/-0.1 eV to similar to0.60 +/-0.1 eV with increasi ng doping in the GaN epilayers. Schottky barrier height calculated from the measurements of BSBH on n-GaN agrees very well with results from previous studies. We have also estimated the surface state density for GaN based on the measured values of BSBH. The semiconductor "work function" at the Al0.3 5Ga0.65N surface (in heterostructure samples) is observed to decrease by si milar to0.60 eV with increase in barrier layer thickness from similar to 50 to similar to 440 A. A simple model considering the presence of a uniform density of charged acceptors in the Al0.35Ga0.65N layer is proposed to expl ain the observed decreasing trend in work function. (C) 2001 American Insti tute of Physics.