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

Surface potentials on GaN epilayers and ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N/GaN}$ heterostructures have been studied by scanning Kelvin probe microscopy (SKPM) in conjunction with noncontact atomic force microscopy. The dependence of the surface potential on doping in GaN films, as well as the variation of surface potential with ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ barrier layer thickness has been investigated. The bare surface barrier height (BSBH), as measured by SKPM, is observed to decrease from ∼1. 40±0.1 eV to ∼0.60±0.1 eV with increasing 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 ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ surface (in heterostructure samples) is observed to decrease by ∼0.60 eV with increase in barrier layer thickness from ∼50 to ∼440 Å. A simple model considering the presence of a uniform density of charged acceptors in the ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ layer is proposed to explain the observed decreasing trend in work function.