Characterization of unintentional doping in nonpolar GaN

Unintentional doping in nonpolar a-plane (1120) gallium nitride (GaN) grown on r-plane (1102) sapphire using a three-dimensional (3D)-two-dimensional (2D) growth method has been characterized. For both 2D only and 3D-2D growth, the presence of an unintentionally doped region adjacent to the GaN/sapphire interface is observed by scanning capacitance microscopy (SCM). The average width of this unintentionally doped layer is found to increase with increasing 3D growth time. By using an intentionally doped GaN:Si staircase structure for calibration, it is shown that the unintentionally doped region has an average carrier concentration of (2.5 +/- 0.3)x10(18) cm(-3). SCM also reveals the presence of unintentionally doped features extending at 60 degrees from the GaN/sapphire interface. The observation of decreasing carrier concentration with distance from the GaN/sapphire interface along these features may suggest that the unintentional doping arises from oxygen diffusion from the sapphire substrate. Low temperature cathodoluminescence spectra reveal emission peaks at 3.41 and 3.30 eV, which are believed to originate from basal plane stacking faults (BSFs) and prismatic stacking faults (PSFs), respectively. It is shown that the inclined features extending from the GaN/sapphire interface exhibit both enhanced BSF and PSF emission. We suggest that enhanced unintentional doping occurs in regions around PSFs. Where BSFs intersect this doped material their emission is also enhanced due to reduced nonradiative recombination. Transmission electron microscopy confirms the presence of PSFs extending through the film at 60 degrees from the GaN/sapphire interface.