Unintentional doping in GaN assessed by scanning capacitance microscopy

Using scanning capacitance microscopy (SCM) we observe a layer of unintentional n-type conductivity in GaN/sapphire epilayers in a region adjacent to the GaN/sapphire interface. It is shown to have a carrier concentration of (1.66 +/- 0.08) x 10(18) cm(-3). Secondary ion mass spectroscopy (SIMS) data from a similar sample suggests that the n-type conductivity in this region results from unintentional oxygen incorporation. Various doped regions are also observed in epitaxial lateral over-growth (FLOG) samples. GaN initially grows through a SiNx mask forming faceted stripes, which exhibit unintended n-type conductivity. Magnesium is used to enhance stripe coalescence, resulting in p-type regions. The top layer, grown following coalescence, appears non-conducting. The different electron concentrations found within the FLOG cross-section could correspond to the different facets formed during growth which may indicate that the changes in carrier concentration reflect the rate of impurity incorporation on each facet. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.