Undoped a-plane GaN layers grown by metal-organic vapour phase epitaxy onto sapphire (10•2) substrates using low-temperature GaN seed layers and in situ SiN masks were characterized by Hall-effect measurements, CV-characteristics and photovoltage spectroscopy. With increasing deposition time of the SiN masks the electron concentrations of the GaN layers were enhanced. The dominant activation energy between 14 and 22meV determined by temperature-dependent Hall effect was very similar to the donor silicon on gallium site. Two other activation energies at 30meV and between 50 and 70meV were found to correspond well with OGa and VN defects, respectively. The depth profiles of the net donor densities showed a strong increase towards the substrate/low-temperature GaN/high-temperature-GaN interface indicating diffusion of silicon from the SiN mask towards the surface. Therefore, the Si doping was attributed to the dissolution of the SiN masks during the following high-temperature GaN layer growth. The Si doping from the SiN masks also explained the deterioration of the band bending within the low-temperature-GaN/high-temperature-GaN junction found by photovoltage spectroscopy.

Unintentional Doping of a-Plane GaN by Insertion of in situ SiN Masks. H.Witte, M.Wieneke, A.Rohrbeck, K.M.Guenther, A.Dadgar, A.Krost: Journal of Physics D, 2011, 44[8], 085102