Abstract
Defects in pulsed-laser deposition grown have been investigated using thermal admittance spectroscopy and secondary ion mass spectrometry (SIMS). A film was grown on either a tin-doped indium oxide or an aluminum-doped zinc oxide buffer layer on a sapphire substrate functioning as back contact layer in vertical diode structures. In both sample types, a distinct signature in the capacitance signal was observed in the temperature range of 150–260 K. The corresponding defect charge-state transition level, labeled , was found to exhibit an activation energy of 0.21 eV. Potential candidates for the level were investigated using a combination of SIMS and hybrid-functional calculations. SIMS revealed the main impurities in the sample to be tin, silicon, and iron. The hybrid-functional calculations predict the acceptor levels of substitutional iron to lie 0.7–1.2 eV below the conduction band minimum depending on Ga-site, making an unlikely candidate for the level. Furthermore, Si as well as Sn substituting on the sixfold coordinated Ga2 site and the fivefold coordinated Ga3 and Ga4 sites are all shallow donors in -, similar to that of -. Sn substituting on the fourfold Ga1 site is, however, predicted to have levels in the bandgap at 0.15 and 0.24 eV below the conduction band minimum, in accordance with the extracted activation energy for . Thus, we tentatively assign as the origin of the level.