On the investigation of electronic defect states in ZnO thin films by space charge spectroscopy with optical excitation

Electronic defect states in a n-type conducting zinc oxide thin film sample were investigated by means of space charge spectroscopy focussing on levels in the midgap region as well as on hole traps. To overcome the experimental difficulties arising from the wide bandgap and the lack of p-type conduction, optical excitation was employed to measure the emission of trapped charge carriers from these levels. Therefore – besides deep-level transient spectroscopy measurements – photo-capacitance, optically chopped photo-current, minority carrier transient spectroscopy, and optical capacitance–voltage experiments were conducted. In doing so, a midgap level labelled T4, and hole traps labelled TH1 and TH2 were detected. In the case of T4 and TH1 the photo-ionisation cross-section spectra were determined.

► We investigated electronic defect states the entire bandgap of a ZnO thin film. ► A new hole trap TH2 was detected by minority carrier transient spectroscopy. ► TH2 exhibits a huge capture cross-section for holes. ► By photo-capacitance we detected the midgap level T4 and the hole trap TH1. ► The photo-ionisation cross-section spectra for T4 and TH1 were measured.