Photocurrent spectroscopy of deep levels in ZnO thin films

Epitaxial ZnO(0001) thin films have been grown by pulsed-laser deposition on a-Al2O3 and investigated by deep level transient spectroscopy (DLTS) and by Fourier transform infrared photocurrent (FTIR-PC) spectroscopy in the midinfrared wavelength range. FTIR-PC spectra of undoped ZnO layers show several well-resolved spectral features between 100 and 500 meV due to transitions from deep defect states either to the conduction band or to the valence band. They include the commonly observed intrinsic deep defects E1 at ~110 meV and E3 at ~320 meV. DLTS and FTIR-PC measurements were repeated after annealing the samples either in vacuum, under oxygen, or nitrogen atmospheres. Based on annealing effects, the possible microscopic origin of major deep levels in the ZnO samples is discussed. Further FTIR-PC investigations of Co- and Mn-doped ZnO reveal defect levels at ~270, ~380, and ~450 meV and are compared with corresponding DLTS data.