Characterization of point defects in ZnO thin films by optical deep level transient spectroscopy

Abstract

Abstract We have applied a special variant of optical DLTS to deep levels found in ZnO thin films. The optical emission rates of charge carriers from deep levels into a band have been measured over a range of photon energies from 0.7 to 3.0 eV and photoionization cross‐section spectra calculated from this data. Two of the defects – among these the well‐known E3 – showed no optical emission in the investigated range of photon energies. The experimental photo cross‐section spectra of two optically active defects were compared and discussed using a model [Chantre, Vincent, and Bois, Phys. Rev. B 23 (10), 5335 (1981)] that yields information about the band structure and defect properties. DLTS signal simulations were compared to the experimental data for the case that E4 has two states within the bandgap. These simulations revealed that a two‐state model where both states independently allow electron emission is in accordance with the experimental data. The thermal DLTS peak of E4 was observed to be broadened and has been simulated under the assumption of a distribution of the carrier capture cross‐section. The agreement between the simulation and the measured DLTS peak possibly explains why one of the photo‐cross section spectra appears more broadened than can be simulated by the model cited above.