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 photocross section spectra appears more broadened than can be simulated by the model cited above.

Defects in a nitrogen implanted and thermally annealed zinc oxide thin film (n-type conducting) and reference samples were studied. Space charge regions realised by fabrication of semitransparent palladium Schottky contacts enabled the application of capacitance spectroscopic methods and photocurrent measurements. We report on the formation of a deep level, in the following labelled TN1. It is 580 meV below the conduction band edge, probably related to nitrogen, and must be distinguished from the well known intrinsic deep level E4 at almost equal energetical depth. Capacitance measurements in combination with optical excitation, conducted at different temperatures, as well as photo-current measurements revealed the existence of two states approximately 60 meV and 100 meV above the valence band edge for the nitrogen implanted sample. These states cause an acceptor compensation degree larger than 0.9. The thermal emission of holes from these states into the valence band was observed by optical deep level transient spectroscopy.

Defects in zinc-implanted and thermally annealed ZnO thin films were investigated by means of capacitance-voltage spectroscopy (C-V), thermal admittance spectroscopy (TAS), deep level transient spectroscopy (DLTS), and photoluminescence (PL) spectroscopy. The authors report on the formation of two donor states approximately 35 and 190meV below the conduction band edge, observed by TAS and DLTS, respectively. In the PL spectra of a reference sample a peak at 3.366eV was present, which diminished after the implantation, while a new peak at 3.364eV was observed only in the spectrum of the implanted sample. Since only intrinsic ions were implanted, the authors consider the defects formed by the zinc implantation and annealing to be intrinsic.

Electronic defects in nickel-doped zinc oxide thin films have been investigated by means of capacitance spectroscopy. The samples were grown by pulsed laser deposition on a-plane sapphire substrates. Nickel was introduced into the films (a) during growth and (b) by implantation of Ni ions and subsequent thermal annealing. From deep-level transient spectroscopy it was concluded that a nickel-related trap, TNi2, with an energy level approximately 540 meV below the conduction band edge was formed. Photo-capacitance (PCAP) measurements performed on the nickel-implanted sample proved the existence of a further nickel-related trap, TNi1, in the midgap. The photo-ionisation cross-section spectra of this state were calculated from the PCAP transients and gave evidence that TNi1 and TNi2 are two levels of the same defect, TNi, which is possibly nickel on a tetrahedral lattice site. A model for TNi is proposed.

We investigated the electronic properties of the T2 deep-level in zinc oxide thin films. It was found that T2 preferentially forms under zinc-rich conditions and can be generated by either annealing the samples at reduced oxygen partial pressures (p O 2 < 1 bar) or implanting zinc or copper ions, respectively. A strong dependence of its activation energy and high temperature limit of its cross-section for electron capture on the T2 concentration in the sample is reported. Double DLTS measurements showed that the T2 activation energy decreases with increasing electric field due to phonon assisted tunnelling. Furthermore T2 can be photo-ionised with a threshold photon energy of about 700 meV. Depth-resolved concentration profiles of the T2 level in the samples were measured by optical capacitance-voltage spectroscopy.