Optoelectrical studies of ZnO

Abstract

The temperature dependence of the band structure of ZnO has been studied on epitaxial films and bulk crystals with the methods of temperature dependent photoluminescence, photoconductivity, reflectivity and transmission spectroscopy. A major question investigated was the intriguing detail that could be resolved in band edge photoconductivity spectra of both high quality ZnO bulk crystals as well as epitaxial films. The connection of these spectral details in photoconductivity to the excitonic band structure of ZnO was made by comparison to the other spectroscopic methods which have a better understood relation to the semiconductor band structure.

Photoluminescence spectroscopy enabled us to get a direct and reliable feedback about the energy fine structure of emitting levels in ZnO. Comparison of the emitting levels of epitaxial films with the emitting levels of high quality bulk material allowed the identification of dominating defect structures and impurities in the epitaxial films. The investigation of the effect of annealing on these emission lines finally allowed us to get a better understanding of the effects of annealing on the crystal and electric structure of epitaxially grown heterostructural films and allowed the determination of the optimum temperature range to be used for improved crystal quality.

It has been investigated if temperature dependent reflectivity can serve as a simple tool for the examination of the temperature dependence of the band structure of ZnO. The appeal of reflectivity is its enhanced sensitivity only to free excitonic transitions. This proved a valuable simplification compared to the methods of photoluminescence and photoconductivity: Photoluminescence is limited by phonon-broadening of the multitude of emission levels in the band gap region of ZnO, and photoconductivity has a multitude of processes that are potentially contributing to its spectra, making the identification of their relation to the band structure less reliable. Therefore the applicability of reflectivity for the deduction of the temperature dependence of the band structure has been investigated, by measuring the temperature dependence of the energy positions of the characteristic reflectivity features, with particular focus on the effect of phonon broadening and interaction of close lying resonator levels.

The investigation of the temperature dependence of photoconductive centres was enabled through the resulting possibility of directly relating the purely excitonic reflectivity spectra to the complex features in photoconductivity. The temperature dependent evolution of the spectra obtained by photoconductivity then revealed that there are at least two types of photoconductive processes that have to be distinguished: features in photoconductivity that are directly related to the band structure proved to be distinguishable from slow defect related processes in terms of their response speed. For the samples of bulk ZnO as well as epitaxial films, the peaks in photoconductivity only had a meaningful position in regard to the band structure for the cases of spectra that are dominated by fast processes. The spectra dominated by slow processes showed a meaningful temperature dependence of respective dips in the spectra. The strong response of fast photoconductive levels in bulk ZnO allowed us to directly observe the A- and B-free excitons by photoconductivity. Additional fine structure could be observed that is likely to be related to the narrow photo emission lines of neutral as well as ionized donor bound excitons and the upper polariton branch of the A-free exciton. These findings agree with the temperature dependence of related Anti-Stokes phonon replica levels that allow a first estimate of the activation energies of the zero-phonon lines.

The energy and temperature dependent lateral transport properties of ZnO are expected to be of importance in ZnO device technology