Novel ZnO-based Ternary Oxides for Optoelectronic Applications

Abstract

Zinc oxide (ZnO) has been used in a wide range of products for many years, including, among others, varistors, surface acoustic wave devices and cosmetics. Besides these established applications, ZnO and its ternary alloys are now also being considered as potential materials for optoelectronic applications, such as light emitting diodes, photovoltaics, sensors, displays, etc. Unlike other materials, which could be used alternatively, ZnO has the advantage of being inexpensive, chemically stable and relatively plentiful. In spite of the long research history, fabrication of defect free ternary alloys and stable p-type ZnO is still challenging. The aim of this work was therefore to provide a better understanding of ZnO ternary alloys, so that - based on the gained knowledge - their optical properties can be further improved and, in a second step, optoelectronic applications based on these materials can soon be commercialized. The work carried out in this thesis was two-fold: the first part aimed at identifying the origin of defect related luminescence phenomena in ZnMgO, and the second part was dedicated to the exploration of a novel ZnCdO-based heterostructure photovoltaic applications. In the case of ZnMgO, luminescence properties of deep level defects were studied by photoluminescence (PL) spectroscopy and a model was proposed to explain the changes in the deep band emission with increasing Mg content. In this model, the observed trends can be understood by considering interaction of native zinc and oxygen defects of the ZnO sublattice with Mg interstitials (Mgi). In summary, the deep level bands at 3.0 and 2.8 eV, which show a blueshift with increasing Mg content, were assigned to free-to-bound type transitions between zinc interstitials (Zni) with the valence band edge and between the conduction band edge with zinc vacancies (VZn), respectively. A red band at 2.0 eV, which does not show an apparent shift of the peak energy for increasing Mg content, is associated with the oxygen vacancies (VO). Two luminescence bands at 2.3 and 2.5 eV, which are redshifted for higher Mg concentrations, were assigned to transitions between zinc and oxygen interstitials and between zinc interstitials and zinc vacancies, respectively. The redshift is interpreted in terms of a competing supply of electrons from slightly deeper Mgi donor states. The ZnMgO band gap diagram, which the model is based on, has contributed to gain valuable information about the nature of the deep defects both in ZnO and ZnMgO and is therefore of fundamental interest. In the second part of this work, focused on ZnCdO, a stacked heterostructure was designed for application in a photoelectrochemical cell, which is used for hydrogen production by photoelectrolysis using the semiconductor as an absorber. Optical and photoelectrochemical measurements led to the conclusion that the optical emission band for the ZnCdO heterostructures is broadened compared to a ZnO single layer. The broadened emission could be explained by combined excitonic recombination from the individual layers in the structure. The carrier dynamics in the structures were further investigated by time-resolved photoluminescence spectroscopy. A comparison of recombination parameters in ZnCdO heterostructures and in ZnO single layer films suggests a higher density of non-radiative recombination centers in the heterostructures. Furthermore, the effect of built-in fields on the carrier dynamics was assessed by investigating carrier recombination processes in a variety of different heterostructure geometries. The study does not only provide knowledge necessary to understand the origin of limiting factors in the proposed ZnCdO structure, but is also of general interest as the insight can be applied to a variety of other graded band gap type structures. Finally, photoelectrochemical testing of the ZnCdO structures confirmed the optical activity of the films, thus providing a proof of concept for the suitability of ZnCdO heterostructures as photoanodes in photoelectrochemical cells.