Schottky Contact Formation to Bulk Zinc Oxide

Abstract

Zinc oxide is a II-VI semiconductor with considerable potential for optoelectronic and power-electronic applications in the UV spectrum, due to its wide direct band gap (3.35 eV at 300 K), high exciton binding energy (60 meV), high melting point, and excellent radiation hardness. A key requirement for many device applications is the consistent production of high performance Schottky contacts. Schottky contact formation to n-type ZnO was investigated via systematic studies into the relative performance of different metal and metal oxide Schottky contacts to hydrothermal and melt grown, bulk ZnO. The results of these studies can be explained by the dominating influence of two key mechanisms in the formation of high quality contacts: the removal of the natural hydroxide termination of ZnO and the associated surface accumulation layer, and the minimisation of process induced oxygen vacancies which tend to pin the barrier height of ZnO Schottky contacts in the 0.6 - 0.8 eV range. These investigations also led to the discovery of a new technique for the consistent production of high quality ZnO Schottky contacts, using the deposition of metal oxide films in reactive oxygen ambients. Specifically, silver oxide, iridium oxide, and platinum oxide films were used to consistently produce highly rectifying, very low ideality factor Schottky contacts to bulk ZnO, with figures of merit significantly better than those published in the literature. In addition, a number of previously unreported, surface polarity related effects were discovered in the electrical and optical properties of ZnO, which increase in magnitude with decreasing carrier concentration of the ZnO material. For example, metal oxide Schottky contacts fabricated on the Zn-polar surface of hydrothermal ZnO have significantly higher barrier heights than those on the O-polar surface, and low temperature (4 K) photoluminescence emission, from free excitons and excitons bound to ionised donors, is also significantly stronger from the Zn-polar face of the same material. These effects are thought to be related to the large spontaneous polarisation (-0.057 Cm-2) of ZnO, and indicate that surface polarity is an important variable when comparing experiment results with theoretical models, and in the future design of ZnO based devices.