ZnO Surface Properties and Schottky Contacts

Abstract

With the development of exciting new optoelectronic and microelectronic device applications for ZnO, there is now renewed interest in the understanding and control of its electrical contact properties. Transparent thin film transistors, blue/UV light-emitting diodes (LEDs) and lasers, high electron mobility transistors, electronic nanostructures, and spintronics all require metal contacts and an understanding of metal-ZnO interfaces and processing techniques. Historically, there has been considerable research devoted to the surface physics and chemistry of ZnO, in large part due to the strong effect that surface and polarity have on ZnO interface charge transfer. This chapter will examine the results of surface and interface research produced over the past fifty years in the context of Schottky barriers.The challenge of ZnO surfaces and interfaces is evident from the wide and variable range of Schottky barriers measured for the same metal on the ZnO surface. In the case of Au/ZnO diodes, Schottky barrier heights F SB can range from 0 to 1.2 eV, depending on the experimental conditions. Similarly wide n-type F n SB energy ranges are observed for Pt/ ZnO and Ta/ZnO diodes. Furthermore, high work function metals exhibit lower than expected F n SB . Since the ZnO electron affinity x ZnO ¼ $4.2 eV, F n SB should be F M Àx ZnO ¼ 5.65À4.2 ¼ 1.45 eV for Pt, whereas measured F SB are limited to 0.96 eV for air-exposed surfaces and 0.75 eV for high vacuum-cleaved surfaces. Wide barrier variations are evident for Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, First Edition. Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins.