Unusual Electronic Structure and Properties of Transparent Conducting Oxide Semiconductors
Oxide semiconductors have become of great technological interest and importance in recent years with opportunities to improve both the existing materials and their potential device applications. This is particularly true for a sub-group of materials that display both optical transparency and high electrical conductivity, so-called transparent conducting oxides (TCOs). The fact that some of these materials, such indium tin oxide (ITO) have been around for many years and seen significant industrial use as transparent conductors in a relatively low quality form, has perhaps contributed to the belated recognition of using these materials as semiconductors in their own right. Here, examples from the surface and bulk electronic properties of several epitaxially grown oxide semiconductors (In2O3, CdO and ZnO) will be discussed along with the effects of modifying their surfaces by controlled adsorption. The valence band density of states and the surface electronic properties of these TCO’s have been studied using high-resolution synchrotron radiation angle-resolved photoemission (SRARPES) and core-level photoemission spectroscopy with hard x-rays (HAXPES), and these data are compared with theoretical DFT band structure calculations. A common property of these oxide semiconductors is the presence of a significant electron accumulation layer at the surface. While this is similarly found at the surfaces of materials such as InN and In-rich InGaN, it is in marked contrast to the electron depletion typically observed at the surfaces of conventional III-V, II-VI and Group IV semiconductor materials. More unusual still is the quantized nature of this surface 2D electron gas. The origins of this phenomenon will be discussed in terms of the band structure and intrinsic properties of these materials.
Professor Chris McConville, RMIT University