High-mobility transparent conductive oxide layers

Abstract

Transparent conductive oxides (TCOs) are wide band gap degenerated semiconductors popularly used in devices. Most commonly, TCO films are employed as transparent electrodes in flat-panel displays, thin-film solar cells, light-emitting diodes, and electrochromic windows. The development of high-performance optoelectronic devices has stimulated research on TCO films; namely, the improvement of the electrical and/or optical properties of existing TCO materials, the development of alternative TCO materials composed of naturally abundant and low-cost metals, and the development of new multifunctional TCOs that can improve device performance. TCO materials can be binary (In2O3, ZnO, SnO2, CdO, and Ga2O3) or multicomponent (In-Zn-O, Zn-Sn-O, and In-Ga-Zn-O). To develop and refine TCOs, we must understand their electron transport mechanisms and optical properties. The dielectric functions of TCO films can be elucidated by spectroscopic ellipsometry (SE). Moreover, by analyzing the dielectric functions at infrared wavelengths, we can evaluate the optical effective mass and the relaxation time of free electrons in TCO films. From the relationship between effective mass and carrier density, we can determine the curvature of the energy band as a function of Fermi energy, which reveals the fundamental nature of the material. Meanwhile, the relaxation time helps us to understand the dominant scattering mechanism of free carriers in the films and how to improve the film fabrication process. This chapter presents an analytical SE procedure for TCO layers, focusing on high-mobility TCOs and their advantages. The superior electrical transport and optical properties of TCO thin films and solar cells installed with TCO electrodes are also discussed.