Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

Defects in ZnO

Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO : From basics towards applications

ZnO is presently experiencing a research boom with more than 2000 ZnO-related publications in 2005. This phenomenon is triggered, for example, by hope to use ZnO as a material for blue/UV optoelectronics as an alternative to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits that are transparent in the visible or for semiconductor spintronics. Currently, however, the main problem is to achieve high, reproducible and stable p-doping. Herein, we critically review aspects of the material growth, fundamental properties of ZnO and ZnO-based nanostructures and doping as well as present and future applications with emphasis on the electronic and optical properties including stimulated emission.

ZnO: Material, Physics and Applications

Chemical design to find a new transparent conductive oxide having p-type conductivity has been proposed. Following the chemical design, we have selected CuGaO2 and CuAlO2 as candidate materials. CuGaO2 thin films were prepared on silica glass substrates by RF sputtering method. The optical band gap of the film was estimated to be ∼3.4 eV. Positive sign of Seebeck coefficient demonstrated the p-type conductivity of the film. The dc conductivity of the film was 5.6 × 10−3S·cm-1 and the activation energy was 0.22 eV at room temperature. Because of rough texture of the film, the observed conductivity was not an intrinsic property of the material. Further, CuAlO2 thin films were prepared by laser ablation. The film deposited in O2 atmosphere of 1.3 Pa at 690°C showed higher optical transmission in visible and near-infrared regions than previously reported. Contribution of Cu 3d components to upper edge of valence band in CuGaO2 and CuAlO2 were confirmed by photoemission spectroscopic measurements.

Chemical Design and Thin Film Preparation of p-Type Conductive Transparent Oxides

To study the effects of ions diffused from the glass substrate on TiO2 photocatalysis, TiO2 thin films were prepared on different kinds of glass substrates at different heating temperatures. The TiO2 films were characterized using X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), ellipsometry, and absorption spectroscopy. The photocatalytic activities of the TiO2 thin films were evaluated using the degradation reaction of trichloroethylene (TCE). Diffused ions such as Na+ and Si4+ raised the anatase formation temperature and increased the particle size of TiO2 thin films (rc). The photocatalytic activity of the thin films largely depended on the particle size of TiO2; the reciprocal number of the half-life of the TCE degradation, t1/2-1, was roughly proportional to rc-1.8. The conventional recombination center hypothesis for the Na+ ions might not hold for the sol−gel TiO2 films.

F. Gärtner

Papers

Paramagnetic and optical properties of Na-doped ZnO single crystals