Although critically important in many electronic applications, oxides are still confined to playing mostly passive roles in actual devices. This state of affairs is set to change dramatically in the near future as a result of increased appreciation of the potential offered by a very large family of semiconductor materials based on oxides. Spanning a broad range of electrical and optical properties, oxide semiconductors are of interest for a number of device applications including detectors, solar cells, light emitters and both transparent and high speed transistors.Reference King and Veal1 Most prominent of these is perhaps ZnO, an ultraviolet band gap semiconductor readily available as very high-quality bulk substrates. The subject of many review articles already,Reference Özgür, Alivov, Liu, Teke, Reshchikov, Doğan, Avrutin, Cho and Morkoç2 it is readily synthesized by many techniques in both thin film and nanostructure form. A number of high-quality devices have already been fabricated from this semiconductor, including Schottky-based diodesReference Mead3, Reference Allen, Durbin and Metson4 and transistors,Reference Bayraktaroglu, Leedy and Neidhard5, Reference Grundmann, Frenzel, Lajn, Lorenz, Schein and von Wenckstern6 although achieving reproducible p-type material has been surprisingly challenging. Still, despite the fact that our understanding of the optical and electrical properties of this material has improved greatly over the past several years, there remain many open questions, particularly regarding both impurities and fundamental defects, as well as unusual polarity effects.Reference Allen, Zemlyanov, Waterhouse, Metson, Veal, McConville and Durbin7
Additional oxide semiconductors attracting interest for their device potential include CdO, In2O3, Ga2O3 and both SnO and SnO2. Intriguingly, SnO tends to be p-type as-grown, whereas SnO2 is known to have a strong tendency to be n-type, similar to ZnO, In2O3 and CdO. These compounds are also readily synthesized by a number of conventional deposition techniques including sputtering, pulsed laser deposition, chemical vapor deposition and molecular beam epitaxy, but do not yet benefit from high-quality bulk crystals suitable for substrates. Consequently, our understanding of these materials is limited, and better control of carrier concentration is an important goal. Rather than studying these materials in isolation, however, it is clear that a better approach is to improve our understanding of them as a family, and this should ultimately lead to devices with a broad range of applications. It is with this intention that we selected the papers which comprise this Focus Issue.
It is also worth noting that more recently, considerable attention is being given to thin film indium gallium zinc oxide– often referred to as IGZO – which is of significant interest for thin film transistors used in flat panel displays. This material can be compositionally tuned and is a serious rival to amorphous silicon, the traditional material of choice. Device performance is improving dramatically, driven by significant commercial interest, and it is likely that increased attention will be directed to developing a more detailed understanding of its fundamental properties and ultimate performance potential.
Finally, we would like to offer our sincere thanks to both the authors and reviewers of the many high-quality manuscripts submitted to this JMR Focus Issue on Oxide Semiconductors, as well as the efficient and talented editorial staff who were always willing to provide much appreciated assistance throughout the process.
