P-Type Transparent Semiconducting Delafossite CuAlO[subscript]2+x Thin Film

Copper based delafossite transparent semiconducting oxide thin films have recently gained tremendous interest in the field of optoelectronic technology, after the discovery of p-type conductivity in a transparent thin film of copper aluminium oxide (CuAlO2). Most of the well-known and widely used transparent conducting oxide thin films such as ZnO, SnO2, ITO etc. and their doped versions are n-type material, but corresponding p-type transparent conducting oxides were surprisingly missing for a long time until the fabrication of above-mentioned p-CuAlO2 thin film have been published (Nature 1997, 389, 939). This has opened up a new field in opto-electronics device technology, the so-called "Transparent Electronics", where a combination of the two types of transparent conducting oxides in the form of a p-n junction could lead to a 'functional' window, which transmits visible portion of solar radiation yet generates electricity by the absorption of UV part of it. Non-stoichiometric and doped versions of various new types of p-type transparent conducting oxides with improved optical and electrical properties have been synthesised in the last few years in this direction. Wide range of deposition techniques have been adopted to prepare the films. But fabrication of device quality films by cost-effective deposition techniques such as sputtering, chemical vapour deposition, wet-chemical dip-coating technique etc. are the need of the hour for large-scale production of these films for diverse device applications. Here the authors have discussed the fabrication and opto-electrical characterisation of p-CuAlO2+x thin films by cost-effective and scalable deposition routes such as sputtering and wet-chemical dip-coating technique. The authors have also discussed briefly some of the new developments in the field of p-type transparent conducting oxide thin film technology and an up-to-date and comprehensive description of different Cu-based p-type transparent conducting oxide thin films is presented. Also the origin of p-type conductivity in these transparent oxides has been dealt with considerable attention. Fabrication of all-transparent junctions is also discussed which is most important in the development of 'Transparent Electronics'. Field emission properties of thin films are currently of much interest due to the potential application in field emission displays (FEDs), which are considered to be strong candidate for low-power panel applications. The low-threshold field emission properties of wide-bandgap CuAlO2 thin films have been investigated for its potential applications in FED technology. The films showed considerable low turn-on field. This finding might open up a new direction in the field-emission technology, and a new group of materials (such as, different transparent conducting oxides) might become a promising candidate for low-threshold field emitter. Also, recently, the research on nanostructured materials generates great interest in the scientific community and offers tremendous opportunities in the field of science and technology. Here, the authors have also discussed in brief, the formation of nanocrystalline p-CuAlO2 films, which may open up an extremely important and interesting field of research for the fabrication of all-transparent nano-active devices. This will not only give a new dimension in the field of 'Transparent Electronics', but new avenues may open up in the nanoparticle research keeping an eye on its tremendous applications in optoelectronics technology.