Transparent p -Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions
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p-Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions Hiroshi Kawazoe, Hiroshi Yanagi, Kazushige Ueda, and Hideo Hosono
Introduction Inorganic solids with wide bandgaps are usually classified as electrical insulators and are used in industry as insulators, dielectrics, and optical materials. Many metallic oxides have wide bandgaps because of the significant contribution of ionic character to the chemical bonds between metallic cations and oxide ions. Their ionic nature simultaneously suppresses the formation of easily ionizable shallow donors or acceptors and enhances the localization of electrons and positive holes. Thus it is understandable that interest in these wide-gap oxides as conductive materials has not been strong. Some oxides with wide bandgaps are exceptionally conductive:1 the oxides of p-block heavy-metal cations with ns0 electronic configurations (where n stands for the principal quantum number)—such as ZnO, CdO, Ga2O3, In2O3, Tl2O3, SnO2 , PbO2 , Sb2O5, and their mixed oxides—can be changed to n-type conductors by electron doping. The conductivity of commercially available In2x SnxO3 (indium tin oxide, ITO) thin films can be as high as 0.9 10 4 S cm1, which is almost equivalent to those of typical metals.2 In ITO thin films, the electron concentration is 2 10 21 cm3, with optical transparency still surviving. Their unique characteristics as transparent electrodes have led to the exclusive utilization of ITO thin films in flat-panel displays and solar cells. The conductivity of transparent conducting oxide (TCO) materials can be con-
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trolled across an extremely wide range such that they can behave as insulators, semiconductors, or metals. Surprisingly, only a limited number of studies have been done on these materials as semiconductors. Transparent thin-film field-effect transistors (FETs) have been fabricated by using n-type TCOs.3,4 This may be due to the lack of available p-type TCO materials. Without p-type materials, the fabrication of p-n junctions exclusively from TCO materials was impossible. Needless to say, the junction is an essential structure in a wide variety of semiconductor devices, thus without this capability, it is not surprising that interest in semiconducting TCOs has been low. Considering the unique optoelectronic properties and important, but as yet limited, applications of TCO materials, we have started to explore the chemical design of p-type TCO materials with a view toward TCOs as unique optoelectronic semiconductors. Some new p-type conducting TCOs have been found recently.5–7 As a demonstration, transparent diodes with rectifying behavior have been fabricated using TCOs.8 Very recently, UV emission from the diode by current injection to the p-n heterojunction at room temperature has been reported.9 In this article, we review our investigations into chemical design, fabrication of thin films, the optical and electrical properties of the thin films, and a trial to fabricate p-n heterojunctions.
Chemical Design of p-Type Conducting Wide-Gap O
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