Electrical and Photonic Functions in Transparent Oxide Semiconductors: Utilization of Built-in Nanostructure
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Electrical and Photonic Functions in Transparent Oxide Semiconductors : Utilization of Built-in Nanostructure Hideo Hosono1,2 Materials and Structures Laboratory, Tokyo Institute of Technology Nagatsuta 4259, Midori-ku, Yokohama 226-8503, JAPAN 2 Transparent Electro-Active Materials Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, KSP-C1232, Sakato3-2-1, Takatsu-ku, Kawasaki 213-0012, JAPAN 1
Abstract We review distinct photonic/electronic properties originating from built-in nano-structures in transparent oxide based materials, emphasizing potential of nanostructures hidden in crystal structure. Materials focused are oxychalcogenides LaCuOCh (Ch=chalcogen ion) and homologous oxides InGaO3(ZnO)m(m=integer) having naturally formed multi-quantum well structures. Novel functions and devices arising from the built-in nanostructure are: (1) modulation doping of positive holes and room temperature stable exciton in LaCuOCh, (2) high performance transparent field-effect transistor fabricated in InGaO3(ZnO)5 epitaxial thin films, and (3) conversion of insulator to persistent electronic conductor by carrier doping in 12CaOï½¥ 7Al2O3 (C12A7). 1. Introduction Oxide ceramics is probably among the oldest of man-made materials owing to the abundance and easy availability of the ingredients. Although most oxides are optically transparent, important for optical applications, it has been believed that active functions based on excited electrons, such as in crystalline semiconductor materials, are not possible. For example, alumina and glasses, which are representative oxides, are optically transparent but electrically insulating. If novel active functionalities utilizing both optical transparency and electron activity in oxide materials are realized, one can expect that a new frontier of materials science will be opened in front of us because these oxides are abundant and environmentally compatible. On the basis of this belief, we started our research project "Transparent Electro-Active Materials (TEAM)" from October 1999 (to September 2004) within a framework of Exploratory Research for Advanced Technology (ERATO) under the sponsorship of Japan Science and Technology Agency (JST). The purpose of this project is to cultivate the new frontier of transparent oxides as functional materials by exploring novel active properties based on excited electrons. We have two major focal points: band engineering and defect engineering in order to design an efficient electron-transfer system. This project is composed of four major subjects, transparent oxide semiconductors1 nanoporous materials, optical oxides for vacuum/deep ultraviolet (UV) lasers 2, and nano-fabrication of transparent dielectrics by interfered femtosecond laser pulses 3. In this paper we briefly review our works on electronic/photonic properties found in transparent materials based on oxides, which have build-in periodic nanostructures. Materials we focused are LnCuOCh (Ln:lanthanide, Ch:chalcogen ion), InGaO3(ZnO)m, and
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