Electrical Conduction Mechanism of Highly Transparent and Conductive ZnO Thin Films
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Electrical Conduction Mechanism of Highly Transparent and Conductive ZnO Thin Films Tadatsugu Minami, Shingo Suzuki and Toshihiro Miyata Optoelectronic Device System R&D Center, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501 JAPAN ABSTRACT In this paper, we describe the underlying theory along with experiments concerning the electrical conductivity of transparent conducting ZnO films with a carrier concentration of 1019-1021 cm-3. The experimentally determined mobility as a function of carrier concentration in the range of 1019-1021 cm-3 could be quantitatively referenced to a theoretically calculated mobility that is dominated by not only grain boundary scattering but also ionized impurity scattering using the Brooks-Herring-Dingle theory with both degeneracy and nonparabolicity of the conduction band taken into account. Concerning nonparabolicity, the conduction band effective mass as a function of carrier concentration was theoretically analyzed and experimentally determined. INTRODUCTION
Highly transparent indium tin oxide (ITO) thin films with a resistivity of 1-2 10-4 cm are widely used for applications such as flat panel displays and photovoltaic devices. However, transparent conducting ZnO thin films doped with an impurity such as Al or Ga [1,2] have been recently attracting much attention as an alternative because indium is a scarce and expensive material, whereas ZnO is abundant, inexpensive and nontoxic. In addition, it has been recently reported that a resistivity on the order of 1 10-4 cm can be obtained in ZnO:Al and ZnO:Ga films prepared by various deposition methods [1,2]. However, a fundamental understanding of such things as the electrical conduction mechanism as well as the development of high rate and large area deposition techniques are essential to widen the use of these ZnO films.
Transparent conducting oxide (TCO) films such as ZnO and ITO films with a carrier concentration on the order of 1020 to 1021 cm-3 are degenerated semiconductors. For example, optical properties such as optical band-gap in ZnO films are modified by many body effects resulting from carrier-carrier and carrier-impurity interactions as well as the Burstein-Moss effect [3-5]. We have also reported that electrical properties such as the conduction mechanism are dominated by both ionized impurity scattering with nonparabolicity in the conduction band taken into account and grain boundary scattering [2,6,7]. However, the electrical properties in TCO films, such as the conduction mechanism, are dependent on the kind of TCO material as well as preparation conditions and methods. In this paper, we describe the underlying theory along with experiments concerning the electrical conductivity of transparent conducting ZnO films with a carrier concentration in the range of 1019-1021 cm-3. RESULTS AND DISCUSSION Electrical properties of impurity-doped ZnO films ZnO films were prepared by conventional r.f. or d.c. planar magnetron sputtering using sintered Al-doped ZnO targets with an Al2O3 conten
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