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Takahiro Nagata National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan; and Materials Department, University of California, Santa Barbara, California 93106

Mark E. White Materials Department, University of California, Santa Barbara, California 93106

Min-Ying Tsai Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106

James S. Speck Materials Department, University of California, Santa Barbara, California 93106 (Received 28 February 2012; accepted 2 May 2012)

The transport properties of doped and undoped, high quality, plasma-assisted molecular beam epitaxy grown tin dioxide (SnO2) thin films are reviewed. Intentional doping can vary the SnO2 resistivity over more than seven orders of magnitude from a transparent conducting oxide-like conductivity up to the semi-insulating range. A region of high unintentional n-type conductivity was identified in the substrate interface region and had to be accounted for. Sb was a well-behaved shallow donor up to the regime of conducting oxides. In and Ga were too deep acceptors to achieve p-type conductivity but were suitable to render SnO2 semi-insulating. While the surface accumulation layer strongly influenced contact properties, its conductance was negligible. The methodology used here for studying the transport can also be applied to other semiconducting oxides.

I. INTRODUCTION 1

Tin dioxide, SnO2, is widely used as gas sensor material and highly Sb- or F-doped as transparent contact.2 For these traditional applications, SnO2 is prepared with a low material quality—typically polycrystalline. To render SnO2 a transparent semiconductor in its own right and to study its intrinsic material properties, a significantly higher material quality is mandatory: electrons at moderate—semiconductor like— donor concentrations are sensitive to scattering and/or compensation by grain boundaries, crystal defects, and impurities, which is far less the case in transparent oxide-based contacts with high intentional donor concentration that clearly exceeds the concentration of other impurities and defects. This paper reviews results on n-type doping, the difficulty to achieve p-type conductivity, the role of the interface region, and that of the surface electron accumulation layer (SEAL) for electron transport in high-quality SnO2 films. Taking into account, the interface carrier system in the films was vital for the interpretation of the transport measurement results. The methodology used here can also be applied to the

transport analysis in other thin-film materials such as other transparent semiconducting oxides. Our semiconductor-like approach aimed at the basic understanding (rather than targeting an application) distinguishes this work from the tremendous body of work on SnO2 nanoparticles, transparent contacts, and gas sensors. Many published results on n-type (and even p-type) doping of SnO2 films were achieved with cost-effective low-quality material (e.g., prepared by spray pyrolysis) and doping concentrations well in