Textured Zinc Oxide by Expanding Thermal Plasma CVD: the Effect of Aluminum Doping
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V3.9.1
Textured zinc oxide by expanding thermal plasma CVD: the effect of aluminum doping R. Groenen, E.R. Kieft, J.L. Linden1, M.C.M. van de Sanden Eindhoven University of Technology, Dept. of Applied Physics, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands 1 TNO TPD, Division Materials Research and Technology, P.O. Box 595, NL-5600 AN Eindhoven, The Netherlands
ABSTRACT Aluminum doped ZnO films are deposited on glass substrates at a temperature of 200°C by expanding thermal plasma CVD. Surface texture, morphology and crystal structure have been studied by AFM, SEM and XRD. A rough surface texture, which is essential for application as front electrode in thin film solar cells, is obtained during deposition. The addition of aluminum as a dopant results in distinct differences in film morphology, a transition from large, rounded crystallites to a more pyramid-like structure is observed. The structure of films is hexagonal with a preferred crystal orientation in the faces (002) and (004), indicating that films are oriented with their c-axes perpendicular to the substrate plane. In addition, spectroscopic ellipsometry is used to evaluate optical and electronic film properties. The presence of aluminum donors in doped films is confirmed by a shift in the ZnO band gap energy from 3.32 to 3.65 eV. In combination with reflection and transmission measurements in the visible and NIR ranges, film resistivities have been obtained from the free-carrier absorption. These results are consistent with direct measurements. Resistivities as low as 6.0 10-4 Ωcm have been obtained. INTRODUCTION Zinc oxide (ZnO) is a transparent conducting oxide (TCO) of considerable technological interest for amongst others application in thin film solar cells. It allows for superior transparency, lower deposition temperature, lower costs and less environmental impact compared to the widely used fluorine doped tin oxide (SnO2:F) which shows an excellent surface texture enabling effective light trapping properties [1]. Recently, a new method has been presented for low temperature deposition of surface textured ZnO utilizing an expanding thermal argon plasma created by a cascaded arc [2]. It has been shown that high quality material is obtained, showing excellent performance in thin film a-Si:H solar cells [3,4]. Here, the effect of aluminum doping on the ZnO film properties is investigated. The structural properties will be discussed briefly. Both optical and electronic film properties are studied using spectroscopic ellipsometry in combination with reflection and transmission measurements in the visible and NIR ranges.
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V3.9.2
EXPERIMENTAL Undoped and aluminum doped ZnO films are deposited on Corning 1737F glass substrates (100x50 mm2) by the expanding thermal plasma chemical vapor deposition process as described in d
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