Influence of Oxygen Addition and Substrate Temperature on Textured Growth of Al-Doped ZnO Thin Films Prepared by RF Magn
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Influence of Oxygen Addition and Substrate Temperature on Textured Growth of AlDoped ZnO Thin Films Prepared by RF Magnetron Sputtering P. Kuppusami1, K. Diesner, I. Sieber, K.Ellmer Hahn-Meitner-Institut, dept. Solar Energetics, D-14109 Berlin, Glienicker Str. 100, Germany 1 Indira Gandhi Centre for Atomic Research, Kalpakkam, India ABSTRACT Sputtering of aluminium doped zinc oxide thin films from a ceramic ZnO:Al target requires a controlled addition of oxygen to the sputtering atmosphere in order to obtain films with low resistivity and high transparency. In this paper the influence of the oxygen addition and of the substrate temperature on the structural, morphological and electrical properties of ZnO:Al films is investigated. The oxygen addition leads to a minimum resistivity when the oxygen content during sputtering is 0.2%. This small amount of oxygen not only improves the transparency of the films, it also induces to a significant grain growth as revealed by scanning electron microscopy. A further increase of the oxygen content leads to highly resistive films, due to a complete oxidation of the dopant Al. As expected, higher substrate temperatures from about 373 to 673 K improve the of crystallinity and hence the resistivity. The lowest resistivity achieved was about 1.2.10-3 Ωcm. At still higher temperatures the resistivity increases which seems to be due to an outdiffusion of sodium into the ZnO:Al films from the soda lime glass, compensating part of the donors. INTRODUCTION Transparent conducting oxide (TCO) films have attracted the attention of many researchers due to their wide range of applications as transparent electrodes in flat panel displays, solar cells or light emitting diodes [1, 2]. These oxides, which are wide-band gap semiconductors, can be doped to very high charge carrier densities leading to high electrical conductivity (80 %) in the visible and high reflectance in the infrared region [1-3]. The increasing use of TCO films has also promoted the development of materials such as zinc oxide (ZnO) and tin oxide (SnO2) in place of the expensive indium-tin oxide (ITO). Compared to ITO and tin oxide, ZnO has the following advantages: (i) ZnO is abundant, cheap and non-toxic, (ii) it can be prepared with low resistivities at low substrate temperatures without additional treatments and (iii) it can be easily etched in order to form fine electrodes. Various chemical and physical deposition techniques can be used for the preparation of ZnO:Al films. Among these techniques, magnetron sputtering has become the dominant technique, due to its inherent advantages such as high deposition rate, low substrate temperature and scalability to large areas [4]. The electrical and optical properties of ZnO:Al films are determined by an interplay between the texture, the microstructure and the mechanical film stress as well as the degree of oxidation of the zinc oxide host and the dopant aluminium. These properties are strongly influenced by process parameters of the specific deposition technique such as depositio
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