Applied Surface Science 256 (2010) 4601–4605 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Oxygen influence on sputtered high rate ZnO:Al films from dual rotatable ceramic targets H. Zhu a,b,∗ , J. Hüpkes b , E. Bunte b , S.M. Huang a a Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, East China Normal University, 200062 Shanghai, PR China b IEF5-Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany article info Article history: Received 15 January 2010 Received in revised form 15 February 2010 Accepted 18 February 2010 Available online 25 February 2010 Keywords: ZnO:Al films Ceramic targets Sputtering Surface structures abstract In this study, the influence of oxygen on high rate (up to 110 nm m/min) sputtered aluminum doped zinc oxide films (ZnO:Al) was systematically investigated. Different oxygen gas flows from 0 sccm to 8 sccm were inputted into the chamber during the preparation of ZnO:Al films from dual rotatable ceramic targets under high discharge power (14 kW). The resistivity increases from 4.2 × 10 -4  cm to 4.3 × 10 -2  cm with the rising oxygen gas flow. While both the carrier concentration and mobility drop by one order of magnitude from 3.4 × 10 20 cm -3 to 2.5 × 10 19 cm -3 and from 43.5 cm 2 /V s to 5.6 cm 2 /V s, respectively. The as-grown ZnO:Al films and after-etched ZnO:Al films after a chemical wet etching step in diluted HCl solution (0.5%) exhibit different surface structures. All films show high light transmission and low light absorption but different light scattering properties (diffusion and haze) because of different sur- face structures. Moreover, ZnO:Al films display different optical bandgaps between 3.51 eV and 3.27 eV, which are corresponding to different carrier concentrations. The variation of mobility and morphology is related with chemisorption of oxygen in the grain boundaries as well as high energetic oxygen ions bombardment. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Aluminum doped zinc oxide thin films (ZnO:Al) are already used in thin film solar cells as an attractive transparent front electrode due to its high transparency in visible region and low electricity as well as high carrier mobility [1]. There are many fabrication meth- ods for the preparation of the ZnO:Al thin films such as magnetron sputtering [2–5], evaporation [6,7], and sol–gel method [8,9]. For reactive magnetron sputtering of ZnO:Al films, oxygen works as a reactive gas to form ZnO films together with sputtered zinc atoms. Different oxygen gas flows lead to the sputtering carrying out at different reactive regions, which are mainly comprised of metallic region, oxide region and transition region [4,5]. While for non- reactive sputtering from ceramic targets, a good base pressure is necessary for preparation of high quality ZnO films. Apart from the negative influence of nitrogen partial pressure inside the chamber, oxygen partial pressure is also a considerable factor to determine the film properties. For fast magnetron sputtering technologies, the deposition conditions of ZnO:Al films should be much severer to achieve good homogeneity as well as good optical and electri- cal properties. In this study, we study systematically the oxygen ∗ Corresponding author at: IEF5-Photovoltaik, Forschungszentrum Jülich GmbH, Leo-Brandt-Straße D-52425 Jülich, Germany. Tel.: +49 02461 611550. E-mail address: hongbing1982@hotmail.com (H. Zhu). influence on high rate sputtered ZnO:Al films from dual rotatable ceramic targets, with different flows of oxygen into the chamber during sputtering. In this paper, optical and electrical properties were investigated. Besides, morphologies of as-grown as well as after-etched ZnO:Al films after a chemical wet etching step in diluted hydrochloric acid solution (HCl, 0.5%) were checked with scanning electron microscope (SEM). More details about the prop- erties of prepared films and relative causes will be discussed. For dual rotatable ceramic targets, the utilization of target material can be greatly enhanced up to 80%. Since by rotating the whole areas of targets can be sputtered and therefore no erosion zones appear on the target surfaces, which usually happen on planar targets. In addition, the deposition rates of films are easily increased simply by rising discharge power, and meanwhile the high rate films keep good electrical properties [2]. However, the binding and prepa- ration techniques for rotatable targets need strict requirements compared to that for planar targets. 2. Experimental details ZnO:Al films were prepared on corning glass substrates (Corning Eagle 2000) in a vertical in-line sputtering system for 30 × 30 cm 2 substrates (VISS 300, by von Ardenne Anlagentechnik, Dresden, Germany) as shown in Fig. 1. Dual rotatable magnetron cath- odes with ZnO:Al 2 O 3 (99.5:0.5 wt%) ceramic rotatable targets were 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.057