Effects of substrate position and oxygen gas flow rate on the properties of ZnO: Al films prepared by reactive co-sputtering D. Horwat ⁎ , A. Billard Laboratoire de Science et Génie des Surfaces (UMR 7570) – Ecole Nationale Supérieure des Mines de Nancy – Parc de Saurupt – CS 14 234 – 54042 Nancy Cedex, France Received 9 May 2006; received in revised form 23 November 2006; accepted 8 December 2006 Available online 20 January 2007 Abstract Al-doped zinc oxide (ZnO: Al) thin films are deposited at room temperature on rotating glass substrates by direct current co-sputtering of metallic targets under various oxygen partial pressures in the range 0.05–0.067 Pa. The films are polycrystalline with wurtzite structure and show preferential (001) orientation when they are transparent. The electrical resistivity is strongly influenced by sample position with the lowest value of 6.6 × 10 - 4 Ω cm far from the magnetron axis, where it is directly linked to grain size. As the oxygen gas flow rate is enhanced, the optical transparency rises up and both the electrical conductivity facing the magnetron axis and its lateral homogeneity decrease. A significant reduction in heterogeneity as the draw distance increases suggests an influence of the energy of impinging metal atoms, the instantaneous deposition rate and oxygen reactivity on the electrical behaviour. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO: Al; Transparent conductor; Reactive co-sputtering; Lateral homogeneity 1. Introduction The performances of electro-optical systems such as solar cells and electrochromic devices are linked to the electrical and optical properties of transparent semi-conducting oxide electro- des. ITO (Indium Tin Oxide In 2 O 3 : Sn) is the most widely studied material of this category [1–4] and can achieve elec- trical resistivity as low as 2.10 - 4 Ω cm [5]. Nevertheless, it is difficult to obtain simultaneously high transparency and con- ductivity without heating the substrate. An alternative consists in Al-doped ZnO (ZnO: Al) films whose constitutive elements are less polluting and cheaper. The magnetron sputtering tech- nology is well suited to deposit these compounds at room temperature as thin and dense films. Moreover, this method can be dedicated for industrial treatment of large surface areas. In this paper is proposed an investigation of the structural, electrical and optical behaviours of ZnO: Al films prepared by reactive co-sputtering of Zn and Al targets under Ar + O 2 atmo- spheres. A particular attention is paid to the properties evolution in relation with oxygen partial pressure and sample position along a radius of the rotating substrate-holder. 2. Experimental procedure ZnO: Al films have been deposited on glass substrates by co- sputtering of Zn and Al metallic targets in the presence of argon–oxygen reactive gas mixtures. The experimental device is a 40-l sputtering chamber pumped down via turbo-molecular pump allowing a base vacuum of 10 - 4 Pa (Fig. 1). It is equipped with two targets mounted on unbalanced magnetrons and separated by 120 mm from each other. The substrates are placed on a rotating substrate-holder parallel to the targets surfaces at various distances from the substrate-holder axis (Dsha), at draw distances (Dts) reported in Table 1. The targets, 50 mm in diameter, are powered by Advanced Energy generators (MDX D.C. supply for Zn and pulsed D.C. Pinnacle+ for Al). The argon and oxygen flow rates are controlled with MKS flow- meters and the pressure is measured using a MKS Baratron absolute gauge. Deposition rates are determined by thickness measurements using the step method with a Talisurf profilometer, allowing an Thin Solid Films 515 (2007) 5444 – 5448 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +33 3 83 58 42 56. E-mail address: David.horwat@mines.inpl-nancy.fr (D. Horwat). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.188