Structural and electrical properties of transparent conducting Al 2 O 3 -doped ZnO thin films using off-axis DC magnetron sputtering Sunghwan Lee a,n , Seung-Hyun Kim a , Youngha Kim b , Angus I. Kingon a , David C. Paine a , Kwangsoo No b a School of Engineering, Box D, Brown University, Providence, RI 02912, USA b Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea article info Article history: Received 16 May 2012 Accepted 20 June 2012 Available online 3 July 2012 Keywords: Al 2 O 3 -doped ZnO DC magnetron sputtering Transparent conducting oxide c-axis orientation Off-axis deposition abstract Structural and electrical properties of off-axis DC magnetron sputtered Al 2 O 3 -doped ZnO (AZO) films were systematically investigated as a function of deposition distance from the center. With increasing distance, the AZO films showed an enhanced crystallinity and a denser microstructure with a smooth surface. The AZO film sputtered at the edge of the deposition stage ( 6 cm away from the center) showed the highest mobility ( 10.1 cm 2 /V s) and the lowest resistivity (  2  10 3 O cm) due to the high plasma and thermal power density, which was suitable for transparent conducting oxide applications. & 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, there has been a growing interest in transpar- ent conducting oxide (TCO) systems because of their potential uses as transparent electrodes in optoelectronic devices [1], thin film transistor applications [2,3] and thin film solar cells [4,5]. Indium tin oxide (ITO) is mostly used as TCO materials since it can produce high visible light transmittance ( 485%), low electrical resistivity (  10 4 O cm), and good adhesion strength to various substrates. However, this material shows the chemical instability in a reducing atmosphere (typically in the use of hydrogen) and needs high deposition temperature ( 4300 1C). In addition, the material cost is relatively high. Currently, all of these become major concerns for commercial manufacture and applications. Thus a worldwide effort to identify an effective replacement system is underway [6]. We propose the thermally and chemically stable Al 2 O 3 -doped ZnO (AZO) system as a replacement for the conventional ITO system. AZO is a well known high transparent conducting material with high electron carrier density ( 4  10 20 /cm 3 ) [7] which is attributed to the presence of intrinsic defects, such as oxygen vacancies (V O dd ) and Zn interstitials (Zn i dd ) [8]. In the present work, 2 wt% Al 2 O 3 -doped ZnO was selected to investigate since a high concentration of Al 2 O 3 dopant ( 43 wt%) can act as an electrically inactive site and an energy barrier limiting carrier transport due to the segregation of excessive atoms into grain boundaries of the film [1,7]. An off-axis sputtering configuration is applied to produce high quality thin films by minimizing neutral bombardment during the deposition process since this technique generally enhances the crystallinity and the densifica- tion of the films under optimized condition [9]. In this paper, we also report the spatial variation of the structural and the electrical characteristics of AZO films as a function of deposition distance from the center. 2. Experimental AZO thin films (  200 nm) were sputtered on square glass substrates (Corning Eagle 2000) using an off-axis DC magnetron sputtering system, with the target axis inclined by 451 to the substrate normal from a 98 wt% ZnO–2 wt% Al 2 O 3 (AZO) sintered ceramic target (Samsung Corning Co.). The film deposition was performed at a temperature of 200 1C and a DC power of 200 W with a working pressure of 4  10 3 Torr using high purity Ar (99.99%) as a sputtering gas. For the investigation of spatial variations of film properties, the samples were labeled from 0 (for the sample at the center of the deposition stage) to 6 (for the sample 6 cm away from the center), increasing the distance from the center in 1 cm steps. The crystalline structures of AZO films were evaluated via X-ray diffraction patterns (XRD), using a y–2y coupled mode scan and Cu Ka radiation (l ¼ 1.542 nm) at 40 kV and 40 mA (Rigaku D/ MAX-RC). The density of AZO thin films was determined via X-ray reflectivity (XRR) with o–2y scan and o scan (Rigaku RS3000). For microstructure and thickness analysis, a scanning electron Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.094 n Corresponding author. Tel.: þ1 401 863 6833; fax: þ1 401 863 7677. E-mail address: Sunghwan_Lee@brown.edu (S. Lee). Materials Letters 85 (2012) 88–90