Influence of Al dopant on microstructure and optical properties of ZnO thin films prepared by sol–gel spin coating method G. Srinivasan a,b , R.T. Rajendra Kumar c , J. Kumar a, * a Crystal Growth Centre, Anna University, Chennai 600 025, India b RMD Engineering College, Kavaraipettai, Tamilnadu 601 206, India c School of Physical Sciences, Dublin City University, Dublin 9, Ireland Received 22 July 2006; received in revised form 26 September 2006; accepted 9 November 2006 Available online 21 February 2007 Abstract Zinc oxide thin films are prepared on c-plane sapphire substrates using the sol–gel spin coating method. After coating the samples were annealed at 873 K. Effects of Al doping on the morphological, microstructural and optical properties were examined. Both the pure and Al doped ZnO samples are (0 0 0 2) oriented. Undoped ZnO samples posses film-like morphology where as Al doping resulted in the formation of disconnected spherical grains. Cathodoluminescence spectroscopy measured at room temperature shows a strong ultra vio- let exciton emission around 382 nm along with deep level green emission due to defects around 500 nm for undoped ZnO film. Excitonic emission blue shifted to 380 nm and increase in deep level emission were observed for Al doped ZnO samples possibly due to size effects. Ó 2007 Published by Elsevier B.V. PACS: 61.10.Nz; 68.55.Ln; 81.20.Fw; 81.40.Ef Keywords: Thin films; Sol–gel; Cathodoluminescence; Scanning electron microscopy; Zinc oxide 1. Introduction A well-known wide-band gap wurtzite structured zinc oxide with four-fold tetrahedral coordination lies in the border between ionic and covalent semiconductors. Recently pure and doped zinc oxide films have been redis- covered as a subject of considerable interest in research due to their physical properties and a wide range of possible applications. A special care is directed to optical and mag- netic memory devices, blue light emitting diodes, solar cells and sensors. Also nanoparticles or quantum dots have received considerable attention due to the quantum phe- nomena resulting from an increase in band gap. Nano- structural ZnO providing a different shape and higher surface area might exhibit some interesting physical and chemical properties unattainable by other nanostructures. There have been several reports on the growth of highly quality ZnO samples doped with Al, Sn and In [1]. In cases of ZnO with different dopants, the morphology is modified either by thermal treatment [2] or by the concentration and the type of dopants [3]. The optical properties are influ- enced by surface morphology [2]. The electrical properties of ZnO are changed as function of indium concentration between 1 wt% and 3 wt% and the resistivity is reduced with 1 wt% indium [4]. The resistivity is reduced by control- ling the UV-light intensity and the amount of boron [5]. Also, electrical properties are modified by thermal treat- ment and a minimum d.c. film resistivity of 0.22 X cm was obtained at 350 °C for 3 h [6]. Extremely sharp c-axis orientated ZnO thin films were grown at 600 °C [2]. The more pronounced electrical and optical changes were observed when the doping concentration of Al is 1 at% 0925-3467/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.optmat.2006.11.075 * Corresponding author. Tel.: +91 44 2220 3573; fax: +91 44 2235 2870. E-mail address: marsjk@annauniv.edu (J. Kumar). www.elsevier.com/locate/optmat Optical Materials 30 (2007) 314–317