Role of film thickness on the properties of ZnO thin films grown by sol-gel method Vinod Kumar a,b,f, ⁎, Neetu Singh c,d , R.M. Mehra e , Avinashi Kapoor c , L.P. Purohit b , H.C. Swart f a Materials Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110 067, India b Department of Physics, Gurukula Kangri University, Haridwar-249 404, India c Department of Electronic Science, University of Delhi South Campus, New Delhi-110 021, India d Department of Electronics, Keshav Mahavidyalaya, University of Delhi-110 034, India e School of Engineering & Technology, Sharda University, Greater Noida-201 306, India f Department of Physics, University of the Free State, Bloemfontein, ZA-9300, South Africa abstract article info Article history: Received 24 October 2012 Received in revised form 14 May 2013 Accepted 17 May 2013 Available online 25 May 2013 Keywords: Sol-gel Zinc oxide Thickness Surface morphology Solar cell and gas sensors This paper reports the effect of thickness on the structural, morphological and optical properties of zinc oxide (ZnO) films. Thickness of ZnO films varied from 98 to 366 nm with an increase in the number of deposition cycles. Surface morphological studies showed that the increase in the film thickness causes an increase in the grain size. Roughness of the films has increased from 5.8 to 47 nm with an increase in the film thickness from 98 to 366 nm. The band gap is observed to vary from 3.33 to 3.24 eV with change in the film thickness from 98 to 366 nm. Thickness of the film affected the overall properties of the ZnO films significantly. The large surface roughness makes ZnO films to be potentially used as electrode in solar cells and gas sensing applications. © 2013 Elsevier B.V. All rights reserved. 1. Introduction ZnO is a group II–VI compound n-type semiconductor, with hex- agonal wurtzite structure. ZnO has a direct band gap of 3.37 eV [1] and a large exciton binding energy of 60 meV at room temperature [2]. The microscopic surface topology and grain structure of transpar- ent conducting films strongly affect the performance of solar cells. For such applications, the development of low resistive transparent conducting oxide (TCO) thin films along with textured surface is very important [3,4]. Presence of texture is an advantage for applica- tion in solar cells because scattering of light into the active layer of the cell is enhanced. This increases the optical path length [5] and conse- quently the generation of free carriers. ZnO is a promising material for optoelectronics [6,7], photovoltaics [8], sensors [9], data storage [10], biochemical/chemical sensors [11,12] and solid state lighting sources [13]. Nowadays, many physical and chemical techniques to grow ZnO thin film have been used such as spray pyrolysis [14], metal organic chemical vapor deposition [15], pulsed laser deposition [16], RF sputtering [17] and sol-gel technique [18]. Amongst the different avail- able techniques, the sol-gel technique has the advantage of coating on large areas with easy control of the doping level, solution concentration and homogeneity, without using expensive and complicated equip- ments compared with the other methods. In the sol-gel process, the de- posited films are in amorphous state, which are transformed into crystalline state during the annealing process. There are many factors affecting the crystallinity of the films such as the substrate, solution chemistry, heat treatment conditions and thickness etc. The optical and electrical properties are closely related to the crystallite size and orientation, which are affected by the film thickness. Myoung et al. [19] have reported that the crystalline quality, electrical and optical properties of the ZnO films depend on the thickness. Reddy et al. [20] have reported the effect of film thickness on the structural, mor- phological and optical properties of ZnO films prepared by RF magnetron sputtering. For application in optoelectronic devices, normally the opti- mum film thickness is chosen for the best device performance. Therefore, it is very important to study the effect of film thickness on structural, morphological and optical properties of ZnO film. With this view in mind, in this paper, we present the effect of thickness variation on the structural, morphological and optical properties of ZnO films deposited on glass substrates by the sol-gel spin coating technique. 2. Experimental details ZnO films on Corning (1737) substrates were deposited by the sol-gel method using the spin coating technique. Precursor solution of Thin Solid Films 539 (2013) 161–165 ⁎ Corresponding author at: Department of Physics, University of the Free State, Bloemfontein, ZA9300, South Africa. Tel.: +27 848705159. E-mail address: vinod.phy@gmail.com (V. Kumar). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.05.088 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf