Effect of sol concentration on the properties of ZnO thin films prepared by sol–gel technique M. Dutta, S. Mridha, D. Basak * Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received 4 June 2007; received in revised form 12 July 2007; accepted 8 October 2007 Available online 11 October 2007 Abstract ZnO thin films are deposited on the glass substrates by sol–gel drain coating technique by varying the concentration of the sol. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were used to investigate the effect of sol concentration on the crystallinity and surface morphology of the films. The results show that with increase in sol concentration, the value of full width at half maximum (FWHM) of (0 0 2) peak decreases while the strain first increases and then decreases. The sol with higher concentration results in the increase in the grain size. The studies on the optical properties show that the band gap value increases from 3.27 to 3.3 eV when the sol concentration changes from 0.03 to 0.1 M. The photoconductivity studies reveal that the film for 0.05 M sol shows the maximum photoresponse for ultraviolet (UV) wavelength (<400 nm) which is co-related with the deep-level defects. The growth and decay of the photocurrent is found to be slowest for the same film. # 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Sol–gel; Sol concentration; Optical properties; Photoconductivity 1. Introduction ZnO is a wide and direct band gap (3.37 eV) semiconducting material with a large exciton binding energy (60 meV) [1]. It is an attractive and promising material for many applications in surface acoustic wave devices (SAW) [2], transparent electrode [3], blue and ultraviolet (UV) light emitters [4], solar cell windows [5], gas sensors [6], photovoltaic device [7] and room temperature ultraviolet lasers [4]. Various growth techniques such as chemical vapor deposition [8], r.f. magnetron sputtering [9], pulsed laser deposition (PLD) [3], evaporation [10], spray pyrolysis [11], photo-atomic layer deposition [12], metal oxide chemical vapor deposition (MOCVD) [13], molecular beam epitaxy (MBE) [4] and sol–gel process [14] have been used for ZnO film. The sol–gel process has several advantages due to its simplicity, easy control of the film composition, safety, low cost of the apparatus and raw materials. In fact, sol–gel derived ZnO films might be promising for low cost optoelectronic devices [14]. The characteristics of the films are generally influenced by the preparation conditions such as the deposition method, post- deposition annealing temperature, types of substrates, etc. [15]. It has already been observed that the optoelectronic properties of sol–gel ZnO films are influenced largely by the thickness of the film [16]. Especially, for a polycrystalline film, the grain boundary-related micropores which depend on the film thickness, control the photoconduction properties [16]. For a sol–gel film, the thickness can be varied by the number of coatings and also by the concentration of the sol. The latter affect the thickness via the viscous drag on the substrate by the moving liquid. In this paper, we have investigated the effect of sol concentration on the structural, optical and optoelectronic properties of the ZnO film grown on glass substrates by a sol– gel drain coating technique. It is found that the structural and optical properties are improved as the concentration increases from 0.03 to 0.1 M but the ultraviolet light sensitivity becomes maximum for a film deposited by using a sol of 0.05 M. 2. Experimental procedure ZnO thin films were deposited on glass substrates using sol– gel drain coating method. The glass substrates were cleaned ultrasonically initially first by HCl and KOH then by acetone, methanol and finally with de-ionized water. Different sols of www.elsevier.com/locate/apsusc Available online at www.sciencedirect.com Applied Surface Science 254 (2008) 2743–2747 * Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805. E-mail address: sspdb@iacs.res.in (D. Basak). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.009