Solid State Communications 147 (2008) 465–469 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc The Meyer–Neldel rule in sol–gel derived polycrystalline ZnO:Al thin films Parmod Sagar a,∗ , Manoj Kumar b , R.M. Mehra c a Department of Physics, Shivaji College, University of Delhi, New Delhi-11007, India b Department of New Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea c Department of Electronic Science, University of Delhi South Campus, New Delhi-110021, India article info Article history: Received 21 May 2008 Received in revised form 30 June 2008 Accepted 1 July 2008 by T. Kimura Available online 4 July 2008 PACS: 72.80.Ey 73.50.Bk 73.50.-h Keywords: A. ZnO thin film B. Sol–gel process D. Electronic transport D. Al doping abstract The present paper reports the application of Meyer–Neldel Rule to the observed spread in activation energy in electrical conduction of sol–gel derived ZnO:Al polycrystalline thin films deposited on corning 7059 glass substrate. The experimental results obtained on electrical conductivity over a wide range of temperature (150–600 K) have been analyzed in the light of theory corresponding to the two different transport paths. The conduction mechanism in the high temperature range (400–600 K) is found to be thermally activated whereas at low temperature it is attributed to variable range hopping. The variation in activation energy with Al doping concentration in ZnO has been studied in order to estimate the characteristic MNR temperature where the conductivity process is independent of the activation energy. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction ZnO is a wide band gap semiconducting material exhibiting unique properties useful for various device applications [1–5]. Extensive studies have been made on the structural and optical properties of ZnO thin films grown by various techniques [6– 8]. However, very little efforts have been made towards the measurements on the electrical conductivity especially over a wide temperature range [9,10]. Studies on conduction mechanism of semiconductors are useful for development of functional devices especially for photonic applications. The electrical resistivity of ZnO thin films decreases by doping it with positive trivalent atoms such as (Al, Ga, In, B) at cation site [11,12]. The incorporation of Al in ZnO showed improvement and stability of conductivity of ZnO:Al films. ZnO offers a wide variation in the conductivity from 10 4 to 10 −8  −1 cm −1 depending upon the Al dopant concentration of the film and the growth kinetics [9–12]. The electrical conduction in doped ZnO films above room temperature has been attributed to thermal excitation of electrons from the donor levels originating from native defects or impurity atoms [9,10]. ∗ Corresponding author. Tel.: +91 11 25540316. E-mail addresses: panwarm72@yahoo.com, sagarparmod1123@yahoo.co.in (P. Sagar). In general, the electrical conductivity of a semiconductor is strongly temperature dependent and a spread of activation is ob- served due to creation of defects levels. The temperature depen- dent conductivity in several oxide materials have been analyzed in the prospect of variable range hopping [13–17]. The slow increase in conductivity with temperature in the low temperature region is co related with hopping mechanism between the centers having variable barrier heights [VRH]. Meyer–Neldel Rule (MNR) studied the thermal dependent electrical conductivity of the semiconduc- tors and correlates the spread in activation energy with the ex- ponential pre-factor [18]. It is reported that the process becomes independent of activation energy or dopant concentrations at a characteristic MNR temperature. MNR describes an exponential relation between the activation energy and pre-exponential fac- tor, and has been observed in large range of materials which in- clude single crystals, polycrystalline, amorphous, organic solids and even ionically conducting materials [19–24]. Several mecha- nisms have been proposed for the applicability of MNR in various semiconductors. Jackson [22] reported that whenever a multi- trapping transport process is observed over a fixed distance as a function of temperature, MNR should be followed. Many re- searchers attributed the MNR to the effect of disorder within the material [23,24]. However no efforts have been made to study the applicability of MNR on temperature dependent electrical conduc- tivity data of ZnO thin film. The incorporation of Al in ZnO cre- ates donor levels below the conduction band and influences the 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.07.001