Temperature effect on the electrical properties of undoped NiO thin films A. Hakim, J. Hossain, K.A. Khan * Department of Applied Physics and Electronic Engineering, University of Rajshahi, Rajshahi-6205, Bangladesh article info Article history: Received 10 June 2008 Accepted 10 May 2009 Available online 16 June 2009 Keywords: NiO thin films E-beam technique Temperature dependence of resistivity Size effect Activation energy Thermoelectric power abstract Undoped NiO thin films have been prepared onto glass substrate by e-beam evaporation of the element Ni in vacuum at w2  10 4 Pa. The as-deposited Ni films were then oxidized in air by heating about 2 h at a temperature of 470 K and then the oxidized Ni films are turned into NiO thin films. From the deposition time and film thickness after annealing in air, an effective deposition rate of NiO thin films was about 6.67 nms 1 . X-ray diffraction (XRD) study shows the NiO films are amorphous in nature. SEM studies of the surface morphology of NiO films exhibit a smooth and homogeneous growth on the entire surface. The elemental composition of NiO films is estimated by Energy Dispersive Analysis of X-rays (EDAX) method. The effects of temperature on the electrical properties of NiO thin films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat-treatment in air. Thickness dependence of conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with activation energy. The thickness dependence of activation energy as well as thermopower studies was done within 293–473 K temperature range, respectively. Thermopower study indicates the NiO films a p-type semiconductor. Optical study in the wavelength range 0.3 < l<1.2 mm range exhibits a high transmittance in the visible as well as in the near infra-red. Calculation from the optical data, the NiO sample exhibits a band gap at 3.11 eV, which does agree well with earlier reported values. These studies may be of importance for the application of this material in energy efficient surface coating devices. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Research and development on thin films has led to the conclu- sion that different classes of materials are of particular interest for different applications. Nickel oxide is of particular interest because the material has in recent a variety of applications in electronic devices [1–4], energy efficient smart windows [5], automobile mirrors [6], building glazings [4,7], and hetrojunction solar cells [8]. NiO is a wide band gap [9], low cost, promising ion storage mate- rials in terms of cyclic stability [10]. It has an excellent durability and electrochemical stability with large span optical density and possibility of manufacturing it by a variety of techniques. Literature reports indicate that thin films of NiO have been produced by a number of techniques by a number of researchers. These include the vacuum evaporation [11], electron beam evaporation [12], sputterings [13], spray pyrolysis [14], chemical deposition [15], sol– gel [16] and reactive pulsed laser ablation technique [17]. Most of the reports address themselves to the structural aspects of these films [4] emphasizing the effect of substrate temperature on elec- trochromic property, cyclic durability, surface morphology and sensing property [18] of the films, electronic response during potential switching and the degree of coloration [2] as well as of their practical use in electrochemical and photovoltaic cells [8]. Although there have been a number of investigations on the elec- trical, optical and structural properties of the films, no systematic study appears to have been done on the electrical and optical properties at varying deposition conditions. Moreover, there is a considerable lack of understanding [4,7] regarding the layer degradation and slow switching time as well as the material also exhibits a very different electrochemical and optical properties in terms of its deposition conditions and its applications in using different measuring techniques. Different microstructures caused by different deposition conditions could be the probable reasons for the lacks in understanding. Hence, there is a need to study how varying deposition conditions affect the physical prop- erties of NiO films to assess its usefulness in energy-oriented device applications. In this paper, we present and discuss the effect of pressure on the deposition condition of NiO thin films as well as the * Corresponding author. Tel.: þ880 721 750254; fax: þ880 721 750064. E-mail address: kakhan_ru@yahoo.ca (K.A. Khan). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.05.014 Renewable Energy 34 (2009) 2625–2629