Effect of annealing temperature on the optical and electrical properties of aluminum doped ZnO films

Abstract

abstract Aluminum doped ZnO thin films were successfully deposited on the silicon substrates by spin coatingmethod. The effects of an annealing temperature on electrical and optical properties were investigatedfor 1.5 at.% of aluminum. Refractive index profile has been obtained for the film annealed at 350 C usingellipsometry and it has shown minimum refractive index of 1.95 and maximum value of 2.1. Thicknessprofile shows quite good uniformity of the film having minimum thickness value of 30.1 nm and maxi-mum value of 34.5 nm. Maximum conductivity value obtained was 4.63 X 1 -cm 1 for the film annealedat 350 3C. Maximum carrier density of 2.20 10 17 cm was deduced from the Hall measurement andFourier transform infrared spectroscopy clearly reveals major peak at 407 cm 1 in the spectra associatedwith the ZnO bond. 2009 Elsevier B.V. All rights reserved. 1. IntroductionTransparent conducting oxide (TCO) films are of prominentinterest due to their potential in optoelectronic devices such astouch panels, flat panel displays (FPD), and thin film solar cells[1,2]. For display applications, TCO films of high optical transmit-tance in the visible region and high electrical conductivity are re-quired. Indium tin oxide (ITO) has been commercially used forthe flat panel displays [3,4]. However, impurity-doped zinc oxides(ZnOs) serves as an alternative for ITO-based TCO because of itsadvantages of low cost, resource availability and nontoxicity. Zincoxide is accepted as wide band gap semiconductors as an alterna-tive to GaN because it has band gap of 3.37 eV at room tempera-ture, large free exciton binding energy (60 meV), and highmechanical and thermal stabilities. With enhancement in the qual-ity andcontrolof conductivityinbulkZnO [5] foundits potentialasa material for short wavelength light emitters and transparentelectronics [6,7].Generally, the undoped ZnO is showing a n-type conductivity.Particularly, an Al doped ZnO transparent conductive oxide (TCO)filmhashightransmittanceinthevisibleregionwithalowresistiv-ity and by doping it with aluminum its electrical properties can beimproved[8,9].Asaresult,theAldopedZnOshowsexcellenttrans-parency over the entire visible spectrum and has better transportproperties than tin oxide due to higher electron mobility. Thereare several methods used for the fabrication of ZnO thin films suchas pulsed laser deposition [10–12], sputtering [13,14], molecularbeam epitaxy [15], and sol–gel process [16], etc. Among thesemethods, the sol–gel technique has several advantages over theothers such as low cost, simplicity; homogeneity and large areafilms can be prepared at lower temperature.Many factors can affect the properties of aluminum doped ZnOfilms prepared by sol–gel method. Among them, Aluminum com-position and annealing temperature have significant effect on theoptical and electrical properties of the film. Present work reportsaluminum-doped zinc oxide films deposited on silicon substrateusing sol–gel spin coating process. We have already reported effectof aluminum doping on the properties of the films [9]. Here, wehave investigated effect of annealing temperature on the Alumi-num doped ZnO films prepared by sol–gel method on the siliconsubstrate by considering 1.5 at.% of aluminum. The main goal ofthe paper is to investigate effect of annealing temperature on theelectrical and optical properties of the aluminum doped ZnO filmsby keeping aluminum concentration fixed.