Characterization of structural, optical and electrical properties of pure and al doped Zno thin films prepared by spray pyrolysis

Abstract

Pure and aluminum (Al) doped zinc oxide (AZO) thin films were prepared by spray pyrolysis on glass substrate at 3500C substrate temperature. Zinc acetate dihydrate (Zn (CH3COO) 2) and Aluminium chloride hepahydrate (AlCl3.6H2 O) are used as a precursors to synthesize the ZnO and AZO thin films. The Al (dopant) concentration was varied from 0 to 2.0 at% in thin films of AZO. The surface morphological, structural, optical and electrical properties of the as-deposited films are studied as a function of different doping level. The thickness of the synthesized films was measured by Fizeau fringes interference method which varied from 260 to 280nm. From X-ray diffraction (XRD) it confirmed that all the ZnO films having polycrystalline nature and preferential orientations are along (100), (002), (101), (102) and (110).The lattice parameters (a=3.2120Å and c= 5.1722Å) were calculated for pure ZnO and varies (a=3.1870Å to 3.2569Å and c=5.2148 to 5.2339Å) due to different doping level of Al. The films were found in typical hexagonal wurtzite structure with crystallite size varying between 3.67 to 7.32 nm. The scanning electron microscopy (SEM) micrographs of as-deposited films show uniform surface and deposition covers the substrate well. Nano fibers are observed around the nucleation centre of the as-deposited thin films. The quantitative analysis of the as-deposited ZnO and Al doped ZnO thin films carried out by Energy Dispersive X-ray (EDX). EDX results clearly show that the samples are typically comprised of both Zn and O for pure and Zn, O, Al for Al doped films. From EDX data it is found that the atomic weight % of Al increased with the increase of Al concentration in Al doped ZnO thin films and Zn1-xAlx O films are stoichiometric. The optical constants such as absorbance, transmittance, refractive index and dielectric constant of the films have been studied for as deposited thin films in the wavelength ranges from 250 to 1100nm. For direct transition the observed optical band gap of Zn1-xAlxO depends on the doping concentration and varies from 3.24 eV to 3.16 eV. The average transmittance of all the films is higher than 90% in a wide wavelength 400-1100 nm. The activation energy of Zn1-xAlx O films is found to increase with the incorporation of low concentration of Al in the solution and start to decrease again for high doping concentration. The refractive index was found to decrease with the doping concentration and decreases gradually with the wavelength. The resistivity, • , decreases with increasing of doping concentration and temperature (300K-430K) but at higher doping concentration resistivity starts to increase.