Tailoring of ZnO thin films: effect of number of coating and sample ageing

Abstract

In this work, the effect of number of coating and sample ageing on the properties of ZnO thin films prepared by dip coating technique using 0.1 M aqueous solution of zinc acetate and methanol on the glass substrate was experimentally investigated. The linear four probe configuration has been used to measure sheet resistance of the deposited samples whereas laboratory purpose Hall-effect measurement apparatus has been used to measure carrier concentration and Hall mobility. The dark sheet resistance (180.37 kΩ/□ to 41.69 kΩ/□) and normal sheet resistance (163.15 kΩ/□ to 31.72 kΩ/□) of ZnO thin films decreases significantly as the sample age increased from 79 to 167 days, particularly for films deposited with 8, 12, 16, and 20 coats. It is also found that as the number of coating increases, thickness of the thin films (from 199.7 nm to 211.5 nm) and carrier concentration increases whereas the Hall mobility decrease. The spectrophotometer (USB2000, photonics) has been used to investigate optical properties of deposited films. All the ZnO thin films demonstrate remarkable transparency, exhibiting transmittance lies between 61% and 85% across the wavelength range of 450 nm to 950 nm. The optical transmittance is observed to decrease with increase in number of coatings in the wavelength range 650 to 950 nm. The optical band gap value increases slightly from 3.22 eV to 3.25 eV for corresponding number of coatings from 8 to 20. This slight change in optical band gap indicates that it does not depend significantly on number of coating and thickness of the films.