Optoelectronic properties of Cr doped ZnO thin films deposited by RF magnetron sputtering using a powder target

Abstract

In this work Cr doped ZnO thin films were deposited onto glass substrates by RF magnetron sputtering using a powder target at room temperature in order to study the effect of Cr concentrations on structural, optical and electrical properties.X-ray diffraction analysis show that the undoped ZnO thin films crystallize in the hexagonal wurtzite structure and show a preferential orientation along the (002) plane. For the Cr doped ZnO thin films, we found a gradual change of orientation from the (002) plane to the (101) plane as the Cr content increases.The optical transmittance in the visible region for undoped and Cr doped ZnO thin films achieve 88% and the optical band gap energy decreases from 3.35 eV to 3.31 eV.Hall Effect measurements at room temperature showed that the conduction is n-type for all samples. We have found that doping increases the concentration of carriers, so the resistivity of ZnO which is relatively low (3.42 × 10−2 Ω cm) decreases by an order of magnitude by inserting the dopant into the ZnO matrix. The electrical conductivity and the conduction mechanism were also studied by the impedance spectroscopy in the temperature range which extends from 633 K to 703 K. The AC conductivity of the Cr doped ZnO thin films increases with the angular frequency ω. The exponent S decreases with increasing temperature and remains less than unity. Such behavior suggests the validity of the CBH model. The activation energy obtained from the relaxation frequency and the DC conductivity are in good agreement confirming that the conduction mechanism is thermally activated by jumping between the localized states.