A comparative study of transport properties of copper doped cadmium selenide thin films at two dopant concentrations

Abstract

We have explored the effect of Cu dopant concentration on the electrical transport properties of Cu doped CdSe thin films at two concentrations of Cu (at 1 and 5 at.%). Structural, morphological and elemental analysis has been carried out by using X-ray Diffraction (XRD), Scanning electron microscopy (SEM) and Energy Dispersive X-ray analysis (EDX). XRD analysis reveals the hexagonal (wurtzite) structure of nc-CdSe:Cu 1% and nc-CdSe:Cu 5% thin films. EDX analysis determines the atomic weight percentage of Cu in these thin films. SEM studies reveal that the grains are uniformly distributed all over the surface of the substrates for nc-CdSe:Cu 1% and nc-CdSe:Cu 5% thin films. Dark conductivity measurements are made on nc-CdSe:Cu 1% and nc-CdSe:Cu 5% thin films in the temperature range 110–370 K in order to determine the effect of Cu concentration. The obtained results reveal that dark conductivity increases as the Cu dopant concentration increases as compared to undoped nc-CdSe thin films. Dark conductivity graphs show two distinct regions at high and low temperatures with decreasing activation energies. Low temperature data is analyzed using Mott’s variable-range hopping model which indicates hopping in localized states near the Fermi level. Various Mott parameters such as the density of localized states N(EF), characteristic temperature (To), hopping distance (R) and hopping energy (W) near the Fermi level are calculated. The concentration of charge carriers and carrier mobility are calculated for nc-CdSe:Cu 1% and nc-CdSe:Cu 5% thin films by using Hall measurements. Constant photocurrent measurement is also performed on Cu 1% and Cu 5% doped CdSe thin films to determine occupied density of states, Urbach parameter and density of defect states. Obtained results show that occupied density of states increases for nc-CdSe:Cu 5% as compared to nc-CdSe:Cu 1% thin films.