Characterization of cadmium doped zinc oxide (Cd : ZnO) thin films prepared by spray pyrolysis method

Thin films of cadmium doped zinc oxide (Cd : ZnO) with different cadmium concentrations have been prepared by the spray pyrolysis method on different substrates at 400 deg. C. The physical properties of the films were studied as a function of increasing cadmium dopant concentration (up to 25 wt%). The films were characterized by different methods to understand their structural, optical and electrical properties. Energy dispersive x-ray diffraction spectroscopy analysis showed that the Cd concentration in the solid film is slightly lower than that of the starting solution. The x-ray diffraction analysis revealed that the films are polycrystalline in nature having a hexagonal wurtzite type crystal structure with a preferred grain orientation in the (0 0 2) direction. Due to Cd doping, the degrees of polycrystallinity increased simultaneously and the orientation of the grains in the (0 0 2) axis is found to be deteriorated. Atomic force microscopy measurements reveal that the surface morphology of the films changes continuously with a decrease in the grain size due to Cd doping. Both photoluminescence and optical measurements showed that the band gap decreases from 3.12 to 2.96 eV with increasing Cd concentration. Increasing the Cd concentration also leads to the broadening of the emission peak and degrading the crystalline quality. The films are highly transparent in the visible region and the absorption edge showed a red shift. The sheet resistance of pure ZnO has been determined as 650 Ω sq-1 cm-1 and is decreased for higher concentrations of Cd doping. The characterization studies clearly indicate the incorporation of Cd into ZnO; hence the observed decrease in the optical band gap and electrical resistivity can be directly attributed to the effect of Cd ion incorporation into the ZnO lattice.