Structural and optical investigation of nanocrystalline Zn1−xNixS diluted magnetic semiconductor thin films

Abstract

We have investigated the structural and optical properties of nanocrystalline Zn1−xNixS (0.005≤x≤0.2) diluted magnetic semiconductor thin films synthesized by electron beam deposition technique. All the deposited films crystallize in zincblend type structure. The structure parameters of the nanosize deposited films are extracted by X-ray diffraction and atomic force microscopy techniques and found to vary from 14nm to 17.5nm. The X-ray diffraction measurements show that the highest solubility limit of Ni in ZnS matrix is found to be ≈15%. In a wide spectral range, the refractive index and refractive index dispersion of the nanocrystalline Zn1−xNixS films have been calculated from transmission or reflection spectra using wavenumber method and found to increase with increasing Ni concentration. The increase of the refractive index dispersion of Zn1−xNixS with Ni concentration has been ascribed to the corresponding change in the net polarizability of the whole system. Through the higher wavenumber region (2×10+4cm−1 to 2.5×10+4cm−1), the group velocity factor is found to increase with increasing Ni concentration. In addition, for fixed Ni concentration (x=0.005) the refractive index and refractive index dispersion are found to increase with increasing film thickness. Wemple and DiDomenico single oscillator model was used to explain the increases of the refractive index dispersion with Ni concentration based on variation of coordination number of the Zn1−xNixS system. The optical transition is found to be direct transition with optical energy gap Egopt decreases with increasing Ni concentration. The reduction of Egopt with increasing Ni concentration is explained by considering the sp–d interaction of the Ni ions in ZnS matrix. The results reported here show that Ni doped ZnS nanocrystalline films can be employed in the fabrication of nanoscale optical and magneto-optical devices.