EFFECTS OF Co CONCENTRATION ON THE STRUCTURAL AND OPTICAL PROPERTIES OF Zn1-xCoxS FILMS

Amorphous and crystalline Zn1-xCoxS (x = 0.1, 0.3, 0.5) thin films were grown on sapphire (Al2O3) substrates by pulsed laser deposition at substrate temperature of 25 degrees C and 800 degrees C, respectively. The X-ray diffraction results show that the crystalline film has a cubic zinc blende structure and the crystalline quality decreased with increasing Co-doping concentration. The X-ray diffraction and X-ray photoelectron spectroscopy spectra reveal that the samples reached an overdoping state at Co-doping concentration of x = 0.5. The absorbance of films increases and the absorption edge shifts to longer wave length direction with increasing Co-doping concentration. The redshift of the band gap energy depends on the Co composition associating with the Urbach energy. Furthermore, the refractive index and dielectric constant increase with increasing Co-doping concentration. The dispersion parameters, such as dispersion energy (E-d), oscillator energy (E-o), static refractive index (n(0)), static dielectric constant (epsilon(0)), interband transition strength moments (M-1 and M-3), oscillator strength S-o and oscillator wavelength lambda(o), have been analyzed by Wemple-DiDomenico single oscillator model. All these parameters were found to be dependent upon the Co-doping concentration in the Zn1-xCoxS thin films.