Optical properties of Co-doped ZnSe thin films synthesized by pulsed laser deposition

Effects of Co concentration on the properties of ZnSe:Co films were investigated. The amorphous and crystalline (ZnSe)(1-x):Co-x (x = =0.1, 0.3, 0.5) thin films were grown on sapphire (Al2O3) substrates with temperature of 25 degrees C and 800 degrees C respectively by pulsed laser deposition. The X-ray diffraction analyses indicate that, with increasing Co concentration, the average grain size decrease whereas the microstrain and dislocation density increase. Meanwhile, the further investigation for crystalline thin films shows that the crystalline phase transform from single cubic zinc blende structure to a mixture structure containing a small amount of hexagonal wurtzite phase. The Raman spectra and X-ray photoelectron spectroscopy reveal that the Co atoms were incorporated into ZnSe lattice and the samples reached an overdoping state when the x value surpassed 0.3. With increasing Co concentration, the average transmittance of films decreased due to the Co impurities un-incorporating into ZnSe lattice. Meanwhile, the band gap E-g values of films increased from 3.17 eV to 3.50 eV for T-S = =25 degrees C and from 2.86 eV to 3.34 eV for T-S = =800 degrees C. The large E-g values with regard to that of undoped bulk ZnSe (similar to 2.7 eV) can be attributed to the Co doping and quantum confinement effect. Moreover, the refractive index of the films increased by improving Co concentration. The dispersion energy E-d, oscillator energy E-o, static refractive index n(0), static dielectric constant epsilon(0), oscillator strength S-o and oscillator wavelength lambda(o) are analyzed by a single oscillator model. All these parameters were found to be dependent upon the Co concentration in the (ZnSe)(1-x):Co-x thin films.