Effect of Annealing Temperature on Excitonic Binding Energy and Other Optical Properties of Mn-Ni Co-Doped Transparent ZnO Thin Films

In this work, low cost sol-gel spin coating technique was successfully employed to deposit Mn-Ni co-doped ZnO (MNZO) thin films on glass substrates. Structural, topographical and optical properties of MNZO thin films were studied in the light of XRD spectra, Raman spectroscopy, SEM, AFM, UV-visible spectroscopy and photoluminescence spectroscopy. XRD study revealed that all annealed ZnO thin films have hexagonal wurtzite structure with (002) preferred orientation along c-axis. Hexagonal wurtzite structure for all films has been confirmed by Raman spectra which show optical phonon modes. Grain size, micropores and RMS surface roughness showed an increasing trend with the rise in annealing temperature as revealed by SEM and AFM images. The experimental data of absorption coefficient was analysed in the light of hydrogenic excitonic model to estimate excitonic binding energy (R) and other optical parameters of the prepared thin films. The excitonic binding energy decreased from 71.55 to 49.2 meV on increasing annealing temperature from 200° to 500°C. Peaks corresponding to UV, violet, green, yellow and orange were clearly observed in photoluminescence spectra for all annealed thin films. The ultraviolet peak shifted towards lower wavelength (blue-shifted) as the annealing temperature was increased. All the peaks in visible region were attributed to transition between complex intrinsic and extrinsic defects. Such films can be a potential candidate for efficient lasers and LEDs and for variety of optoelectronic device applications.