Properties of undoped ZnO and Mg doped ZnO thin films by sol-gel method for optoelectronic applications

Abstract

Magnesium-doped zinc oxide (MgZO) is one of the potential optical materials in enhancement of the band gap in solid state lighting. Undoped and Mg-doped ZnO thin film with various Mg concentrations (0.03 M:Mg, 0.05 M:Mg, 0.07 M:Mg) were synthesized using sol gel method on n-type silicon (111) and undergone annealing treatment at 450 °C and 550 °C using nitrogen gas for 1 h. The surface and optical properties of the thin films were investigated by field emission scanning electron microscopy (FESEM), Raman, UV-Vis spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The undoped and Mg-doped ZnO thin film has hexagonal wurtzite structure with ZnO peaks (100), (002), and (101) that were identified by XRD analysis. The increment of Mg concentration resulted in the decrease of the intensity of the ZnO peaks. However, the intensity of the ZnO peaks increased with the increment of temperature. 0.07 M:Mg samples from both annealing temperatures have the largest crystallite size and the lowest dislocation density than that of the others. The crystallite size increased but the dislocation density decreased as annealing temperature was increased from 450 to 550 °C. FESEM results demonstrated that grain size of thin films reduced with increased doping concentration within the range of 25 to 65 nm and 26 to 44 nm for 450 °C and 550 °C annealing samples, respectively. Other than that, Raman spectra showed the decreased intensity of the mode vibration at peak 0.03 M:Mg but increased slowly with higher doping concentration. The influence of the annealing treatment also affect the mode position and intensity of Raman peak for undoped and Mg-doped ZnO thin films. The UV-Vis spectra revealed that no changes in band gap values at 450 °C for increased Mg doping concentration, but increment in band gap values within the range 3.35 ~3.60 eV was observed for samples annealed at 550 °C which occurs due to the improvement of crystallinity in samples. Characteristic vibrational mode of ZnO was observed in the absorption band in FTIR spectrum. Through this work, it is suggested that Mg-doped ZnO thin film could be a promising material in solid state and optoelectronic application.