Weakening of excitonic screening effects in TixZn1-xO thin films

Excitonic transitions of TixZn1-xO thin films have been studied using spectroscopic ellipsometry in the photon energy range of 0.5-6.5 eV. ZnO-based thin films were deposited on Si substrates by DC-unbalanced magnetron sputtering with Ti concentrations of 0, 1, and 3 at.%. By using a combination of Tauc-Lorentz, Drude, and Gaussian oscillators, the complex dielectric function and its derivatives were obtained and analyzed. In pure ZnO, the excitonic transitions were observed at ~ 3.35 eV through critical point analysis of the complex dielectric function. Mid-gap states, which acted as an electronic screening of excitons, were found in the photon energy region of 1.50-3.35 eV. Upon 1 at.% Ti doping, the amplitude of the mid-gap states decreased from 0.26 to 0.04 while the amplitude of excitonic transitions increased from 0.22 to 0.28, as compared to the pure ZnO. We explain such phenomena due to the weakening of excitonic screening effects. The results show that Ti plays an important role in the electronic structure and excitonic effects of ZnO.