Band Gap Shift and the Optical Nonlinear Absorption of Sputtered ZnO-TiO₂ Films

ZnO-TiO₂ composite films with different Zn/Ti atomic ratios were prepared with radio frequency reactive sputtering method. The Zn percentage composition (f _Zn) dependent optical band gap and optical nonlinear absorption were investigated using the transmittance spectrum and the Z-scan technique, respectively. The results showed that composite films with f _Zn in the range of 23.5%–88.3% are poor crystallized and their optical properties are anomalous which exhibit adjustable optical band gap and large optical nonlinear absorption. The optical absorption edge shifted to the blue wavelength direction with the increasing of f _Zn and reached the minimum value of 285 nm for the sample with f _Zn = 70.5%, which has the largest direct band gap of 4.30 eV. Further increasing of f _Zn resulted in the red-shift of the optical absorption edge. The maximum optical nonlinear absorption coefficient of 1.5 × 10³ cm/GW was also obtained for the same sample with f _Zn = 70.5%, which is more than 40 times larger than those of pure TiO₂ and ZnO films.