Optical characterization of band gap graded ZnMgO films

We investigated the optical properties of compositionally graded Zn1−xMgxO (g-ZnMgO) films using spectroscopic ellipsometry. The g-ZnMgO and ZnO films were grown on Pt/Ti/SiO2/Si substrates by ultrasonic spray pyrolysis. We simulated a uniformly graded optical band gap layer on the Pt substrate to reproduce the experimental result. The band gap of the bottommost layer of the g-ZnMgO film was estimated to be ∼3.22 eV, the same as the undoped ZnO film. Then we considered a linearly increasing band gap with the film composition, and obtained a band gap of ∼3.56 eV for the topmost layer of the film. In addition, the exciton peak showed a strong increase for the topmost layer of the film suggesting an important role of doping.