Recovering the optical transitions in tin oxide thin films at room temperature using electroreflectance

Observing the optical transitions in Tin Oxide at room temperature is a difficult task because of its forbidden bandgap nature in bulk form. Moreover, it possesses the well know low emission efficiency issue in its reduced dimensional thin film structures. In this work, we use Electroreflectance (ER) method to recover the optical transitions in tin oxide thin films. A metal-oxide-metal geometry, with SnO2 film was sandwiched between two metal electrodes, was illuminated by the light under an external perturbation voltage. This results the well-enhanced critical points in the ER spectra with a suppressed featureless background. The characteristics of observed critical points are found using the third derivative functional form model. The points are identified as defect transition and surface exciton transition originated due to the presence of defect states formed by the oxygen vacancies. In addition to this, the free exciton transition, from deep of the valence band to the conduction band, is explicitly revealed by ER spectra of r-SnO₂ film. A built-in electric field driven by external voltage is supposed to be the main factor for enhancing the critical points associated with optical transitions.