Optical properties of sodium niobate thin films

NaNbO$_3$ thin films were deposited under different conditions by rf magnetron sputtering of ceramic target. Spectral transmission of the deposited films was measured in the UV-Visible-near IR range. Films deposited at 300$^\circ$ C showed more absorption, and films annealed at 300$^\circ$ C showed less absorption than those deposited at room temperature (RT), which was found to be consistent with their X-ray diffraction (XRD) patterns. From the observed transmission spectra, refractive index, optical band gap, absorption coefficient, extinction coefficient and film thickness were calculated for the deposited films. Refractive index at 550 nm wavelength was found to be 2.11, 2.01 and 2.34 for the films deposited at RT, 300$^\circ$ C and annealed at 300$^\circ$ C, respectively. The refractive index was found to be almost constant with respect to frequency for the films annealed at 300$^\circ$ C. Optical band gap was found 3.82, 3.7 and 3.81 eV for the films deposited at RT, 300$^\circ$ C, and annealed at 300$^\circ$ C, respectively. Film thickness was shown to decrease with annealing. Absorption and extinction coefficients decreased with increasing wavelength, in all the samples. Band gaps associated with different interactions have been calculated for the deposited films. Phonon assisted indirect forbidden transition was most favorable in the deposited films.