Thickness and oxygen pressure dependent optical properties of niobium oxide thin films

The optical properties of niobium oxide thin films in the thickness range between 75 and 200 nm have been studied. The films were deposited by de magnetron sputtering and it was found that the refractive index, extinction coefficient and the absorption edge were all strongly dependent on the oxygen pressure during sputtering as well as the thickness of the deposited films. In general, the low thickness films had a lower refractive index than the high thickness films. The highest refractive index obtained was 2.46 at a wavelength of 650 nm for the film deposited at an oxygen pressure of 2 mTorr and to a thickness of 200 nm. The low thickness (similar to 120 nm) films showed an initial decrease in refractive index, with oxygen pressure increasing above a critical value. The thicker films, however showed the opposite behaviour, increasing initially and decreasing marginally above the same critical pressure. The absorption edge showed a critical value of thickness above and below which it decreased and a critical value of oxygen pressure during sputtering above and below which it increased. The band gap values varied from 3.3 to 3.6 eV with the thinner films showing the higher band gap. The observed behaviour was explained using the single effective Lorentzian oscillator model.