Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin filmswere studied in this paper. The films were characterized by Fouriertransform infrared spectroscopy, UV--visible transmittance andreflection spectra. h-BN thin films with a wide optical band gapE_g (5.86~eV for the as-deposited film and 5.97~eV for theannealed film) approaching h-BN single crystal were successfullyprepared by radio frequency (RF) bias magnetron sputtering andpost-deposition annealing at 970~K. The optical absorption behaviourof h-BN films accords with the typical optical absorptioncharacteristics of amorphous materials when fitting is made by theUrbach tail model. The annealed film shows satisfactory structurestability. However, high temperature still has a significanteffect on the optical absorption properties, refractive index n,and optical conductivity σ of h-BN thin films. Theblue-shift of the optical absorption edge and the increase of E_g probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive indexclearly exhibits different trends in the visible and ultraviolet regions.Previous calculational results of optical conductivity of h-BNfilms are confirmed in our experimental results.