Structure and optical characteristics of barium–strontium niobate films on Al$_2$O$_3$ substrates

The structure and optical characteristics of thin films of relaxor ferroelectric Ba$_{0.5}$Sr$_{0.5}$Nb$_{2}$O$_{6}$ grown by RF sputtering in an oxygen atmosphere on an Al$_2$O$_3$ substrate ($c$ cut) have been studied. X-ray diffraction analysis shows that Ba$_{0.5}$Sr$_{0.5}$Nb$_{2}$O$_{6}$ films are $c$-oriented and unit-cell parameter $c$ is 3.948(1) $\mathring{\mathrm{A}}$. Ellipsometric measurements confirm that SBN-50 films are characterized by a natural growth direction, which coincides with the direction of the optical crystal axis. An analysis of ellipsometric results shows that there is no transition layer at the film/substrate interface; the damaged layer on the free film surface is 7.5 nm thick, and the volume filling factor is estimated to be 0.625.