Dielectric properties of nanometer-thick barium-strontium titanate films

Using submillimeter and infrared spectroscopies, the reflectance R(ν) and transmittance T(ν) spectra of heteroepitaxial barium-strontium titanate films of different thicknesses on MgO substrates are taken for the first time in the frequency range 10 < ν < 3000 cm−1. By modeling the experimental spectra by the Fresnel formulas for layered media, the spectra of complex permittivity ɛ*(ν) = ɛ′(ν) + iɛ″(ν) of the films are determined. It is shown that when the film thicknesses decrease down to 10 nm, there appear tensile stresses in the direction parallel to the substrate surface. As a result, the dielectric contribution of a low-frequency soft mode becomes several times larger than before.