Thickness dependence of the soft ferroelectric mode in SrTiO3 thin films deposited on MgO

We have measured the complex dielectric constants of SrTiO3 thin films deposited on MgO substrate, by using the broadband terahertz time-domain spectroscopy. The dielectric dispersion of SrTiO3 thin films with thickness of 1046, 460 and 55 nm has been observed in the frequency range from 0.1 to 8 THz. The dispersion mainly consists of the TO1 ferroelectric soft mode with a slight absorption of the TO2 phonon mode. From the analysis of the obtained dispersion, we found that the soft mode frequency hardens as the thickness of the film becomes thinner, and is extremely large compared with the bulk crystals. The damping of the soft mode is also larger than that of bulk SrTiO3, which suggests the extrinsic nature of the broadening of the soft mode dispersion. In the thinnest film of 55 nm, even the shape of the dielectric dispersion changes, which may be related to integrated defects near the interface.