Observation of Strain Effects of SrTiO₃Thin Films by Terahertz Time-Domain Spectroscopy with a 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Emitter

We measured strain effects of SrTiO 3 (STO) thin films by terahertz time-domain spectroscopy (THz-TDS) with a 4-dimethylamino- N -methyl-4-stilbazolium tosylate (DAST) crystal that was expected to be a broadband THz emitter. STO thin films were deposited on MgAl 2 O 4 , MgO, and LSAT substrates, and lattice constants of the films changed from the value of unstrained STO bulk because of lattice mismatches between STO and the substrates. The dielectric dispersions of the STO thin films deposited on MgO and MgAl 2 O 4 were measured up to 6.0 THz by the broadband THz-TDS system and all the features of dielectric peaks of soft modes responsible for ferroelectricity were clearly observed. The soft-mode frequencies of the films shifted from the original value of STO bulk owing to the strain effects. In particular, the softening of the soft mode of the film on MgAl 2 O 4 , which implied that the in-plane tensile strain enhanced ferroelectric fluctuations, was of interest.