Abstract
We measured strain effects of SrTiO3 (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 MgAl2O4, 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 MgAl2O4 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 MgAl2O4, which implied that the in-plane tensile strain enhanced ferroelectric fluctuations, was of interest.
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