Enhancement of quantum ferroelectricity in SrTi¹⁸O₃ thin film

We fabricated ¹⁸O-exchanged SrTi¹⁸O_0.03x ¹⁶O_0.03(100-x) (STO18) thin films on SrTi ¹⁶O₃(001) single-crystal substrates by pulsed laser deposition from a SrTi¹⁸O₃ single-crystal target at various growth temperatures and ¹⁶O₂ partial pressures. The diffusion of ¹⁶O from the substrate could be successfully suppressed by growing films under high ¹⁶O₂ partial pressures, and a homogeneously ¹⁸O-exchanged STO18 thin film (x=60) was then obtained. The STO18 film became ferroelectric at a critical temperature higher than that expected from the corresponding STO18 bulk sample by ∼10 K, at which the Raman spectra suggested that the first-order transition occurred, different from the case of the STO18 bulk.