Ferroelectricity in Strain-Free ${\mathrm{SrTiO}}_3$ Thin Films

Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as ${\mathrm{SrTiO}}_3$. By a direct comparison of the strained and strain-free ${\mathrm{SrTiO}}_3$ films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all ${\mathrm{SrTiO}}_3$ films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.

Figure 1

(a),(b) Temperature dependence of (a) dielectric constant and (b) remanent polarization for a 60-nm-thick STO film on NGO (strained) and a 100-nm-thick STO film on STO (strain-free). (c) SHG signals for the strained STO film and a STO single crystal. The latter is magnified $\sim 12$ times for clarity. The SHG vs temperature of a 100 nm STO film on STO is indistinguishable from the SHG of a (100) STO single crystal. Insets: (a) fits to the Curie-Weiss law and the Vogel-Fulcher law, (b) $P\mathrm{\text{−}}E$ hysteresis loops of the strained film, and (c) SHG polar plots show tetragonal symmetry.

Figure 2

(a) Amplitude-contrast PFM images as poled (0 h), after 2 h, and after 4 h for a 50-nm-thick strained STO film on NGO. (b) Correlation function, $C(r)=\sum _{r}I(r+\Delta r)I(r)$, calculated for (a) as a function of distance in the $y$ direction. The solid lines are fits to $C(r)={\sigma }^2\exp \mathbf{(}-(r/\xi {)}^{2h}\mathbf{)}$, where ($\sigma$, $\xi$, $h$) for 0, 2, and 10 h are, respectively, ($\sim 0.82$, $\sim 1044$, $\sim 2.25$), ($\sim 0.71$, $\sim 1164$, $\sim 2.37$), and ($\sim 0.11$, $\sim 100$, $\sim 0.91$). (c) PFM hysteresis loops for 50-nm-thick strain-free and strained STO films. The loops are measured at room temperature in air with a 100 nm biased tip by applying voltage pulses of 40 ms for domain reversal, followed by variable pulse-off periods during which time the PFM signal is measured to avoid electrostatic contributions. (d) Change of PFM signal before and after poling with $-25\mathrm{V}$. (e) PFM signal as a function of time after poling with $-25\mathrm{V}$ for BFO, PMN-PT, strained, and strain-free STO films. Solid black lines are fits to the power-law decay.

Figure 3

Vacuum PFM images (left: amplitude contrast, right: phase contrast) of polarization domains recorded at 50 K by scanning the tip biased at $\pm 8–10\mathrm{V}$ across the surface of the (a) strain free and (b) strained STO films.

Figure 4

(a) Raman spectra of a bulk STO single crystal, a near-stoichiometric STO film grown on STO by molecular-beam epitaxy (MBE), an intentionally Sr-deficient film with composition ${\mathrm{Sr}}_{0.9}{\mathrm{TiO}}_{3-x}$ grown on STO by MBE, and nominally stoichiometric 50 and 1000 nm thick STO films grown on SRO/STO by PLD at 10 K. (b) Temperature-dependent Raman spectra of the 50 nm thick STO/SRO/STO film.