Room-temperature relaxor ferroelectricity and photovoltaic effects in tin titanate directly deposited on a silicon substrate

Tin titanate ($\mathrm{SnTi}{\mathrm{O}}_3$) has been notoriously impossible to prepare as a thin-film ferroelectric, probably because high-temperature annealing converts much of the $\mathrm{S}{\mathrm{n}}^{2+}$ to $\mathrm{S}{\mathrm{n}}^{4+}$. In the present paper, we show two things: first, perovskite phase $\mathrm{SnTi}{\mathrm{O}}_3$ can be prepared by atomic-layer deposition directly onto $p$-type Si substrates; and second, these films exhibit ferroelectric switching at room temperature, with $p$-type Si acting as electrodes. X-ray diffraction measurements reveal that the film is single-phase, preferred-orientation ferroelectric perovskite $\mathrm{SnTi}{\mathrm{O}}_3$. Our films showed well-saturated, square, and repeatable hysteresis loops of around $3\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ remnant polarization at room temperature, as detected by out-of-plane polarization versus electric field and field cycling measurements. Furthermore, photovoltaic and photoferroelectricity were found in $\mathrm{Pt}/\mathrm{SnTi}{\mathrm{O}}_3/\mathrm{Si}/\mathrm{SnTi}{\mathrm{O}}_3/\mathrm{Pt}$ heterostructures, the properties of which can be tuned through band-gap engineering by strain according to first-principles calculations. This is a lead-free room-temperature ferroelectric oxide of potential device application.

Figure 1

(a) Polarization–electric-field (P-E) hysteresis loop and switching current at different applied voltages between 1 and 10 V measured at room temperature. The inset shows the direction of a hysteresis loop as a function of time. (b) Schematic diagram of the Pt/$\mathrm{SnTi}{\mathrm{O}}_3/p$-Si/$\mathrm{SnTi}{\mathrm{O}}_3$/Pt capacitor structure used for electrical measurements. (c) Frequency dependence of P-E hysteresis loops.

Figure 2

(a) $I-V$ characteristics of Pt/$\mathrm{SnTi}{\mathrm{O}}_3/p$-Si/$\mathrm{SnTi}{\mathrm{O}}_3$/Pt capacitors at various temperatures. (b) $\ln I-\ln V$ plots at various temperatures. (c) $\ln I-\ln V$ plot at 300 K exhibits trap-assisted space-charge-limited current (SCLC) behavior of current transportation. (d) Arrhenius plot for capacitor structure.

Figure 3

(a) Temperature dependence of the dielectric constant. (b) Extrapolated freezing temperature was observed to be ∼452 K. The inset shows the linear fitting of experimental data obtained from modified Curie-Weiss law. We note that for any relaxor, as the probe frequency is lowered, the temperature corresponding to dielectric constant peaks must decrease and the peak magnitude must increase; in any viscous material, the susceptibility (both electrical and mechanical) must increase as frequency decreases.

Figure 4

(a) $I-V$ characteristics of a Pt/$\mathrm{SnTi}{\mathrm{O}}_3/p$-Si/$\mathrm{SnTi}{\mathrm{O}}_3$/Pt capacitor in dark and under white light illumination. The inset shows the portion of the $I-V$ curves around zero field, revealing $J_{\mathrm{SC}}$ and $V_{\mathrm{OC}}$ to be 3 μA and 0.13 V, respectively. (b) The time dependence of $J_{\mathrm{SC}}$ and $V_{\mathrm{OC}}$ was measured under multiple on/off light cycles. (c) The evolution of remnant polarization with the number of bipolar cycles in dark and light. The inset shows the PE hysteresis loops after ${10}^6$ switching cycles.

Figure 5

(a) ${\left(\alpha E\right)}^2$ vs photon energy plot gives the direct band gap of 2.6 eV for $\mathrm{SnTi}{\mathrm{O}}_3$ thin films grown on $p$-Si substrate. A gray line is an extrapolation line to estimate the band gap. (b)–(d) Electronic band dispersion (left panel) and ion and $l$ quantum-number-resolved electronic density of states (EDOS) (right panel) calculated for $\mathrm{SnTi}{\mathrm{O}}_3$ for the biaxial misfit strain $\varepsilon$ of (b) $-0.33\%$, (c) $0\%,$ and (d) $0.33\%$ within the local density approximation (LDA). The location of the bands is referenced with respect to the low-lying O $2s$ state, which is assumed to be undisturbed by any structural distortions.