Switchable two-dimensional electron gas based on ferroelectric Ca:$\mathrm{SrTi}{\mathrm{O}}_3$

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a “ferroelectric” 2DEG whose transport properties can be electrostatically switched in a nonvolatile way. We generate a 2DEG by depositing a thin Al layer onto a $\mathrm{SrTi}{\mathrm{O}}_3$ single crystal in which 1% of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

Figure 1

(a) Reflection high-energy electron diffraction (RHEED) pattern acquired on the pre-annealed Ca:STO substrate, revealing its high crystal quality. (b) X-ray photoelectron spectroscopy (XPS) of the Ti $2p$ core levels of bare preannealed Ca:STO substrate, showing no presence of $\mathrm{T}{\mathrm{i}}^{3+}$. (c) Atomic force microscopy image of an as-grown Al//Ca:STO sample confirms the surface cleanliness. (d) XPS Ti $2p$ spectra of a Al//Ca:STO sample, revealing a significant presence of $\mathrm{T}{\mathrm{i}}^{3+}$ amounting to 32.8% of the total ${\mathrm{Ti}}^{3+}+{\mathrm{Ti}}^{4+}$ area.

Figure 2

(a) Measured current (top) and polarization (bottom) loops as a function of the electric field for the Al//Ca:STO sample at 2 K are consistent with the existence of a ferroelectric state $\left(f=100\mathrm{Hz}\right)$. (b) Temperature dependence of the remanent polarization $P_R$ after the application of a maximum electric field of ±1.4 kV/cm, showing a critical transition temperature of ∼25 K. (c) Raman spectra of Al//Ca:STO obtained for temperatures between 10 and 40 K, displaying a clear phonon mode present at a shift of $20\mathrm{c}{\mathrm{m}}^{-1}$ at 10 K. (d) The temperature dependence of the relative intensity of the mode at $20\mathrm{c}{\mathrm{m}}^{-1}$ in the Raman spectra confirms a critical transition temperature of $\sim 25\mathrm{K}$.

Figure 3

(a) Temperature dependence of the sheet resistance $R_s$ of the Al//Ca:STO sample between 2–40 K (red curve, left $y$ axis) as well as the sheet resistance derivative with respect to temperature (blue curve, right $y$ axis). Both curves point towards a transition temperature around 25 K. The inset shows the $R_s\left(\mathrm{T}\right)$ between 2–300 K which is representative of usual 2DEG metallic behavior (above the ferroelectric Curie temperature). (b) The Hall carrier density, $n_s$, extracted between 2 and 40 K confirms the presence of two different electronic regimes with a transition temperature of ∼25 K. The solid lines are guides to the eye.

Figure 4

(a) Polarization loops at 2 K measured in the ferroelectric state for different increasing maximum values of the back-gate voltage $V_g$. The curves are shifted by 1.25 µC/cm for clarity. (b) Electric field dependence of $R_s$ for different maximum values of $V_g$ at 2 K. The curves are shifted by 150 Ω/sq for clarity.