Switchable two-dimensional electron gas based on ferroelectric Ca: SrTiO3

Abstract

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 spinorbit 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 non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO3 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.

Metal/ SrTiO3 two-dimensional electron gases for spin-to-charge conversion

“…The tunability of the 2DEG properties, their simple elaboration procedure and their efficient spin charge conversion capability not only make STO 2DEGs appealing for MESO-type devices but should also motivate experiments to probe further charge-spin conversion and spin-orbit torque. More generally, our results expand the research on STO 2DEGs for spintronics and open avenues towards the design of STO 2DEGs based on other reactive metals displaying functional properties [37][38][39].…”

SrTiO3—Glimpses of an Inexhaustible Source of Novel Solid State Phenomena

“…PFM yields stable hysteresis loops at room temperature, as shown in Figure 6b, where low and high laser fluences during PLE clearly demonstrate their key role in creating FE polarization. Similar propositions were made by the Barthélémy-Bibes group, which invoked both an electric fieldswitchable two-dimensional electron gas emerging in ferroelectric SCT films [32] and the non-volatile electric control of spin-charge conversion in an STO Rashba system [33]. Only recently has another insight into the ferroelectric state of compressively strained STO become available from high-angle annular dark-field imaging in scanning transmission electron microscopy.…”

Mechanism for Switchability in Electron-Doped Ferroelectric Interfaces

“…This assumption is supported by the observation of a large perpendicular polarization component in Al/Sr 1−x Ca x TiO 3 bilayers. [37] Our model comprises an electronic Hamiltonian, describing the 2DEG, and an ionic Hamiltonian, describing the lattice polarization. The lattice and electronic degrees of freedom are coupled through the self-consistently determined electric field.…”

“…More recently, Bréhin et al demonstrated that a switchable 2DEG forms in the perovskite layer of an Al/Sr 0.99 Ca 0.01 TiO 3 bilayer. [37] In both of these systems, the perovskite substrate is intrinsically insulating, and charge carriers are provided by the cap layer.…”