A switchable two-dimensional electron gas based on ferroelectric Ca:SrTiO$_3$
Julien Bréhin, Felix Trier, Luis M. Vicente-Arche, Pierre Hemme, Paul Noël, Maxen Cosset-Chéneau, Jean-Philippe Attané, Laurent Vila, Anke Sander, Yann Gallais,… 展开作者
Julien Bréhin, Felix Trier, Luis M. Vicente-Arche, Pierre Hemme, Paul Noël, Maxen Cosset-Chéneau, Jean-Philippe Attané, Laurent Vila, Anke Sander, Yann Gallais, Alain Sacuto, Brahim Dkhil, Vincent Garcia, Stéphane Fusil, Agnès Barthélémy, Maximilien Cazayous, Manuel Bibes
发表信息：
Published on April 16, 2020 as Phys. Rev. Materials 4, 041002(R) (2020)
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 non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO$_3$ single crystal in which 1 percent 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.
Spontaneous polarization and bulk photovoltaic effect (BPVE) are two concomitant physical properties in ferroelectric materials. The flipping of ferroelectric order usually accompanies with the switching of BPVE as both of them are reversed under the inversion symmetry. In this study, we report the distinctive BPVE characters in two-dimensional (2D) interlayer sliding ferroelectric materials featuring unswitchable in-plane BPVE (light-induced photocurrent in the xy plane) and switchable out-of-plane BPVE (light-induced polarization along the z-direction). Symmetry analysis within abstract bila…
Nanoscaled room-temperature ferroelectricity is ideal for developing advanced non-volatile high-density memories. However, reaching the thin film limit in conventional ferroelectrics is a long-standing challenge due to the possible critical thickness effect. Van der Waals materials, thanks to their stable layered structure, saturate interfacial chemistry and weak interlayer couplings, are promising for exploring ultra-thin two-dimensional (2D) ferroelectrics and device applications. Here, we demonstrate a switchable room-temperature ferroelectric diode built upon a 2D ferroelectric α-In2Se3 la…
SrTiO$_{3}$ is known to be an incipient ferroelectric. It is thought that ferroelectric stability in SrTiO$_{3}$ is suppressed by a delicate competition with quantum fluctuation and antiferrodistortion. The ferroelectric phase can, however, be stabilized by doping, isotope manipulation and strain engineering etc. Till date ferroelectricity in SrTiO$_{3}$ thin films was observed to exist up to room temperature -- that was when the films were grown on specially engineered substrates using complex growth techniques. It was possible to write and erase ferroelectric domains on the specially enginee…
The temperature-dependent polarization of SrTiO_3 thin films is investigated using confocal scanning optical microscopy. A homogeneous out-of-plane and inhomogeneous in-plane ferroelectric phase are identified from images of the linear electrooptic response. Both hysteretic and non-hysteretic behavior are observed under a dc bias field. Unlike classical transitions in bulk ferroelectrics, local ferroelectricity is observed at temperatures far above the dielectric permittivity maximum. The results demonstrate the utility of local probe experiments in understanding inhomogeneous ferroelectrics.
We use density functional theory to explore the interplay between octahedral rotations and ferroelectricity in the model compound SrTiO$_3$. We find that over the experimentally relevant range octahedral rotations suppress ferroelectricity as is generally assumed in the literature. Somewhat surprisingly we observe that at larger angles the previously weakened ferroelectric instability strengthens significantly. By analysing geometry changes, energetics, force constants and charges, we explain the mechanisms behind this transition from competition to cooperation with increasing octahedral rotat…
Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects using hexagonal ferrite (h-RFeO3) thin films having narrow band gaps of ~1.2 eV, where R denotes rare-earth ions. FPVs with narrow band gaps suggests their potential applicability as photovolt…
We investigated a switchable ferroelectric diode effect and its physical mechanism in Pt/BiFeO3/SrRuO3 thin-film capacitors. Our results of electrical measurements support that, near the Pt/BiFeO3 interface of as-grown samples, a defective layer (possibly, an oxygen-vacancy-rich layer) becomes formed and disturbs carrier injection. We therefore used an electrical training process to obtain ferroelectric control of the diode polarity where, by changing the polarization direction using an external bias, we could switch the transport characteristics between forward and reverse diodes. Our system …
Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO$_3$. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows that the non-oscillatory second harmonic generation signal can b…
Artificial PbTiO$_{3}$/SrTiO$_{3}$ superlattices were constructed using off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO$_{3}$ to SrTiO$_{3}$ was changed. For PbTiO$_{3}$ layer thicknesses larger than the 3-unit cells SrTiO$_{3}$ thickness used in the structure, the polarization is found to be reduced as the PbTiO$_{3}$ thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For …
Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO$_3$ by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy,…
In this paper, an efficient computational material design approach (cluster expansion) is employed for the ferroelectric $PbTiO_3$/$SrTiO_3$ system. Via exploring a conguration space including over $3 \times 10^6$ candidates, two special cation ordered configurations: either perfect or mixed 1/1 (101) superlattice, are identified with the mostly enhanced ferroelectric polarization by up to about 100% in comparison with the (001) superlattice. Analyzing these two special configurations reveals the exotic couplings between the antiferrodistortive distortion (AFD) and ferroelectric polarization (…
Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are introducing new directions in the 2D materials landscape. The coexistence of ferroelectricity and spin-orbit interactions play a key role in their optoelectronic properties. We perform a detailed study on a recently synthesized ferroelectric 2D-HOIP, (AMP)PbI$_4$ (AMP = 4-aminomethyl-piperidinium). The calculated polarization and Rashba parameter are in excellent agreement with experimental values. We report a striking new effect, i.e., an extraordinarily large Rashba anisotropy that is tunable by ferroelectric polarization:…
Here we demonstrate a method to tune ferroelectric imprint, which is stable in time, based on the coupling between the non-switchable polarization of ZnO and switchable polarization of PbZrxTi(1-x)O3. SrRuO3/PbZrxTi(1-x)O3 /ZnO/SrRuO3 heterostructures were grown with different ZnO thicknesses. It is shown that the coercive voltages and ferroelectric imprint varies linearly with the thickness of ZnO. It is also demonstrated that the ferroelectric imprint remains stable with electric field cycling and electric field stress assisted aging.
Artificially induced asymmetric tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe where down domains having lower friction coefficient than up domains can be used as smart masks as they show slower wear rate than up domains. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribol…
Results from dielectric and structural measurements on epitaxial SrTiO$_3$/BaZrO$_3$ superlattices reveal properties that cannot be explained simply in terms of those measured on single films of the constituent materials. For large superlattice periodicities (20/20 and 38/38 structures, i.e. samples in which each SrTiO3 and BaZrO3 layer are 20 or 38 unit cells thick, respectively), the capacitance-voltage curves indicate room-temperature ferroelectricity. For smaller periodicities (7/7 and 15/15), anti-ferroelectric-type behavior is observed, suggesting strong coupling between individual polar…
A monolayer of ZrO$_{2}$ has recently been grown on the Si(001) surface and shown to have ferroelectric properties, which signifies the realization of the lowest possible thickness in ferroelectric oxides [M. Dogan et al., Nano Lett., 18 (1) (2018)]. In our previous computational study, we reported on the multiple (meta)stable configurations of ZrO$_{2}$ monolayers on Si, and how switching between a pair of differently polarized configurations may explain the observed ferroelectric behavior of these films [M. Dogan and S. Ismail-Beigi, arXiv:1902.01022 (2019)]. In the current study, we conduct…
SrTiO3 is an incipient ferroelectric that is believed to exhibit a prototype displacive, soft mode ferroelectric transition when subjected to mechanical stress or alloying. We use high-angle annular dark-field imaging in scanning transmission electron microscopy to reveal local polar regions in the room-temperature, paraelectric phase of strained SrTiO3 films, which undergo a ferroelectric transition at low temperatures. These films contain nanometer-sized domains in which the Ti columns are displaced. In contrast, these nanodomains are absent in unstrained films, which do not become ferroelec…
In epitaxially strained ferroelectric thin films and superlattices, the ferroelectric transition temperature can lie above the growth temperature. Ferroelectric polarization and domains should then evolve during the growth of a sample, and electrostatic boundary conditions may play an important role. In this work, ferroelectric domains, surface termination, average lattice parameter and bilayer thickness are simultaneously monitored using in-situ synchrotron x-ray diffraction during the growth of BaTiO$_3$/SrTiO$_3$ superlattices on SrTiO$_3$ substrates by off-axis RF magnetron sputtering. The…
"Hidden phases" are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. These phases can host exotic properties in otherwise conventional materials and hence may enable novel functionality and applications, but their discovery and access are still in early stages. Using intense terahertz electric field excitation, we show that an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in the quantum paraelectric SrTiO$_3$. The induced lowering in crystal symmetry yields dramatic changes in the phonon excitation…
In the framework of Landau--Ginzburg--Devonshire (LGD) phenomenological theory, the influence of misfit strains, surface energy, and finite-size effects on phase diagrams, polar properties, and hysteresis loops has been calculated for multilayered thin films of the type ferroelectric/virtual ferroelectric. The influence of elastic deformations that arise at the interface thin film--substrate owing to a mismatch between the lattice constants in the film and the substrate on the phase diagrams of multilayered thin films virtual ferroelectric SrTiO$_{3}$/ferroelectric BaTiO$_{3}$ has been studied…
We report first-principles calculations on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions, can be used to control the orientation of the main axis of the system. The electrostatic constrains at the interface facilitate the rotation of the polarization and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those that would be required considering only s…
Phase Switchable Photocatalytic Water Splitting via a Paraelectric-Ferroelectric Transition in Zr2Ge2S6 Monolayer: A Comprehensive Theoretical Insights
The effect of epitaxial strain on the phonon spectra, crystal structure, spontaneous polarization, dielectric, piezoelectric, and elastic properties of (001)-oriented ferroelectric (BaTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices ($m = n = {}$1-4) was studied using the first-principles density-functional theory. The ground state of free-standing superlattices is the monoclinic $Cm$ polar phase. Under the in-plane biaxial compressive strain, it transforms to tetragonal $P4mm$ polar phase, and under the in-plane biaxial tensile strain, it transforms to orthorhombic $Amm2$ polar phase. When changing…
We demonstrate how the quantum paraelectric ground state of SrTiO$_3$ can be accessed via a microscopic $ab~initio$ approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temper…
Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping ($D^+$) and the corresponding bound polaron ($e^-$).A dipole of 6.15 Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of ${\rm Sr_3Ti_2O_7}$. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard $U$ interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectric-like behavior report…
SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO$_3$ thin films…
We present an inexpensive first-principles approach for describing quantum paraelectricity that combines density functional theory (DFT) treatment of the electronic subsystem with quantum mechanical treatment of the ions through solution of the single-particle Schrödinger equation with the DFT-calculated potential. Using BaTiO$_3$, SrTiO$_3$ and KTaO$_3$ as model systems, we show that the approach can straightforwardly distinguish between ferroelectric, paraelectric and quantum paraelectric materials, based on simple quantities extracted from standard density functional and density functional …
We explore computationally the formation of tri-layer superlattices as an alternative approach for combining ferroelectricity with magnetism to form magnetoelectric multiferroics. We find that the contribution to the superlattice polarization from tri-layering is small compared to typical polarizations in conventionalferroelectrics, and the switchable ferroelectric component is negligible. In contrast, we show that epitaxial strain and ``negative pressure'' can yield large, switchable polarizations that are compatible with the coexistence of magnetism, even in materials with no active ferroele…
The effect of a variety of intrinsic defects and defect clusters in bulk and thin films of SrTiO$_3$ on ferroelectric polarization and switching mechanism is investigated by means of density-functional-theory (DFT) based calculations and the Berry phase approach. Our results show that both the titanium Ti$_\mathrm{Sr}^{\bullet \bullet}$ and strontium Sr$_\mathrm{Ti}^{"}$ antisite defects induce ferroelectric polarization in SrTiO$_3$, with the Ti$_\mathrm{Sr}^{\bullet \bullet}$ defect causing a more pronounced spontaneous polarization and higher activation barriers of polarization reversal tha…
We study a model SrTiO$_3$ interface in which conduction $t_{2g}$ electrons couple to the ferroelectric (FE) phonon mode. We treat the FE mode within a self-consistent phonon theory that captures its quantum critical behavior, and show that proximity to the quantum critical point leads to universal tails in the electron density of the form $n(z) \sim (λ+z)^{-2}$, where $λ\sim T^{2-d/\mathfrak{z}}$, with $d=3$ the dimensionality and $\mathfrak{z}=1$ the dynamical critical exponent. Implications for the metal-insulator transition at low electron density are discussed.
Ferroelectrics are characterized by domain structures as are other ferroics. At the nanoscale though, ferroelectrics may exhibit non-trivial or exotic polarization configurations under proper electrostatic and elastic conditions. These polar states may possess emerging properties not present in the bulk compounds and are promising for technological applications. Here, the observation of rotational polarization topologies at the nanoscale by means of aberration-corrected scanning transmission electron microscopy is reported in BaTiO3/SrTiO3 superlattices grown on cubic SrTiO3(001). The transiti…
The switching of electric polarization induced by electric fields -a fundamental functionality of ferroelectrics- is closely associated with the motions of the domain walls that separate regions with distinct polarization directions. Therefore, understanding domain-walls dynamics is of essential importance for advancing ferroelectric applications. In this Letter, we show that the topology of the multidomain structure can have an intrinsic impact on the degree of switchable polarization. Using a combination of polarization hysteresis measurements and piezoresponse force microscopy on a uniaxial…
The coexistence of metallicity and ferroelectricity has been an intriguing and controversial phenomenon as these two material properties are considered incompatible in bulk. We clarify the concept of ferroelectric metal by revisiting the original definitions for ferroelectric and metal. Two-dimensional (2D) ferroelectrics with out-of-plane polarization can be engineered via layer stacking to a genuine ferroelectric metal characterized by switchable polarization and non-zero density of states at the Fermi level. We demonstrate that 2D ferroelectric metals can serve as electrically-tunable, high…
Strong spin-orbit coupling in noncentrosymmetric materials and interfaces results in remarkable physical phenomena, such as nontrivial spin textures, which may exhibit Rashba, Dresselhaus, and other intricated configurations. This provides a promising basis for nonvolatile spintronic devices and further implications. Here, we simulate from first principles a two-dimensional electron gas in ultrathin platinum and palladium layers grown on ferroelectric PbTiO$_3$(001). The latter allows, in principle, to switch and control the spin-to-charge conversion by the polarization reversal. We show how t…
We show that in the antiferromagnetic ferroelectric BaNiF4 the Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic moments, away from the easy axis, resulting in a noncollinear magnetic structure. The canting corresponds to an additional "weak" antiferromagnetic order parameter whose orientation is determined by the polar structural distortion and can be reversed by switching the ferroelectric polarization with an electric field. Our results point the way to a more general coupling mechanism between structural distortions and magnetic order parameters in magnetoelectric …
Ferroelectric and photovoltaic properties of BiFeO3/LaFeO3 superlattices grown by pulsed laser deposition have been investigated being the bilayer thickness). For a high concentration of BiFeO3 a ferroelectric state is observed simultaneously with a switchable photovoltaic response. In contrast for certain concentration of LaFeO3 a non-switchable photovoltaic effect is evidenced. Such modulation of the PV response in the superlattices is attributed to the ferroelectric to paraelectric phase transition which is controlled with the increase of x. Remarkably, concomitant to this change of PV mech…
Generating and controlling spin current (SC) are of central interest in spin physics and applications. To date, the spin-orbit interaction (SOI) is an established pathway to generate SC through the spin-charge current conversion. We predict an efficient spin-light conversion via the Rashba and higher-order cubic Dresselhaus SOIs in ferroelectrics. Different from the known Edelstein effect, where SC is created by the nonequilibrium spin density, our predicted spin-polarized current is from direct interactions between light and unique spin textures generated by SOI in ferroelectrics. Using first…
The simultaneous presence of seemingly incompatible properties of solids often provides a unique opportunity to address questions of fundamental and practical importance. The coexistence of ferroelectric and topological orders is one such example. Ferroelectrics, which have a spontaneous macroscopic polarization switchable by an applied electric field, usually are semiconductors with a well-developed wide band gap with a few exceptions. On the other hand, time-reversal symmetric $Z_2$ topological insulators (TI), characterized by robust metallic surface states protected by the topology of the …
