Polar Discontinuity Research Articles

Electronic switching by metastable polarization states in BiFeO3 thin films

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of $\mathrm{BiFe}{\mathrm{O}}_{3}$. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of $\ensuremath{\sim}1\text{--}10\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$, approaching the $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$ threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, $\mathrm{BiFe}{\mathrm{O}}_{3}$ locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as $\ensuremath{\sim}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions.

Structural stability and electronic properties of the (0 0 0 1) inversion domain boundary in III-nitrides

A structural investigation of (0 0 0 1) plane inversion domain boundaries (IDBs) in group III-nitrides (GaN, AlN and InN) has been carried out by means of Monte Carlo (MC) simulation of Stillinger-Weber empirical potential. Eight possible IDB configurations were found to be stable during the structural searching process. Their energetics, chemical bonding properties as well as electronic structures were further investigated using first-principle calculations based on density functional theory (DFT). The comparison of relative energetic stability revealed that the H4 configuration is the most stable structure among H (Head-to-Head type) IDBs except in AlN; as for T (Tail-to-Tail type) IDBs, T2 is more energetic favorable within all materials. The electron localization function (ELF) and the Bader population analysis clearly point out 2-dimensional hole gas (2DHG) in H IDBs and 2-dimensional electron gas (2DEG) in T IDBs. And this is ascribed to the polarization discontinuity. A detailed analysis of Projected Density of States (PDOS) shows a metallic character in all IDBs. The hybridization states at the valance band edge crossed the Fermi level in H boundaries which acts as a p dopant. For T type IDBs, the PDOS is largely extended in density with the Fermi level shifted up above the conduction band maximum (CBM) which suggests an electron excess in boundaries.

Energetics and optimized ferroelectric polarization with metallic electronic states in ATiO3 (A = Ba and Pb): Layer doping strategy

Density functional theory is used to investigate the formation energetics (Ef), the spontaneous polarization (SP), and electronic properties of layer doped ATiO3 (A = Ba and Pb) systems having 10 dopants at A-site. We found that Ef and SP show a strong interdependence of dopant sizes (atomic radii) and both increases gradually with the decrease of dopant sizes. Along with this, the energy band gap substantially reduced for lengthening bond lengths. Interestingly, our results show that P@Pb, As@Pb, and Sb@Pb PTO doped systems are promising candidates for device applications because of optimized SP due to large octahedral distortion and n-type conductivity as compared to the pristine one. Furthermore, Bi@Pb PTO doped system also exhibits metallicity but have little lower magnitude of SP than that of undoped. The metallic electronic states induced due to charge transfer from (P+3O−2)+1, (As+3O−2)+1, (Sb+3O−2)+1, and (Bi+3O−2)+1 polar doped to (Ti+4O−4)0 non-polar undoped layers owing to the polar discontinuity. Hence, our findings open a new perspective to enhance SP with conducting electronic states in perovskite oxides by layer doping strategy.

The role of strain and polar discontinuity in magnetism in LaMnO3/SrTiO3/LaAlO3 (0 0 1) heterostructures

The role of strain and polar discontinuity in magnetism in LaMnO3/SrTiO3/LaAlO3 (0 0 1) heterostructures

Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces.

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two‐dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1− x(Sr,Ti)xO3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field‐effect devices reveals, in agreement with ab initio calculations and self‐consistent Poisson–Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

Chemical intermixing at oxide heterointerfaces with polar discontinuity

The stability of the (4 × 4) reconstruction is studied on SrTiO3(111) and LaTiO3(111) surfaces by scanning tunneling microscopy. While the reconstruction may effectively compensate the surface polarity, the associated large lattice distortion on SrTiO3 with relatively strong polarity has to be relaxed by extra adsorbates. Alternatively, substituting Sr by La can reduce the polarity and stabilize the clean reconstructed surface. This provides the driving force for La segregating towards the surface during the heteroepitaxy of SrTiO3 on LaTiO3-covered SrTiO3(111), and thus, the interface sharpness will be deteriorated. To improve the quality of heterointerfaces where the surface polarity of substrates tends to be increased by the epitaxial films, the growth kinetics must be optimized to play the major role.

Removal of the Magnetic Dead Layer by Geometric Design

AbstractThe proximity effect is used to engineer interface effects such as magnetoelectric coupling, exchange bias, and emergent interfacial magnetism. However, the presence of a magnetic “dead layer” adversely affects the functionality of a heterostructure. Here, it is shown that by utilizing (111) polar planes, the magnetization of a manganite ultrathin layer can be maintained throughout its thickness. Combining structural characterization, magnetometry measurements, and magnetization depth profiling with polarized neutron reflectometry, it is found that the magnetic dead layer is absent in the (111)‐oriented manganite layers, however, it occurs in the films with other orientations. Quantitative analysis of local structural and elemental spatial evolutions using scanning transmission electron microscopy and electron energy loss spectroscopy reveals that atomically sharp interfaces with minimal chemical intermixing in the (111)‐oriented superlattices. The polar discontinuity across the (111) interfaces inducing charge redistribution within the SrTiO3 layers is suggested, which promotes ferromagnetism throughout the (111)‐oriented ultrathin manganite layers. The approach of eliminating problematic magnetic dead layers by changing the crystallographic orientation suggests a conceptually useful recipe to engineer the intriguing physical properties of oxide interfaces, especially in low dimensionality.

Spontaneous Hall effects in the electron system at the SmTiO3/EuTiO3 interface

Magnetotransport and magnetism of epitaxial SmTiO3/EuTiO3 heterostructures grown by molecular beam epitaxy are investigated. It is shown that the polar discontinuity at the interface introduces ∼3.9 × 1014 cm−2 carriers into the EuTiO3. The itinerant carriers exhibit two distinct contributions to the spontaneous Hall effect. The anomalous Hall effect appears despite a very small magnetization, indicating a non-collinear spin structure, and the second contribution resembles a topological Hall effect. Qualitative differences exist in the temperature dependence of both Hall effects when compared to uniformly doped EuTiO3. In particular, the topological Hall effect contribution appears at higher temperatures and the anomalous Hall effect shows a sign change with temperature. The results suggest that interfaces can be used to tune topological phenomena in itinerant magnetic systems.

Controlled growth of complex polar oxide films with atomically precise molecular beam epitaxy

At heterointerfaces between complex oxides with polar discontinuity, the instability-induced electric field may drive electron redistribution, causing a dramatic change in the interfacial charge density. This results in the emergence of a rich diversity of exotic physical phenomena in these quasi-two-dimensional systems, which can be further tuned by an external field. To develop novel multifunctional electronic devices, it is essential to control the growth of polar oxide films and heterointerfaces with atomic precision. In this article, we review recent progress in control techniques for oxide film growth by molecular beam epitaxy (MBE). We emphasize the importance of tuning the microscopic surface structures of polar films for developing precise growth control techniques. Taking the polar SrTiO3 (110) and (111) surfaces as examples, we show that, by keeping the surface reconstructed throughout MBE growth, high-quality layer-by-layer homoepitaxy can be realized. Because the stability of different reconstructions is determined by the surface cation concentration, the growth rate from the Sr/Ti evaporation source can be monitored in real time. A precise, automated control method is established by which insulating homoepitaxial SrTiO3 (110) and (111) films can be obtained on doped metallic substrates. The films show atomically well-defined surfaces and high dielectric performance, which allows the surface carrier concentration to be tuned in the range of ~1013/cm2. By applying the knowledge of microstructures from fundamental surface physics to film growth techniques, new opportunities are provided for material science and related research.

Tuning Interfacial Magnetic Ordering via Polarization Control in Ferroelectric SrTiO3/PbTiO3 Heterostructure.

The electromagnetic properties at the interface of heterostructure are sensitive to the interfacial crystal structure and external field. For example, the two-dimensional magnetic states at the interface of LaAlO3/SrTiO3 are discovered and can further be controlled by electric field. Here, we study two types of heterostructures, TiO2/PbTiO3 and SrTiO3/PbTiO3, using first-principle electronic structure calculations. We find that the ferroelectric polarization discontinuity at the interface leads to partially occupied Ti 3d states and the magnetic moments. The magnitude of the magnetic moments and the ground-state magnetic coupling are sensitive to the polarization intensity of PbTiO3. As the ferroelectric polarization of PbTiO3 increases, the two heterostructures show different magnetic ordering that strongly depends on the electron occupation of the Ti t2g orbitals. For the TiO2/PbTiO3 interface, the magnetic moments are mostly contributed by degenerated d yz/d xz orbitals of interfacial Ti atoms and the neighboring interfacial Ti atoms form ferromagnetic coupling. For SrTiO3/PbTiO3 interface, the interfacial magnetic moments are mainly contributed by occupied d xy orbital because of the increased polarization intensity, and as the electron occupation increases, there exists a transition of the magnetic coupling between neighboring Ti atoms from ferromagnetism to antiferromagnetism via the superexchange interaction. Our study suggests that manipulating the polarization intensity is one effective way to control interfacial magnetic ordering in the perovskite oxide heterostructures.

Discovery of a magnetic conductive interface in PbZr0.2Ti0.8O3 /SrTiO3 heterostructures

Emergent physical properties often arise at interfaces of complex oxide heterostructures due to the interplay between various degrees of freedom, especially those with polar discontinuities. It is desirable to explore if these structures may generate pure and controllable spin currents, which are needed to attain unmatched performance and energy efficiency in the next-generation spintronic devices. Here we report the emergence of a spin-polarized two-dimensional electron gas (SP-2DEG) at the interface of two insulators, SrTiO3 and PbZr0.2Ti0.8O3. This SP-2DEG is strongly localized at the interfacial Ti atoms, due to the interplay between Coulomb interaction and band bending, and can be tuned by the ferroelectric polarization. Our findings open a door for engineering ferroelectric/insulator interfaces to create tunable ferroic orders for magnetoelectric device applications and provide opportunities for designing multiferroic materials in heterostructures.

Reconstruction-stabilized epitaxy of LaCoO3/SrTiO3(111) heterostructures by pulsed laser deposition

Unlike widely explored complex oxide heterostructures grown along [001], the study of [111]-oriented heterointerfaces is very limited thus far. One of the main challenges is to overcome the polar discontinuity that hinders the epitaxy of atomically sharp interfaces. Here, by taking LaCoO3/SrTiO3(111) as a prototype, we show that the reconstruction, which effectively compensates the surface polarity, can stabilize the epitaxy of the heterostructure with polar discontinuity. A reconstructed substrate surface is prepared, while the growth is controlled to form reconstruction on the film surface. To suppress the chemical diffusion across the interface, the growth is interrupted between each unit cell layer to allow the lattice relaxation at a lowered temperature. In this way, high quality two-dimensional growth is realized and the heterointerfaces exhibit sharpness at the atomic scale. Our work provides a path to precisely control the growth of complex oxide heterostructures along polar orientations that exhibit emergent quantum phenomena.

Influence of In-Gap States on the Formation of Two-Dimensional Election Gas at ABO3/SrTiO3 Interfaces

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO3/SrTiO3 interface and the influence of substitution layers on 2DEG.

Variational study of the 2DEG wave function in InAlN/GaN heterostructures

The variational method has been widely used to study the electronic structures of heterostructure materials in spite of this method being less accurate than the numerical method, because analytical formulas for some electrical parameters can be derived using this method. However, effects of surface states on the two-dimensional electron gas (2DEG) have not been taken into account in the variational studies of GaN-based heterostructures. In the present study, analytical formulas for the electron wave function and ground state energy level of the 2DEG in InAlN/GaN heterostructures are derived using the variational method, and the influences of structural parameters of InAlN/GaN heterostructures on the electrical properties are discussed. In the theoretical model, evenly distributed surface states below the conduction band are assumed to be the origin of the 2DEG, and the polarization charges at the InAlN surface and the InAlN/GaN interface due to spontaneous and piezoelectric polarization effects in InAlN/GaN heterostructures are taken into account. A trial envelope wave function with two variational parameters is used to derive the expectation value of the total energy per electron. The variational parameters are determined by minimizing the expectation value. The model predicts a linear conduction band profile in InAlN barrier layer and an approximately triangular-shaped potential well on the GaN side of the InAlN/GaN interface. Electrons released from the surface states are confined in the potential well, forming the 2DEG. The 2DEG sheet density for the lattice-matched InAlN/GaN heterostructure with a 15 nm InAlN layer is 1.961013 cm-2, and the average distance from the InAlN/GaN interface of electrons is 2.23 nm. The 2DEG sheet density increases rapidly with InAlN thickness increasing when the InAlN layer exceeds the critical thickness, and starts to be saturated above 15 nm. The dependence of the calculated 2DEG sheet density on the InAlN thickness quantitatively agrees with recently reported experimental data. The increasing 2DEG sheet density results in increasing the ground state energy level and Fermi energy, and the energy spacing between the two also increases for containing more electrons. The polarization discontinuity at the InAlN/GaN interface decreases with increasing In mole fraction, causing the 2DEG sheet density to decrease, and thus the ground state energy level and the Fermi energy to decrease. This model is conducive to understanding the electrical behaviors of InAlN/GaN heterostructures and providing readily applicable formulas for studying the electron transport and optical transitions.

Quantum confined two-dimensional electron/hole gas switching by facet orientation of perovskite oxides.

The Polar discontinuity at heterointerface and the bare surface reconstructs the electronic phase of perovskite oxides. This gives rise to confined free electrons which intrinsically transit material from band insulator to metal. However, the insulator–metal transition induced by free holes has not been investigated so far due to the challenge in obtaining free hole state in oxides. Here, we propose a simple method whereby free holes can be obtained via polar facet reorientation. In the high polarity case, free holes can be supported by the lift up of O 2p subbands, which split into three independent subbands (one heavy hole subband and two light hole subbands) due to strong quantum confinement. Results show that both of the free electron and hole states are confined in a two dimensional quantum well, subjecting to the confined energy (E − EF) and occupied density of states around the Fermi level, indicating a finite thickness for preserving the metal states.

Design of two-dimensional electron gas systems via polarization discontinuity from large-scale first-principles calculations

This work demonstrates an efficient approach to design perovskite-oxide-based two dimensional electron gas systems using large-scale first-principles calculations.

Prediction of a mobile two-dimensional electron gas at the LaScO3/BaSnO3 (001) interface

Two-dimensional electron gases (2DEG) at oxide interfaces, such as $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (001), have aroused significant interest due to their high carrier density $(\ensuremath{\sim}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2})$ and strong lateral confinement (\ensuremath{\sim}1 nm). However, these 2DEGs are normally hosted by the weakly dispersive and degenerate $d$ bands (e.g., $\mathrm{Ti}\text{\ensuremath{-}}3d$ bands), which are strongly coupled to the lattice, causing mobility of such 2DEGs to be relatively low at room temperature $(\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs})$. Here, we propose using oxide host materials with the conduction bands formed from $s$ electrons to increase carrier mobility and soften its temperature dependence. Using first-principles density functional theory calculations, we investigate $\mathrm{LaSc}{\mathrm{O}}_{3}/\mathrm{BaSn}{\mathrm{O}}_{3}$ (001) heterostructure and as a model system, where the conduction band hosts the $s$-like carriers. We find that the polar discontinuity at this interface leads to electronic reconstruction resulting in the formation of the 2DEG at this interface. The conduction electrons reside in the highly dispersive $\mathrm{Sn}\text{\ensuremath{-}}5s$ bands, which have a large band width and a low effective mass. The predicted 2DEG is expected to be highly mobile even at room temperature due to the reduced electron-phonon scattering via the inter-band scattering channel. A qualitatively similar behavior is predicted for a doped $\mathrm{BaSn}{\mathrm{O}}_{3}$, where a monolayer of BaO is replaced with LaO. We anticipate that the quantum phenomena associated with these 2DEGs to be more pronounced owing to the high mobility of the carriers.

Appearance and disappearance of ferromagnetism in ultrathin LaMnO3 on SrTiO3 substrate: A viewpoint from first principles

The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science \textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultra-thin LaMnO$_3$ films as well as superlattices. Starting from the very precise descriptions of both LaMnO$_3$ and SrTiO$_3$, it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within LaMnO$_3$ layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for LaAlO$_3$/SrTiO$_3$ heterostructures. Our study not only explains the long-term puzzle regarding the magnetism of ultra-thin LaMnO$_3$ films, but also shed light on how to overcome the notorious magnetic dead layer in ultra-thin manganites.

Energetic and Electronic Properties of (0001) Inversion Domain Boundaries in ZnO

In this work, the eight possible configurations of (0001) inversion domain boundaries (IDBs) in wurtzite ZnO have been investigated systematically by first‐principle calculations based on density‐functional theory (DFT). The energetic stability revealed that H4 are the most stable among the Head‐to‐Head type (H) IDBs, whereas for the Tail‐to‐Tail type (T) IDBs, T1 and T2 IDBs have lower formation energies. Their electronic properties were investigated using the electron localization function (ELF) and the projected density of states (PDOS). The results revealed that all the boundaries present a metallic character with the hybridization bands crossing the Fermi level; they are mainly dominated by Zn:3d and O:2p states in H IDBs and Zn:4s states in T IDBs, respectively. In particular, owing to the polarization discontinuity, electron accumulation occurs at all the T IDB regions with the conduction band minimum (CBM) shifting down below the Fermi level.

Strain-induced polar discontinuities in two-dimensional materials from combined first-principles and Schrödinger-Poisson simulations

The local application of mechanical stress in piezoelectric materials gives rise to boundaries across which the electric polarization changes. Polarization charges appear along such polar discontinuities and the ensuing electric fields drive a charge reconstruction with the accumulation of free carriers at the boundaries. This is particularly relevant for two-dimensional materials that can sustain very large strains and display record piezoelectric responses. Here we show by first-principles simulations the emergence of one-dimensional wires of free electrons and holes along strain interfaces, taking SnSe as a paradigmatic material. We complement this by developing a Schr\"odinger-Poisson approach specifically designed for two-dimensional materials that it is able to reproduce the ab-initio results and also to extend them to regimes of parameters and system sizes that would be unaffordable in first principles calculations. This model allows us to assess the degree of tunability for the free charge in the wires coming from strain values and profiles, and to obtain the critical size at which the interfaces start to be metallic.