Creating Two-Dimensional Electron Gas in Polar/Polar Perovskite Oxide Heterostructures: First-Principles Characterization of LaAlO3/A(+)B(5+)O3

Density functional theory study on the formation mechanism and electrical properties of two-dimensional electron gas in biaxial-strained LaGaO 3 /BaSnO 3 heterostructure

The two-dimensional electron gas (2DEG) in BaSnO3 -based heterostructure (HS) has received tremendous attention in the electronic applications because of its excellent electron migration characteristic. We modeled the n-type (LaO)+ /(SnO2 )0 interface by depositing LaGaO3 film on the BaSnO3 substrate and explored strain effects on the critical thickness for forming 2DEG and electrical properties of LaGaO3 /BaSnO3 HS system using first-principles electronic structure calculations. The results indicate that to form 2DEG in the unstrained LaGaO3 /BaSnO3 HS system, a minimum thickness of approximately 4 unit cells of LaGaO3 film is necessary. An increased film thickness of LaGaO3 is required to form the 2DEG for -3%-biaxially-strained HS system and the critical thickness is 3 unit cells for 3%-baxially-strained HS system, which is caused by the strain-induced change of the electrostatic potential in LaGaO3 film. In addition, the biaxial strain plays an important role in tailoring the electrical properties of 2DEG in LaGaO3 /BaSnO3 HS syestem. The interfacial charge carrier density, electron mobility and electrical conductivity can be optimized when a moderate tensile strain is applied on the BaSnO3 substrate in the ab-plane.

Evidence of linear and cubic Rashba effect in non-magnetic heterostructure

TheLaAlO3/KTaO3system serves as a prototype to study the electronic properties that emerge as a result of spin-orbit coupling (SOC). In this article, we have used first-principles calculations to systematically study two types of defect-free (0 0 1) interfaces, which are termed as Type-I and Type-II. While the Type-I heterostructure produces a two dimensional (2D) electron gas, the Type-II heterostructure hosts an oxygen-rich 2D hole gas at the interface. Furthermore, in the presence of intrinsic SOC, we have found evidence of both cubic and linear Rashba interactions in the conduction bands of the Type-I heterostructure. On the contrary, there is spin-splitting of both the valence and the conduction bands in the Type-II interface, which are found to be only linear Rashba type. Interestingly, the Type-II interface also harbors a potential photocurrent transition path, making it an excellent platform to study the circularly polarized photogalvanic effect.

KTaO3 -The New Kid on the Spintronics Block

Long after the heady days of high-temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO₃ (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO₃ (KTO), a material with all the "good" properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin-orbit coupling. In this state-of-the-art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO-based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin-polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

Two-dimensional conducting states in infinite-layer oxide/perovskite oxide hetero-structures

Heterointerfaces sandwiched by oxides of dissimilar crystal structures will show strong interface reconstruction, leading to distinct interfacial effect arising from unusual physics. Here, we present a theoretical investigation on the interfaces between infinite-layer oxide and perovskite oxide (SrCuO₂/SrTiO₃and SrCuO₂/KTaO₃). Surprisingly, we found well-defined two-dimensional electron gas (2DEG), stemming from atomic reconstruction and polar discontinuity at interface. Moreover, the 2DEG resides in both the TiO₂and CuO₂interfacial layers, unlike LaAlO₃/SrTiO₃for which 2DEG exists only in the TiO₂interfacial layer. More than that, no metal-to-insulator transition is observed as the SrCuO₂layer thickness decreases to one unit cell, i.e., the metallicity of the new interface is robust. Further investigations show more unique features of the 2DEG. Due to the absence of apical oxygen at the SrCuO₂/SrTiO₃(KTaO₃) interface, the conducting states in the interface TiO₂(TaO₂) layer follows thedxy<d3z2-r2<dxz/yzorbital order rather than thed_xy<d_xz/yzorbital order of paradigm LaAlO₃/SrTiO₃(KTaO₃), exhibiting enhanced interfacial conduction. This work suggests the great potential of heterointerfaces composed of non-isostructural oxides for fundamental research.

The CdTiO3/BaTiO3 superlattice interface from first principles

The oxide interface has been studied extensively in the past decades and exhibits different physical properties from the constituent bulks. Using first-principles electronic structure calculations, we investigated the interface of CdTiO₃/BaTiO₃ (CTO/BTO) superlattice with ferroelectric BaTiO₃. In this case, the conduction bands of CdTiO₃ are composed of Cd-5s orbitals with low electron effective mass and nondegenerate dispersion, and thus expected to have high mobility. We predicted a controllable conductivity at the interface, and further analyzed how the polarization direction and strength affect the conductivity. We also explored the relationship between two components: thickness and polarization. Intriguingly, the total polarization in CTO/BTO might be even larger than that of ferroelectric bulk BaTiO₃. Therefore, we found a way to maximize the superlattice polarization by increasing the fraction of the CdTiO₃ layers, based on the interesting dependence of the total polarization and CTO/BTO ratio.

2DEG and 2DHG in NaTaO3 polar thin films: thickness and strain dependency

Two-dimensional (2D) carrier gases in perovskite surfaces and interfaces have been intensely studied since their properties are attractive to many functional devices and applications. Here, we demonstrate through ab initio DFT calculations that surface 2D carries gases can be found in NaTaO3 ultrathin films. Furthermore, we show the thickness dependence of such phenomenon and how it can be tuned when biaxial in-plane strain is applied. Tensile does not alter the valence and conduction character of the films but promotes 2D electron and hole gases in the (TaO2)+ and (NaO)− surfaces, respectively. Because of the competition between surface and strain effects to deal with the cleavage-induced polarity, biaxial compression is able to generate 2D hole gases in the (TaO2)+ surface instead. Such carrier-type and layer switching are explained through changes in the electrostatic potential balancing along the [001] direction and (Na,Ta) cations displacements. The presented results concern not only nanoelectronics but also catalytic applications where modulating bandgap and valence/conduction states is desired.

Two-dimensional polar metals in KNbO3/BaTiO3 superlattices: first-principle calculations

Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions. By using first-principle electronic structure calculations, we explored the possibility of producing metallic states in the polar/nonpolar KNbO₃/BaTiO₃ superlattice (SL) composed of two prototypical ferroelectric materials: BaTiO₃ (BTO) and KNbO₃ (KNO). Two types of polar/nonpolar interfaces, p-type (KO)⁻/(TiO₂)⁰ and n-type (NbO₂)⁺/(BaO)⁰, which can be constituted into two symmetric NbO₂/BaO–NbO₂/BaO (NN-type) and KO/TiO₂–KO/TiO₂ (PP-type) SL, as well as one asymmetric KO/TiO₂–NbO₂/BaO (PN-type) SL. The spatial distribution of ferroelectric distortions and their conductive properties are found to be extraordinarily sensitive to the interfacial configurations. An insulator-to-metal transition is found in each unit cell of the symmetric interfacial SL models: one exhibiting quasi-two-dimensional n-type conductivity for NN-type SL, while the other being quasi-two-dimensional p-type conductivity for PP-type SL. The anisotropic coexistence of in-plane orientation of free carriers and out-of-plane orientation of ferroelectric polarization in KNO/BTO SL indicates that in-plane free carriers can not eliminate the out-of-plane dipoles. Our results provide a road map to create two-dimensional polar metals in insulating perovskite oxide SL, which is expected to promote applications of new quantum devices.

Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions.

First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO₃)_m/(KNbO₃)_n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations.

Thickness Control of the Spin-Polarized Two-Dimensional Electron Gas in LaAlO3/BaTiO3 Superlattices

We explored the possibility of increasing the interfacial carrier quantum confinement, mobility and conductivity in the (LaAlO₃)_n/(BaTiO₃)_n superlattices by thickness regulation using the first-principles electronic structure calculations. Through constructing two different interfacial types of LaAlO₃/BaTiO₃ superlattices, we discovered that the LaO/TiO₂ interface is preferred from cleavage energy consideration. We then studied the electronic characteristics of two-dimensional electron gas (2DEG) produced at the LaO/TiO₂ interface in the LaAlO₃/BaTiO₃ superlattices via spin-polarized density functional theory calculations. The charge carrier density of 2DEG has a magnitude of 10¹⁴ cm⁻² (larger than the traditional system LaAlO₃/SrTiO₃), which is mainly provided by the interfacial Ti 3d_xy orbitals when the thicknesses of LaAlO₃ and BaTiO₃ layers are over 4.5 unit cells. We have also revealed the interfacial electronic characteristics of the LaAlO₃/BaTiO₃ system, by showing the completely spin-polarized 2DEG mostly confined at the superlattice interface. The interfacial charge carrier mobility and conductivity are found to be converged beyond the critical thickness. Therefore, we can regulate the interfacial confinement for the spin-polarized 2DEG and quantum transport properties in LaAlO₃/BaTiO₃ superlattice via controlling the thicknesses of the LaAlO₃ and BaTiO₃ layers.

First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram

First-principles calculations predict a surface phase stability diagram for the polar perovskite KTaO₃.

Comparison Studies of Interfacial Electronic and Energetic Properties of LaAlO3/TiO2 and TiO2/LaAlO3 Heterostructures from First-Principles Calculations

By using first-principles electronic structure calculations, we studied electronic and energetic properties of perovskite oxide heterostructures with different epitaxial growth order between anatase TiO₂ and LaAlO₃. Two types of heterostructures, i.e., TiO₂ film grown on LaAlO₃ substrate (TiO₂/LaAlO₃) and LaAlO₃ film grown on TiO₂ substrate (LaAlO₃/TiO₂), were modeled. The TiO₂/LaAlO₃ model is intrinsically metallic and thus does not exhibit an insulator-to-metal transition as TiO₂ film thickness increases; in contrast, the LaAlO₃/TiO₂ model shows an insulator-to-metal transition as the LaAlO₃ film thickness increases up to 4 unit cells. The former model has a larger interfacial charge carrier density (n ∼ 10¹⁴ cm^-2) and smaller electron effective mass (0.47m_e) than the later one (n ∼ 10¹³ cm^-2, and 0.70m_e). The interfacial energetics calculations indicate that the TiO₂/LaAlO₃ model is energetically more favorable than the LaAlO₃/TiO₂ model, and the former has a stronger interface cohesion than the later model. This research provides fundamental insights into the different interfacial electronic and energetic properties of TiO₂/LaAlO₃ and LaAlO₃/TiO₂ heterostructures.

First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) Heterostructures

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO₃/SrTiO₃ (A = Ca, Sr, and Ba) heterostructures. Two types of nonpolar/nonpolar interfaces, (AO)⁰/(TiO₂)⁰ and (HfO₂)⁰/(SrO)⁰, each with AO and HfO₂ surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition is found in all the heterostructures with HfO₂ surface termination: for (AO)⁰/(TiO₂)⁰ interfaces, predicted critical film thickness for an insulator-to-metal transition is about 7, 6, and 3 unit cells for CaHfO₃/SrTiO₃, SrHfO₃/SrTiO₃, and BaHfO₃/SrTiO₃, respectively; for (HfO₂)⁰/(SrO)⁰ interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the heterostructures with AO surface termination, CaHfO₃/SrTiO₃ exhibits a much larger critical film thickness about 11-12 unit cells for an insulator-to-metal transition; while SrHfO₃/SrTiO₃ and BaHfO₃/SrTiO₃ do not show any polarization behavior even film thickness increases up to 20 unit cells. The strain-induced polarization behavior was well-elucidated from energy versus polarization profile. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO₃/SrTiO₃ HS.