Maziar Behtash

Enhancing interfacial conductivity and spatial charge confinement of LaAlO3/SrTiO3 heterostructures via strain engineering

We explored the possibility of enhancing interfacial conductivity and spatial charge confinement of LaAlO3/SrTiO3 (LAO/STO) heterostructure (HS) via strain engineering using first-principles electronic structure calculations. We found that applying a tensile strain on the STO substrate along the ab-plane can significantly enhance the interfacial conductivity, magnetic moments, and the spatial charge confinement of the HS system. In contrast, a compressive strain can dilute the interfacial charge carrier density, make the mobile charges transfer to deep STO substrate, and weaken the spatial charge confinement along the c-axis. Hence, we propose that applying a tensile strain can be an effective way to enhance the interfacial conductivity and magnetism of STO-based HS systems.

Electronic structures and formation energies of pentavalent-ion-doped SnO2: First-principles hybrid functional calculations

We studied the electronic properties and relative thermodynamic stability of several pentavalent-ion (Ta, Nb, P, Sb, and I) doped SnO2 systems using first-principles hybrid density functional theory calculations, in order to evaluate their potential as transparent conducting oxides (TCOs). I-doped SnO2, though conductive, shows a narrowed optical band gap with respect to the undoped system due to the formation of gap states above the valence band. Nb-doped SnO2 forms localized impurity states below the conduction band bottom, suggesting that the Nb dopant exists as an Nb4+-like cation, which is consistent with the recent experimental finding of the formation of the impurity level below the conduction band bottom [Appl. Phys. Express 5, 061201 (2012)]. Ta- and Sb-doped SnO2 display n-type conductivity, high charge carrier density, and widened optical band gap. P-doped SnO2 shows similar n-type electronic properties with that of Sb- and Ta-doped systems, and thus P-doped SnO2 is proposed as a promising cand...

High-Throughput Design of Two-Dimensional Electron Gas Systems Based on Polar/Nonpolar Perovskite Oxide Heterostructures

The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.

Interface Energetics and Charge Carrier Density Amplification by Sn-Doping in LaAlO3/SrTiO3 Heterostructure.

Tailoring the two-dimensional electron gas (2DEG) at the n-type (TiO2)(0)/(LaO)(+1) interface between the polar LaAlO3 (LAO) and nonpolar SrTiO3 (STO) insulators can potentially provide desired functionalities for next-generation low-dimensional nanoelectronic devices. Here, we propose a new approach to tune the electronic and magnetic properties in the n-type LAO/STO heterostructure (HS) system via electron doping. In this work, we modeled four types of layer doped LAO/STO HS systems with Sn dopants at different cation sites and studied their electronic structures and interface energetics by using first-principles electronic structure calculations. We identified the thermodynamic stability conditions for each of the four proposed doped configurations with respect to the undoped LAO/STO interface. We further found that the Sn-doped LAO/STO HS system with Sn at Al site (Sn@Al) is energetically most favorable with respect to decohesion, thereby strengthening the interface, while the doped HS system with Sn at La site (Sn@La) exhibits the lowest interfacial cohesion. Moreover, our results indicate that all the Sn-doped LAO/STO HS systems exhibit the n-type conductivity with the typical 2DEG characteristics except the Sn@La doped HS system, which shows p-type conductivity. In the Sn@Al doped HS model, the Sn dopant exists as a Sn(4+) ion and introduces one additional electron into the HS system, leading to a higher charge carrier density and larger magnetic moment than that of all the other doped HS systems. An enhanced charge confinement of the 2DEG along the c-axis is also found in the Sn@Al doped HS system. We hence suggest that Sn@Al doping can be an effective way to enhance the electrical conduction and magnetic moment of the 2DEG in LAO/STO HS systems in an energetically favorable manner.

Creating Two-Dimensional Electron Gas in Polar/Polar Perovskite Oxide Heterostructures: First-Principles Characterization of LaAlO3/A+B5+O3

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) at the polar/polar (LaO)(+)/(BO2)(+) interface in the LaAlO3/A(+)B(5+)O3 (A = Na and K, B = Nb and Ta) heterostructures (HS). Unlike the prototype polar/nonpolar LaAlO3/SrTiO3 HS system where there exists a least film thickness of four LaAlO3 unit cells to have an insulator-to-metal transition, we found that the polar/polar LaAlO3/A(+)B(5+)O3 HS systems are intrinsically conducting at their interfaces without an insulator-to-metal transition. The interfacial charge carrier densities of these polar/polar HS systems are on the order of 10(14) cm(-2), much larger than that of the LaAlO3/SrTiO3 system. This is mainly attributed to two donor layers, i.e., (LaO)(+) and (BO2)(+) (B = Nb and Ta), in the polar/polar LaAlO3/A(+)B(5+)O3 systems, while only one (LaO)(+) donor layer in the polar/nonpolar LaAlO3/SrTiO3 system. In addition, it is expected that, due to less localized Nb 4d and Ta 5d orbitals with respect to Ti 3d orbitals, these LaAlO3/A(+)B(5+)O3 HS systems can exhibit potentially higher electron mobility because of their smaller electron effective mass than that in the LaAlO3/SrTiO3 system. Our results demonstrate that the electronic reconstruction at the polar/polar interface could be an alternative way to produce superior 2DEG in the perovskite-oxide-based HS systems.

The role of uniaxial strain in tailoring the interfacial properties of LaAlO3/SrTiO3 heterostructure

The two-dimensional electron gas (2DEG) at the (LaO)+1/(TiO2)0 n-type interface in LaAlO3/SrTiO3 (LAO/STO) heterostructures (HS) opens up new opportunities in next-generation nanoelectronic devices because of its unique and tunable physical properties. In this respect, strain plays an important role in tailoring the 2DEG properties of STO-based HS systems. Our first-principles calculations reveal that a uniaxial strain along the [100]-direction of the STO substrate significantly affects the chemical bonding of LAO and STO near the interfacial region, which can further alter the interfacial electronic properties. We find that uniaxial tensile strain can considerably increase the interfacial charge carrier density, which is consistent with the recent experimental findings in [Moler et al., Nat. Mater., 2013, 12, 1091]. Our results also indicate that uniaxial compressive strain on STO can lead to higher interfacial charge carrier density and larger magnetic moments than uniaxial tensile strain, and further that the 2DEG in the compressively strained systems displays superior charge confinement in the c-direction. For the sake of comparison, the effects of ab-plane biaxial compressive and tensile strains applied on the STO substrate were also investigated. It is found that biaxial tensile strain improves the interfacial electronic properties and magnetic moments, while biaxial compressive strain shows the opposite effect.