LaAlO3 Interface Research Articles

Acoustoelectric charge transport at the LaAlO3/SrTiO3 interface

The two-dimensional electron system (2DES) formed at the interface of LaAlO3 (LAO) and SrTiO3 (STO), both band insulators in bulk, exhibits properties not easily attainable in conventional electronic materials. The extreme shallowness of the 2DES, only a few nanometers below the surface, opens up unique possibilities such as tunneling spectroscopy, local electronic sensing, and in situ patterning by manipulating the surface properties. It is particularly tempting to manipulate the charge carriers with surface acoustic wave (SAW) phonons, which are confined to the surface. However, the absence of intrinsic piezoelectricity in both LAO and STO complicates the electric generation of SAWs, as well as the induction of an acoustoelectric current. Here, we present robust acoustoelectric coupling between SAWs and the LAO/STO 2DES by using electrostriction in STO, induced by a dc electric field. Electromechanical coupling to the carriers is provided by phonon-induced modulation of the 2DES potential well, leading to SAW-induced carrier transport. The ability to control charge carriers with SAWs brings the versatile LAO/STO 2DES into reach of quantum acoustics, opening possibilities to study the interplay of nanoscale mechanical waves and the rich physics exhibited by nonpiezoelectric complex oxides, including superconductivity, magnetism, and correlated insulator states.

Transport and excitations in a negative-U quantum dot at the LaAlO3/SrTiO3 interface

In a solid-state host, attractive electron–electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high Tc superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect.

Patterning the two dimensional electron gas at the LaAlO3/SrTiO3 interface by structured Al capping

We demonstrate an approach for patterning a quasi-two dimensional electron gas (q-2DEG) at the interface of LaAlO3 (LAO) and SrTiO3 (STO) utilizing a structured Al capping layer. The capping of Al enables the formation of q-2DEG at the interface of 1–3 unit cells (uc) of LAO on STO, which was originally insulating before capping. The properties of the q-2DEG induced by the Al capping layer are essentially the same as those of q-2DEG without Al. Therefore, we can pattern q-2DEG by simply patterning the Al film on LAO (2 or 3 uc)/STO using a one-step liftoff process. Our approach circumvents the difficulty of direct patterning of oxide materials and provides a simple and robust patterning method for future device applications based on complex oxide interfaces.

Elastically strained and relaxed La0.67Ca0.33MnO3 films grown on lanthanum aluminate substrates with different orientations

Structure of 40-nm thick La0.67Ca0.33MnO3 (LCMO) films grown by laser evaporation on (001) and (110) LaAlO3 (LAO) substrates has been investigated using the methods of medium-energy ion scattering and X-ray diffraction. The grown manganite layers are under lateral biaxial compressive mechanical stresses. When (110)LAO wafers are used as the substrates, stresses relax to a great extent; the relaxation is accompanied by the formation of defects in a (3-4)-nm thick manganite-film interlayer adjacent to the LCMO-(110)LAO interface. When studying the structure of the grown layers, their electro- and magnetotransport parameters have been measured. The electroresistance of the LCMO films grown on the substrates of both types reached a maximum at temperature T (M) of about 250 K. At temperatures close to T (M) magnetoresistance of the LCMO/(110)LAO films exceeds that of the LCMO/(001)LAO films by 20-30%; however, the situation is inverse at low temperatures (T < 150 K). At T < T (M) , the magnetotransport in the grown manganite films significantly depends on the spin ordering in ferromagnetic domains, which increase with a decrease in temperature.

Electronic and magnetic phenomena at the interface of LaAlO3 and Ru doped SrTiO3

We have investigated the effect of Ru doping the SrTiO3 (STO) side of the LaAlO3/STO (LAO/STO) interface. The metallic behavior at the interface is remarkably robust to defects and disorder. Despite spin moment contribution from Ru ions, we see no evidence of magnetic ordering at the Ti L3,2 edge in either doped or undoped interfaces using X-ray magnetic circular dichroism. Magnetotransport measurements also do not show any evidence of magnetic scattering beyond that observed in undoped LAO/STO interfaces. Insertion of more than 7 unit cells of Ru doped STO at the interface suppresses metallic conductivity with a surprisingly sharp metal insulator transition. A similar metal-insulator transition is observed when a homoepitaxial STO film is grown on the single crystal substrate before LAO deposition. Together, our results indicate that ferromagnetism is not intrinsic to the interface, magnetic Ru dopants are not significant sources of scattering, and that cation vacancy formation alone cannot explain the insulating behavior observed in thick homoepitaxial LAO/STO/STO bilayers.

Electron microscopy of iron chalcogenide FeTe(Se) films

The structure of Fe1 + δTe1 − xSex films (x = 0; 0.05) grown on single-crystal MgO and LaAlO3 substrates has been investigated by transmission and scanning transmission electron microscopy. The study of Fe1.11Te/MgO structures has revealed two crystallographic orientation relationships between the film and substrate. It is shown that the lattice mismatch between the film and substrate is compensated for by the formation of misfit dislocations. The Burgers vector projection is determined. The stresses in the film can partially be compensated for due to the formation of an intermediate disordered layer. It is shown that a FeTe0.5Se0.5 film grown on a LaAlO3 substrate is single-crystal and that the FeTe0.5Se0.5/LaAlO3 interface in a selected region is coherent. The orientation relationships between the film and substrate are also determined for this case.

Oxide-based platform for reconfigurable superconducting nanoelectronics

We report quasi-1D superconductivity at the interface of LaAlO3 and SrTiO3. The material system and nanostructure fabrication method supply a new platform for superconducting nanoelectronics. Nanostructures having line widths w ∼ 10 nm are formed from the parent two-dimensional electron liquid using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3, placing them in the quasi-1D regime. Broad superconducting transitions versus temperature and finite resistances in the superconducting state well below Tc ≈ 200 mK are observed, suggesting the presence of fluctuation- and heating-induced resistance. The superconducting resistances and V–I characteristics are tunable through the use of a back gate. Four-terminal resistances in the superconducting state show an unusual dependence on the current path, varying by as much as an order of magnitude. This new technology, i.e., the ability to ‘write’ gate-tunable superconducting nanostructures on an insulating LaAlO3/SrTiO3 ‘canvas’, opens possibilities for the development of new families of reconfigurable superconducting nanoelectronics.

Electron transport at interface of LaAlO3 and SrTiO3 band insulators

Electron transport properties at n-type LaAlO3/SrTiO3 interfaces have been investigated numerically. Carrier distributions, band structures, and sheet density have been calculated by solving Schrödinger equations with Poisson equation in a self-consistent manner for various LaAlO3/SrTiO3 interfaces with and without atomic interdiffusions. It was found that the interface with A-site atom interdiffusion has the critical thickness of 4 unit cells below which it remains insulating. Most electrons are localized within 10 nm from the interface forming two-dimensional electron gas and multi-subbands are occupied indicating the multi-channel conduction. Electron mobility along the A-site atom interdiffused interface has been calculated using the linearized Boltzmann transport equation including scattering mechanisms of acoustic phonon, polar optical phonon, interface roughness, and net charged layers. At low temperature, the mobility is limited by the interface roughness and net charged layers. At room temperature, the polar optical phonon is the dominant electron scattering mechanism and the mobility is almost independent from the thickness of LaAlO3 film.

Single-Mode Superconductivity in LaAlO3/SrTiO3 Nanostructures

ABSTRACTThe properties of superconductors at the extreme limits of dimensionality are of fundamental interest. The interface of LaAlO3 and SrTiO3 hosts a quasi-two-dimensional superconductor below Tc≈200 mK. Here we report superconductivity in nanowire-shaped structures created at the LaAlO3/SrTiO3 interface using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3 (w &lt;&lt;ξSC∼100 nm), placing them in the quasi-1D regime. The ability to “write” fully superconducting nanostructures on an insulating LaAlO3/SrTiO3 “canvas” opens possibilities for the development of new families of superconducting nanoelectronics. Four-terminal transport measurements suggest that in some devices both the normal and superconducting states are confined to a single quantum channel.

Anisotropic electrical transport properties of a two-dimensional electron gas at SrTiO3–LaAlO3 interfaces

Experimental evidence of strong in-plane anisotropy in electrical properties of the confined electron gas at the SrTiO3–LaAlO3 interface on top of (LaAlO3)0.3(Sr2AlTaO3)0.7 substrates is provided by detailed transport measurements. Structured measurement geometries in multiple directions are used to show dependence of the sheet resistance with the in-plane angle θ, which is fitted with a sine function with a period of 180°. The carrier density remains constant and a directional dependence of the carrier mobility of more than one order of magnitude is determined with respect to the orientation of the unit cell height steps present at the SrTiO3–LaAlO3 interface.

Shubnikov–De Haas Oscillations inSrTiO3/LaAlO3Interface

Quantum magnetic oscillations in SrTiO3/\LaAlO3 interface are observed. The evolution of their frequency and amplitude at various gate voltages and temperatures is studied. The data are consistent with the Shubnikov de-Haas theory. The Hall resistivity rho exhibits nonlinearity at low magnetic field. It is fitted assuming multiple carrier contributions. The comparison between the mobile carrier density inferred from the Hall data and the oscillation frequency suggests multiple valley and spin degeneracy. The small amplitude of the oscillations is discussed in the framework of the multiple band scenario.

Paramagnetic point defects in (100)Si∕LaAlO3 structures: Nature and stability of the interface

The atomic nature of the interface in (100)Si∕LaAlO3 structures with nanometer-thin amorphous LaAlO3 layers of high dielectric constant (κ), deposited directly on clean (100)Si by molecular beam deposition at ∼100°C, was assessed through probing of paramagnetic point defects. On the as-grown samples K-band electron spin resonance indicated the absence of a Si∕SiO2-type interface in terms of the archetypal Si-dangling bond-type Si∕SiO2 interface defects (Pb0, Pb1). With no Pb-type defects observed, this state is found to persist during subsequent annealing (1atm N2 or 5% O2 in N2 ambient) up to the temperature Tan∼800°C, referring to a thermally stable abrupt Si∕LaAlO3 interface, quite in contrast with other high-κ metal oxide∕Si structures. However, in the range Tan∼800–860°C a Si∕SiO2-type interface starts forming as evidenced by the appearance of Pb0 defects and, with some delay in Tan, the EX center—a SiO2 associated defect, attesting to significant structural∕compositional modification. The peaking of the defect density versus Tan curves indicates the SiOx nature of the interlayer to break up again upon annealing at Tan⩾930°C, possibly related to crystallization and∕or degrading silicate formation. No specific LaAlO3-specific point defects could be traced.

Nature and stability of the (100)Si∕LaAlO3 interface probed by paramagnetic defects

Electron spin resonance analysis of (100)Si∕LaAlO3 structures reveals the absence of a Si∕SiO2-type interface in terms of archetypal Si-dangling bond-type Si∕SiO2 interface defects (Pb0,Pb1). With no Pb-type defects observed, this state is found to persist during subsequent annealing (5% O2+N2 ambient) up to Tan∼800°C, indicating a thermally stable and abrupt Si∕LaAlO3 interface. In the range Tan∼800–860°C, however, a Si∕SiO2-type interface starts forming as evidenced by the appearance of Pb0 defects and, with some delay in Tan, the EX center (a SiO2 associated defect) attesting to significant structural/compositional modification. The peaking of the defect density versus Tan curves indicates that the interlayer with SiOx nature breaks up upon annealing at Tan⩾930°C, possibly related to crystallization and silicate formation. No LaAlO3-specific point defects could be detected.

Electron energy barriers at interfaces of GaAs(100) with LaAlO3 and Gd2O3

Electron energy barriers at the interfaces of GaAs(100) with Gd2O3 appear to be insensitive to the Fermi level pinning indicating that charges at interface states are of marginal importance for the band alignment at semiconductor/insulator interfaces. The inferred conduction band offset of 1.6±0.1eV for GaAs(100)∕Gd2O3 is close to that measured at the GaAs(100)∕LaAlO3 interface which is consistent with the 5.8 and 5.7eV wide band gaps of these two insulators. However, the defects revealed by photoionization measurements exhibit a distinctly different in-depth distribution. In GaAs∕LaAlO3 most of the traps are located close to the semiconductor surface, while in GaAs∕Gd2O3 case they are found distributed across the entire oxide layer.

Spin-dependent tunneling through high-k LaAlO3

We report on the use of the LaAlO3 (LAO) high-k dielectric as a tunnel barrier in magnetic tunnel junctions. From tunnel magnetoresistance (TMR) measurements on epitaxial La2∕3Sr1∕3MnO3∕LAO∕La2∕3Sr1∕3MnO3 junctions, we estimate a spin polarization of 77% at low temperature for the La2∕3Sr1∕3MnO3∕LAO interface. Remarkably, the TMR of La2∕3Sr1∕3MnO3∕LAO∕Co junctions at low bias is negative, evidencing a negative spin polarization of Co at the interface with LAO, and its bias dependence is very similar to that of La2∕3Sr1∕3MnO3∕SrTiO3∕Co junctions. We discuss possible reasons for this behavior.

High-dielectric-tunability of ferroelectric (Pb,Sr)TiO3 thin films on (001) LaAlO3

Ferroelectric (Pb,Sr)TiO3 (PSTO) thin films were epitaxially grown on (001) LaAlO3 (LAO) by using pulsed laser deposition. Microstructural characterizations with x-ray diffraction and transmission electron microscopy indicate that the as-grown films have excellent single crystalline quality and a (001)PSTO∕∕(001)LAO and [100] PSTO∕∕ [100] LAO interface relationship. Dielectric property measurements reveal that the as-grow films have a very high dielectric constant value of 3100 and very large dielectric tunability of 48% at 40V∕cm at room temperature. These excellent results suggest that the highly epitaxial ferroelectric (Pb,Sr)TiO3 thin films can be developed for room-temperature tunable microwave elements in wireless communication applications.

Fabrication of stress-induced SrRuO3 nanostructures by pulsed laser deposition

The compressive stress originated in the coherent epitaxial-SrRuO3/LaAlO3 interface has been used as a nano-patterning tool for the creation of island and wire arrays in ultrathin (<20 nm) SrRuO3 films. The nanostructure, stoichiometry, and electrical properties of these arrays have been studied by X-ray diffraction, atomic force microscopy, Rutherford back-scattering spectroscopy and resistivity vs temperature measurements. The influence of the shadowing effect produced by the non-perpendicular incidence of the evaporated particles on the array morphology is discussed.

Structure and properties of the YBa2Cu3O7−x/LaAlO3 superlattices

Superconductor/insulator superlattices have been prepared using pulsed laser deposition. Two different [(YBa2Cu3O7−x)n/LaAlO3)m]×N superlattices with N=15 and N=30 have been studied by high resolution electron microscopy. Both supperlattices show a zero resistivity transition at T=82 K and T=86 K, respectively. High resolution transmission electron microscopy leads us to propose two models for the YBa2Cu3O7−x–LaAlO3 interface layers. These models describe the interfaces as a stacking of CuO–AlO layers and BaO–AlO layers. High resolution transmission electron microscopy observations also reveal the presence of antiphase boundaries in the lanthanum aluminate deposited layers. Their formation is attributed to partial stress relaxation in the superlattice.