SrTiO3 Barrier Research Articles

Symmetry-controlled SrRuO3/SrTiO3/SrRuO3 magnetic tunnel junctions: spin polarization and its relevance to tunneling magnetoresistance

Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO3/SrTiO3/SrRuO3(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO3(001) electrodes and evanescent states in the SrTiO3(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO3/SrTiO3/SrRuO3(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs.

All Oxide-Based Magnetic Tunnel Junctions Fabricated by Focused Ion Beam Nanomachining Technique

This study offers a comprehensive investigation into the fabrication of all-oxide-based magnetic tunnel junctions (MTJs) utilizing the focused ion beam (FIB) nanomachining method. The oxide thin films of SrTiO3 (STO), La[Formula: see text]CaxMnO3 (LCMO) and La[Formula: see text]SrxMnO3 (LSMO) were grown using pulsed laser deposition (PLD). Magnetization measurements at 5K revealed a notable difference in coercivity between LCMO and LSMO films, highlighting their suitability as electrodes in MTJ device fabrication. The deposited trilayer structure of LCMO/STO/LSMO exhibited heteroepitaxial growth, enabling the independent magnetic switching of electrode layers in the MTJ with the STO barrier layer. Conventional photolithography and ion milling were used to create a series of 4[Formula: see text][Formula: see text]m Hall bar tracks in the deposited films. Nanopillar devices, sized 250[Formula: see text]nm[Formula: see text]×[Formula: see text]250[Formula: see text]nm, were successfully fabricated on these 4[Formula: see text][Formula: see text]m tracks by optimizing the FIB nanomachining technique. The fabricated MTJs demonstrated a high tunnel magnetoresistance (TMR) response, leading to the conclusion that the FIB nanomachining technique holds significant promise for the development of high-performance oxide-based spintronic devices.

Metal–insulator transitions in dimensionality controlled LaxSr1−xVO3 films

Carrier doping into two dimensional (2D) Mott insulators is one of the prospective strategies for exploring exotic quantum phenomena. Although ultra-thin oxide films are one such target, it is vitally important to fabricate well-defined and clean samples to extract intrinsic properties. In this study, we start from establishing the growth of clean SrVO3 films with a low residual resistivity (∼4 × 10−7 Ω cm) and a high mobility (∼103 cm2/V s). By confining them with SrTiO3 barrier layers, the Mott insulator state appears at the thickness below 3 unit cells (u.c.). By the electron doping in the form of LaxSr1−xVO3 for such two dimensional systems (2 and 3 u.c), metallic-like phases appear in a narrow x region around x = 0.17, indicating a collapse of the Mott insulator state. This study demonstrates that artificial 2D systems of clean oxides are a promising playground for exploring novel Mott physics in confined systems.

Huge tunneling magnetoresistance in magnetic tunnel junction with Heusler alloy Co2MnSi electrodes

Magnetic tunnel junction with a large tunneling magnetoresistance has attracted great attention due to its importance in the spintronics applications. By performing extensive density functional theory calculations combined with the nonequilibrium Green’s function method, we explore the spin-dependent transport properties of a magnetic tunnel junction, in which a non-polar SrTiO3 barrier layer is sandwiched between two Heusler alloy Co2MnSi electrodes. Theoretical results clearly reveal that the near perfect spin-filtering effect appears in the parallel magnetization configuration. The transmission coefficient in the parallel magnetization configuration at the Fermi level is several orders of magnitude larger than that in the antiparallel magnetization configuration, resulting in a huge tunneling magnetoresistance (i.e. > 106), which originates from the coherent spin-polarized tunneling, due to the half-metallic nature of Co2MnSi electrodes and the significant spin-polarization of the interfacial Ti 3d orbital.

The Role of Ferroelectric Polarization in Resistive Memory Properties of Metal/Insulator/Semiconductor Tunnel Junctions: A Comparative Study.

Recently, tunnel junction devices adopting semiconducting Nb:SrTiO3 electrodes have attracted considerable attention for their potential applications in resistive data storage and neuromorphic computing. In this work, we report on a comparative study of Pt/insulator/Nb:SrTiO3 tunnel junctions between ferroelectric BaTiO3 and nonferroelectric SrTiO3 and LaAlO3 barriers to reveal the role of polarization in resistance switching properties. Although hysteretic behaviors appear in current-voltage measurements of all devices regardless of the barrier character, significantly improved current ratios by more than three orders of magnitude are observed in the Pt/BaTiO3/Nb:SrTiO3 tunnel junctions due to the dominance of polarization in modulation of junction barrier profiles between the low and high resistance states. The switchable polarization also gives rise to enhanced resistance retention since the electron diffusion that smears the barrier contrast of the bistable resistance states is suppressed by the polar BaTiO3/Nb:SrTiO3 interface associated with the ferroelectric bound charges. These polarization-induced effects are absent in the nonferroelectric Pt/SrTiO3/Nb:SrTiO3 and Pt/LaAlO3/Nb:SrTiO3 devices in which serious resistance state decay, described by Fick's second law, is observed since there are no switchable interface charges on SrTiO3/Nb:SrTiO3 and LaAlO3/Nb:SrTiO3 to block the electron diffusion. In addition, the Pt/BaTiO3/Nb:SrTiO3 device also exhibits an excellent switching endurance up to ∼4.0 × 106 bipolar cycles. These enhancements indicate the importance of ferroelectric polarization for achieving high-performance resistance switching and suggest that metal/ferroelectric/Nb:SrTiO3 tunnel junctions are promising candidates for nonvolatile memory applications.

Improving the Josephson energy in high-Tc superconducting junctions for ultra-fast electronics

We report the electrical transport of thin vertically-stacked Josephson tunnel junctions. The devices were fabricated using 16 nm thick GdBa2Cu3O7−δ electrodes and 1–4 nm SrTiO3 as an insulating barrier. The results show Josephson coupling for junctions with SrTiO3 barriers of 1 and 2 nm. Subtracting the residual current in the Fraunhofer patterns, energies of 3.1 mV and 5.7 mV at 12 K are obtained for STO barriers of 1 nm and 2 nm, respectively. The residual current may be related to the contribution of pinholes and thickness fluctuations in the STO barrier. These values are promising for reducing the influence of thermal noise and increasing the frequency operation rate in superconducting devices using high-temperature superconductors.

Depletion Mode MOSFET Using La-Doped BaSnO3 as a Channel Material.

The high room-temperature mobility that can be achieved in BaSnO3 has created significant excitement for its use as channel material in all-perovskite-based transistor devices such as ferroelectric field effect transistor (FET). Here, we report on the first demonstration of n-type depletion-mode FET using hybrid molecular beam epitaxy grown La-doped BaSnO3 as a channel material. The devices utilize a heterostructure metal-oxide semiconductor FET (MOSFET) design that includes an epitaxial SrTiO3 barrier layer capped with a thin layer of HfO2 used as a gate dielectric. A field-effect mobility of ∼70 cm2 V-1 s-1, a record high transconductance value of >2mS/mm at room temperature, and the on/off ratio exceeding 107 at 77 K were obtained. Using temperature- and frequency-dependent transport measurements, we quantify the impact of the conduction band offset at the BaSnO3/SrTiO3 interface as well as bulk and interface traps on device characteristics.

Electrical transport across nanometric SrTiO3 and BaTiO3 barriers in conducting/insulator/conducting junctions

We report the electrical transport properties of conducting/insulator/conducting heterostructures by studying current–voltage IV curves at room temperature. The measurements were obtained on tunnel junctions with different areas (900, 400 and 100 μm2) using a conducting atomic force microscope. Trilayers with GdBa2Cu3O7 (GBCO) as the bottom electrode, SrTiO3 or BaTiO3 (thicknesses between 1.6 and 4 nm) as the insulator barrier, and GBCO or Nb as the top electrode were grown by DC sputtering on (100) SrTiO3 substrates For SrTiO3 and BaTiO3 barriers, asymmetric IV curves at positive and negative polarization can be obtained using electrodes with different work function. In addition, hysteretic IV curves are obtained for BaTiO3 barriers, which can be ascribed to a combined effect of the FE reversal switching polarization and an oxygen vacancy migration. For GBCO/BaTiO3/GBCO heterostructures, the IV curves correspond to that expected for asymmetric interfaces, which indicates that the disorder affects differently the properties at the bottom and top interfaces. Our results show the role of the interface disorder on the electrical transport of conducting/insulator/conduction heterostructures, which is relevant for different applications, going from resistive switching memories (at room temperature) to Josephson junctions (at low temperatures).

Low noise all-oxide magnetic tunnel junctions based on a La0.7Sr0.3MnO3/Nb:SrTiO3 interface

All-oxide magnetic tunnel junctions with a semiconducting barrier, formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 and n-type semiconductor SrTi0.8Nb0.2O3, were designed, fabricated, and investigated in terms of their magneto-transport properties as a function of applied bias and temperature. We found that the use of the heavily Nb-doped SrTiO3 as a barrier results in significant improvement in the reproducibility of results, i.e., of large tunnel magnetoresistance (TMR) ratios, and a spectral noise density reduced by three orders of magnitude at low temperature. We attribute this finding to a considerably decreased amount of point defects in SrTi0.8Nb0.2O3, especially oxygen vacancies, compared with the conventional insulating SrTiO3 barrier.

Investigation of the tunnel magnetoresistance in junctions with a strontium stannate barrier

We experimentally investigate the structural, magnetic, and electrical transport properties of La0.67 Sr0.33MnO3 based magnetic tunnel junctions with a SrSnO3 barrier. Our results show that despite the high density of defects in the strontium stannate barrier, due to the large lattice mismatch, the observed tunnel magnetoresistance (TMR) is comparable to tunnel junctions with a better lattice matched SrTiO3 barrier, reaching values of up to 350% at T=5 K. Further analysis of the current-voltage characteristics of the junction and the bias voltage dependence of the observed tunnel magnetoresistance show a decrease of the TMR with increasing bias voltage. In addition, the observed TMR vanishes for T>200 K. Our results suggest that by employing a better lattice matched ferromagnetic electrode, and thus reducing the structural defects in the strontium stannate barrier, even larger TMR ratios might be possible in the future.

Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers.

Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.

High transition temperature superconductor/insulator bilayers for the development of ultra-fast electronics

High transition temperature superconductor (HTc)/SrTiO3 (STO) bilayers were fabricated by sputtering deposition on (100) STO substrates. Their transport and morphological properties were characterized using conductive atomic force microscopy. The STO barriers present good insulating properties, with long attenuation lengths (λ ∼ 1 nm) which reduce the junction resistance and increase the operating critical current. The samples present roughness values smaller than 1 nm, with an extremely low density of surface defects (∼5 × 10−5 defects/μm2). The high control of the barrier quality over large defect free surfaces is encouraging for the development of microelectronics devices based in HTc Josephson junctions.

Enhanced magnetic performance of CoFe2O4/BaTiO3 multilayer nanostructures with a SrTiO3 ultra-thin barrier layer

Multi-functional thin films with both magnetic CoFe2O4 and piezoelectric BaTiO3 components show limitations for novel device applications due to a severe reduction in magnetic properties in nanostructures. A systematic study of this system revealed a strong correlation between the number of CoFe2O4/BaTiO3 interfaces and the observed reduction in magnetization. We elucidate a new way to address this problem by depositing four unit cells of a SrTiO3 barrier layer at each interface, preventing chemical interaction between the two layers.

Room temperature magnetoresistance in CoFeB/SrTiO3/CoFeB magnetic tunnel junctions deposited by ion beam sputtering

Room temperature transport properties are reported in polycrystalline SrTiO3-based magnetic tunnel junctions deposited by ion beam sputtering. The junctions comprise CoFeB electrodes and the SrTiO3 barrier with thickness varied between 0.9 and 1.9 nm. Resistance area product values between 3 Ω.μm2 and 22 kΩ.μm2 have been measured with a tunnel magnetoresistance ratio ranging from 3.1 to 13% at room temperature. At low barrier thickness (1.2 nm), ferromagnetic coupling between electrodes is observed, indicating the presence of defects in the structure. A post-oxidation step was found to improve transport properties at lower barrier thickness.

Effects of electrodes and space charges on the tunneling electroresistance in the ferroelectric tunnel junction with a SrTiO3/BaTiO3 composite barrier

Using the lattice model for strained nanoscale ferroelectric films and quantum tunneling theory, we present a theoretical investigation of tunneling electroresistance (TER) in the ferroelectric tunnel junction (FTJ) with a SrTiO3/BaTiO3 composite barrier. The exact profile of local polarizations within the composite barrier is calculated from first principles. It is found that the TER will sensitively depend on the choice of electrode with the increase in SrTiO3 barrier’s thickness. The space charges, which exist at the interface between the two barriers, will enhance the TER, and provide an effective approach to achieving large TER in FTJs.

SrTiO3 Recombination-Inhibiting Barrier Layer for Type II Dye-Sensitized Solar Cells

Type II dye-sensitized solar cells (DSSCs) differ from conventional DSSCs by their mechanism of light absorption and electron injection. Instead of photoexcitation of the dye, followed by electron injection to the semiconductor conduction band, in type II DSSCs, there is a direct electron injection from the HOMO level of the sensitizer into the conduction band of the semiconductor. The main drawback of such cells is their extremely rapid back-electron-transfer rate. Herein, we present a new approach for inhibiting back electron transfer in a catechol-sensitized type II DSSC using a thin layer barrier coating of SrTiO3 between the semiconductor and the sensitizer. A 70% improvement in charge collection efficiency is reported. A proposed mechanism for the operation of the SrTiO3 barrier layer is presented.

Electrical properties of epitaxial SrTiO3 tunnel barriers on (001) Pt/SrTiO3 substrates

Coherent, epitaxial Pt films with root-mean-square surface roughness values of less than one unit cell were grown on (001) SrTiO3 substrates by dc sputtering. These Pt films served as bottom electrodes for epitaxial SrTiO3 tunnel barriers grown by rf magnetron sputtering. The SrTiO3 barriers were free of pinholes and showed mean surface roughness values of less than one unit cell. Barriers with a thickness of 4.5 nm showed excellent insulating properties and nonlinear current-voltage characteristics. At high bias fields, a reproducible hysteresis and deviation from the ideal tunneling behavior were observed.

Space-charge trap mediated conductance blockade in tunnel junctions with half-metallic electrodes

A conductance blockade effect has been observed in the magnetic tunnel junction consisting of La0.7Sr0.3MnO3 electrodes and a SrTiO3 barrier. The blockade effect is correlated with the space-charge trap states in the barrier. The blockade threshold eVB=128meV is significantly greater than Coulomb charging energy EC=11meV. The blockade can be lifted with a magnetic field, accompanied by a very large magnetoresistance up to 10 000%. The intriging blockade behavior is distinctly different from the conventional Coulomb blockade effect, showing a unique spin-dependent tunneling process mediated by the localized charge trap states.

Magnetotransport properties of manganite based magnetic tunnel junctions

We have studied spin-dependent tunneling in La2/3Ba1/3MnO3−δ based trilayer magnetic tunnel junctions fabricated by pulsed laser deposition. In the heteroepitaxial trilayer structures, a strained SrTiO3 layer of different thickness was used as a tunneling barrier. For the junctions with a SrTiO3 barrier, the measured tunneling characteristics deviated considerably from the ideal characteristics expected for elastic tunneling through an ideal barrier. The data indicate that inelastic multistep tunneling as described by the Glazman-Matveev model is present. Our analysis indicates that the SrTiO3 barrier layer consists of two different parts: The interface region with the manganite electrodes containing a high density of structural defects, and the central barrier layer with a small density of defects. The observed temperature dependence of the junction resistance can be explained by a series connection of the resistance contributions from the interfacial and central barrier part. The magnetic field dependence of the junction resistance is found to depend strongly on the barrier thickness and can show complicated behavior due to both a complex magnetic state in the junction electrode and magnetic interactions between the ferromagnetic electrodes. A very large tunneling magnetoresistance up to ΔR/Rp≃1200% is observed that, however, depends sensitively on the magnetic history.

Tunneling magnetoresistance with positive and negative sign inLa0.67Sr0.33MnO3∕SrTiO3∕Cojunctions

We have investigated the effects of modification of the SrTiO3 /Co interface as well as the SrTiO3 barrier on the tunnel magnetoresistance TMR of La0.67Sr0.33MnO3 /SrTiO3 /Co junctions. Modification was realized by the introduction of one atomic layer of either TiO2 or SrO at the SrTiO3 /Co interface. Barriers with different oxygen content were also studied. In these structures we have observed positive as well as negative TMR, with a trend towards positive TMR for junctions with interfacial SrO and/or more oxygen-deficient barriers. This work offers more insight into the SrTiO3 /Co tunnel spin polarization and its sign.