Nb-doped SrTiO3 Research Articles

Growth and ferroelectric properties of highly c-oriented epitaxial Aurivillius BaBi4Ti4O15 thin films

We report the growth and ferroelectric properties of epitaxial Aurivillius BaBi4Ti4O15 (BBTO) thin films with complex crystal structures deposited via a pulsed laser deposition (PLD) method. The BBTO thin films tended to grow in the c-orientation on the Nb-doped (100) SrTiO3 substrate. These highly c-oriented BBTO thin films could be grown by reducing the PLD deposition rate. The physical properties of the BBTO thin films were analyzed by measuring the leakage current, ferroelectric hysteresis loop, piezoelectric d33 hysteresis loop, and fatigue. The leakage current characteristics were improved with the growth of the highly c-oriented BBTO thin films, but its ferroelectric polarizations were observed to be reduced owing to the crystal structure of BBTO. The study of the ferroelectric domain structures showed that the domain wall energy was increased by the growth of the highly c-oriented BBTO thin films.

Improved piezoelectricity and energy storage performance simultaneously achieved in [001]-preferentially oriented Bi0.5Na0.5TiO3–BaTiO3–BiMnO3 thin films grown on Nb-doped SrTiO3 single-crystalline substrates

The control of grain orientation by introducing Nb-doped SrTiO3 single crystalline substrates is an effective way to improve piezoelectric and ferroelectric properties of thin films. Therefore, in this work, 0.99(0.94Bi0.5Na0.5TiO3–0.06BaTiO3)–0.01BiMnO3 thin films were coated on Pt(111)/TiOx/SiO2/Si(001) polycrystalline substrates and Nb-doped SrTiO3 single crystalline substrates, respectively. The Nb-doped SrTiO3 single crystalline substrates can affect the orientation of the BNT–BT–0.01BMO thin films, promote the grain growth, reduce the leakage current, and form an interfacial layer between the thin film and the substrate, and a comparatively larger converse piezoelectric coefficient (46.49 pm·V−1), a high energy storage density (56.5 J cm-3), and a high energy storage efficiency (79.4 %) were therefore simultaneously achieved in the thin films grown on Nb-doped SrTiO3 single crystalline substrates, promising the oriented thin films broad application prospects.

Interface Structure and Transport Properties of Thin-Film Solid-State Batteries with an Epitaxial Cathode

A problem of solid-state Li ion batteries is a relatively low ionic conductivity at the interface between electrodes and electrolytes that limits the charging/discharging rate. For designing the solid-state batteries, understanding the mechanism behind the low interface conductivity is crucial. Investigation of the atomic structure of the interfaces and its relation to the ionic conductivity would be helpful for this purpose. However, atomic-scale observation of the buried interfaces is often difficult in practical batteries which consist of grain aggregates. Recent progress in thin-film deposition techniques of electrodes and electrolyteshas opened a path to study the interface quantitatively. By using an all-in-vacuum battery fabrication system [1], we succeeded in fabricating a very low solid-electrolyte/electrode interface resistance in thin film batteries with an epitaxial cathode. The well-defined crystalline interfaces allow the analysis of atomic-scale structure and its transport properties. For the structure analysis, we used X-ray crystal truncation rod (CTR) scattering which enables the atomic-scale analysis in a non-destructive manner.The analysis of two interfaces will be presented: a solid-electrolyte/electrode interface and an electrode/current collector interface. In the former case, we studied Li(negative electrode)/Li3PO4(solid electrode)/LiCoO2(positive electrode) thin-film Li batteries with low and high (more than 10 times) interface resistances. It turned out that the low-resistance interface has an atomically well-ordered LiCoO2 surface and a disordered surface CoO2 layer can results in the high-resistance interface, demonstrating the crucial importance of the structural ordering in the interfacial layer for realizing a low-resistance interface [2]. In the second topic, we focused on electrode/current collector interface that can also affect the battery performance. We found a drastic improvement of the charging/discharging property by inserting 1.2 nm-thick band insulator LaAlO3 into the LiCoO2 (electrode)/Nb-doped SrTiO3 (current collector) interface. The CTR analysis revealed that a formation of electric dipole at the LaAlO3/Nb-doped SrTiO3 interface, which causes a energy band alignment preferable for the electronic conductance between the electrode and the current collector.

Atomic-scale identification of invisible cation vacancies at an oxide homointerface

Cation vacancies play an important role in creating new functionalities in complex oxides. Directly identifying the cation vacancies at the atomic scale is thus key to addressing quantum phenomena related to acceptor states. Here, atomic-scale identification of invisible cation vacancies at an oxide interface via energy-dispersive X-ray spectroscopy (EDX) spectrum imaging is demonstrated. At the homointerface of SrTiO3 (STO) film and a Nb-doped SrTiO3 substrate, the veiled behavior of cation vacancies of Sr and Ti is revealed by the approach for the first time; they are found to reside on their sublattices within the first three or four unit cells of the film in the absence of oxygen vacancies. Theoretical calculations show that two-dimensional electron gas with three unit-cells at the Nb:STO/STO interface is formed by the charge transfer, which leads to the spontaneous formation of cation vacancies for charge compensation, and induces the lattice distortion as well. The results suggest that our EDX approach is useful for obtaining atomic-site-specific information on point defect chemistry with unparalleled precision, which facilitates a path towards atomically precise defect engineering.

Effects of Post-deposition Annealing on Epitaxial CoO/Fe3O4 Bilayers on SrTiO3(001) and Formation of Thin High-Quality Cobalt Ferrite-like Films

In order to explore an alternative pathway to prepare ultrathin CoFe2O4 films, epitaxial CoO/Fe3O4 bilayers with varying film thickness of the CoO film were grown on Nb-doped SrTiO3(001) substrates...

Interfacial Polarons in van der Waals Heterojunction of Monolayer SnSe2 on SrTiO3 (001).

Interfacial polarons have been demonstrated to play important roles in heterostructures containing polar substrates. However, most of polarons found so far are diffusive large polarons; the discovery and investigation of small polarons at interfaces are scarce. Herein, we report the emergence of interfacial polarons in monolayer SnSe2 epitaxially grown on Nb-doped SrTiO3 (STO) surface using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). ARPES spectra taken on this heterointerface reveal a nearly flat in-gap band correlated with a significant charge modulation in real space as observed with STM. An interfacial polaronic model is proposed to ascribe this in-gap band to the formation of self-trapped small polarons induced by charge accumulation and electron-phonon coupling at the van der Waals interface of SnSe2 and STO. Such a mechanism to form interfacial polaron is expected to generally exist in similar van der Waals heterojunctions consisting of layered 2D materials and polar substrates.

Resistive switching behavior in epitaxial brownmillerite SrFeO2.5/Nb:SrTiO3 heterojunction

Epitaxial brownmillerite SrFeO2.5 thin films were deposited on conductive Nb-doped SrTiO3 single crystal (001) and (111)-oriented substrates using pulsed laser deposition, and their resistive switching behavior was investigated. In both the (001) and (111) device configuration, the bottom SrFeO2.5/Nb:SrTiO3 interface exhibited Ohmic behavior, while the top Au/BM-SrFeO2.5 interface displayed Schottky-like contact. Unfortunately, no resistive switching behavior was observed in the (001) devices, where the oxygen vacancy channels of SrFeO2.5 are ordered along the in-plane direction of the device. Conversely, the (111)-grown SrFeO2.5 thin films with out-of-plane oriented ordered oxygen vacancy channels displayed excellent resistive switching behavior with a high on/off ratio (∼104). The discrepancies between the (001) and (111) devices were explained in terms of their oxygen dynamics and corresponding topotactic phase transition in the SrFeO2.5 switching layer.

Quantum well electronic states in spatially decoupled 2D Pb nanoislands on Nb-doped SrTiO3(0 0 1)

Two-dimensional (2D) Pb nanoisland has established an ideal platform for studying the quantum size effects on growth mechanism, electronic structures as well as high-temperature superconductivity. Here, we investigate the growth and quantum well electronic states of the 2D Pb nanoislands on Nb-doped SrTiO3(001) by scanning tunneling microscopy and spectroscopy. In contrast to Pb/Si(111), Pb/Cu(111) and Pb/Ag(111), there is no wetting layer of Pb formed on the Nb-doped SrTiO3(001) surface, resulting in isolated Pb nanoislands with an apparent height of 4 atomic layers as the building blocks for the island growth. According to the thickness-dependent quantum well states resolved in both occupied and unoccupied energy regions, the constant group velocity vg = 1.804 × 106 m/s and the Fermi wavevector kF = 1.575 Å−1, have been extracted from a linear fit of the Pb(111) band dispersion along the Γ-L direction. In addition, the energy-dependent scattering phase shift φ(E) obtained by means of the phase accumulation model shows a metallic-like scattering interface analogous to Pb/Ag(111). These spatially decoupled 2D Pb nanoislands thus realize an opportunity to explore the intrinsic quantum confinement phenomena in nanoscale superconductors on the doped titanium-oxide-type substrate.

Investigation of the magnetoresistance in EuS/Nb:SrTiO3 junction* *Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB28000000 and XDB07000000), the National Key Research and Development Program of China (Grant No. 2016YFA0300600), and the Fund from the Beijing Municipal Science & Technology Commission (Grant No. Z191100007219012).

EuS is one of typical ferromagnetic semiconductor using as spin filter in spintronic devices, and the doped one could be a good spin injector. Herein, we fabricate a spin-functional tunnel junction by epitaxially growing the ferromagnetic EuS film on Nb-doped SrTiO3. The improvement of Curie temperature up to 35 K is associated with indirect exchange through additional charge carriers at the interface of EuS/Nb:STO junction. Its magnetic field controlled current–voltage curves indicate the large magnetoresistance (MR) effect in EuS barriers as a highly spin-polarized injector. The negative MR is up to 60% in 10-nm EuS/Nb:STO at 4 T and 30 K. The MR is enhanced with increasing thickness of EuS barrier. The large negative MR effect over a wide temperature range makes this junction into a potential candidate for spintronic devices.

Thermoelectric properties of Nb-doped SrTiO3 films prepared by co-sputtering

ABSTRACT SrTiO3 (STO), a member of perovskite structure materials, exhibits a large thermoelectric figure of merit at high temperatures. Theoretically, its property can be significantly improved by niobium (Nb) dopant. So far, the synthesis method of Nb-doped STO and the evaluation of their thermoelectric properties at low temperatures have not been explored fully. Here, our development of highly Nb-doped STO films using the co-sputtering with STO and Nb targets at room temperature (298 K) is introduced. A rapid annealing process (30 min, at 1073 K) is applied to further crystallize these films. Through measurements at 300 ~ 600 K, a significant improvement of the thermoelectric performance of STO films upon 2% Nb-doping is observed. Particularly, the working temperature of Nb-doped STO begins at 350 K instead of 475 K of STO. The figure of merit of Nb-doped STO can reach 0.53 at 600 K. Through TEM-EDS and TOF-SIMS characterizations, the mechanism of Nb-doping effect on the STO properties is revealed and discussed in detail. Our doping method at room temperature is potential, as compared to other synthesis methods, regarding the uniform incorporation of Nb dopants in STO films. Overall, our work showed another way for further improvement of perovskite-based thermoelectric materials.

Deposition-temperature dependence of structure, ferroelectric property and conduction mechanism of BCZT epitaxial films

Ba0·85Ca0·15Zr0·1Ti0·9O3 (BCZT) epitaxial films were successfully deposited on (001) Nb-doped SrTiO3 substrate, and the effects of deposition temperatures on the structure, ferroelectric property, and conduction mechanism were studied. The results of XRD and AFM verified the well epitaxial structure with smooth surfaces of the deposited BCZT epitaxial films. BCZT epitaxial films deposited at 650 °C exhibited the best ferroelectric property (2Pr = 22.91 μC/cm2, 2EC = 1702.11 kV/cm) and relatively low leakage current density (7.91 × 103 A/cm2). The conduction mechanism of BCZT films corresponds to the space-charge-limited current (SCLC) mechanism and the Fowler-Nordheim (F–N) tunneling mechanism in the low and high electric field range, respectively.

Oxygen vacancy enhanced ferroelectricity in BTO:SRO nanocomposite films

The enhancement of ferroelectric properties in lead-free ferroelectric is usually achieved by strain engineering. Here, we report a surprising polarization enhancement effect in an isostructural ferroelectric nanocomposite system composited by the ferroelectric material of BaTiO3 and metallic non-ferroelectric oxide of SrRuO3. BaTiO3:SrRuO3 (BTO:SRO) ferroelectric nanocomposite films with the volume ratio of nanogranular SRO ranging from 0 to 16% grown on the Nb-doped SrTiO3 (NSTO) single-crystal substrates by pulsed laser deposition (PLD) are investigated. Robust ferroelectric polarization is observed with a remanent polarization of about 40 μC/cm2, comparable to those found in Pb(ZrxTi1-x)O3 thin films. By combining X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculations, and phase-field simulation, a hypothesis has been proposed that the polarization enhancement may be mainly attributed to the accumulation of oxygen vacancies at the BTO/SRO interface rather than lattice mismatch strain. The novel mechanism of polarization enhancement opens new possibilities for designing future ferroelectric devices.

Large tunnel magnetoresistance in a fully epitaxial double-barrier magnetic tunnel junction of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3

We report large tunnel magnetoresistance (TMR) ratios of up to 219% at 300 K and 366% at 3.7 K obtained for a high-quality fully epitaxial double-barrier magnetic tunnel junction (MTJ) composed of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3. The obtained TMR ratios are among the highest values reported in Fe/MgO/Fe structures. This result may be attributed to the small in-plane wave vectors of the tunneling electrons injected from the Nb-doped SrTiO3 electrode with a small carrier density, demonstrating good compatibility between the Fe-based MTJ and SrTiO3.

Highly improved thermoelectric performance of Nb-doped SrTiO3 due to significant suppression of phonon thermal conduction by synergetic effects of pores and metallic nanoparticles

A remarkable thermoelectric performance in oxide materials was achieved by Nb and Ni co-doped strontium titanate (STNNO) containing pores and Ni nanoparticles. A combined structure of macroporus matrix and the Ni nanoparticles was realized by an exsolution reaction of metallic Ni from the porous STNNO matrix. The porous nanocomposite structure led to a substantial decrease in the thermal conductivity down to 1.4 W m−1 K−1 at 800 °C, and a moderate improvement of the electrical properties. As a result, the porous nanocomposite structure containing the metallic Ni nanoparticles significantly decreased the phonon thermal conductivity to 0.91 W m−1 K−1 at 800 °C, resulting in a remarkable ZT of 0.60 at 800 °C for STNNO reduced in 20% H2/N2.

Positive temperature dynamics of near-band-edge photoluminescence in Nb-doped SrTiO3

The positive temperature dynamics in the near-band-edge (NBE) photoluminescence (PL) of Nb-doped SrTiO3 single crystal have been investigated in the temperature range of 10 K–120 K. Intensity of the NBE PL emission with a peak at 3.21 eV has shown increase with increasing temperature. This NBE PL consists of two emission lines with peaks at 3.208 and 3.167 eV. The thermal dynamic in the intensity of the 3.21 eV emission was mainly defined by the temperature behavior of the 3.208 eV transition. The increase in the intensity of the 3.208 eV line with increasing temperature was associated with an increase in the number of electrons thermally excited from the surface. Additionally, a critical point in intensity of the PL at 3.21 eV can be caused by the phase transition, which occurs in Nb-doped SrTiO3 at 115 K.

Critical points in photoluminescence spectra and their relation with phase transition in Nb-doped SrTiO3

Luminescence spectra are extremely sensitive to variations in structural environment; thus, result of structural change, such as phase transition, can be observed via luminescence intensity. The temperature dynamic of photoluminescence in the Nb-doped SrTiO3 demonstrates two critical points at 115 and 160 K, which correspond to temperatures of structural phase transitions for the bulk and the surface of SrTiO3, respectively. The absence of a hysteresis effect in the photoluminescence emission points out the correspondence of the critical points to a second-order phase transition. Similar critical behaviours were also observed in oxygen-deficient SrTiO3, confirming a relationship between the PL and phase transition. The existence of peaks in the temperature coefficient of resistivity at the same temperatures also confirms the correlation between photoluminescence and phase transition in the Nb-doped SrTiO3, providing a simple non-contact method to detect phase transitions in luminescence materials.

Microstructure of coated conductors with La- or Nb-doped SrTiO3 conductive buffer

A novel configuration for coated conductors (CCs) was proposed to reduce material cost and consisted of YBa2Cu3Oy (YBCO) / conductive buffer(s) / Ni-electroplated {100} <001> textured Cu, and SUS316 lamination tape. Conductive buffers are a key subject to reduce material cost of CCs. In the CC using the conductive buffer of Nb-doped SrTiO3 (Nb-STO), a high-J C YBCO (2.5 MA/cm2 at 77 K and self-field) was successfully obtained. Moreover, a clean interface between the buffer and the metal tape without oxides was confirmed using TEM analysis. However, the resistivity of the Nb-STO buffer increased to be several Ω·cm after the YBCO deposition, which was beyond expectations. Therefore, the La-doped STO (La-STO) conductive buffer was incorporated into CCs as a novel conductive buffer. Two CCs with the different conductive buffers were compared in terms of microstructure analysis using transmission electron microscopy.

An in situ electrical transport measurement system under ultra-high vacuum.

Low-dimensional materials exhibit exotic properties and have attracted widespread attention. However, many low-dimensional materials are highly sensitive to air, making it challenging to investigate their intrinsic properties with ex situ measurements. To overcome such challenges, here, we developed a system combined with sample growth, electrode deposition, and in situ electrical transport measurement under ultra-high vacuum condition. The in situ deposition of electrodes enables desired ohmic electrical contacts between the probes and samples, which allows continuous temperature dependent resistance (R-T) measurements. Combined with a scanning tunneling microscope, surface morphology, electronic structure, and electrical transport properties of the same sample can be systematically investigated. We demonstrate the performance of this in situ electrical transport measurement system with three-unit-cell thick FeSe films grown on Nb-doped SrTiO3(001) substrates, where a low-noise R-T curve with a zero-resistance superconducting transition temperature of ∼30 K is observed.

Dependence of the inverse spin Hall effect in Sr(Nb[formula omitted]Ti[formula omitted])O[formula omitted] on the Nb concentration

We measured the spin rectification effect and the inverse spin Hall effect in Nb-doped SrTiO3 by pumping the spins from the ferromagnetic thin films into SrTiO3. The spin injection is increased when the doping level is increased. However, the spin to charge conversion efficiency which is measured by the spin Hall angle decreases when Nb concentration is more than ∼0.02 wt%. We argued that it is because that the electrons contributed to the inverse spin Hall effect are from the d−orbitals of Nb instead of Ti beyond this concentration. Our work points to the importance of orbital occupations in the (inverse) spin Hall effect. We may explore controllable spin and charge interconversion in oxide spintronics.

Manipulating leakage behavior via thickness in epitaxial BaZr0.35Ti0.65O3 thin film capacitors

High quality, epitaxial Ba(Zr0.35Ti0.65)O3 thin films were fabricated via radio frequency magnetron sputtering technique. The leakage current density vs electric field (J–E) tests were conducted using the device structures with Pt top electrodes and Nb-doped SrTiO3 bottom electrodes. The conduction mechanism of the films and the J–E characteristic that changed from the asymmetric structure to the relative symmetric structure were investigated. Analysis of the thin film leakage behavior suggests that, at low electric field, the conduction mechanisms are mainly bulk-limited Ohmic and space-charge-limited current. When the electric field is higher, the interface-limited Schottky emission and the Fowler–Nordheim (FN) tunneling as the dominant conduction mechanism occur for the films with a thickness below 166 nm and only FN tunneling for the films with the thickness of 300 and 400 nm. In addition, the asymmetric structure is strongly dependent on the large relative value of the Schottky barrier height between the positive and negative biases. Moreover, increasing the electric field range used to generate the bulk-limited conduction behavior and decreasing the current density induced by the interface-limited conduction through increasing the film thickness are beneficial for the films to obtain a high breakdown strength and a large energy storage density, as well as excellent efficiency.