Nb-doped SrTiO3 Research Articles

Temperature-modulating rectifier behavior and photovoltaic response of VOx /Nb:SrTiO3 heterojunctions

Vanadium oxide (VOx) is a promising candidate as a material for smart windows, and 0.7 wt% Nb-doped SrTiO3 (NSTO) is frequently used in electrodes for electro-optic devices. Here, we demonstrate that a rectifying VOx/Nb: SrTiO3 heterojunction has a photovoltaic response. It can be used in thermochromic photovoltaic modulating devices. The crystalline structure and electrical characteristics of VOx thin film deposited on NSTO were investigated. It was found that an asymmetric hysteresis current between increasing and decreasing voltages occurs at 80, 300 and 360 K. The photovoltaic effect was observed under irradiation by a pulsed laser.

Epitaxial growth of high-k BaxSr1−xTiO3 thin films on SrTiO3 (001) substrates by atomic layer deposition

Atomic layer deposition (ALD) offers a viable route for the growth of thin and conformal films over 3D topographies and is becoming attractive as a method to grow films thin enough, and with sufficient dielectric constants (k), for the fabrication of next-generation dynamic random memories. The authors used ALD to grow thin (≤15 nm) BaxSr1 − xTiO3 (BST) films that are epitaxially integrated to SrTiO3 (001) (STO) and Nb-doped SrTiO3 (001) (Nb:STO). Films of three compositions, which are x ∼ 0.7, 0.5, and 0.3, and thicknesses of 7.8–14.9 nm were grown at 1.05 Torr and 225 °C using barium bis(triisopropylcyclopentadienyl), strontium bis(triisopropylcyclopentadienyl), titanium tetraisopropoxide, and H2O. Film compositions were controlled by changing cycle ratios (Ba:Sr, Ba:Ti, and Sr:Ti) and confirmed by in situ x-ray photoelectron spectroscopy. Films were amorphous as-deposited and required postdeposition vacuum annealing at 650–710 °C to crystallize. Epitaxy was confirmed with x-ray diffraction and transmission electron microscopy. Only BST (00l) out-of-plane diffraction signals were detected. Capacitance-voltage (C-V) measurements revealed that BST thin films grown by ALD have dielectric constant values ranging from 210 for Ba0.71Sr0.26TiO3 to 368 for Ba0.48Sr0.43TiO3.

Study of the intensity asymmetry in Brillouin light scattering from magnons in FeNi thin films

The Stokes–anti-Stokes intensity asymmetry, observed by Brillouin light scattering from surface magnetic excitations in thin FeNi films, has been studied by replacing substrates with different dielectric constant as well as varying the applied magnetic field and the laser incident angle. The experimental results show that the Stokes–anti-Stokes intensity ratio depends upon the applied magnetic fields, the wave vectors and also the material of the substrates used to support the FeNi films. Most interesting is that for a fixed magnetic field and a fixed wave vector, the intensity ratio is much more significant for the FeNi film deposited on Nb-doped SrTiO3 substrate than that deposited on surface oxidized Si substrate. The phenomenon of the different asymmetry is depending on the dielectric constant of the substrates. The tendency is in agreement with the theory.

Atomic-Scale insight into the reversibility of polar order in ultrathin epitaxial Nb:SrTiO3/BaTiO3 heterostructure and its implication to resistive switching

Ferroelectric heterostructures with bi-stable state of polarization are appealing for data storage as well as tunable functionalities such as memristor behavior. While an increasing number of experimental and theoretical studies suggest that polarization persists in ultrathin epitaxial heterostructures approaching just a couple of unit cells, the switching of such polar order is much less well understood, and whether polarization can be reversed in ultrathin ferroelectric heterostructures remains to be answered. Here we fabricate high-quality 7-unit cell thick BaTiO3 (BTO) films on Nb-doped single crystalline SrTiO3 (NSTO) substrate, and demonstrate their apparent yet unambiguously false polarization reversal due to charge injection using comprehensive piezoresponse force microscopy (PFM) studies. The presence of weak polar order consistent with linear piezoelectricity is confirmed at the atomic scale by high resolution integrated differential phase contrast (IDPC) of scanning transmission electron microscopy (STEM) as well as macroscopic second harmonic generation (SHG), while the lack of polarization reversal under the voltage applied is supported by density functional theory calculation showing the persistence of dead layer on the surface. Nevertheless, poling-induced electric conduction differing by two orders of magnitude is observed, demonstrating resistive switching in ferroelectric heterostructure in the absence of polarization reversal, even with weak polar order. Our finding has technological implications on emerging memristor applications with potentially more accessible states than bi-stable polarization modulated mechanism, and raises technical challenges to unambiguously demonstrate polarization switching in ultrathin films at their critical size limit.

The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 thin films

0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 (BFGMO–BTO) thin films have been epitaxially grown on (0 0 1)-oriented Nb-doped SrTiO3 (NSTO) single-crystal substrates by the pulsed laser deposition. X-ray diffraction, selected area electron diffraction, magnetic hysteresis loop, and piezoresponse force microscopy results show that the structural and magnetic properties of the BFGMO–BTO epitaxial films can be progressively tuned by the oxygen pressure (PO2) during film deposition. The films prepared atPO2 = 5 Pa phase separated into BFGMO and BTO phases, however, with increasing PO2 to 20 Pa the films evolve from two phases to single phase with the best crystalline quality, largest saturation magnetization, lowest coercive field and relatively easy-switchable ferroelectric domains.

High-performance ferroelectric non-volatile memory based on La-doped BiFeO3 thin films.

An ultrathin (6.2 nm) ferroelectric La0.1Bi0.9FeO3 (LBFO) film was epitaxially grown on a 0.7 wt% Nb-doped SrTiO3 (001) single-crystal substrate by carrying out pulsed laser deposition to form a Pt/La0.1Bi0.9FeO3/Nb-doped SrTiO3 heterostructure. The LBFO film exhibited strong ferroelectricity and a low coercive field. By optimizing the thickness of the LBFO film, a resistance OFF/ON ratio of the Pt/LBFO (∼6.2 nm)/NSTO heterostructure of as large as 2.8 × 105 was achieved. The heterostructure displayed multi-level storage and excellent retention characteristics, and showed stable bipolar resistance switching behavior, which can be well applied to ferroelectric memristors. The resistance switching behavior was shown to be due to the modulating effect of the ferroelectric polarization reversal on the width of the depletion region and the height of the potential barrier of the LaBiFeO3/Nb-doped SrTiO3 interface.

Thickness Dependence and Strain Effects in Ferroelectric Bi2FeCrO6 Thin Films

We report on the ferroelectric properties of epitaxially grown Bi2FeCrO6 (BFCO) films with thicknesses of 7.5, 49, and 98 nm obtained by pulsed laser deposition. Because of the strains induced by the Nb-doped SrTiO3(001) substrate, the films exhibit a variable Fe–Cr order along the growth axis, with a disordered phase located near the interface and an increased order at the top of the films. This is first evidenced by X-ray diffraction and UV–vis–NIR absorption measurements as the ordered/disordered phases show different lattice parameters and band gaps. The strain effects which depend on the film thickness are found to strongly impact the ferroelectric properties. For the 49 nm thick film, piezoresponse force microscopy (PFM) reveals an out-of-plane intrinsic polarization orientation, effect which is absent for 98 nm thick films. The polarization anisotropy increases when reducing the thickness to 7.5 nm. The intrinsic polarization of the as-deposited 49 nm films induces a significant shift of the curren...

Controlled Synthesis of Nanocrystalline Nb:SrTiO3 Electron Transport Layers for Robust Interfaces and Stable High Photovoltaic Energy Conversion Efficiency in Perovskite Halide Solar Cells

Perovskite halides are of great attraction as efficient light absorption materials for solid-state solar cells, but the stability and photovoltaic energy conversion efficiency of perovskite solar cells (PSCs) are still limited by the interface structures and defects between their light-absorbing perovskite halides and electron transport layers (ETLs). Here, we report the ultraviolet (UV) light-induced degradation mechanism at the interfaces between perovskite halide and conventional TiO2 ETL materials and provide a solution to overcome this drawback. UV-induced degradation is shown to be attributed to the formation of oxygen vacancies formed at the perovskite halide–ETL interface under UV light illumination, where the oxygen atoms released at the interface accelerate the decomposition of perovskite halide by inducing chemical reactions. Meanwhile, nanocrystalline SrTiO3 (STO) ETLs are revealed to be tunable in enabling high performance in PSCs under UV light illumination. Indeed, tuning the electronic structure of STO ETLs by Nb doping, in combination with the controllable removal of SrO phases segregated on the Nb-doped STO ETL surfaces, is exhibited to enable robust interface stability and stable high photovoltaic energy conversion efficiency for PSCs. Furthermore, we demonstrate that STO-based PSCs have no hysteresis due to low defect concentrations at the perovskite halide–STO ETL interfaces.

Electro and photon double-driven non-volatile and non-destructive readout memory in Pt/Bi0.9Eu0.1FeO3/Nb:SrTiO3 heterostructures

Ferroelectric resistive switching has recently attracted considerable attention as a promising candidate for next-generation non-volatile memory. In this work, we report an electro and photon double-driven bipolar resistive switching behavior in Pt /Bi0.9Eu0.1FeO3 (BEFO) /Nb-doped SrTiO3 (NSTO) heterostructures prepared via pulsed laser deposition. In addition to the polarization-based control of the resistive memory, a switchable photovoltaic effect is observed that can be used to detect the polarization direction non-destructively. Significantly, the electric field-modulated interfacial barrier can be further affected by photon-generated carriers. This phenomenon is attributed to the barrier modulation in the Pt /BEFO and BEFO /NSTO interfaces by electric field and photon excitation. These results indicate the feasibility of non-volatile and non-destructive readout from ferroelectric memory.

Orientation-modulated multiferroic properties of BCZT/LCMO bilayer heterostructures

In this paper, orientation-modulated multiferroic properties of Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3 (BCZT/LCMO) bilayer heterostructures grown on (001), (110), and (111)-oriented Nb-doped SrTiO3 single-crystal substrates are investigated. Epitaxially grown heterostructures with sharp and clear interfaces are verified by TEM analysis. Results show the strong orientation dependence of multiferroic properties for the heterostructures. The optimal ferroelectric and dielectric properties lie in the (001) direction, whereas the improved saturation magnetization and piezoelectric response are achieved in (111)-oriented heterostructure. The maximum magnetoelectric coupling is obtained in (111)-oriented heterostructure with a αE31 value of 207 mV/cm·Oe.

Microstructure of Candidate Conductive Buffer and Superconducting Layers in a Coated Conductor Using {100} <001> Textured Cu Tape

To develop a low-cost coated conductor, a new wire architecture using conductive buffer layers and a metal tape with high conductivity has been proposed. Nb-doped SrTiO 3 (Nb-STO) was chosen as the conductive buffer layer. A high JC of 2.5 MA/cm 2 at 77 K under a self-magnetic field was demonstrated using a Nb-STO buffer layer and a cube-textured Cu and stainless-steel laminate tape as a metal tape with high conductivity. Considering the industrial production of wires, it is preferable to have many suitable materials. Thus, Nb-doped TiO 2 (Nb-TiO 2 ) was examined as a new conductive buffer layer. However, it was found to be difficult to grow a well biaxially aligned YBCO layer on a single Nb-TiO 2 buffer. Therefore, in this paper, buffer layers combining both Nb-STO and Nb-TiO 2 were prepared and an YBCO layer was deposited on this buffer layer by PLD with the purpose of realizing a superior conductivity characteristic to that of the single Nb-STO buffer layer. It was found that the combined buffer layer has potential use as a conductive buffer layer. However, the process must be optimized to increase the benefit of adding Nb-TiO 2 to Nb-STO.

Residual Strain-Mediated Multiferroic Properties of Ba0.85Ca0.15Zr0.9Ti0.1O3/La0.67Ca0.33MnO3 Epitaxial Heterostructures.

In this study, artificial multiferroic Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.67Ca0.33MnO3(BCZT/LCMO) epitaxial heterostructures were deposited on Nb-doped SrTiO3 substrates using pulsed laser deposition. The epitaxial growth of the heterostructures on the substrate was demonstrated by XRD, RSM, and TEM analyses, which displayed decreasing residual strain with increasing BCZT layer thickness. The electrical, magnetic, and magnetoelectric properties of the epitaxial heterostructures were investigated in detail, and they were sensitive to the varying BCZT layer thickness in terms of residual strain. The multiferroic nature of the heterostructures was demonstrated by ferroelectric and ferromagnetic hysteresis loops. Strain-mediated magnetoelectric behavior was observed in the epitaxial heterostructures. Finally, dielectric properties were enhanced in the heterostructures over the BCZT single layer film, and the maximum magnetoelectric coefficient of the heterostructures was 206.5 mV/cm·Oe.

Origin of Resistance Switching and Regulation of the Resistance Stability by the Interface State Density on the Pt/Nb:SrTiO3 Interface

The surface defect and carrier concentrations of Nb‐doped SrTiO3 (NSTO) single crystals play an important role in its resistance switching (RS) properties. Herein, the RS of both 0.05 and 0.7 wt% Nb‐doped NSTO single crystals are studied. The hole defects on NSTO, which are characterized by electron paramagnetic resonance, increase with the enhanced Nb‐doping level. The hole defects with positive charges directly affect the RS characters of the Pt/NSTO junction by electron trapping/detrapping. The stability and discrimination of the resistance states are correlated with the interface state density and carrier concentration, which is controlled by the Nb‐doping level. The fundamental physical processes are proposed according to the experimental results.

Observation of Resistance Switching in BiMnO₃ Thin Film Grown on Nb-doped SrTiO₃ (110) Substrate

Observation of Resistance Switching in BiMnO₃ Thin Film Grown on Nb-doped SrTiO₃ (110) Substrate

Giant Electroresistance in Ferroionic Tunnel Junctions

SummaryOxide-based resistive switching devices, including ferroelectric tunnel junctions and resistance random access memory, are promising candidates for the next-generation non-volatile memory technology. In this work, we propose a ferroionic tunnel junction to realize a giant electroresistance. It functions as a ferroelectric tunnel junction at low resistance state and as a Schottky junction at high resistance state, due to interface engineering through the field-induced migration of oxygen vacancies. An extremely large electroresistance with ON/OFF ratios of 5.1×107 at room temperature and 2.1×109 at 10 K is achieved, using an ultrathin BaTiO3-δ layer as the ferroelectric barrier and a semiconducting Nb-doped SrTiO3 substrate as the bottom electrode. The results point toward an appealing way for the design of high-performance resistive switching devices based on ultrathin oxide heterostructures by ionic controlled interface engineering.

BiFe1–xCrxO3 Ferroelectric Tunnel Junctions for Neuromorphic Systems

We report on the fabrication of ferroelectric tunnel junctions of BiFe0.45Cr0.55O3 as a tunneling barrier, Nb-doped (111) SrTiO3 as a bottom electrode, and platinum as a top electrode. BiFeO3 is a ...

Tuning Photovoltaic Performance of Perovskite Nickelates Heterostructures by Changing the A-Site Rare-Earth Element.

Perovskite rare-earth nickelates (RNiO3) have attracted much attention because of their exotic physical properties and rich potential applications. Here, we report systematic tuning of the electronic structures of RNiO3 (R = Nd, Sm, Gd, and Lu) by isovalent A-site substitution. By integrating RNiO3 thin films with Nb-doped SrTiO3 (NSTO), p-n heterojunction photovoltaic cells have been prepared and their performance has been investigated. The open-circuit voltage increases monotonically with decreasing A-site cation radius. This change results in a downward shift of the Fermi level and induces an increase in the built-in potential at the RNiO3/NSTO heterojunction, with LuNiO3/NSTO showing the largest open-circuit voltage. At the same time, the short-circuit current initially increases upon changing the A-site element from Nd to Sm. However, the larger bandgaps of GdNiO3 and LuNiO3 reduce light absorption which in turn induces a decrease in the short-circuit current. A power conversion efficiency of 1.13% has been achieved by inserting an ultrathin insulating SrTiO3 layer at the SmNiO3/NSTO interface. Our study illustrates how changing the A-site cation is an effective strategy for tuning photovoltaic performance and sheds light on which A-site element is the best for photovoltaic applications, which can significantly increase the applicability of nickelates in optoelectric devices.

Effect of strain on the modifications in electronic structure and resistive switching in Ca-doped BiFeO3 films

Strain-induced modifications in the structure, electronic structure, electrical, and ferroelectric properties of the Bi0.90Ca0.10FeO3 (BCFO)/Nb-doped SrTiO3 (100) films have been systematically studied in light of variation in film thickness. X-ray diffraction and ɸ-scan measurements confirm the single phase, (100) oriented epitaxial growth of all films. Room temperature absorption spectra show the presence of asymmetric broad peak around ∼2.5 eV, which is indicative of the presence of defect states inside the bandgap and is attributed to the oxygen vacancies. Improvement in the bipolar resistive switching behavior with a decrement in oxygen vacancies and improvement in ferroelectric properties with increasing film thickness suggest the crucial role of oxygen vacancies and strain in modifying the electrical properties of the BCFO films. Improvement in the ferroelectric behavior is attributed to the increment in the Fe 3d-O 2p hybridization, localization of Fe 3deg/Bi 6s-O 2p orbitals, and reduction in the oxygen vacancies with an increase in the film thickness. Observed stable retention and large ON/OFF switching ratio in BCFO films make them a promising candidate for application in the non-volatile memory device.

FE-SEM observation of chains of nanohillocks in SrTiO3 and Nb-doped SrTiO3 surfaces irradiated with swift heavy ions

Single crystals of SrTiO3 (1 0 0) and 0.05% Nb-doped SrTiO3 (1 0 0) were irradiated with 200 MeV 197Au32+ swift heavy ions and 350 MeV 129Xe26+ ions, respectively. The angle between the sample surface and the beam was less than 2°. The irradiated surfaces were examined by field-emission scanning electron microscopy (FE-SEM). On the SrTiO3 surface, a chain of periodic nanohillocks is created along the ion path. In contrast, black lines accompanied by hillocks are observed on the Nb-doped SrTiO3 surface. The observation of the edge effect in FE-SEM strongly suggests that the black lines observed for Nb-doped SrTiO3 can be attributed to groove formation. The dark contrast between the adjacent hillocks observed for ion-irradiated SrTiO3 suggests that the area correspond to the depressed region. On this basis, a formation process for the hillock chains in the framework of Rayleigh instability is proposed.

Synthesis and characterization of (111)-oriented BaTiO3 thin films

The synthesis of (111)-oriented BaTiO3 thin films on Nb-doped SrTiO3 and bilayers of BaTiO3 and La0.7Sr0.3MnO3 on SrTiO3 are investigated. With increasing thickness the films are found to exhibit a decreasing out-of-plane lattice parameter and increased surface roughness. The BaTiO3 films on doped SrTiO3 are found to be relaxed with indications of increasing defect density with increasing thickness. Through piezoresponse force microscopy, pyroelectric measurements, and tunneling electroresistance measurements, indications of ferroelectric behavior are found in (111)-oriented BaTiO3 down to a thickness of 5 nm.