Nb-doped SrTiO3 Single Crystals Research Articles

Colossal dielectric behavior of Nb-doped SrTiO3 single crystal

Crystals are usually used to explore the origin of material properties due to the fact that they have no grain boundaries and no pores. Although doped SrTiO3 crystal have remarkable dielectric properties, the origin of this improvement is still not fully understood. Herein, Nb5+ has been selected as a donor ion to study the dielectric properties of SrTiO3 due to its radius close to Ti4+. It has been found that the dielectric permittivity of the Nb-doped crystal is greater than 94,500 and the dielectric loss is maintained below 0.002 in the frequency range from 20 Hz to 0.2 MHz. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicate that the dielectric properties of doped crystals were improved due to the existence of oxygen vacancies and there was no effect of interface polarization. It was beneficial for the Nb-doped crystal to be annealed in a N2 atmosphere resulting in oxygen loss more oxygen vacancies, and a higher concentration of Ti3+. The electron can be bound by VO•• and Ti4+ and form the defect dipole [Ti4+∙e − VO•• − Ti4+∙e], which showed a higher permittivity and lower dielectric loss. In summary, the origin and optimization of dielectric properties of SrTiO3-based materials can be deeply understood when from doped SrTiO3 crystals.

On the flexoelectric-like effect of Nb-doped SrTiO3 single crystals

The enhanced flexoelectric-like effect in oxide semiconductors has outstanding properties that compare favorably with those of high-K materials, and this finding has stimulated research aimed at enhancing flexoelectricity of such materials. However, the effective flexoelectric coefficient approaches limits under different doping methods, which implies that there are still some fundamental questions that need to be addressed. In this context, we revisit the origin of the flexoelectric-like effect in oxide semiconductors, which differs significantly from flexoelectricity in dielectrics. The effective flexoelectric coefficient of Nb-doped SrTiO3 single crystals increases with a larger doping concentration of Nb and is linearly proportional to the reciprocal of the depletion layer width, which is consistent with the theoretical model. More interestingly, due to the doping with Nb, the sign of the flexoelectric-like effect is reversed, which emphasizes the distinction from intrinsic flexoelectricity. Despite the contribution of electrons, frequency-scan measurements indicate that the presence of oxygen vacancies has a crucial impact on flexoelectricity at low frequencies. These results provide insights into semiconductor flexoelectricity and provide a strategy for enhancing the flexoelectric-like effect by optimizing the depletion layer.

Anomalous dielectric relaxation peak in Nb-doped SrTiO3 single crystals

Anomalous dielectric relaxation appears in Nb-doped SrTiO3 (SNT) single crystal with relaxation rate displaying unusual slowing down with increasing temperature. Dielectric measurements show that the resistivity increases with temperature. Raman spectrum and the spherical aberration-corrected transmission electron microscope (STEM) show the existence of polar nano regions (PNRs) in the crystal. The temperature-dependence of DC conductivity and current-voltage (I–V) curves suggest that this anomalous behavior happens at the interfaces of PNRs. The evolution of the anomalous relaxation peak in the annealed samples indicates that this effect is simultaneously affected by the carrier concentration and the interface structure. The anomalous dielectric relaxation is reminiscent of the positive temperature coefficient of resistance (PTCR) effect in a polycrystalline material. The resistive switching effect makes such materials to have potential application value in switches, sensors, and device miniaturization.

Giant permittivity in Nb-doped SrTiO3 single crystal: Compositional gradient and local structure

Nb-doped SrTiO3 single crystal exhibited a giant permittivity (>6.5 × 105) with an acceptably low dielectric loss (<10−1) in a wide temperature range from −120 to 200 °C, making this material a good candidate for energy storage devices and modern microelectronics components. The mechanisms responsible for the giant permittivity of Nb-doped SrTiO3 single crystals were studied by means of microstructure characterizations, dielectric measurements, and density-functional theory calculations. A chemical compositional gradient extending from the surfaces was found, forming the internal barrier layer capacitance (IBLC) effect. Polar nanoregions (PNRs) were observed because of local fluctuations in distributions of Nb and oxygen vacancies. While both compositional gradients and local chemistry fluctuations increased polarization of the Nb-doped SrTiO3 single crystals, the local fluctuations dominated enhanced polarizability. This work suggests that optimizing local structures and chemistries in dielectrics is an effective way to tailor the desired dielectric performance.

Properties of self-oxidized single crystalline perovskite N : BaTiO3 oxynitride epitaxial thin films

Epitaxial perovskite self-oxidized oxynitride N : BaTiO3 thin films, deposited on 1% Nb-doped SrTiO3(001) single crystals, were obtained by atomic nitrogen assisted molecular beam epitaxy without supplying additional oxygen gas.

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.

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.

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.

Thermal Transport Measurements of LaCoO3 and SrTiO3

Thermal conductivity and thermopower measurements as functions of temperature were carried out on SrTiO3 single crystals below room temperature and compared to those obtained for LaCoO3 polycrystalline samples. The maximum value of thermal conductivity for SrTiO3 (110) and Nb-doped SrTiO3 single crystals was 1.09 and 0.91 W/mK, respectively, at 300 K. The thermopower measurements were − 5.47 and − 41.17 μ V/K, respectively, at 300 K. This indicates the predominance of negative charge carriers or electrons in the material. Measurements on LaCoO3 polycrystalline samples yielded maximum values of 0.37 W/mK and 620 μ V/K for thermal conductivity and thermopower, respectively. These positive values suggest that positive charge carriers are predominant or that the compound is slightly hole-doped. We studied the effect of Nb doping on the thermopower and thermal conductivity of SrTiO3 and contrasted the results obtained with similar measurements on LaCoO3.

Intrinsically Activated SrTiO3: Photocatalytic H2 Evolution from Neutral Aqueous Methanol Solution in the Absence of Any Noble Metal Cocatalyst.

Noble metal cocatalysts are conventionally a crucial factor in oxide-semiconductor-based photocatalytic hydrogen generation. In the present work, we show that optimized high-temperature hydrogenation of commercially available strontium titanate (SrTiO3) powder can be used to engineer an intrinsic cocatalytic shell around nanoparticles that can create a photocatalyst that is highly effective without the use of any additional cocatalyst for hydrogen generation from neutral aqueous methanol solutions. This intrinsic activation effect can also be observed for SrTiO3[100] single crystal as well as Nb-doped SrTiO3[100] single crystal. For all types of SrTiO3 samples (nanopowders and either of the single crystals), hydrogenation under optimum conditions leads to a surface-hydroxylated layer together with lattice defects visible by transmission electron microscopy, electron paramagnetic resonance (EPR), and photoluminescence (PL). Active samples provide specific defects identified by EPR, PL, and electron-energy loss spectroscopy as Ti3+ states in a defective matrix-this is in contrast to the inactive defects formed in other reductive atmospheres. In aqueous media, active SrTiO3 samples show a significant negative shift of the flatband potential (in photoelectrochemical as well as in capacitance data) and a lower charge-transfer resistance for photoexcited electrons. We therefore ascribe the remarkable cocatalyst-free activation of the material to a synergy between thermodynamics (altered interface energetics induced by hydroxylation) and kinetics (charge transfer mediation by suitable Ti3+ states).

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact with Lithium Metal.

Understanding ionic transport across interfaces between dissimilar materials and the intrinsic chemical stability of such interfaces is a fundamental challenge spanning many disciplines and is of particular importance for designing conductive and stable solid electrolytes for solid-state Li-ion batteries. In this work, we establish a surface science-based approach for assessing the intrinsic stability of oxide materials in contact with Li metal. Through a combination of experimental and computational insights, using Nb-doped SrTiO3 (Nb/STO) single crystals as a model system, we were able to understand the impact of crystallographic orientation and surface morphology on the extent of the chemical reactions that take place between surface Nb, Ti, and Sr upon reaction with Li. By expanding our approach to investigate the intrinsic stability of the technologically relevant, polycrystalline Nb-doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Nb0.5O12) system, we found that this material reacts with Li metal through the reduction of Nb, similar to that observed for Nb/STO. These results clearly demonstrate the feasibility of our approach to assess the intrinsic (in)stability of oxide materials for solid-state batteries and point to new strategies for understanding the performance of such systems.

Electrode effect regulated resistance switching and selector characteristics in Nb doped SrTiO3 single crystal for potential cross-point memory applications

Current-voltage characteristics of Nb doped SrTiO3 single crystals are detected at room temperatures. Results reveal that modifications of electrode configurations play a significant role on current-voltage behaviors. The asymmetric selector characteristics and symmetrical selector behaviors are the first reported in Nb doped SrTiO3 single crystals, which can be controlled by changing electrode configurations. A new model of Ag nanofilament can explain selector characteristics. Ohmic and space charge limited current mechanisms controlled typical resistance switching behaviors with good cyclic properties are also observed. Schottky barrier electrical conduction mechanism related typical diode behaviors are observed, while voltage is applied on same top electrodes Au.

Large phonon-drag enhancement induced by narrow quantum confinement at theLaAlO3/SrTiO3interface

The thermoelectric power of the two-dimensional electron system (2DES) at the LaAlO3/SrTiO3 interface is explored below room temperature, in comparison with that of Nb-doped SrTiO3 single crystals. For the interface we find a region below T =50 K where thermopower is dominated by phonon-drag, whose amplitude is hugely amplified with respect to the corresponding bulk value, reaching values ~mV/K and above. The phonon-drag enhancement at the interface is traced back to the tight carrier confinement of the 2DES, and represents a sharp signature of strong electron-acoustic phonon coupling at the interface.

Investigation of dislocations in Nb-doped (100) SrTiO3 single crystals and their impacts on resistive switching

Nb-doped SrTiO3 showed interesting resistive switching phenomena, which could be modulated by either thermal treatment or doping level. The impacts of oxygen vacancies and dislocations on resistive switching were investigated by comparing the switching behaviors of as-prepared and air-annealed crystals with a variety of doping levels. It was found that both oxygen vacancies and dislocations may have effects on the switching depending on the doping level. It was dominated by oxygen vacancies for low doping and dislocations for high doping.

Optically controlled electroresistance and electrically controlled photovoltage in ferroelectric tunnel junctions.

Ferroelectric tunnel junctions (FTJs) have recently attracted considerable interest as a promising candidate for applications in the next-generation non-volatile memory technology. In this work, using an ultrathin (3 nm) ferroelectric Sm0.1Bi0.9FeO3 layer as the tunnelling barrier and a semiconducting Nb-doped SrTiO3 single crystal as the bottom electrode, we achieve a tunnelling electroresistance as large as 105. Furthermore, the FTJ memory states could be modulated by light illumination, which is accompanied by a hysteretic photovoltaic effect. These complimentary effects are attributed to the bias- and light-induced modulation of the tunnel barrier, both in height and width, at the semiconductor/ferroelectric interface. Overall, the highly tunable tunnelling electroresistance and the correlated photovoltaic functionalities provide a new route for producing and non-destructively sensing multiple non-volatile electronic states in such FTJs.

Comparison of acid- and non-acid-based surface preparations of Nb-doped SrTiO3 (001)

High-resolution angle-resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy (XPS) were used to study the relative effectiveness of acid- and non-acid-based surface preparations of Nb-doped SrTiO3 (STO) single crystals. ARPES measurements show that boiling STO in deionized water produces surfaces of similar quality to those etched with buffered HF (Kawasaki method), or HCl/HNO3 (Arkansas method). XPS measurements indicate this water-based surface preparation is more effective than acid-based methods at removing SrOx crystallites and leaving the surface TiO2-terminated.

Instrument for stable high temperature Seebeck coefficient and resistivity measurements under controlled oxygen partial pressure

The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO2) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO2-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O2 sensor and the transient behavior of the resistance as a proxy. A pO2 range of 10−25–100 atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt–Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO2 on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. An approximately 11 % increase in power factor over a pO2 range of 10−19–10−8 atm at 973 K for the donor-doped single crystals is observed.

Voltage-driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films

Abstract Multiferroic Co/BaTiO3 layered composite thin films were grown on Nb-doped SrTiO3 single crystal by pulse laser deposition, in which the polycrystalline Co film was annealed under magnetic field to induce in-plane uniaxial magnetic easy axis. Voltage-induced magnetization changes along and perpendicular to the easy axis were measured by Magneto-Optical Kerr effect (MOKE) magnetometer without applied magnetic field. These changes in magnetization could be due to magnetoelectric coupling between Co and BaTiO3, and to possibly electro-optical effect of BTO if the MOKE laser could penetrate the top Co film. After excluding the electro-optical interference by analyzing the experimental results within an optical model, a hysteric loop of magnetization versus voltage is identified with a relative change in magnetization of about 8%.

Nanoscale Resistive Switching Schottky Contacts on Self-Assembled Pt Nanodots on SrTiO3

A nanoscale Schottky diode using Pt nanodisks on a Nb-doped SrTiO3 (Nb:STO) single crystal was fabricated, and resistive switching (RS) was demonstrated with conductive atomic force microscopy at ultrahigh vacuum. Pt disks with diameters on the order of 10 nm were formed using colloidal self-assembled patterns of silica nanospheres, followed by evaporation of the Pt layers on the Nb:STO single crystal. Here we show that the reproducible bipolar RS behavior of the nanoscale Pt/Nb:STO Schottky junction was achieved by utilizing local current-voltage spectroscopy. The conductance images, obtained simultaneously with topographic images, show the homogeneous current distribution of selected triangular-shaped Pt nanodisks during repetitive resistive switching between the high-resistance state (HRS) and low-resistance state (LRS). The endurance characteristics of the Pt/Nb:STO junction exhibit reliable switching behavior. These results suggest that the rectifying and resistive Pt/Nb:STO junction can be scaled down to the 10 nm range and their position can be controlled.

Surface electronic structure for various surface preparations of Nb-doped SrTiO3 (001)

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to study the surface electronic structure of Nb-doped SrTiO3 (STO) single crystals prepared using a variety of surface preparations. ARPES measurements show that simple degreasing with subsequent anneal in vacuum is not an adequate surface preparation of STO, rather, preparations consisting of etching with buffered HF or HCl, and to a lesser extent, simple water leaching resulted in surfaces with much less disorder. A non-dispersing, mid-gap state was found ∼800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present for vacuum-annealed and water-leached samples, as well as for STO thin films grown using molecular beam epitaxy. Theoretical modeling using density functional theory suggests that this mid-gap state is not related to the SrO- and TiO2-terminated surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching.