Doped SrTiO3 Substrates Research Articles

Dielectric properties of sub 20 nm homoepitaxial SrTiO3 thin film grown by molecular beam epitaxy using oxygen plasma

Epitaxial SrTiO 3 thin films were grown by molecular beam epitaxy (MBE) using an oxygen plasma source. SrO and TiO2 layers were alternately deposited on 0.5 wt% (001) Nb doped SrTiO3 substrates whose surface were terminated with TiO2. A two-dimensional (2-D) layer-by-layer growth mode was confirmed by the reflection high energy electron diffraction (RHEED) oscillation during the growth and the well-terraced SrTiO3 surface was obtained after growth. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) analysis showed that the obtained SrTiO3 thin films had epitaxially (001) oriented and stoichiometric crystal structures without any interfacial layer. With simple MIS (Metal-Insulator-Semiconductor) capacitors using Ir top electrodes, the dielectric constants of the SrTiO3 thin film with 10, 15, 20, 100 nm were measured at room temperature, 10 kHz and zero bias.

Enhanced multiferroic properties of tetragonally strained epitaxial BiMnO3 thin films grown on single crystal Rh substrates

Epitaxial BiMnO3 (BMO) thin films with tetragonally strained structures were fabricated and their multiferroic properties were investigated in order to improve the leakage current characteristics of the BMO thin films and to secure the excellent multiferroic properties. The x-ray diffraction experiment confirmed that the BMO thin films grown on the Rh substrates had noticeably tetragonally strained structure compared to the BMO thin films on the Nb doped SrTiO3 substrates. As a result, the leakage current characteristics of the BMO thin films were improved, and the ferroelectric hysteresis loops were also excellent enough to be used for the nonvolatile memory devices. In addition, it was confirmed that the BMO thin films had a multiferroic property by measuring the ferromagnetic hysteresis loops of the BFM thin films, and it was confirmed that the ferromagnetic properties were improved by the tetragonal strain. Further, the band gaps of the BMO thin films were measured and it was confirmed that the band gap of the strained BFO thin film was increased.

Selfrectifying Memristor Device Based on Polarization Switching and Space Charge Distribution Via External Electric Field

Strong interest in resistive switching phenomena is driven by a possibility to develop electronic devices with novel functional properties not available in conventional systems. Bistable resistive devices are characterized by two resistance states that can be switched by an external voltage. Recently, ferroelectric memristors divices with tunable resistive behavior—have emerged as a new paradigm for nonvolatile memories and adaptive electronic circuit elements. In this work, we fabricated the tunable ferroelectric tunnel junction memristor device as future memory device. Ca doped BiFeO3/BiFeO3 thin films grown on Nb doped SrTiO3 substrate (conductive electrode) by pulsed laser deposition are studied by X-ray diffraction and I-V measurement to characterize their structure and memristive properties. These are simultaneously realized that the change of the resistance of the semiconductor substrate due to the remanent polarization of the ferroelectric thin film and the change of the anisotropic resistance due to the distribution of the space charge in the thin film were simultaneously realized. The results indicate that the self-rectifying resistive switching properties, and this device can be applied as memory device integrating the function of a selector in 3D cross point array.

Strong composition dependence of resistive switching in Ba1-xSrxTiO3 thin films on semiconducting substrates and its thermodynamic analysis

In this work, we report on the variability of the Schottky effect in solution processed Ba1-xSrxTiO3 films (BST, x = 0, 0.5) grown on 0.5% Nb doped SrTiO3 substrates with top Pt electrodes (NSTO/BST/Pt). The films display leakage currents accompanied by varying degrees of hystereses in the current-voltage measurements. The magnitude of the leakage and hystereses depend on the Sr content. We focus on the current-voltage (I-V) behavior of our samples in the light of thermodynamic theory of ferroelectrics coupled with equations of semiconductors. Our calculations allowed us to unambigously determine the electronic character of the defects and related band bending effects in our samples. The extent of asymmetry and the hystereses in the I-V curves for x = 0 and 0.5 are shown to be controlled by the polarization in qualitative agreement with our calculations. Amplitude of the ferroelectric polarization, which is a function of composition here, has a strong impact on leakage currents in forward bias while this effect is much weaker under negative bias. The latter occurs as polarization pointing away from the NSTO semiconducting substrate causes depletion of carriers at the NSTO side of the NSTO/BST interface, increasing resistance to current flow through the stack. Such an occurence also increases the energy gap between the Fermi level and the conduction bands of the films, thereby reducing the bulk conduction through the film as well. The dependence of leakage currents on polarization direction points out to the possibility of a non-destructive read-out route in ferroelectric films much thicker than tunnel junctions.

Structural, electrical and magnetic properties of (110)-oriented BF-BZT-ST Films

Highly (110)-oriented BiFeO3-Bi(Zn/Ti)O3-SrTiO3 thin films were prepared on conductive Nb doped SrTiO3 substrates by pulsed laser deposition. The results demonstrate that the films show a pure perovskite phase with R3c symmetry. The films have a low dielectric loss, and a typical multiferroics character, possessing both of ferroelectric and ferromagnetic properties. The reduced dielectric loss is attributed to thermodynamic stabilization and charge compensation mechanisms in the BiFeO3 system. The remnant polarization (Pr) and the remnant magnetization (Mr) are ~ 46.2 μC/cm2 and ~ 4.6 emu/cm3 respectively.

Sodium‐potassium niobate nanorods with various crystal structures and their application to nanogenerator

Abstract(Na1−xKx)NbO3 (NKN) nanorods with an orthorhombic (OR) structure, a rhombohedral (RH) structure, or a polymorphic phase (PP) structure, which contains both OR and RH structures, are prepared. The presence of RH and PP NKN nanorods is explained by the existence of OH− defects at the O2− sites of the NKN nanorods. The PP NKN nanorods grown on a Nb5+‐doped SrTiO3 substrate show the largest piezoelectric strain constant of 175 pm/V because they have more directions for dipole rotation than OR and RH NKN nanorods. Piezoelectric nanogenerators (NGs) are synthesized using composites consisting of NKN nanorods with various structures and polydimethylsiloxane. The largest open‐circuit output voltage is 35 V, and the short‐circuit current is 5.0 μA, which are obtained using the NG containing 0.7 g of PP NKN nanorods. Moreover, this NG shows a maximum output power of 16.5 μW for an external load of 10.0 MΩ.

Deposition and dielectric study as function of thickness of perovskite oxynitride SrTaO2N thin films elaborated by reactive sputtering

The present study concerns the deposition of perovskite oxynitride SrTaO2N films and their dielectric characterization at low frequencies. Those radio frequency sputtered thin films have been obtained under a reactive plasma (92.3vol.% Ar/7.7vol.% N2) for substrate temperatures ranging from 600 to 900°C. As shown by X-rays diffraction and band-gap measurements, the deposition temperature (TS) determines the film structure and leads to films with band-gap and cell volume approaching the ones of the SrTaO2N bulk material with increased TS. The dielectric study has been performed on polycrystalline, textured and epitaxial SrTaO2N layers deposited on conductive niobium doped SrTiO3 substrates and thickness of films ranging from 30 to 900nm. The related permittivities vary from 66.5 to 90 (@10kHz, room temperature). These low values do not point out an effect of the crystallographic strain of films, due to their thickness, on the permittivity values. The latter remains irrespective of the frequency up to 100kHz and does not vary upon the application of an external DC voltage.

Carrier tuning the metal-insulator transition of epitaxial La0.67Sr0.33MnO3 thin film on Nb doped SrTiO3 substrate

La0.67Sr0.33MnO3 (LSMO) thin films were deposited on (001)SrTiO3(STO) and n-type doped Nb:SrTiO3(NSTO) single crystal substrates respectively. The metal to insulator transition temperature(TMI) of LSMO film on NSTO is lower than that on STO, and the TMI of LSMO can be tuned by changing the applied current in the LSMO/NSTO p-n junction. Such behaviors were considered to be related to the carrier concentration redistribution in LSMO film caused by the change of depletion layer thickness in p-n junction which depends greatly on the applied electric field. The phenomenon could be used to configure artificial devices and exploring the underlying physics.

Parallel Band and Hopping Electron Transport in SrTiO3

SrTiO3 (STO) is a model system for studying oxide electronic devices. This work examines the electronic transport through a heterostructure comprising an acceptor (Fe)‐doped STO layer on a donor (Nb)‐doped STO substrate. This is done by fitting the steady‐state current–voltage (I–V) curve measured at ambient temperature to numerical solutions of the drift‐diffusion equations of itinerant (i.e., free) electrons and holes (band conduction) and localized electrons hopping through defect states within the bandgap (hopping conduction). The analysis shows that at reverse bias and small forward bias the current is carried mostly by hopping electrons that give rise to unexpectedly high currents. At forward bias above 1 V, most of the current is due to electron transport through the conduction band. The transition from hopping to band conduction occurs when the quasi Fermi levels of electrons and holes depart from the energy level of the defect states through which the electrons hop. These observations shed new light on the transport properties of STO‐based devices at ambient temperatures wherein ionic defects such as oxygen vacancies are immobile and holes are trapped, for the most part.

Nano LaAlO3 buffer layer-assisted tunneling current in manganite p–n heterojunction**Project supported by the National Natural Science Foundation of China (Grant No. 10804089).

An oxide p–n heterojunction composed of a 150-nm La0.67Ca0.33MnO3 (LCMO) film, 0.05 wt% Nb doped SrTiO3 substrate (STON), and sandwiched 5-nm LaAlO3 (LAO) thin film is fabricated with the pulsed laser deposition technique and the interfacial transport properties are experimentally studied. The rectifying behavior of the junction is in agreement with Newman’s equation, indicating that tunneling is the dominant process for the carriers to pass through the interface while thermal emission is the dominant transport model of an LCMO/STON heterojunction with no LAO buffer layer.

BaTiO3 based relaxor ferroelectric epitaxial thin-films for high-temperature operational capacitors

The epitaxial growth of 0.6[BaTiO3]–0.4[Bi(Mg2/3Nb1/3)O3] (BT–BMN) relaxor ferroelectric thin-films on (100) Nb doped SrTiO3 substrates has been achieved and the structure is investigated for high-temperature capacitor applications. The post growth annealing decreases the oxygen vacancy and other defects in BT–BMN films, resulting in the enhancement of dielectric constant. An insertion of intermediate SrRuO3 layers as an electrode instead of Pt, sandwiching the film, is found to be more effective in enhancing the dielectric constant. A very high dielectric constant exceeding 400 was achieved from high-temperature annealed film and the film showed an excellent dielectric constant stability of below 11% in the temperature range of 80–400 °C. This will enable smaller, high-temperature tolerant, monolithically integrated thin-film capacitors on power semiconductor devices.

The influence of the local oxygen vacancy concentration on the piezoresponse of strontium titanate thin films.

In this study, the influence of the local oxygen vacancy concentration on piezoresponse force microscopy (PFM) measurements was investigated. Ultra-thin single-crystalline SrTiO3 thin films were deposited on niobium doped SrTiO3 substrates and analyzed using a combined PFM and local conductive atomic force microscopy (LC-AFM) measurement setup. After applying different polarization voltages between ±2 V and ±5 V to the thin films, we simultaneously observed an anomalous contrast in the piezoresponse amplitude and phase signal as well as a changed local conductivity in the exact same region. Since classic ferroelectricity can be excluded as the reason for the observed contrast, an influence of the local oxygen vacancy concentration on the piezoresponse is considered. Additionally, the surface potential was measured using Kelvin probe force microscopy (KPFM) revealing a change in surface potential in the regions of the applied voltage. The observed relaxation of the surface potential over time was fitted to a local oxidation reaction of the previously reduced regions of the ultra-thin SrTiO3 film. We propose a model that relates the local oxygen vacancy concentration to the surface potential. The influence of the oxygen vacancy concentration on the PFM measurements is explained.

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH− and H2O in the KN unit cell, tetragonal KNbO3 (KN) nanowires were formed when the synthesis was carried out at 120°C for 48h. However, when the fabrication was conducted at high temperatures (⩾150°C) or at 120°C for a long period of time (⩾72h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120°C for 60h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb5+-doped SrTiO3 substrate, and their d33 values were measured for the first time. A tetragonal KN nanowire exhibited a d33 value of 23.5pmV−1, which is larger than that of the orthorhombic KN nanowire (11.6pmV−1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d33 value of 40.0pmV−1. The d33 values of KN nanowires increased to 104.5, 137.1 and 146.0pmV−1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [100] direction by application of a DC voltage of 10V.

Ferroelectric domain structure of highly textured BiFeO3 microcrystal films prepared by hydrothermal method

Microcrystal BiFeO3 films have been prepared on Nb doped SrTiO3 substrate with the hydrothermal method. The microcrystals in the film are highly textured and c-oriented with the (101)/(011) planes as their preferential side growth facets. These crystal facets intersect the film plane along either x or y direction. Microscopic domain structures are studied by combining the information of both in-plane and out-of-plane piezoresponse microscopy (PFM) measurements. Both 71° and 109° domain walls have been observed. Ordered stripe domains are found in the valley areas between two adjacent grains, which are ascribed to the compressive strain exerted during growth by the neighboring grains.

The study on the rectifying properties of oxygen non-stoichiometric La0.9Ba0.1MnO3-δ/SrTiO3:Nb p-n heterojunction

The La0.9Ba0.1MnO3-δ/SrTiO3:Nb p-n heterojunctions have been fabricated by growing the oxygen nonstoichiometric La0.9Ba0.1MnO3-δ thin film on the Nb doped SrTiO3 substrate with pulsed laser deposition. The good rectifying curves have been demonstrated in the temperature range of 20—300K. The diffusion voltage VD exhibits a maximum near the transition temperature of La0.9Ba0.1MnO3-δ thin film, which is different to the observation in the oxygen stoichiometric La0.9Ba0.1MnO3/SrTiO3:Nb p-n heterojunction. The interesting phenomenon has been well explained with the resistance-temperature experiment of La0.9Ba0.1MnO3-δ thin film and energy band calculations.

Effects of film thickness and preferred orientation on the dielectric properties of (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 films

(Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 (BZN) films with different thicknesses and preferred orientations have been fabricated on Nb doped SrTiO3 substrates by pulsed laser deposition. As the thickness increases, the permittivity increases, and the dielectric loss decreases, while the tunability only has a little variation. The asymmetric behaviour of the electric field dependent permittivity reduces gradually with the increasing thickness, which should be attributed to the decrease in the effect of the interfacial layer between the dielectric film and substrate (electrode). Furthermore, compared with the (1 0 0) oriented BZN film, BZN film with (1 1 1) preferred orientation exhibits high dielectric loss.

C-axial oriented (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 thin film grown on Nb doped SrTiO3 substrate by pulsed laser deposition

A c-axial oriented (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 thin film has been grown on a (0 0 1) Nb doped SrTiO3 substrate by pulsed laser deposition. The permittivity, dielectric loss and tunability of the c-axial oriented film are 187, 0.002 and 6% (at 750 kV cm−1 biasing), respectively, indicating a figure of merit of 30. Moreover, an asymmetry behaviour is observed in the dc electric field dependence of permittivity, which could be attributed to the asymmetry of top and bottom electrodes.

Strain-assisted tunneling current through TbMnO3∕Nb-1 wt %-doped SrTiO3 p–n junctions

Simple oxide heterostructures have been fabricated by growing multiferroic TbMnO3 thin film on Nb-1 wt %-doped SrTiO3 substrate. In addition to the beneficial rectifying characteristics in a temperature range from 350 to 30 K, the intriguing observation is that at constant reverse bias the current increases with decreasing temperature below 275 K. By analyzing the band diagram, the anomalous increasing current with decreasing temperature was ascribed to the strain-assisted tunneling current through TbMnO3∕Nb-doped p–n junctions.

Temperature stability of permittivity and dielectric relaxationin multilayered thin films of (Ba0.80Sr0.20)(Ti1−xZrx)O3with a compositionally graded layer

Mutilayered thin (Ba0.80Sr0.20)(Ti1−xZrx)O3 (BSTZ) films with various compositional graded layers (CGL) have been successfully fabricated on Nb doped SrTiO3 substrates by pulsed-laser deposition technique with four BSTZ ceramic targets (x=0.36,0.18,0.08,0). The gradients of compositions are artificially tailored in multilayered thin films by varying the CGL, and x-ray diffraction indicates that the internal stress is modulated in the multilayered films. Influence of the composition gradient on the dielectric properties has been investigated at the temperature range from 120 to 440K. Temperature stability of permittivity of the multilayered films is found to be improved with the increase of the gradients of compositions. Moreover, a dielectric relaxation process with activation energy of 1.02eV is observed, which is also related to the composition gradient, and can be described to motion of oxygen vacancies. The results show that the temperature stability of permittivity can be tailed by the design of multilayered film with CGL, and the internal stress induced by the gradients of composition could influence the relaxation process.

PZT Electro-Optic Waveguide Devices Fabricated by Solid-Phase Epitaxy

ABSTRACTHigh quality epitaxial PZT optical waveguides have been grown by solid-phase epitaxy based on metal alkoxide solution process. Optical propagation loss was 4 dB/cm in epitaxial PZT thin film optical waveguides grown on SrTiO3 substrates. Epitaxial PZT optical waveguides were grown on Nb doped conductive SrTiO3 substrates, since considerable reduction in drive voltage will be expected when top electrode / optical waveguide / conductive substrate structures are realized. Propagation loss was relatively large, as compared with the structure using non-dope insulative substrates. Preliminary electrooptic deflection devices were fabricated by preparing prism electrodes on the surface of the PZT optical waveguides. Efficient deflection/switching of coupled laser beam in the PZT optical waveguides as large as 26 mrad was observed by applying 70 volts between prism electrode and Nb doped SrTiO3 substrates.