Nb-doped SrTiO3 Single-crystal Substrates Research Articles

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current

Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10−3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices.

Schottky-to-Ohmic switching in ferroelectric memristors based on semiconducting Hf0.93Y0.07O2 thin films

We demonstrate resistive switching and memristive behavior in devices consisting of ultrathin (4.5 nm) semiconducting, epitaxial ferroelectric Hf0.93Y0.07O2 (YHO) films on La0.7Sr0.3MnO3-buffered, Nb-doped SrTiO3 single crystal substrates with Au top electrodes. Unlike the tunneling-driven current–voltage characteristics of ferroelectric tunnel junctions which utilize ultrathin insulating (fully depleted) ferroelectric films, the semiconducting nature of our YHO films, i.e., the presence of free charge carriers introduced by Y doping, results in radically different current–voltage characteristics. Current–voltage measurements indicate a polarization-modulated transition from Schottky-barrier-controlled charge transport to Ohmic conduction in the YHO devices, which results in a large on/off ratio of up to 540. Moreover, voltage pulse train measurements reveal a broad range of accessible resistance states, which indicates the memristive behavior of the devices. Our results represent an important step toward the development of future nonvolatile memory and brain-inspired neuromorphic computing applications based on ultrathin semiconducting ferroelectric films.

Ferromagnetic properties of conducting filament nanodots formed on epitaxial BiFeO3 thin film

We investigate the ferromagnetic properties of the conducting filament (CF) nanodots formed on epitaxial BiFeO3 (BFO) thin film, which exhibit resistive switching behavior. 50-nm thick epitaxial BFO thin film was grown on Nb-doped SrTiO3 single crystal substrates through pulsed laser deposition. The ferroelectric microcapacitor fabricated using the epitaxial BFO thin film shows excellent ferroelectric properties with a remanent polarization of approximately 32 μC/cm2 and unipolar resistive switching behavior. CF nanodots with a diameter of approximately 16 nm are formed in the local region of the BFO thin film through conductive atomic force microscopy. Generally, ferromagnetic materials exhibit superparamagnetism, on a scale of tens of nanometers, as a result of the thermal energy of the magnetic moments. In this study, magnetic force microscopy demonstrates that the CF nanodot possesses ferromagnetism. It is suggested that exchange coupling between the CF nanodot and surrounding BFO causes the ferromagnetism of the nanodot. Our study provides new insight on the design of multistate memory cells with resistive switching and magnetic spin ordering, for application in high-density information storage media.

Characterization of PN junctions of doped Mott insulators

A heterojunction comprising a La1.9Sr0.1CuO4 thin film on a 0.5 wt. % Nb doped SrTiO3 single-crystal substrate was fabricated using magnetron sputtering deposition and ion milling etching techniques. Rectification has been observed in the current–voltage characteristics of the heterojunctions from 30 to 300 K. The temperature evolution of the ideality factor implies the important role of barrier inhomogeneity at the junction interface. A systematic study of capacitance as a function of frequency under different bias voltages at room temperature was also carried out. The capacitance can be explained by deep-level defects in the space charge region.

Frequency and Temperature Dependence of Dielectric Properties of Orthorhombic HoMnO<sub>3</sub> Thin Films

Orthorhombic HoMnO3 films were prepared epitaxially on Nb-doped SrTiO3 single crystal substrates by using pulsed laser deposition technique. The films showed perfectly a-axis crystallographic orientations. X-ray diffraction and atomic force microscopy were used to characterize the films. The complex dielectric properties were measured as functions of frequency (40 Hz~1 MHz) and temperature (80 K~300 K) with a signal amplitude of 50 mv. The respective dielectric relaxation peaks shifted to higher frequency as the measuring temperature increased, with the same development of real part of the complex permittivity. The cole-cole diagram was obtained according to the Debye model, and the effects of relaxation process were discussed.

Structural, magnetic and dielectric properties of La2NiMnO6 thin film by chemical solution deposition method

Double perovskite La2NiMnO6 thin film was successfully prepared on (001)-oriented 0.7 wt% Nb-doped SrTiO3 single crystal substrate by chemical solution deposition method. The microstructures as well as properties were investigated. The results show that the derived thin film is an admixture of Ni/Mn ordering, and two ferromagnetic transitions are observed with Curie temperatures of 289 and 110 K, respectively. A crossover from positive to negative magnetodielectric effect is observed near room temperature. The success preparation of La2NiMnO6 thin film by chemical solution deposition will provide a useful method to deposit double perovskite multiferroic thin films with low-cost.

Large rectifying leakage current in Pt/BaTiO3/Nb:SrTiO3/Pt structure

20-nm-thickness epitaxial BaTiO3 (BTO) thin films were prepared on the Nb-doped SrTiO3 single-crystal substrates by RF magnetron sputtering technology. The rectifying characteristic current-voltage curves were observed and was discussed using the Schottky barrier model taking into account the movement of oxygen vacancies. The leakage current mechanisms were interpreted by the Schottky emission and Poole-Frenkel emission near the Pt/BTO interface at lower bias and higher bias, respectively.

Fast photovoltaic effects tuned by vicinal interface microstructure in manganite-based all-perovskite-oxide heterojunctions

Interfacial microstructure tunable photovoltaic effects have been reported in heterojunctions of La(1-x)Ca(x)MnO(3) thin films and tilted or exact cut Nb-doped SrTiO(3) single crystal substrates under irradiation of UV pulsed laser. The photoresponse times of vicinal junctions and films were obviously faster than those of exact cut samples. The behaviors can be explained by the slope of the built-in electric field in the 10° tilted heterojunction and the interfacial terrace microstructure of La(1-x)Ca(x)MnO(3) films.

Oriented growth and ferroelectric anisotropy of sol–gel derived Bi3.15Nd0.85Ti3O12 thin films on Nb‐doped SrTiO3

AbstractFerroelectric Bi3.15Nd0.85Ti3O12 (BNdT) thin films were grown through a cheap and simple sol–gel process on Nb‐doped SrTiO3 single crystal substrates of (001) and (011) orientations. The films containing 30% (100)/(010)‐oriented plus 18% (119)‐oriented columnar grains were obtained on Nb:SrTiO3(011) using a rapid heating during crystallization, while the films containing 19% c‐axis‐oriented prolate grains on Nb:SrTiO3(001) using a low heating rate. Anisotropic ferroelectric properties were determined, with a remanent polarization 2Pr = 28.4 μC/cm2 in the film with predominant 100/010/119 orientations, but only 2Pr = 5.7 μC/cm2 in the predominantly c‐axis‐oriented film. The ferroelectric anisotropy confirms that the polarization vector of BNdT is close to the a‐axis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Proton Conducting Oxide Epitaxial Thin Films and Evaluation of Their Characteristics and Electrical Properties

Epitaxial thin films of 5 mol% Y-doped BaZrO3 (BZY5) proton conducting oxide were fabricated on MgO and Nb-doped SrTiO3 single crystal substrates using PLD method. Crystal structure, microstructure and electrical properties in in-plane and perpendicular directions of those films were studied. BZY5 is grown epitaxially over 700oC, and its epitaxial relationship is typical “Cube on Cube type”. TEM and AFM observations reveal that the films consist of columnar structure. Electrical properties in in-plain direction show significant lower conductivity and higher activation energy than that in perpendicular direction, and the properties are close to that of grain boundary in ceramics, while the electrical properties in perpendicular direction are similar to that of bulk in ceramics. These results suggest that the interfaces between columnar crystals even in epitaxial thin films prevent proton migration as the grain boundaries in ceramics do.

Epitaxial growth of sol-gel derived BiScO3–PbTiO3 thin film on Nb-doped SrTiO3 single crystal substrate

Bi Sc O 3 – Pb Ti O 3 (BSPT) thin film was grown on electrical conductive Nb-doped SrTiO3 (100) (Nb-STO) single crystal substrate by a sol-gel method. The x-ray diffraction and high resolution transmission electron microscopy proved the epitaxial growth relation between the BSPT thin film and Nb-STO substrate. The [001]-epitaxial BSPT thin film exhibited considerable high remanent polarization of 74μC∕cm2 and effective piezoelectric coefficient d33* of 130pm∕V, which were much higher than those of the (100)-oriented BSPT thin film grown on traditional silicon substrate.

Relaxation of nanopatterns on Nb-doped SrTiO3 surface

By atomic force microscopy (AFM), we have realized nanolithography and changed the physical properties within a desired nanosized region on Nb-doped SrTiO3 single-crystal substrate. Importantly, a considerable relaxation of these written patterns was observed directly, which evidently indicates that a significant chemical transport occurs during and after the AFM lithography. Furthermore, we found that defects introduced by Ar bombardment can stabilize these patterns, which is quite effective for the application of AFM lithography in perovskite oxides.