Oriented Strontium Titanate Research Articles

Structure and Properties of Thin Magnetron Films of Cadmium Arsenide on Various Substrates

Purpose of the study. Synthesis of cadmium arsenide magnetron films on various substrates and study of their structure, composition, optical and electrical properties.Methods. The deposition of thin films of cadmium arsenide was carried out by the method of non-reactive highfrequency magnetron sputtering in an argon atmosphere. The structure and composition of the films were studied using X-ray phase analysis, scanning electron microscopy, energy dispersive analysis, and small-angle X-ray diffractometry. Optical studies were performed using Raman spectroscopy. The results of a study of the electrical properties of thin films of cadmium arsenide are presented.Results. On silicon, sapphire, and strontium titanate substrates, thin films of the Dirac semimetal, cadmium arsenide, were obtained with a thickness of about 40 nm. A study of their structure and composition showed a significant effect of annealing in an argon atmosphere following deposition on the crystallinity of the film. After annealing, regardless of the crystal structure of the substrate, partial orientation of the film with the (112) texture axis. The films closest to the stoichiometric composition were obtained by deposition followed by annealing onto an oriented strontium titanate substrate, and to the crystal structure of cadmium arsenide single crystals of a film on a sapphire substrate. Annealing also leads to a smoothing of the film surface, a decrease in structural defects, and the transition of the fractal dimension of its topology to two-dimensional from close to three-dimensional immediately after deposition. The optical properties after annealing also change, which indicates their transition from a polycrystalline (amorphous) state to a single-crystal (textured).Conclusion. Experimental studies of the structure and properties performed by various methods made it possible to establish that single-crystal or textured cadmium arsenide films suitable for studying the manifestation of topological properties can be obtained by controlled annealing.

Magnetic transitions and coercive field variations in La1−xSrxMnO3 thin films (0.04 ≤ x ≤ 0.50)

We have investigated the electronic and magnetic ground state of La1−xSrxMnO3 thin films, especially around doping x = 0.50 in comparison to the well studied system of x = 0.33 and the low doped system of x = 0.04. The films discussed here were grown by molecular-beam epitaxy, epitaxially strained to (001) oriented strontium titanate substrates. Apart from resistivity and magnetization measurements probing the electronic and magnetic ground state, we also study the temperature dependence of the coercive field of these samples. Our measurements reveal that the coercive field increases as the doping is changed from x = 0.33. The coercive field at 5 K for the x = 0.50 sample is Hc = 0.0450 T compared to Hc = 0.0080 T for the x = 0.33 sample. The temperature dependent coercive field measurements on the x = 0.50 sample show a dramatic cusp around 100 K that is coincident with more subtle features observed in both magnetization and resistivity data at this temperature.

Optimization of Pulsed Laser Deposition Parameters for Single-Crystalline (011) Oriented Tetragonal Pzt Thin Films

Recent advances in nanotechnology applications require the processing of high-quality thin films. A systematic study of the optimization of Pulsed deposition parameters for (110) oriented tetragonal PZT thin films is reported in this work. Prior to film deposition, a highly tetragonal Pb(Zr0.5, Ti0.95) or PZT 05/95 was selected to be deposited on (110) oriented Strontium titanate, SrTiO3 substrate, STO(110) to insure small compressive misfit strain. Unlike SRO, the LSOM bottom electrode was then successfully grown at 620 °C, 1.0 J/cm2 and a target-substrate distance of 3.9 cm. Finally, deposition parameters of PZT were progressively optimized through monitoring film morphological changes. Monocrystalline PZT films of different thicknesses with layer–by–layer growth mode were finally realized at a deposition temperature of 550 °C, laser energy of 1.0 J/cm2, background pressure of 200 mTorr O2, while deposition rate was 3 Hz and target-substrate distance of 3.1 cm. The optimal film growth rate was finally estimated to be 0.3 nm/100 Pulses.

Doping dependent electronic and magnetic ordering in mixed-valent La1−x Sr x MnO3 thin films

We have investigated the collective electronic and magnetic orderings of a series of La1−x Sr x MnO3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films.

We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca0.5Sr0.5TiO3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from ∼1 to ∼10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin "vibrational bridge" layer between two solids.

Symmetry breaking in single-crystal SrTiO3 plates: EPR manifestations

Results of the electron paramagnetic resonance (EPR) studies of Fe3+ and Mn4+ impurity centers in high-quality single-crystal SrTiO3 plates with thickness are presented. Impurities served the paramagnetic probes of the crystal structure. Analysis of the collected data indicates that the symmetry of the crystal structure of thin (001) and (110) oriented strontium titanate plates above lowers in the bulk from the cubic to tetragonal. The structure of this high-temperature tetragonal state is different from that of the conventional low-temperature antiferrodistorsive phase and has not been reported for SrTiO3 before. It is shown that the effect does not originate from the residual stress; its magnitude and particular manifestations depend on plate orientation, surface quality and geometry of a sample. Peculiarities of the observed phenomenon are presented and the possible scenario of its realization is discussed.

Электрополевой эффект в спектрах ЭПР центров Fe-=SUP=-3+-=/SUP=- и Mn-=SUP=-4+-=/SUP=- в тонких пластинках SrTiO-=SUB=-3-=/SUB=-

The results of the study of the electric field effect in the electron paramagnetic resonance spectra of the Fe3+ and Mn4+ impurity ions in thin oriented strontium titanate single-crystal plates are presented. The crystal structure of plates at T> 105 K turned out to be tetragonal, different from both the cubic and the antiferrodistortive tetragonal, inherent to SrTiO3 at T < 105 K. It is shown that for the studied centers, the electric field effect is quadratic in the applied electric field and has the same sign and order of magnitude, indicating the nonpolar origin of the observed high temperature tetragonal state. Analysis of the dependences of the axiality value on the applied field indicates that the electric field effect does not originate from the electrostriction and is due to the modification the wavefunction of the ground state of the impurity centers.

The Facet Structure and Photochemical Reactivity of Arbitrarily Oriented Strontium Titanate Surfaces

AbstractSrTiO3 polycrystalline ceramics with polished surfaces are annealed at 1250 °C in air. This treatment causes the flat surfaces to break up into facets meeting at sharp edges and corners. An analysis of the orientations and topography of the faceted surfaces demonstrates that all are either {100} or {110} oriented. The {100} surfaces are photocathodically active and reduce Ag+ to Ag metal. The {110} surfaces are photoanodically active and oxidize Mn2+ and Pb2+ to Mn4+ and Pb4+, respectively. The chemical properties of both surfaces appear to be uniformly photocathodic or photoanodic. However, after annealing at 1100 °C with Sr3Ti2O7, the {110} facets have a combination of photocathodic and photoanodic terraces. The results show that the photocathodic‐to‐photoanodic surface area ratio, which influences the overall rate of a photochemical reaction, can be controlled for arbitrarily oriented SrTiO3 surfaces by using thermal treatments to create low index facets and to control the chemical terminations on these facets.

Tuning the Surface Electrochemistry by Strained Epitaxial Pt Thin Film Model Electrodes Prepared by Pulsed Laser Deposition

A novel approach for tailoring strained Pt model electrocatalysts using pulsed laser deposition is presented. The physical properties of Pt films grown on single‐crystalline (100)‐ and (111)‐oriented strontium titanate depend significantly on the various deposition parameters (e.g., fluence, temperature, background atmosphere, postannealing, and number of process steps), as revealed by scanning electron microscopy and X‐ray studies. By properly selecting the deposition conditions, thin, epitaxial, and strained (100)‐ and (111)‐oriented Pt films suitable as model electrodes can be prepared. Cyclic voltammetry measurements indicate that the strain in the Pt films changes significantly the Pt surface properties, such as the hydrogen underpotential deposition (Hupd) features. CO oxidation experiments demonstrate that the strain affects strongly also the position of the CO oxidation peak shifting it to more positive potentials. This might be attributed to a modification of the adsorption behavior of oxygenated species on the strained interface. This research clearly highlights the potential of using as‐produced Pt film as model catalysts to study the effect of strain on electrocatalytic reactions while minimizing possible side effects connected to the common preparation methods for strained Pt catalysts, such as alloy formation, alloy leeching, or conductive support contributions.

Spectral identification scheme for epitaxially grown single-phase niobium dioxide

This publication describes how to distinguish under-oxidized and over-oxidized phases of the metal-insulator transition material NbO2 by employing a combination of the Nb 3d and O 1s core-level and valence-band spectra. Niobium oxides (NbOx) are grown under various conditions by molecular beam epitaxy on several different substrates, mostly (111)-oriented strontium titanate (SrTiO3), in order to obtain phase-pure NbO2. The films are characterized by in situ reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy to determine crystallinity, lattice constants, and epitaxial relationships. Niobium metal oxidation states and film stoichiometry are determined with in situ X-ray photoelectron spectroscopy. Asymmetric Nb 3d core-level spectral components are observed in phase-pure NbO2, with a binding energy for Nb 3d5/2 of 206 eV. The effect of substrate type, growth temperature, and oxygen pressure on the film composition is also described.

Unravelling the effect of SrTiO3 antiferrodistortive phase transition on the magnetic properties of La0.7Sr0.3MnO3 thin films

Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films, with different thicknesses ranging from 20 to 330 nm, were deposited on (1 0 0)-oriented strontium titanate (STO) substrates by pulsed laser deposition, with their structure and morphology characterized at room temperature. The magnetic and electric transport properties of the as-processed thin films reveal an abnormal behaviour in the temperature dependent magnetization M(T) below the antiferrodistortive STO phase transition (TSTO), and also an anomaly in the magnetoresistance and electrical resistivity close to the same temperature. Films with thickness ≤100 nm show an in-excess magnetization and pronounced changes in the coercivity due to the interface-mediated magnetoelastic coupling with antiferrodistortive domain wall movement occurring below TSTO. However, in thicker LSMO thin films, an in-defect magnetization is observed. This reversed behaviour can be understood with the emergence in the upper layer of the film, of a columnar structure needed to relax the elastic energy stored in the film, which leads to randomly oriented magnetic domain reconstructions. For enough high-applied magnetic fields, as thermodynamic equilibrium is reached, a full suppression of the anomalous magnetization occurs, wherein the temperature dependence of the magnetization starts to follow the expected Brillouin behaviour.

A Strategy to Prepare Wafer Scale Bismuth Compound Superstructures

Epitaxial wafer scale superstructures of bismuth compounds are synthesized. Single crystalline β-Bi2O3 films are obtained by sputtering amorphous BiOx onto (001)-oriented strontium titanate with a buffer layer, followed by thermal crystallization. This is used as the precursor for the growth of the superstructures. The superstructures of bismuth compounds reveal anisotropic physical properties that are related to their unique morphology.

Capacitor structures based on strontium titanate films

The structural properties of strontium titanate films and the electrical parameters of the related capacitor structures have been investigated. It has been found that the deposition temperature exerts a direct effect on the phase composition of the films, the degree of perfection of the crystal structure, and the internal stresses in the lattice. The strontium titanate films deposited at a temperature of 800°C are single-phase, have a rather perfect crystal structure, and possess minimum internal stresses. It has been shown that the capacitor structures based on oriented strontium titanate films provide a twofold decrease in the level of dielectric loss as compared to similar structures based on barium strontium titanate solid solution films at a frequency of 1.5 GHz.

Epitaxial growth of tin ferrite thin films using pulsed laser deposition technique

Epitaxial thin films of tin ferrite (SnFe 2O 4) were deposited on (0 0 2) oriented strontium titanate (SrTiO 3) substrate using pulsed laser deposition method. The quality and epitaxial nature of the films were investigated by X-ray diffraction technique. The phi scan of the film and the substrate shows four folds symmetry indicating cube-on-cube epitaxial growth of the film on the substrate. The optical bandgap of the film was estimated to be 2.6 eV using optical transmittance data. Magnetic measurements indicate that the coercive field and remnant magnetization of the film decrease with increase in temperature. The presence of hysteresis loop in M vs. H plot at room temperature indicates the ferromagnetic nature of the film.

Crystalline ZnO/SrTiO3 transparent field effect transistor

AbstractWe fabricated field effect transistors which are transparent at visible wavelength, by realizing epitaxial ZnO/SrTiO3 heterostructures. High crystalline quality ZnO layers were grown by pulsed laser deposition on 110 oriented strontium titanate single crystals. By conventional photolithographic techniques we realized micrometric sized devices in both bottom‐gate stacked layers geometry and planar side‐gate configuration. Our transistors show 80% light transmittance, on/off ratios up to 106 and field effect mobilities up to 30 cm2/Vs at 77 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Quality Deposition of Strontium Titanate (STO) Thin Films using Sol-Gel Method

AbstractHighly oriented strontium titanate (STO) thin films were successfully deposited on sapphire substrates using a sol-gel method. Sintering temperature of 1400°C and the use of c-cut sapphire for the substrates are the keys to obtain highly oriented thin films. The sample obtained on c-cut sapphire with a sintering temperature of 1400°C showed quite high (111)-orientation and a narrow full width half maximum (FWHM) value of 0.265°, which indicates a high quality deposition of the STO thin films.We believe this high quality STO deposition technique will bring a new scope of oxide material applications for future electronic devices.

High mobility ZnO thin film deposition on SrTiO 3 and transparent field effect transistor fabrication

Zinc oxide and strontium titanate are among the most interesting materials for oxide electronics due to their semiconducting and dielectric properties respectively. We obtain high electron mobility values in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Values up to 400 cm 2/V s are found below 50 K in samples grown by a two step method: firstly a thin ZnO relaxing layer is deposited on the SrTiO 3 (110) substrate at relatively low temperature (550 ∘C) and then the deposition temperature is raised up to 750 ∘C for the growth of a second ZnO layer. The realized epitaxial ZnO/SrTiO 3 heterostructures are used to fabricate field effect transistors transparent at visible wavelength. By conventional photolithographic techniques we realize micrometric sized devices in planar side-gate configuration. The transistors have an 80% transmittance, on–off ratios up to 10 6, and field effect mobilities up to 30 cm 2/V s.

High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layer

High electron mobility is measured down to low temperature in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Electron mobility is evaluated by both magnetoresistance and resistivity-Hall effect data. Values up to 400cm2∕Vs are found below 50 K in epitaxial thin films grown by a two-step method: first a 100-nm-thick ZnO relaxing layer is deposited on the SrTiO3 (110) substrate at relatively low temperature (550–600 °C) and then the deposition temperature is raised up to 800 °C for the growth of a second ZnO layer. Reflection high energy electron diffraction analysis during the deposition, ex situ x-ray diffraction and AFM morphology studies performed separately on each layer reveal that the first layer grows in a quasi-two-dimensional mode while the increased temperature in the second step improves the crystalline quality of the film. The integration of ZnO transparent semiconductor with high-k dielectric perovskite substrates may lead to a wide variety of new electronic∕optoelectronic devices.

Processing aspects of MOD strontium titanate buffer layers for coated conductor applications

Recent trends in HTS research have been geared toward development of coated conductors. In these conductors, a flexible metallic substrate, typically a nickel alloy, is covered successively with one or more thin buffer layers such as YSZ, STO, MgO, etc., and a relatively thick film of Y123. To make the fabrication of such conductors scalable and cost effective, this paper demonstrates the application of a solution based metallorganic decomposition (MOD) process for deposition of strontium titanate buffer layers. We present results on the effect of precursor chemistry and heat treatment variables on the densification behavior, texture, and microstructure of the strontium titanate films. Highly oriented strontium titanate films with good coverage across the substrate surface have been processed using this method. Thus, this process offers promise to the application of scalable solution deposition techniques to coated conductor fabrication.