SrTiO3 Single Crystalline Substrates Research Articles

Hydrothermal Growth and Orientation of LaFeO3 Epitaxial Films.

LaFeO3 thin films were successfully epitaxially grown on single-crystalline SrTiO3 substrates by the one-step hydrothermal method at a temperature of 320 °C in a 10 mol/L KOH aqueous solution using La(NO3)3 and Fe(NO3)3 as the raw materials. The growth of the films was consistent with the island growth mode. Scanning electronic microscopy, elemental mapping, and atomic force microscopy demonstrate that the LaFeO3 thin films cover the SrTiO3 substrate thoroughly. The film subjected to hydrothermal treatment for 4 h exhibits a relatively smooth surface, with an average surface roughness of 10.1 nm. X-ray diffraction in conventional Bragg-Brentano mode shows that the LaFeO3 thin films show the same out-of-plane orientation as that of the substrate (i.e., (001)LaFeO3||(001)SrTiO3). The in-plane orientation of the films was analyzed by φ-scanning, revealing that the orientational relationship is [001]LaFeO3||[001]SrTiO3. The ω-rocking curve indicates that the prepared LaFeO3 films are of high quality with no significant mosaic defects.

Electrocaloric temperature changes in epitaxial Ba1−xSrxTiO3 films

The basic aim of our study is to investigate the correlation between structural parameters and the electrocaloric effect in lead-free epitaxial Ba1−xSrxTiO3 (BSTO) based thin film architectures. Therefore, BSTO thin films with Sr contents of x = 0 to x = 0.3 were grown on SrRuO3 buffered SrTiO3 single crystalline substrates by pulsed laser deposition. Structural characterization verified an epitaxial growth for all Sr contents with an additional tetragonal distortion compared to bulk material. Temperature and frequency-dependent measurements of dielectric properties revealed increased permittivity values for thicker films with broad maxima indicating a diffuse phase transition. The temperature of maximum permittivity decreases with increasing Sr content, whereas polarization measurements indicate a relaxor-like behaviour in particular above room temperature. Adiabatic temperature changes were determined with the indirect method resulting in |ΔT| values of up to 2.9 K for a 680 nm thick BSTO layer with x = 0.3 at an applied electric field of 750 kV cm−1.

Features in low-temperature electrical resistivity of amorphous Cd3As2 films due to hopping conductivity with changing activation energy

Amorphous Cd3As2 films were deposited on single-crystalline SrTiO3 substrate by high-frequency non-reactive magnetron sputtering. Electrical resistivity of the film, measured within 3-75 K range, increases at cooling. Below ∼70 K, negative transverse magnetoresistance observed. Features in electrical resistivity and magnertoresistance can be attributed to variable-range hopping conductivity of the Mott type with local activation energy, which is temperature-dependent. Two temperature ranges in the hopping conductivity observed. Appearing two ranges is in qualitative accordance with the Mott-Davis energy-band model developed for amorphous semiconductors. High temperature range of the hopping conductivity is attributed to electron hops between localized states, positioned inside band tails, whereas electron hops between localized states, positioned inside Fermi-peak, can be responsible for low-temperature range. At taking into account temperature dependences of local activation energy, universal expression for various mechanisms of the hopping conductivity could be successfully applied to analyze the experimental data.

Thermal conductivity across the van der Waals layers of α-MoO3 thin films composed of mosaic domains with in-plane 90° rotations

α-Molybdenum trioxide (α-MoO3) is a typical two-dimensional metal oxide material, in which the covalently bonded layers are bonded by van der Waals forces. Herein, the thermal conductivities across the van der Waals layers of α-MoO3 thin films with nominal thicknesses of 50, 100, and 125 nm were investigated. The α-MoO3 thin films were fabricated on the (100) plane of a single-crystalline SrTiO3 substrate heated up to 400 °C by DC reactive magnetron sputtering using a Mo metal target. The b-axis-oriented α-MoO3 thin films epitaxially grown on the SrTiO3 (100) plane were confirmed by x-ray diffraction and x-ray pole figure analyses. Electron diffraction patterns and plane-view transmission electron micrographs revealed that the α-MoO3 thin films were composed of mosaic domains with a diameter of ∼2 nm, with each domain rotated in-plane by 90° with respect to the neighboring one. The mean thermal conductivity across the van der Waals layers of the three α-MoO3 thin films was evaluated to be 1.2 ± 0.3 W m−1 K−1, which is comparable to the reported thermal conductivities of layered cobalt oxide thin films. The reduced thermal conductivity is mainly due to phonon scattering at domain boundaries lying in the in-plane direction.

Morphology control of self-organised Sr3V2O8 nanostructures on SrVO3 grown onto single and poly-crystalline subjacent SrTiO3 substrates

SrVO3 (SVO) is a complex oxide with interesting optical and conduction properties as an indium-free transparent conducting oxide for electrode applications. In this paper, we report how the surface of SVO can be designed at the nanoscale from self-organized Sr3V2O8 nanostructures (NS) with different shapes and morphologies depending on crystalline orientations. By combining transmission electron microscopy (TEM) and atomic force microscopy (AFM), we compare the characteristics of the NS for SVO films grown on SrTiO3 (STO) polycrystalline substrates, according to combinatorial substrate epitaxy (CSE), with those of NS observed from SVO films deposited on STO single crystalline substrates with different crystallographic orientations ((1 0 0), (1 1 0), (1 1 1)). We are able not only to show that the obtained morphologies are correlated to the specific substrate orientation due to the epitaxial relationships between the NS, the SVO, and the substrate, but also to establish a NS library and determine the necessary crystalline orientations for a certain NS morphology on demand. Finally, dissolution of the NS in water leads to typical inverse imprints on the surface of the film, offering a fast and easy way to pattern the electrodes. This work presents a new way of patterning surfaces with a specific design of self-organized NS.

Lead-free relaxor-ferroelectric thin films for energy harvesting from low-grade waste-heat

One of the ways to mitigate the world energy crisis is to harvest clean and green energy from waste-heat, which is abundant, ubiquitous, and free. Energy harvesting of this waste-heat is one of the most encouraging methods to capture freely accessible electrical energy. Ferroelectric materials can be used to harvest energy for low power electronic devices, as they exhibit switchable polarization, excellent piezoelectric and pyroelectric properties. The most important characteristic of ferroelectric materials, in the context of energy harvesting, is their ability to generate electric power from a time-dependent temperature change. In this work, we grew highly c-axis oriented heterostructures of BaZr0.2Ti0.8O3 (barium zirconium titanate, BZT)/Ba0.7Ca0.3TiO3 (barium calcium titanate, BCT) on SrRuO3 (strontium ruthenate, SRO) and deposited on SrTiO3 (strontium titanate, STO) single crystalline substrate using pulsed laser deposition (PLD) technique. We investigated the structural, electrical, dielectric, and pyroelectric properties of the above-mentioned fabricated heterostructures. The wide range of θ–2θ X-ray diffraction (XRD) patterns only shows (00l) reflection peaks of heterostructures and the substrate which confirmed that the films are highly c-axis oriented. We are also capable to convert the low-grade waste-heat into electrical energy by measuring various temperature-dependent ferroelectric hysteresis loops of our nanostructure films via pyroelectric Ericsson cycles and the structures show an energy conversion density ~ 10,970 kJ/m3 per cycle. These devices exhibit a large pyroelectric current density of ~ 25 mA/m2 with 11.8 °C of temperature fluctuation and the corresponding pyroelectric coefficient of 3425 μC/m2K. Our research findings suggest that these lead-free relaxor-ferroelectric heterostructures might be the potential candidates to harvest electrical energy from waste low-grade thermal energy.

Improved piezoelectricity and energy storage performance simultaneously achieved in [001]-preferentially oriented Bi0.5Na0.5TiO3–BaTiO3–BiMnO3 thin films grown on Nb-doped SrTiO3 single-crystalline substrates

The control of grain orientation by introducing Nb-doped SrTiO3 single crystalline substrates is an effective way to improve piezoelectric and ferroelectric properties of thin films. Therefore, in this work, 0.99(0.94Bi0.5Na0.5TiO3–0.06BaTiO3)–0.01BiMnO3 thin films were coated on Pt(111)/TiOx/SiO2/Si(001) polycrystalline substrates and Nb-doped SrTiO3 single crystalline substrates, respectively. The Nb-doped SrTiO3 single crystalline substrates can affect the orientation of the BNT–BT–0.01BMO thin films, promote the grain growth, reduce the leakage current, and form an interfacial layer between the thin film and the substrate, and a comparatively larger converse piezoelectric coefficient (46.49 pm·V−1), a high energy storage density (56.5 J cm-3), and a high energy storage efficiency (79.4 %) were therefore simultaneously achieved in the thin films grown on Nb-doped SrTiO3 single crystalline substrates, promising the oriented thin films broad application prospects.

Development of a Fluorine-Free Polymer-Assisted-Deposition Route for YBa2Cu3O7−x Superconducting Films

Polymer assisted deposition (PAD) was used as an environmentally friendly, non-fluorine, growth method for superconducting YBa2Cu3O7−x (YBCO) films. The kinetics of the thermal decomposition of the precursor powder was studied by thermogravimetry coupled with mass spectrometry (TG-QMS). YBCO films were spin coated on (100) SrTiO3 (STO) single crystalline substrates, followed by a single step thermal treatment under wet and dry O2 and O2/N2 mixture. The as-obtained films were epitaxially grown having a [001]YBCO||[001]STO out-of-plane epitaxial relationship and exhibited good superconducting properties with Tc (R = 0) > 88 K, transition widths, ΔT ≈ 2 K and critical current densities as high as 2.3 MA/cm2 at 77 K and self magnetic field.

Single Source Precursor for PAD-LaMnO3 Thin Films

A new lanthanum and manganese ethylenediaminetetraacetate (EDTA) coordination polymer (EDTA4− = [(CH2N)2(CH2-COOH)(CH2COO)4]) was synthesized from La(NO3)3 and Mn(NO3)2 reagents, ethylenediaminetetraacetic acid, and water at room temperature. The structure of the new compound formed, [{La2Mn3(EDTA)3(H2O)11}·12H2O]n, was determined by the single crystal X-ray diffraction technique. The synthesis and characterization of the La(III) and Mn(II) coordination complex, characterized by FTIR spectroscopy, thermogravimetry, and differential thermal analysis of the complex, are envisaged. X-ray crystal structure determination indicates that seven- and four-coordinate modes between La(III)/Mn(Π) and H4EDTA exist. [{La2Mn3(EDTA)3(H2O)11}·12H2O]n crystallizes in the monoclinic space group C2 with unit cell parameters of a = 16.1227(17) Å, b = 14.8049(16) Å, c = 14.8736(16) Å, and β = 116.107(2)°. Using this precursor, LaMnO3 (LMO) epitaxial thin films were grown by the polymer-assisted deposition (PAD) method on (100)SrTiO3 (STO) single crystalline substrates at a temperature of 900 °C. The LMO crystallized films exhibit a (001)LMO/(001)STO out-of-plane epitaxial relationship and a smooth surface morphology.

Effects of Zn doping on the pinning potential and the glass-liquid transition temperature of YBa2Cu3O6+δ films

A pristine YBa2Cu3O6+δ film as well as two Zn-doped YBa2(Cu1−x Zn x )3O6+δ films with doping levels x of 0.2 % and 0.4 %, respectively, grown epitaxially on SrTiO3 single crystalline substrates by pulsed laser deposition, were investigated regarding the effect of the doping on the glass-liquid transition line and the pinning potential distribution. Zn doping did not show any effect on the shape of the transition line but caused a suppression of the transition temperature T GL of about 16 K%−1. Although the doping did not lead to an additional pinning effect, the increase of the critical current density j c towards lower temperatures is mainly driven by the variation of the threshold critical current density, j cm. The width of the critical current density distribution j 0 saturates at low temperatures and only drops above ~ 0.9T GL.

Strain control of phase transition and magnetocaloric effect in Nd0.5Sr0.5MnO3 thin films

Phase transition and the magnetocaloric effect (MCE) in Nd0.5Sr0.5MnO3 (NSMO) epitaxial thin films were tailored through controlling the lattice-mismatch-induced-strain by depositing on (011)—(La0.18Sr0.82)(Al0.59Ta0.41)O3 and SrTiO3 (STO) single crystalline substrates, respectively. The NSMO film grown on STO, exhibiting uniaxial like tensile strain of 1.3% along the in-plane [100] direction, undergoes a paramagnetic to ferromagnetic transition at ∼210 K followed by a ferromagnetic to A-type antiferromagnetic transition at ∼179 K upon cooling; meanwhile, the film grown on LSAT, exhibiting anisotropic in-plane tensile strains of 0.36% along [100] and 0.50% along [01¯1] directions, undergoes further transition to CE-type antiferromagnetic transition at ∼145 K. NSMO/LSAT with such transitions facilitates a strong MCE over a much wider temperature range from 90 to 170 K, with the magnetic entropy change comparable to the recently reported La0.25Ca0.75MnO3 bulk. These findings suggest that control of strain in manganite films with first-order phase transition is a feasible way to broaden their MCE temperature range.

Enhanced ferroelectricity in epitaxial Hf0.5Zr0.5O2 thin films integrated with Si(001) using SrTiO3 templates

SrTiO3 templates have been used to integrate epitaxial bilayers of ferroelectric Hf0.5Zr0.5O2 and La2/3Sr1/3MnO3 bottom electrodes on Si(001). The Hf0.5Zr0.5O2 films show enhanced properties in comparison to equivalent films on SrTiO3(001) single crystalline substrates. The films, thinner than 10 nm, have a very high remnant polarization of 34 μC/cm2. Hf0.5Zr0.5O2 capacitors at an operating voltage of 4 V present a long retention time well beyond 10 years and high endurance against fatigue up to 109 cycles. The robust ferroelectric properties displayed by the epitaxial Hf0.5Zr0.5O2 films on Si(001) using SrTiO3 templates pave the way for the monolithic integration on silicon of emerging memory devices based on epitaxial HfO2.

Anisotropic grain-boundary effect on electronic transport in superconducting FeSe thin films

The engineering of the microstructure of a superconducting FeSe thin film by the choice of a proper substrate and its crystallographic orientation is demonstrated. To this end epitaxial c-axis oriented FeSe thin films are grown on (110) oriented single-crystalline SrTiO3 substrates by means of sputtering. The extraordinary microstructure is characterized by the uniaxial alignment of plate-like crystalline grains along the substrate direction. The alignment of the elongated grains eventuates in a direction-dependent grain-boundary structure associated with an anisotropic grain-boundary density along the two perpendicular and substrate directions. The effect of the grain-boundary structure on the observed anisotropies of the temperature dependent in-plane resistivities and critical current densities in transverse magnetic fields is extensively studied for current transport along and

Self-polarization effect on large photovoltaic response in lead free ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 epitaxial film

An epitaxial lead free ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) thin film is fabricated on a (001)Nb:SrTiO3 single crystalline substrate by the pulsed laser deposition method. The 2.3% lattice mismatch between the BZT-BCT and substrate suggests that the film is grown under a compressively strained state by leaving a large strain gradient near the interface. Polarization versus electric field measurement reveals that the film exhibits a ferroelectric hysteresis character with a large imprint effect indicating the existence of an internal electric field. The origin of the internal electric field is correlated with the strain gradient induced flexoelectric effect and the interfacial built-in field. Consequently, the resultant internal electric field could lead to a self-polarized non-switchable layer at the interface. The evidence for the envisaged self-polarization effect is indeed shown by the piezo force microscopic measurements. Importantly, photovoltaic studies performed on the film display an open circuit voltage of 1.1 V, which is higher than the values reported for many ferroelectric films. The observed photovoltaic response is correlated with the depolarization field and the self-polarization effect. The demonstrated large photo-response illustrates the application potential of the BZT-BCT system in photovoltaic devices.

Tuning the crystallographic orientation and magnetic properties of multiferroic CuO epitaxial film on single crystalline SrTiO3 substrates

We report a comparative study between the crystal structure and magnetic properties of multiferroic epitaxial CuO thin films grown on (001) and (110) oriented SrTiO3 substrates. CuO thin films were successfully grown along the [010] direction, which is also the direction of their ferroelectric polarization, on (110) oriented SrTiO3 substrates. We correlate our structural findings with the magnetic properties. It was observed that the heterostructure with the CuO film grown along the [010] direction shows distinct anomalies in the temperature dependent magnetization plot at 213 K and 230 K, whereas no such anomaly was observed in the case of the heterostructure grown on the (001) SrTiO3 substrate with the [111] CuO film. Buffered MgO layers were used between the CuO layer and the substrate to improve the crystalline quality of the CuO film.

Fabrication of self-assembled spherical Gold Particles by pulsed UV Laser Treatment

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.

Heteroepitaxial growth of self-assembled BaTiO3-Fe2O3 nanostructures

The self-assembled growth of vertically aligned nanostructures of BaTiO3 in the matrix of Fe2O3 is described. The arrays of well controlled and systematically ordered nanostructures of composite phases were grown on MgO and SrTiO3 single crystalline substrate by using the pulse laser deposition technique under optimized conditions. The phase purities of the composite structures were characterized by using x-ray diffraction and Raman spectroscopy. The surface topographical and morphological measurements were carried out for confirming the growth on nanostructures in the composite thin films. The ferroelectric properties of these composite films were probed by electrical polarization versus electric field (P-E) measurements. The electronic and magnetic properties of the composite were studied by employing x-ray absorption spectroscopy and magnetization measurements. The presence of strain state in nanostructures is found to play an important role in modifying the crystal field effects as well as the magnetic properties of the composite compound. It is shown that the vertical self-assembly of nanorods of BaTiO3 can be grown in the matrix layer of Fe2O3 on the MgO substrate by coablation.

Greatly enhanced flux pinning properties of fluorine-free metal–organic decomposition YBCO films by co-addition of halogens (Cl, Br) and metals (Zr, Sn, Hf)

Additive-free YBCO films, as well as those with halogen (X) added, metal (M) added and (X, M) co-added, have been prepared by the fluorine-free metal–organic decomposition method on SrTiO3(100) single crystalline substrates, where X = Cl, Br and M = Zr, Sn, Hf. It was revealed that the addition of both Cl and Br to the starting solution resulted in the generation of oxyhalide, Ba2Cu3O4X2, in the YBCO films, and that the oxyhalide was found to promote the bi-axial orientation of the YBCO crystals. By adding a decent amount of Cl or Br, highly textured YBCO films with high Jc were reproducibly obtained, even when an impurity metal, M, was co-added, while the addition of M without X did not greatly improve Jc owing to the poor bi-axial orientation of the YBCO crystals. Our results suggest that the addition of Br more effectively enhances Jc than the addition of Cl. The pinning force density at 40 K in 4.8 T reached ∼55 GN m−3 with the co-addition of (Br, M). This value is much larger than that of the pure YBCO film, reaching ∼17 GN m−3.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10−4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.

Atomic Resolution Interfacial Structure of Lead-free Ferroelectric K0.5Na0.5NbO3 Thin films Deposited on SrTiO3

Oxide interface engineering has attracted considerable attention since the discovery of its exotic properties induced by lattice strain, dislocation and composition change at the interface. In this paper, the atomic resolution structure and composition of the interface between the lead-free piezoelectric (K0.5Na0.5)NbO3 (KNN) thin films and single-crystalline SrTiO3 substrate were investigated by means of scanning transmission electron microscopy (STEM) combining with electron energy loss spectroscopy (EELS). A sharp epitaxial interface was observed to be a monolayer composed of Nb and Ti cations with a ratio of 3/1. The First-Principles Calculations indicated the interface monolayer showed different electronic structure and played the vital role in the asymmetric charge distribution of KNN thin films near the interface. We also observed the gradual relaxation process for the relatively large lattice strains near the KNN/STO interface, which remarks a good structure modulation behavior of KNN thin films via strain engineering.