LaAlO3 Single Crystal Substrates Research Articles

Epitaxial Growth of La0.7Ba0.3MnO3 Thin Films on SrTiO3 and LaAlO3 Substrates by Metal-Organic Deposition

La0.7Ba0.3MnO3 (LBMO) thin films were successfully prepared on SrTiO3 (STO) and LaAlO3 (LAO) single-crystal substrates by metal-organic deposition (MOD). The microstructure and the crystallinity of the LBMO films were examined in detail by high-resolution transmission electron microscopy and electron diffraction. The lattice mismatch between LBMO and STO is small as -0.1%; thus, the LBMO films were found to be epitaxially well grown. However, on the LAO substrate, which has a lattice mismatch of ∼-3.2% with LBMO, the films exhibited interfacial misfit dislocations along the interface. For an IR sensor application, we calculated the temperature coefficient of resistance (TCR) from the R(T) measurements. The films grown on STO substrates exhibited a TCR of approximately 5%/K at 306 K compared with 7%/K at 253 K exhibited by the films grown on LAO substrates.

Large Thermoelectric Response of Ti-Containing Perovskite Oxides

Nb-content dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on (100)-face of LaAlO3 single crystal substrates by a pulsed-laser deposition. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT reached 0.37 (20%-Nb-doped SrTiO3 epitaxial film) at 1000 K, which is the largest value among n-type oxide semiconductors ever reported.

Thickness dependence of in-plane dielectric and ferroelectric properties of Ba0.7Sr0.3TiO3 thin films epitaxially grown on LaAlO3

The authors have studied the effects of film thickness on the lattice strain and in-plane dielectric and ferroelectric properties of Ba0.7Sr0.3TiO3 thin films epitaxially grown on LaAlO3 (001) single crystal substrates. With increasing film thickness from 20to300nm, the in-plane lattice parameter (a) increased from 0.395to0.402nm while the out-of-plane lattice parameter (c) remained almost unchanged, which led to an increased a∕c ratio (tetragonality) changing from 0.998 to 1.012 and consequently resulted in a shift of Curie temperature from 306to360K associated with an increase of the in-plane remnant polarization and dielectric constant of the film.

Preparation and Characterization of Compositionally Graded Epitaxial Barium Strontium Titanate Thin Films via Scanning Probe Microscopy

Epitaxially graded barium strontium titanate (BaxSr1-x)TiO3 (x = 0.75, 0.8, 0.9, 1.0, abbreviated as BST75, BST80, BST90 and BTO respectively) thin films were fabricated by pulsed laser deposition method on the (La0.7Sr0.3)MnO3 (LSMO)/LaAlO3 (LAO) single crystal substrate. Scanning probe microscopy with a contact mode was used to characterize the temperature dependence of polarization from room temperature to 140°C. Results indicated that the piezo-response signal of the BST graded films had an obvious change with temperature, and that the graded structures had a flatter temperaturedependence of permittivity. Furthermore, the contrasts of the SPM images were lower for the ferroelectric – paraelectric (F-P) phase transition temperatures of BST 75, BST 80, and BST90, but higher for the F-P transition temperature of BTO.

Tl-based superconducting films prepared by aerosol spray deposition and thallinated in an open system

AbstractSuperconducting Tl-based films were prepared on a LaAlO3 single crystal substrate. Spray pyrolysis of Ba, Ca and Cu nitrate solutions was used for deposition of the precursor films. They were subsequently ex-situ thallinated in flowing oxygen (open system). While the superconducting Tl-2212 phase formed at an annealing temperature of 880°C, thallination at 900°C led to the formation of a Tl-2223 /Tl-2212 mixture. The amount of Tl-2223 increased with prolonged thallination, whereas the Tl-2212 phase progressively disappeared. Films prepared in such a manner were c-oriented and contained only low amounts of non-superconducting impurities. The resulting samples were characterized by XRD and SEM and their T C values were determined by resistive four-point measurements. They showed critical temperatures in the range of T ON = 125–135 K, T C0 = 91–93 K. Differences between the composition and properties of the obtained films and those thallinated in closed systems under 50 kPa of oxygen are discussed in this paper. Results show that the Tl-2212 → Tl-2223 transformation proceeds at a slower rate under flowing oxygen than in a closed system.

Quantum Size Effect of 2DEG Confined Within BaTiO3/SrTiO3:Nb Superlattices

AbstractVery recently, we have found that the high density 2DEG (ne ∼1021 cm−3), which is confined within a unit cell layer thickness of SrTiO3, exhibits unusually large Seebeck coefficient (S2DEG/Sbulk ∼5)[1]. In the optimum, extremely high ZT2DEG of ∼2.4 can be obtained at room temperature, while the effective ZTeff. was only ∼0.24 because 9 unit cells of electrically insulating SrTiO3 layers were used to fabricate the 2DEG structure. Thus, high ZTeff can be realized if the insulating layer thickness is reduced significantly. We selected BaTiO3∼SrTiO3:Nb superlattice to reduce insulating layer thickness because dielectric constant of BaTiO3 is one order of magnitude large (∼3,000) as compared to that of SrTiO3 (∼300). We expected that the conduction electrons can be confined much strongly in the SrTiO3:Nb layer by sandwiching between highly dielectric BaTiO3 layers. As a result, we clarified that the critical BaTiO3 layer thickness is 1.2 nm, significantly small as compared to SrTiO3 layer (4 nm). The BaTiO3/SrTiO3:Nb superlattice films were fabricated by a pulsed laser deposition (PLD) method on (001)-face of LaAlO3 single crystal substrate at 900°C. During the film growth, we monitored RHEED intensity oscillation to control layer thickness precisely. Out-of-plane high-resolution X-ray diffraction measurements and cross sectional HAADF-STEM observations revealed that the resultant films were high quality BaTiO3/SrTiO3:Nb superlattice. Hall mobility of the SrTiO3:Nb layer was 0.4 cm2·V−1·s−1, while that of superlattice decreased gradually with increasing BaTiO3 layer thickness most likely due to that intra layer diffusion of Ba2+ ion occurred between BaTiO3 and SrTiO3:Nb layers[2], which was clearly observed by the EELS mapping. Seebeck coefficient |S|300K of SrTiO3:Nb layer was 57 μV·K−1, which corresponds carrier concentration ne of 5×1021 cm−3. The |S|300K value became large with decreasing the SrTiO3:Nb layer thickness (dSrTiO3:Nb) and it reached 305 μV·K−1, which is approximately 5 times larger than that of SrTiO3:Nb bulk. The slope of log |S|- log dSrTiO3:Nb plots was 1/2, suggesting that quantum size effect occurred. Critical BaTiO3 layer thickness for the quantum confinement of the electrons was 1.2 nm (3 unit cells of BaTiO3), which is significantly small as compared to SrTiO3 (4 nm). Thus, BaTiO3/SrTiO3:Nb superlattice would be a promising candidate to realize high ZTeff.

Magneto-transport characteristics of La1.4Ca1.6Mn2O7 thin film deposited by spray pyrolysis

Polycrystalline thin films of double layer manganite La1.4Ca1.6Mn2O7 (DLCMO) have been deposited by nebulized spray pyrolysis on single crystal LaAlO3 substrates. These single phase films, having grain size in the range ∼70–100 nm, exhibit a ferromagnetic transition (FM) at TC ∼ 95 K. The short range FM ordering due to the in-plane spin coherence is shown to occur at a higher temperature around ∼125 K. The metal to insulator transition occurs at a lower temperature TP ∼ 55 K. The transport mechanism above TC is of Mott's variable range hopping type. Below TC the current–voltage characteristics show non-linear behaviour that becomes stronger with decreasing temperature. At lower temperatures below TCA ∼ 50 K, a magnetically frustrated spin glass-like state is known to exist. The DLCMO films exhibit reasonable low field magneto-resistance of ∼5% at 0.6 kOe and ∼13% at 3 kOe at ∼77 K. This has been explained on the basis of complete spin-polarized tunnelling of charge carriers through the insulating (La, Ca)O2 layers between the adjacent MnO2 bilayers.

New MOD solution for the preparation of high Jc REBCO superconducting films

High quality REBa2Cu3Ox (RE=Y,Eu,Gd) films have been successfully fabricated by metalorganic deposition method using dichloroacetic acid. Metal acetates or oxide powders were dissolved in dichloroacetic acid and blue-colored get was obtained by drying. Coating solution was prepared by diluting blue gel in methanol. Coating solution was coated on single crystal (001) LaAlO3(LAO) by dip coating method. Processing parameters such as oxygen partial pressure, water vapor, ramping rate and pyrolysis temperature etc were controlled in order to obtain a good epitaxial film. 0.5μm-thick film was obtained by single coating and with no crack after calcination heat treatment. Oxygen partial pressure was varied in the range of 100–1000ppm and conversion heat treatment was carried out at the temperature range of 725–765°C. A critical transition temperature (Tc0) of 90K and a critical transport current density (Jc) of >0.5MA/cm2 (77K and self-field) were demonstrated for the GdBCO film grown on LAO single crystal substrate with a thickness of 0.5μm. Ic was determined by utilizing a transport measurement. SEM observations showed that the growth behavior of Y123 grains is different from that of TFA–MOD process.

Effects of deposition conditions on the phase formation of YBCO films prepared by spray pyrolysis method

YBa2Cu3Oy superconducting films were deposited on LaAlO3(100) single crystal substrates by spray pyrolysis method. Two types of ultrasonic and concentric nebulizer were used in order to generate fine droplets of metal-inorganic precursor solution. c-Axis oriented films were obtained at deposition temperature of 750–850°C and at working pressures of 100Torr and 500Torr. In case of ultrasonic nebulizer, the films showed rough surface morphology due to the presence of enormous droplets, whereas smooth and dense films were obtained for concentric nebulizer. The good c-axis oriented YBCO films were formed at the wide range of the oxygen partial pressure. Oxygen which is generated via the decomposition of nitrate precursors is considered to participate in the formation reaction of YBCO film. Microstructures of YBCO films varied depending on oxygen partial pressure and rod-like grains were appeared when the oxygen partial pressure was lower than 30Torr. YBCO films were deposited epitaxially on LAO(100) substrate. Δϕ of in-plane and Δω of out-of-plane texture were measured as 3.3° and 1.0°, respectively. A transport Jc value of 0.50MA/cm2 at 77K and self-field was achieved for the YBCO film deposited on LaAlO3(100) single crystal substrate.

Investigation of room temperature electrical resistivities of LaNiO3−δ thin films deposited by rf magnetron sputtering and high oxygen-pressure processing

Highly (100)-oriented electrically conductive LaNiO3−δ (LNO) thin film with perovskite-type structure was deposited on Si(100) substrates by rf magnetron sputtering at substrate temperatures of 200, 300, 450, and 600°C with a series of 0%, 20%, 40%, and 60% oxygen partial pressures, respectively. The room temperature (RT) resistivity of LNO films decreases with decreasing substrate temperature at a fixed oxygen partial pressure and with increasing oxygen partial pressure at a fixed substrate temperature. The lowest RT resistivity of as-sputtered LNO thin films was about 5.3×10−4Ωcm. This value could be as low as ∼1.55×10−4Ωcm by postprocessing called high oxygen-pressure processing at 8MPa and is comparable to the lowest one, 1.5×10−4Ωcm, of epitaxial LNO thin film deposited on lattice-matched SrTiO3, LaAlO3, or sapphire single-crystal substrates.

Microstructure and growth mechanism of stressed complex oxide thin films in strain-modulation

A series of experiments of strain modulations in heterostructures of SrTiO3/LaAlO3 and LaAlO3/SrTiO3 perovskite thin films fabricated by laser molecular beam epitaxy (L-MBE) were performed to study the effect of the compressive stress and tensile stress on the growth and microstructure of the films. The growth process of the films was in-situ monitored by reflective high-energy electron diffraction (RHEED). The morphology of the films was studied by ex-situ atomic force microscopy (AFM). We demonstrated that the compressive stress-induced self-organized SrTiO3 films deposited on LaAlO3 (100) single crystal substrates exhibited a periodic well-ordered ripple-shaped structure, forming a unique nanopatterning tool to fabricate 1D/2D arrays of confined nanostructures (i.e., islands and wires). Small angle X-ray scattering technique was employed to investigate the superstructure. Symmetric satellite peaks were observed, which also revealed the well-aligned self-organized structures. In contrast, the similar superstructure was not observed during the growth of the tensile stress-induced LaAlO3 films on SrTiO3 substrates. Based on the experimental data, the compressive stress was estimated as the main reason of the self-organized growth. A growth model about the formation mechanisms of compressive stress-induced nanostructure was put forward and systematical kinetics elucidations about the growth processes were also discussed to illustrate the effects of different stresses on the growth and microstructures of the films.

Preparation of Polystyrene and Surfactant Co-intercalated MoO3 Nano Hybrids and their VOC Gas Sensing Properties

A new kind of polystyrene and surfactant, didodecyldimethylammonium, co-intercalated MoO3 nano hybrids was prepared by chemical vapor deposition combined with wet process of simple immersion in an organic solution. The hybrid thin films have a characteristic structure of polystyrene and surfactant co-intercalation into MoO3 with b-axis orientation on the LaAlO3 (100) single crystal substrate. The hybrid thin films show a distinct response to acetaldehyde vapor by decreasing the electrical resistance. We discuss the possible sensing mechanism for acetaldehyde gas.

Growth and investigation of La1-xRxMnO3(R- Ba, Ca, Ce) thin films

Thin La1ixRxMnO3 (R n Ba, Ca, Ce) lms (d = 40n400 nm) were grown on single crystal NdGaO3 (100), SrTiO3 (100), LaAlO3 (100), and MgO substrates using magnetron sputtering and pulsed laser deposition techniques. X-ray diffraction patterns (XRD) revealed high crystalline quality of the La2=3Ca1=3MnO3 and La2=3Ba1=3MnO3 lms grown on lattice-matched SrTiO3, LaAlO3, and NdGaO3. Meanwhile, weak ree xes in £n2£ scans demonstrating traces of CeO2 impurities have been indicated for the La2=3Ce1=3MnO3 lms. Series of post-deposition annealing experiments demonstrated a crucial role of annealing temperature and ambience on both electrical and magnetic properties of the La2=3Ce1=3MnO3 lms.

AgTa0.5Nb0.5O3 THIN FILM MICROWAVE COPLANAR WAVEGUIDE TUNABLE CAPACITORS

ABSTRACT 400 nm thick AgTa0.5Nb0.5O3 (ATN) films have been prepared by pulsed laser deposition technique on LaAlO3 (001) and sapphire (Al2O3-0112, r-cut) single crystal substrates. Comprehensive X-ray diffraction analysis showed epitaxial quality of ATN/LaAlO3 films and preferentially (001) orientation of ATN/Al2O3 films. Voltage tunable microwave capacitors were fabricated by lift-off technique on the surface of ferroelectric films. Microwave on-wafer tests were performed in the range from 1 to 40 GHz. Frequency dispersion is about 4.3%, voltage tunability is 4.7% @ 20 GHz and 200 kV/cm, loss tangent ∼ 0.068 @ 20 GHz, K-factor = tunability/tanδ is ranged from 124% @ 10 GHz to 35% @ 40 GHz.

Electric resistance and magnetoresistance of a two-layer epitaxial heterostructure (30 nm)La0.67Ca0.33MnO3/(30 nm)La0.67Ba0.33MnO3

Two-layer epitaxial heterostructures (30 nm)La0.67Ca0.33MnO3/(30 nm)La0.67Ba0.33MnO3 (LCMO/LBMO) have been grown by laser deposition on single crystal (001)LaAlO3 (LAO) substrates. In this system, the upper (LCMO) layer occurs under the action of tensile stresses in the substrate plane, whereas the lower (LBMO) layer exhibits biaxial compression. The formation of a 30-nm-thick LCMO film on the surface of the 30-nm-thick LBMO layer leads to an increase in the level of mechanical stresses in the latter layer. The maximum electric resistivity ρ of the (30 nm)LCMO/(30 nm)LBMO/LAO structure was observed at a temperature 25–30 K below that corresponding to the maximum of the ρ(T) curve for a single (30 nm)LBMO film on the same LAO substrate.

Growth-parameters-dependent magnetoresistance in pulsed-laser-deposited (La0.5Pr0.2)Ba0.3MnO3 thin films

Epitaxial thin films of (La0.5Pr0.2)Ba0.3MnO3 were deposited on LaAlO3 single-crystal substrates using pulsed laser deposition (PLD) technique with different growth parameters. Structural, surface morphological, electrical, and magnetotransport measurements on these films revealed that unoptimized growth parameters during the deposition using the third harmonic of a Q-switched Nd: YAG laser yielded structurally inhomogeneous epitaxial films having a columnar morphology, while the optimized growth parameters using an excimer laser during the PLD resulted in homogeneous epitaxial films with a smooth morphology. Interestingly, at a temperature of 5K, the films with unoptimized growth parameters showed a large high-field magnetoresistance (MR) of ∼90% while the films with optimized growth parameters showed a high-field MR of only ∼15%. It is contemplated that this exceptionally large MR in the unoptimized films might be due to the phase separation and coexistence of metallic and insulating phases.

The influence of growth conditions on the microstructure and critical currents of TFA-MODYBa2Cu3O7 films

The influence of three processing parameters, temperature, gas flow rate and water pressure, on theYBa2Cu3O7 filmgrowth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal filmprocessing conditions to achieve high critical currents have been determined. We have found thatthe growth conditions maximizing the critical current density are those where the nucleation ofa-axis oriented grains is minimized, as determined byμ-Raman spectroscopy. Under these conditions the normal state resistivity is very near tothat of single crystals because a vanishingly small film porosity is achieved. Transmissionelectron microscopy analysis of films quenched from the growth temperature gives somehints for understanding the mechanism linking the film porosity with the concentration ofa-axis grains. A cross-linked influence of different processing parameters,such as temperature and water pressure, or water pressure and gasflow, has been demonstrated. The optimal growth temperatures are790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gasflow rate and water pressure. The optimal processing ranges are a compromise between twodifferent competing phenomena influencing the quality of the films: inhomogeneous filmformation due to HF gas stagnancy at small nominal growth rates (low gas flow rate orwater pressure) and perturbed crystallinity at high gas flow rates or water pressures.

Microstructures and Enhancement of Critical Current Density in<tex>$rm YBa_2rm Cu_3rm O_7$</tex>Thin Films Grown by Pulsed Laser Deposition on Various Single Crystal Substrates Modified by Ag Nano-Dots

YBa/sub 2/Cu/sub 3/O/sub 7/ (Y123) thin films were grown by pulsed laser deposition (PLD) on YSZ (100), SrTiO3 (100), and LaAlO3 (100) single crystal substrates. Prior to the film deposition, a discontinuous layer of Ag nano-dots was deposited on the substrates. The Y123 films grown on such surfaces modified with Ag nano-dots were characterized by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), AC susceptibility and DC magnetization. The effects of the density of Ag nano-dots, which was controlled by the numbers of PLD shots, on the microstructures and resultant critical current density J/sub c/ have been studied systematically. Results showed that at fixed physical deposition conditions J/sub c/ increased monotonically with number of Ag shots, n, for films grown on both STO and LAO substrates. At 77 K, the J/sub c/ increased from 10/sup 6/ to 3.2/spl times/10/sup 6/ A/cm/sup 2/ for LAO and from 8/spl times/10/sup 5/ to 3.5/spl times/106 A/cm/sup 2/ for STO as n increased from 0 to 150. At 5 K, the enhancement of J/sub c/ was approximately four times at both low and high fields. However, for films grown on YSZ substrate, J/sub c/ increased from 2/spl times/10/sup 5/ to 2/spl times/10/sup 6/ A/cm/sup 2/ as Ag shots increased from 0 to 30, and decreased to 9/spl times/10/sup 5/ for n/spl ges/60. Detailed microstructure investigations indicated that the crystallinity and ab alignment gradually improved as the number of Ag-nano-dots increased.

Microstructure of compositionally-graded (Ba1−xSrx)TiO3 thin films epitaxially grown on La0.5Sr0.5CoO3-covered (100) LaAlO3 substrates by pulsed laser deposition

Compositionally-graded (Ba1−xSrx)TiO3 (BST) epitaxial thin films (with x decreasing from 0.25 to 0.0) were deposited by pulsed laser deposition on (100)LaAlO3 (LAO) single-crystal substrates covered with a conductive La0.5Sr0.5CoO3 (LSCO) layer as a bottom electrode. X-ray and electron diffraction patterns demonstrate that the entire graded film has a single-crystal cubic structure. The epitaxial relationship between BST, LSCO, and LAO can be described as (100)BST‖(100)LSCO‖(100)LAO; [001]BST‖[001]LSCO‖[001]LAO. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrodes have sharp interfaces and overall uniform thickness across the entire specimen, and that they grow with a columnar structure. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50nm. High-resolution TEM images reveal aligned rectangular-shaped voids in the graded BST film, with length size of 12–17nm, and width of 5–8nm along the ⟨001⟩ direction in the (100) plane.

YBCO Superconducting Film Coated on LaAlO3 Substrate by TFA-MOD Process

Epitaxial YBCO film was coated on (001) LaAlO3 single-crystal substrate by metalorganic deposition of metal trifluoroacetate precursors. From XRD and EDS analysis, it was observed that the film contains CuO or Cu2BaO2 segregation, but transport measurement found the segregation structure does not severely prevent the percolation of supercurrent-carrying material. The YBCO film’s performance is up to 2.4 MA/cm2 with Ic of 137 A/cm-width at 77 K and 0 T, and superconducting transition temperature Tc reaches around 91.3 K with sharp transition temperature ΔTc of 0.36 K.