LaAlO3 Substrates Research Articles

Preparation of a series of new buffer materials for coated conductors

New buffer layer materials RESbO3 (RE = Y, Sm, Dy, and Ho), Gd1−xCaxBiO3, Gd1−xPbxBiO3, and Eu1−xCaxBiO3 for coated conductors were deposited on a single crystal LaAlO3 substrate through the self-developed polymer-assisted chemical solution deposition method, and YSbO3 was also deposited on CeO2/NiW. Then, the results of x-ray diffraction patterns and SEM indicate high c-axis oriented, smooth, compact, and seamless properties of fabricated new buffer layers, which satisfied the subsequent growth of the superconducting layer. This work offers an effective method to prepare new buffer layers for coated conductors and may provide a new way to search materials with minimal mismatched lattice.

Structural domains in antiferromagnetic LaFeO3 thin films

Abstract We have grown epitaxial LaFeO3 thin films on different oxide substrates by molecular beam epitaxy and investigated them using transmission electron microscopy. The films reveal structural domains that strongly depend on the substrate material, substrate orientation, and the film thickness. On (001) perovskite substrates of SrTiO3 and LaAlO3, the long c-axis of the orthorhombic unit cell was found always to lie in-plane parallel to one of the <100> axes of the perovskite forming 90° rotational twins with the rotation axis parallel to the [110] axis of LaFeO3 and to the substrate normal. In contrast, MgO and MgAl2O4 substrates lead to the in-plane and the out-of-plane orientation of the c-axis and give rise to smaller domains. The results are discussed with respect to the correlation between the crystallographic and the antiferromagnetic structure of LaFeO3.

La0.7Ca0.3MnO3-δ:Ag nanocomposite thin films with large temperature coefficient of resistance (TCR)

In this work, oxygen vacancy-regulated La0.7Ca0.3MnO3-δ:Ag (LCMO:A) nanocomposite thin films on LaAlO3 (001) substrates were investigated to obtain films with large temperature coefficient of resistance (TCR) values. LCMO:A nanocomposite thin films were synthesized using pulsed laser deposition, and oxygen pressures during film deposition and annealing steps were optimized. As oxygen pressures increased, lattice parameter increased from 70 Pa to 100 Pa, Tp increased monotonically from 168 K to 282 K, and average Mn4+ concentration in the film increased as indicated by X-ray photoemission spectroscopy data. Record high TCR value of ∼37% K−1 was achieved in LCMO:A nanocomposite thin film prepared with optimal oxygen pressures, making this film promising candidate for applications in bolometers.

HAADF STEM and Ab Initio Calculations Investigation of Anatase TiO2/LaAlO3 Heterointerface

The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO2 remains a challenging issue. In particular, in TiO2 ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO2, from extrinsic effects, such as those arising from structural defects, dislocations, and the presence of competing phases at the film/substrate interface. It is, therefore, mandatory to unambiguously ascertain the structure of the TiO2/substrate interface. In this work, by combining high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), density functional theory calculations, and multislice image simulations, we have investigated the nature of strainless anatase TiO2 thin films grown on LaAlO3 substrate. In particular, the presence of oxygen vacancies in anatase TiO2 has been proved to stabilize the formation of an extra alloy layer, Ti2AlO4, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO2 directly on LaAlO3 substrate has required the deposition of a TiOx extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO2 to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO2/LAO hetero-interfaces to engineer the 2DEG formation in anatase TiO2 for tailored applications.

Epitaxial Growth of CaMnO3–y Films on LaAlO3(112¯0) by Pulsed Direct Current Reactive Magnetron Sputtering

CaMnO3 is a perovskite with attractive magnetic and thermoelectric properties. CaMnO3 films are usually grown by pulsed laser deposition or radio frequency magnetron sputtering from ceramic targets. Herein, epitaxial growth of CaMnO3–y (002) films on a ‐oriented LaAlO3 substrate using pulsed direct current reactive magnetron sputtering is demonstrated, which is more suitable for industrial scale depositions. The CaMnO3–y shows growth with a small in‐plane tilt of <≈0.2° toward the (200) plane of CaMnO3–y and the () with respect to the LaAlO3 substrate. X‐ray photoelectron spectroscopy of the electronic core levels shows an oxygen deficiency described by CaMnO2.58 that yields a lower Seebeck coefficient and a higher electrical resistivity when compared to stoichiometric CaMnO3. The LaAlO3 substrate promotes tensile‐strained growth of single crystals. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal antiphase boundaries composed of Ca on Mn sites along <101> and <002>, forming stacking faults.

A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn3+/Mn4+ pairs, distortion of MnO6 octahedron, and electrical transport properties of La1-xCaxMnO3 and La1-xSrxMnO3 materials, room-temperature coefficient of resistivity (TCR) of La0.7Ca0.3-xSrxMnO3 (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO3 (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn3+/Mn4+ pairs, and distortion of MnO6 octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K-1 at 296.1 K in La0.7Ca0.18Sr0.12MnO3 films.

Temperature-dependent out-of-plane anisotropy in compressively strained La0.67Sr0.33MnO3 thin films

We studied the temperature and strain dependence of the perpendicular magnetic anisotropy in La0.67Sr0.33MnO3 thin films by performing temperature- and angle-dependent magnetotransport measurements. Three films of similar thickness (14 u.c., 14 u.c. and 15 u.c.) but with different out-of-plane crystallographic strain (1.9%, 0.9% and -0.7%) are studied. The films are grown on LaAlO3 and SrTiO3 substrates by pulsed laser deposition. We observe a clear increase in the out-of-plane magnetic anisotropy with increasing out-of-plane strain in the angle-dependent magnetotransport measurements which is present up to 80 K for the highest (1.9%) strained sample. The deformation of the unit cell, as discussed in earlier reports, point to the magnetocrystalline anisotropy as the main driver altering the magnetic easy axis direction. Our results highlight the utility of the effective magnetocrystalline anisotropy as a tool to control the desired anisotropy in crystalline thin films of La0.67Sr0.33MnO3.

Heteroepitaxial Growth of a Ta3N5 Thin Film with Clear Anisotropic Optical Properties.

Ta3N5 is a promising semiconductor photocatalyst which can generate H2 gas from water under visible light illumination. It is expected that Ta3N5 exhibits a strong anisotropy in its physical properties stemming from its highly anisotropic crystal structure. However, such anisotropic properties have not been verified experimentally due to the difficulty in synthesizing a large single crystal. Here, we report the synthesis of (010)-oriented Ta3N5 single-crystalline thin films by solid phase epitaxy on the (110) plane of perovskite LaAlO3 substrates. The obtained epitaxial thin films of Ta3N5 exhibited clear optical anisotropy (pleochroism) as predicted by previous first-principles calculations. The optical gap for E||[100] polarization (∼2.12 eV) was smaller than that for E||[100] polarization (∼2.27 eV).

Direct Synthesis of Metastable λ-Phase Ti3O5 Films on LaAlO3 (110) Substrates at High Temperatures

Direct Synthesis of Metastable λ-Phase Ti₃O₅ Films on LaAlO₃ (110) Substrates at High Temperatures

The intrinsic Josephson effect of Bi-2212 superconducting thin films prepared by sol-gel method

In this paper, Bi2Sr2Ca1Cu2O8+σ (Bi-2212) superconducting thin films was prepared by using a sol-gel method on a LaAlO3 (LAO) single crystal substrate, with acetates of bismuth, strontium, calcium, and copper as the starting raw materials. A step structure was specifically designed to measure the intrinsic Josephson effect. The microstructure of the Bi-2212 thin films and their epitaxial growth on the LAO substrate were analyzed by a Cs-corrected transmission electron microscope (Cs-TEM) with an atomic-level resolution and the electrical properties were measured. The results showed that the prepared Bi-2212 thin films had good epitaxial growth characteristics and good in-plane and out-plane growth texture and that its critical transition temperature, Tc, was 87 K. The interface between the substrate and the film was well-defined; the films grow epitaxially on the substrate from the CuO layer, with a highly ordered atom arrangement. The I–V characteristic curve of the prepared step micro-bridging sample showed zero-voltage overcurrent phenomenon, and the critical current was 2.3 mA. In addition, multiple current-voltage jumps were observed on the curve, indicating that the prepared biaxial texture thin films showed the intrinsic Josephson effect.

Compressive Strain-Tuned Epitaxial Nature and Physical Properties of Double-Perovskite PrBaCo2O5.5+δ Thin Films

Double-perovskite PrBaCo2O5.5+δ thin films were grown on (001) LaAlO3 substrates for investigating the processing dynamic-tuned epitaxial nature and physical properties under different oxygen conte...

Exchange bias and vertical shift of the magnetic hysteresis loop in Co/BiFeO3 bilayers

To reveal the mechanism of interfacial exchange coupling, the magnetic hysteresis (M-H) loop of Co/BiFeO3 bilayers deposited on (001) LaAlO3 (LAO) substrates by magnetron sputtering was measured. The M-H loop displays a horizontal shift along the field axis coexisting with a vertical shift along the magnetization axis. The frozen uncompensated interfacial spins result in the EB effect, as demonstrated by the similar dependencies between EB and the vertical shift in temperature and BFO thickness. In addition, the M-H loop also shows an enhancement in magnetization, which is considered to result from the induced net moments at the interface.

Large phonon drag thermopower boosted by massive electrons and phonon leaking in LaAlO3/LaNiO3/LaAlO3 heterostructure.

An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO3. The phonon-drag effect, which is not observed in bulk LaNiO3, enhances S for thin films compressively strained by LaAlO3 substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO3 surface termination, a 10 times S enhancement over the bulk value is observed due to large phonon drag S (Sg), and the Sg contribution to the total S occurs over a much wider temperature range up to 220 K. The Sg enhancement originates from the coupling of lattice vibration to the d electrons with large effective mass in the compressively strained ultrathin LaNiO3, and the electron–phonon interaction is largely enhanced by the phonon leakage from the LaAlO3 substrate and the capping layer. The transition-metal oxide heterostructures emerge as a new playground to manipulate electronic and phononic properties in the quest for high-performance thermoelectrics.

Preparation of flexible thin films from epitaxially grown anatase Nb: TiO2 using water‐soluble Sr3Al2O6 sacrificial layer

AbstractWith the aim of developing new electronic functionalities for flexible devices, a transfer process for the preparation of flexible conductive thin films was examined using epitaxially grown anatase Nb: TiO2. Using pulsed laser deposition (PLD), water‐soluble Sr3Al2O6 (SAO) was deposited on a single‐crystalline LaAlO3 (100) substrate (LAO) as a sacrificial layer, followed by the epitaxial growth of anatase Nb: TiO2 on the sacrificial layer. The Nb: TiO2/SAO/LAO multilayer sample bonded onto a flexible polymer sheet via an adhesive tape was soaked into distilled water to transfer the epitaxially grown Nb: TiO2 thin film from SAO/LAO to the polymer sheet. The transferred anatase Nb: TiO2 thin film retained the clear X‐ray diffraction peaks. Moreover, the areal transfer ratio was almost 100%, without the generation of significant impurity phases during the transfer process. On the basis of these results, the examined process was found promising for the preparation of flexible anatase Nb: TiO2 thin films. The voltage drop–current characteristics were measured successfully at room temperature for the transferred anatase Nb: TiO2 thin film using the standard four‐probe method with gold electrodes.

Magnetic order and electronic structure of [110]-oriented LaTiO3 films: A theoretical study

The epitaxial strain effects on the magnetic ground state and electronic structure of [110]-oriented LaTiO3 films have been calculated using the density functional theory. First, the lattice constants of the a-axis and c-axis are fixed to study. For the compressive strain, a magnetic phase transition from the original G-type antiferromagnet to A-type antiferromagnet is predicted when using the SrTiO3, LaGaO3, and LaAlO3 substrates, similar to the [001] case. Interestingly, a new magnetic phase, i.e., the ferromagnetic order, will appear when the larger compressive LaSrAlO4 is used. For the tensile strain, although the G-type antiferromagnetic order is robust as the ground state, the exchange couplings are significantly increased, which will enhance the Néel temperature. Furthermore, the contributions of d yz , d xz and d xy orbitals to the bands near the Fermi level show an obvious difference due to the Jahn-Teller distortions. For comparison, the case with fixed b-axis and c-axis is also tested, which shows that the compressive strained LaTiO3 remains the G-type antiferromagnetic order while the tensile strained LaTiO3 exhibits the A-type antiferromagnetic order. The underlying physical mechanisms are the lattice distortions, including the Ti-O-Ti bond angles and Ti-O bond lengths.

Effect of Oxygen Partial Pressure on Crystal Structure, Oxygen Vacancy, and Surface Morphology of Epitaxial SrTiO3 Thin Films Grown by Ion Beam Sputter Deposition

Epitaxial SrTiO3 (STO) thin films were grown on (001)-oriented LaAlO3 (LAO) substrates at 800 °C by an ion beam sputter deposition (IBSD). Oxygen partial pressure (PO2) was varied at 1.5 × 10−5, 1.5 × 10−4, and 1.5 × 10−3 Torr during the growth. The effects of PO2 on crystal structure, oxygen vacancy, and surface morphology of the STO films were investigated and are discussed to understand their correlation. It was found that PO2 played a significant role in influencing the crystal structure, oxygen vacancy, and surface morphology of the STO films. All STO films grew on the LAO substrates under a compressive strain along an in-plane direction (a- and b-axes) and a tensile strain along the growth direction (c-axis). The crystalline quality of STO films was slightly improved at higher PO2. Oxygen vacancy was favorably created in the STO lattice grown at low PO2 due to a lack of oxygen during growth and became suppressed at high PO2. The existence of oxygen vacancy could result in a lattice expansion in both out-of-plane and in-plane directions due to the presence of Ti3+ instead of Ti4+ ions. The surface roughness of the STO films gradually decreased and was nearly close to that of the bare LAO substrate at high PO2, indicating a two-dimensional (2D) growth mode. The results presented in this work provide a correlation among crystal structure, oxygen vacancy, and surface morphology of the epitaxial STO films grown by IBSD, which form a useful guideline for further study.

Making BaZrS3 Chalcogenide Perovskite Thin Films by Molecular Beam Epitaxy

AbstractThe making of BaZrS3 thin films by molecular beam epitaxy (MBE) is demonstrated. BaZrS3 forms in the orthorhombic distorted‐perovskite structure with corner‐sharing ZrS6 octahedra. The single‐step MBE process results in films smooth on the atomic scale, with near‐perfect BaZrS3 stoichiometry and an atomically sharp interface with the LaAlO3 substrate. The films grow epitaxially via two competing growth modes: buffered epitaxy, with a self‐assembled interface layer that relieves the epitaxial strain, and direct epitaxy, with rotated‐cube‐on‐cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide perovskites. This work sets the stage for developing chalcogenide perovskites as a family of semiconductor alloys with properties that can be tuned with strain and composition in high‐quality epitaxial thin films, as has been long‐established for other systems including Si‐Ge, III‐Vs, and II‐VIs. The methods demonstrated here also represent a revival of gas‐source chalcogenide MBE.

Influencing the structural, vibrational and electronic properties of pulsed laser deposited PrNi0.95Cu0.05O3 thin films by tuning epitaxial strain

The variation of epitaxial strain in thin films via thickness modification is an established way for influencing the structural and transport properties in low dimensional mott materials. To understand the effect of strain on physical properties, we have prepared a series of PrNi0.95Cu0.05O3 thin films on single crystal LaAlO3 (001) substrate, with varied thickness (5 - 25 nm) using pulsed laser deposition technique. The crystallographic structure of these films is highly oriented towards the (001) axis. The c-axis lattice constant and the unit cell volume decreases with increasing thickness, and therefore, the epitaxial strain tends to partially relax. This partial relaxation of the strain causes a substantial softening of the Raman modes. Temperature-dependent Raman spectra are analyzed to estimate two key parameters, i.e., Grüneisen parameter and first-order temperature coefficient. Power-dependent Raman Spectroscopy and the above-mentioned two parameters have been used to estimate the thermal conductivity of the 25 nm thin film. Balkanski model for three phonon processes has been used to understand the anharmonic contribution in the system, which scales with thickness. The theoretical fits to the temperature-dependent resistivity plots suggest that the films exhibit non-Fermi liquid behavior.

Analyses of Substrate-Dependent Broadband Microwave (1–40 GHz) Dielectric Properties of Pulsed Laser Deposited Ba0.5Sr0.5TiO3 Films

Ba0.5Sr0.5TiO3 (BST-0.5) thin films (600 nm) were deposited on single crystal MgO, SrTiO3 (STO), and LaAlO3 (LAO) substrates by pulsed laser deposition at an oxygen partial pressure of 80 mTorr and temperature of 720 °C. X-ray diffraction and in situ reflection high-energy electron diffraction routinely ascertained the epitaxial quality of the (100)-oriented nanocrystalline films. The broadband microwave (1–40 GHz) dielectric properties were measured using coplanar waveguide transmission line test structures. The out-of-plane relative permittivity (ε⏊/) exhibited strong substrate-dependent dielectric (relaxation) dispersions with their attendant peaks in loss tangent (tanδ), with the former dropping sharply from tens of thousands to ~1000 by 10 GHz. Although homogeneous in-plane strain (ϵǁ), enhances ε⏊/ with εMgOBST−0.5⏊/>εSTOBST−0.5⏊/>εLAOBST−0.5⏊/ at lower frequencies, two crossover points at 8.6 GHz and 18 GHz eventually change the trend to: εSTOBST−0.5⏊/>εLAOBST−0.5⏊/>εMgOBST−0.5⏊/. The dispersions are qualitatively interpreted using (a) theoretically calculated (T)−(ϵǁ) phase diagram for single crystal and single domain BST-0.5 film, (b) theoretically predicted ϵǁ-dependent, ε⏊/ anomaly that does not account for frequency dependence, and (c) literature reports on intrinsic and extrinsic microstructural effects, including defects-induced inhomogeneous strain and strain gradients. From the Vendik and Zubko model, the defect parameter metric, ξs, was estimated to be 0.51 at 40 GHz for BST-0.5 film on STO.

Tuning Flux Pinning in GdBa2Cu3O7–x/La0.7Sr0.3MnO3 Epitaxial Bilayers via Substrate-Induced Strain

The epitaxial bilayer systems consisting of GdBa2Cu3O7–x (GdBCO) and La0.7Sr0.3MnO3 (LSMO) grown on (001) SrTiO3 (STO) and LaAlO3 (LAO) substrates have been fabricated to study the correlation of magnetic flux pinning with substrate-induced strain state examined by X-ray absorption spectroscopy (XAS). Strain-dependent distortions of the MnO6 octahedron are clearly observed in both cases. In the bilayer system on the STO substrate, a weak elongation of the in-plane MnO6 octahedron and a reduction in the magnetization anisotropy of LSMO are observed by substrate-induced tensile strain. In contrast, a large elongation of the out-of-plane MnO6 octahedron and a large magnetic anisotropy are observed for the bilayer system on the LAO substrate that is under compressive strain. The flux pinning analyses of the two systems reveal that magnetic pinning effectively works with a small magnetic anisotropy. For the system under strong compressive strain, not only the magnetic pinning induced by LSMO but also additional flux pinning by structural distortion play a role in the pinning mechanism. These findings suggest a possibility of an active tuning of magnetic pinning by the ferromagnetic layer through a controlling of substrate-induced strain state.