LaAlO3 Substrates Research Articles

Strain-induced phase transition at the surface of epitaxial La0.65Sr0.35MnO3 films

The strong coupling between lattice, electron and spin systems in perovskite manganites along with symmetry breaking at interfaces could lead to inherently different electronic and structural phases at the surface of thin manganite films. To study whether the surface reconstruction can be tuned by epitaxy strain, we employed Surface-Enhanced Raman Spectroscopy (SERS) on La0.65Sr0.35MnO3 (LSMO) thin films epitaxially grown on LaAlO3 substrates. A moderate compressive strain (ε < 1 %) did not change the surface state as compared to that found in unstrained LSMO/MgO(200) films. In contrast, a strong in-plane compressive strain (2 < ε < 3.4 %) stabilizes a novel phase at the surface, characterized by a unique SERS spectrum, which reveals a phase transition at T* = 220 K accompanied by an anomalously strong hardening of the Jahn-Teller mode.

Growth of epitaxial (100)-oriented rare-earth nitrides on (100)LaAlO3

Epitaxial growth of (100)-oriented rare-earth nitrides (RENs) SmN, GdN, and DyN on (100)LaAlO3 (LAO) substrates is demonstrated using molecular beam epitaxy. RHEED and ϕ-scans confirm that the cubic RENs grow 45° rotated with respect to the pseudocubic (100)-surface of LAO, which leads to lattice mismatches between −5.8% and −8.7% for the selected RENs. Those lattice mismatches, despite being significant, are smaller than in previously reported epitaxial (100)RENs, with the exception of growth on (100)YSZ (yttria stabilized zirconia), which however causes the formation of an interface oxide layer. The SmN RHEED pattern shows intense streaks, indicating a high quality, flat surface, and the rocking curves are among the narrowest reported for (100)RENs. In contrast to growth on Si, the epitaxial RENs readily form over a wide range of substrate temperatures, without the need of special substrate treatment or intermediate layers. This robust, high quality growth paired with their clear magnetic switching behavior makes epitaxial RENs grown on LAO ideal candidates for future spintronic devices.

Current–voltage characteristics in single layer SrIrO3 films deposited on LaAlO3(100) substrate

Here, we investigate the structural and electrical properties on SrIrO3 films grown on LaAlO3 (100) substrate with varying thickness (18, 25 and 40 nm). The x-ray diffraction shows good quality epitaxial films without any chemical impurity. The out-of-plane lattice parameter increases with the film thickness. All the films show a semiconducting behavior where the resistivity increases with increasing thickness. Our analysis shows at high temperature the charge conduction mechanism follows Mott’s two-dimensional variable-range-hopping model. Detailed current–voltage (I–V) measurements show a linear Ohmic behavior at room temperature, however, a prominent deviation from linearity has been observed at low temperatures where the exponent n () increases with decreasing temperature, reaching 1.5 at low temperature. Analysis of I–V data indicates that the charge conduction has dominant contribution of Poole–Frenkel mechanism rather than Schottky behavior. This evolution of charge transport with temperature is quite intriguing which may be related to the development of low temperature magnetism in films of SrIrO3.

Interplay Between Electron Correlations, Magnetic State, and Structural Confinement in LaNiO&lt;sub&gt;3&lt;/sub&gt; Ultrathin Films

We report a theoretical study of the effects of electron correlations and structural confinement on the electronic properties and magnetic state of LaNiO3(LNO) thin films epitaxially deposited on the(001)LaAlO3(LAO) substrate. Using the DFT + U method we compute the electronic band structure, magnetic properties, and phase stability of the 1.5 unit-cell-thick NiO2-terminated LNO thin films. Our results reveal complex diversity of the electronic states caused by the effects of structural confinement, interfacial charge transfer and electronic correlations. Our calculations suggest the appearance of in-plane (110) charge disproportionation of the Ni ions in the interface NiO2layer of the antiferromagnetically ordered LNO thin films. Moreover, the electronic states of both the antiferromagnetic and ferromagnetic LNO/LAO show a large orbital polarization of the Ni ions in the surface NiO2layers. Our results suggest the crucial importance of oxygen defects to explain the metal-to-insulator phase transition experimentally observed in a few-unit-cell-thick LNO/LAO thin films.

Manipulation of BiFeO3 nanostructure by substrate terrace morphology

Topological domain structures, such as vortex and center domain have been found in BiFeO3 nanoislands fabricated by self-assemble approaches, which exhibit great application potential in nonvolatile storage. However, how to control the self-assembled nanostructure during preparation process is still an open question. Here, we successfully grew BiFeO3 films with nano-stripe, nano-maze and dense nanoisland structures on LaAlO3 (001) substrate by pulsed laser deposition. We found the formation of different nanostructures is related to the terrace morphology of the substrate. Nano-stripe structure with strip domain and high conduction tail-to-tail domain wall preferred to form on wide terrace, nano-maze structure with three-fold or four-fold center-type domain usually form on bunched terrace, while dense nanoisland structure with two/three-fold domain preferred to from on the narrow or rough terrace LaAlO3 substrate. These findings provide guidance for manipulating the ferroelectric topological domain structure during film preparation process.

X-ray absorption near-edge, terahertz and Raman spectroscopies evidence growth-orientation dependent cation order, phase transitions and spin–phonon coupling in half-metallic Ca2FeMoO6 thin films

The disorder due to anti-site cation distribution is intrinsic to the double perovskites wherein the crystal orientations of the substrate template are predicted to offer different degrees of cation order in thin film form. To demonstrate this effect, epitaxial thin films of half-metallic double perovskite Ca2FeMoO6 (CFMO) were prepared on (100) and (111) oriented LaAlO3 substrates in vacuum and nitrogen atmospheres. The findings using X-ray absorption near-edge structure, Terahertz (THz) and Raman spectroscopies, in combination with magnetization show that (111) epitaxial template effectively restricts the Fe–Mo anti-site cation disorder. A resultantly enhanced cation order in (111) films induces dramatic transformations in its properties as follows: (i) significantly enhanced ferromagnetic exchange interactions and saturation magnetization, (ii) a significant increase in the Curie temperatures, (iii) a metallic behavior down to much lower temperature (∼75 K) compared to that down to 200 K for (100) film, (iv) an enhanced spin–phonon coupling. The complex THz optical conductivity spectra evaluated in the framework of Drude and Drude–Smith phenomenological models and the temperature-dependent Raman data fitted to the Balkanski model corroborate well to indicate an enhanced cation order in (111) films. While this study establishes a dominant role of crystallographic orientation in the much-desired control of cation order in double perovskites, a demonstration of the same in room temperature half-metallic CFMO system could reinforce its technological utility both as active and passive components in emergent spintronic functionalities.

Uniaxial magnetic anisotropy of L10-FeNi films with island structures on LaAlO3(110) substrates by nitrogen insertion and topotactic extraction

Tetrataenite (L10-FeNi) has potential for application in next-generation permanent magnets. This compound is a rare-earth-free material comprising Fe and Ni, which are abundant elements. In addition, an extremely high uniaxial magnetocrystalline anisotropy (Ku) is expected for L10-FeNi because of the ordered arrangement of the elements. This study reports the magnetic properties of uniaxially oriented L10-FeNi films with island structures fabricated by a novel method involving the radio-frequency sputter deposition of FeNi alloy films onto a LaAlO3 (LAO)(110) substrate, followed by nitrogen insertion and topotactic extraction. FeNiN(110) films are epitaxially grown on the LAO(110) plane by the nitriding and additional annealing process of the FeNi alloy films at high temperatures under ammonia gas to reduce the volume fraction of crystallographic variants. Uniaxially oriented L10-FeNi alloy films with island structures are obtained after the topotactic denitriding of the FeNiN films. The uniaxial magnetic anisotropy of the L10-FeNi films is determined from the magnetization curves and via magnetic torque measurements of the films. The obtained L10-FeNi film exhibiting a relatively high superlattice order parameter of 0.74 yielded a uniaxial magnetocrystalline anisotropy energy of ∼0.63 MJ/m3 at 300 K, as well as coercivities of 319 and 358 kA/m at 300 and 10 K, respectively. This coercivity is the highest value reported for artificial L10-FeNi to date.

Percolation and the Phase Slip in Granular SmFeAsO1-xFx Thin Films

A set of granular SmFeAsO1-xFx films were synthesized on LaAlO3 substrates with different percentage of the coverage of the substrate surface, and then their electrical properties were studied. At high temperatures, as the substrate coverage is increased, the films go from an insulating to a metallic behavior; and then, at low temperatures, the films that present a resistance RN of few kOhms in the normal state show a transition to the superconducting state with a width in the R(T) curves that changes with the coverage of the substrate surface. The line shape of these curves can be explained by the Ambegaokar-Halperin (AH) model of the resistance produced by the thermal activated phase slippage at the intergranular junctions if the dimensionality for the films is presumed appropriately. For the thicker films, that can be considered as a 3D network of the Josephson junctions, a temperature dependence of the AH parameter gamma given by simA(1 − t)3/2, fits the R(T) curve satisfactorily. However, for the films that cover the surface of the substrates only partially, the AH model fits the R(T) curves only if gamma ~ A´(1 − t2)2, which is suitable for a 2D network of the Josephson junctions.

Domain boundaries and growth manner of a-plane ZnO film on symmetric (0 0 1)LaAlO3 substrate

Non-polar a-plane ZnO film grown on (001)LaAlO3 (LAO) substrate has been systematically characterized by employing transmission electron microscope. On (001)LAO substrate, a-plane ZnO film comprises two main orientation types of domains perpendicular to each other. In one orientation type of a-plane ZnO domains, stacking mismatch boundary (SMB) and inversion domain boundary (IDB) form in the coalescence of two domain islands parallel to each other. SMB is composed of m- and r-planes. IDB extends along m-plane. The in-plane growth of a-plane ZnO domains extends along the direction with ∼45°-rotation to [0001]ZnO. The boundaries of two perpendicular a-plane ZnO domains consist of 45°-boundaries and r-plane twin boundaries. The profile of a-plane ZnO domain appears as L-shape.

Optimization of the electrical transport properties of La0.7Ca0.3MnO3 films by sintering temperature adjustment

With the development of the radiation thermometers, perovskite manganites have been fallen into the focus of highly sensitive infrared bolometers due to its metal-insulator transition properties. In this study, La0.7Ca0.3MnO3 (LCMO) films were prepared on (l00) LaAlO3 substrates using sol-gel spin-coating technology by adjusting different sintering temperatures (Ts, 1000 °C ≤ Ts ≤ 1400 °C). The analysis of the microstructure, surface morphology, ion valence state, and electrical transport properties revealed that the crystalline quality enhanced and the concentration of Mn4+ increased with the increasing of sintering temperature, promoting the DE effect and improving the electrical properties of the LCMO films. At Ts = 1200 °C, the temperature coefficient of resistivity (TCR) value of the LCMO film reached the maximum value (22.95% K−1) and corresponding peak TCR temperature of 246.85 K indicating that the LCMO film has great potential in infrared bolometers.

Study on the nucleation mechanism of carbonitrides on LaAlO3 in GH4742 superalloy

In this study, focused ion beam (FIB) and transmission electron microscopy (TEM) were used to investigate the morphology, composition, crystal structure, and phase interface growth orientation of the LaAlO3-Ti(C,N)-(Ti,Nb,Mo)C complex inclusion in GH4742 alloy. Based on the theory of lattice mismatch and the thermodynamic model of inclusion formation, the formation mechanism of composite inclusions was studied. Crystallographic analysis showed that Ti(C,N) grew on the LaAlO3 substrate, and its growth was restricted by the parallel orientation relationship with the LaAlO3 substrate. The (0006¯)LaAlO3//(1¯11)Ti(C,N) was the lowest lattice mismatch plane between the two phases, with a 1D lattice mismatch of 9.68% and a 2D lattice mismatch of 10.05%. The distance difference between two parallel planes was compensated by dislocations and lattice distortions. Ti(C,N) had lower lattice mismatch with TiC and LaAlO3, making it easier to grow on the LaAlO3 nucleus. Thermodynamic calculations indicate that oxides exist in solid form within the molten alloy. As the temperature decreases, TiN precipitates in the alloy melt, while TiC precipitates during the solidification process. These results provide an explanation for the formation process of the complex three-layer structure of the complex inclusions.

Optimized electrical transport properties of La0.7Ca0.18Sr0.12MnO3 film by adjusting sintering temperature

Perovskite manganites Lax(Ca1-ySry)1-xMnO3 have conspicuous electrical transport properties used for the application of uncooled bolometers. Thus, considerable attention has paid to the researches on electrical transport properties of Lax(Ca1-ySry)1-xMnO3. Temperature coefficient of resistivity (TCR) and the corresponding peak temperature (Tk) are crucial parameters for uncooled bolometers. Optimal La0.7Ca0.18Sr0.12MnO3 (LCSMO) films with highly oriented growth were prepared on (00l) LaAlO3 substrate at different sintering temperature (Ts) using a facile sol-gel spin coating method. Based on the analysis of microstructure, ionic valence, surface morphology and electrical transport properties, LCSMO films exhibit high crystallinity and large room-temperature TCR at higher Ts. Theoretical Curie temperature and the coupling of the electron lattice weaken gradually with the increase in Ts, which is the dominant factor for the optimization of electrical transport properties. Additionally, the results suggest that the active energy and theoretical Curie temperature play a crucial role in adjusting TCR and Tk. With the increase in Ts, the former increases while the latter decreased, resulting in a high TCR value (12.10%/K) at room temperature (297.38 K). The peak TCR value of LCSMO sintered at 1673 K is about 25% higher than that of the film sintered at 1473 K. The resulting LCSMO films demonstrate great potential for application in uncooled bolometers. Further theoretical basis on the effects of Ts on electrical transport properties was provided.

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films.

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it isunambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it isinferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. Thestudy highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.

Self-passivated freestanding superconducting oxide film for flexible electronics

The integration of high-temperature superconducting YBa2Cu3O6+x (YBCO) into flexible electronic devices has the potential to revolutionize the technology industry. The effective preparation of high-quality flexible YBCO films therefore plays a key role in this development. We present a novel approach for transferring water-sensitive YBCO films onto flexible substrates without any buffer layer. Freestanding YBCO film on a polydimethylsiloxane substrate is extracted by etching the Sr3Al2O6 sacrificial layer from the LaAlO3 substrate. In addition to the obtained freestanding YBCO thin film having a Tc of 89.1 K, the freestanding YBCO thin films under inward and outward bending conditions have Tc of 89.6 and 88.9 K, respectively. A comprehensive characterization involving multiple experimental techniques, including high-resolution transmission electron microscopy, scanning electron microscopy, Raman, and x-ray absorption spectroscopy, is conducted to investigate the morphology, structural, and electronic properties of the YBCO film before and after the extraction process where it shows the preservation of the structural and superconductive properties of the freestanding YBCO virtually in its pristine state. Further investigation reveals the formation of a YBCO passivated layer serves as a protective layer that effectively preserves the inner section of the freestanding YBCO during the etching process. This work plays a key role in actualizing the fabrication of flexible oxide thin films and opens up new possibilities for a diverse range of device applications involving thin-films and low-dimensional materials.

Rapid preparation of cobalt doped GdBa2Cu3O7-δ films by non-fluorine chemical solution deposition method

The efficient preparation of superconducting layer is significant for large-scale applications of REBa2Cu3O7-δ coated conductors. In this work, a series of GdBa2Cu3-xCoxO7-δ films were prepared on LaAlO3 (00 l) substrates by non-fluorine metal organic deposition method. The influence of polymers on the pyrolysis process of the gel film was investigated, and the effect of cobalt doping on the epitaxial growth of the precursor film was analyzed. It was found that smooth and dense precursor film was obtained after decomposition at a high rate of 15 ℃/min. The rapid c-axis textured growth of the precursor film was achieved by optimizing the cobalt doping level for only ten minutes of firing. The optimal film possesses better texture, denser microstructure and larger critical current density (77 K, self-field) up to 1.3 MA/cm2.

Enhanced magnetic properties and leakage current mechanism in bismuth lanthanum ferrite thin films coupled with impedance and magnetic studies

A highly crystalline and pure phase Bi0.993La0.007FeO3 (BLFO) thin films have been grown using a pulsed laser deposition technique. The structural analysis confirmed the thin film is well crystallized with a rhombohedral crystal structure with an R3c space group. The BLFO phase and composition are also confirmed by transmission electron microscopy and EDAX analysis. The surface chemistry of thin films, analysed by X-ray photon spectroscopy, reveals Fe's presence in Fe3+ and Fe2+ states. At RT, the deposited film exhibited high dielectric permittivity (∼4 × 102) and low dielectric loss (∼10−1). The isothermal magnetization on highly oriented (001)-MgO and LaAlO3 (LAO) substrates is investigated. The response is compared to the randomly oriented polycrystalline films on low-cost substrates (Pt/TiO2/SiO2/Si and n-type Si substrate). The enhanced saturation magnetization of 57 emu/cm3 and 36 emu/cm3 at 10 K and 300 K are observed at 2.5 kOe applied field for the film grown on the LAO substrate. Additionally, the low leakage current density of the film is obtained around ∼1 × 10−7 A/cm2 at a field of 100 kV/cm. Acquiring single-phase BLFO thin films with enhanced electrical and magnetic properties can be relevant in designing multiferroic materials for multiple-state memories, magnetic field sensors, and spintronics applications.

Transparent and highly conductive La-doped strontium stannate thin film growth by PI-MOCVD method on LaAlO3 and MgO substrates of (100), (110), (111) orientations

Wide band gap (4.6 eV) lanthanum doped SrSnO3 semiconductors of high electron mobility have potential applications in transparent oxide semiconductor technologies. However, there is a lack of data on the electrical properties of La:SrSnO3 both in single crystals and in thin films with an orientation other than (001). We show that after applying a modified metalorganic chemical vapor deposition method it is possible to deposit relaxed thick (001), (110) and (111) oriented La:SrSnO3 films on LaAlO3 and MgO substrates. Thin films on LaAlO3 substrates exhibit comparatively high charge carrier mobility values: µ100=71 cm2V−1s−1, µ110=72 cm2V−1s−1 and µ111=67 cm2V−1s−1. In this article, using the results of X-ray diffraction and crystallite orientation distribution modeling, as well as scanning and transmission electron microscopy data, the link between the obtained charge carrier mobility (µ100 ≈ µ110 > µ111) and film's microstructure and morphology is discussed.

Phase purity and surface morphology of high-Jc superconducting Bi2Sr2Ca1Cu2O8+δ thin films

Bi2Sr2Ca1Cu2O8+δ (Bi-2212) thin films with thicknesses less than 50 nm (<20 unit cells) are grown by pulsed-laser deposition (PLD) onto (001) LaAlO3 (LAO) single crystal substrates. Phase-pure and smooth c-axis oriented Bi-2212 films with optimal oxygen doping, critical temperature Tc0 up to 86 K, and critical current density Jc(60 K) above 1 MA/cm2 are obtained for samples that are annealed in situ at temperatures below 700 °C. At higher temperature Bi-2212 films on LAO substrates partially decompose to non-superconducting impurity phases, while films on MgO and SrTiO3 substrates are stable. The broadening ΔTc of the metal-to-superconductor resistive phase transition in magnetic fields is much larger for thin films of Bi-2212 as compared to YBa2Cu3O7. The magnetic field-induced suppression of Tc0 is stronger for Bi-2212 films containing impurity phases as compared to the phase-pure Bi-2212 films. The degradation of LAO substrate crystals after several steps of deposition and chemical removal of the Bi-2212 layer is investigated. New, commercially prepared substrates provide Bi-2212 films with smallest surface roughness (3 nm) and strong out-of-plane texture. However, thin films of almost the same quality are obtained on re-used LAO substrates that are mechanically polished after the chemical etching.

Preparation and electrical properties of ultra-thin Bi2212 films by sol-gel method

In order to achieve flexible applications of superconducting thin film materials in devices, the surface quality of the film is critical. The thickness of the film is one of the important factors affecting its surface quality. In this paper, Bi2212 superconducting thin films of different thicknesses were prepared on LaAlO3(100) and SrTiO3(100) single crystal substrates by sol-gel method and the effects of film thickness and different heterogeneous structures on their crystallographic, morphological and superconducting properties were investigated. The results showed that the roughness gradually increased with increasing Bi2212 thickness at the initial stage of film growth and stabilized after reaching a certain thickness, while the superconducting properties were destroyed at very small sample thickness, which was consistent in both single-crystal substrate films. In the Bi2212/LaAlO3 structure, the surface quality of the sample was excellent with a roughness of 5.4 nm at a thickness of about 18 nm for Bi2212, and the growth of the precipitated phase was inhibited to a greater extent. The film also had good superconductivity. This work provided a further research basis for the preparation of high quality Bi2212 films and offers further possibilities for their practical application.

Thermal-strain driving sharp metal-to-insulate transition and island-grain growth of solution-derived NdNiO3 epitaxial thin films

An ultra-sharp metal-to-insulate transition (MIT) of 1.24 K−1 in the epitaxial perovskite NdNiO3 thin films was derived by the chemical solution deposition on the LaAlO3 substrates. The thermal strains from shrink, grain growth, and thermal expansion coefficient misfit play a key role in the film microstructure and electrical properties. The originally theoretical in-plane compressive epitaxial strain changes into a tensile one caused by the thermal driving force. It relaxes with improved grain growth via decreased oxygen vacancies with increasing annealing temperature, while the concurrently enhanced tensile strain from the thermal expansion coefficient misfit between the films and the substrate leads to the destabilization of Ni3+ and the higher MIT temperature. Nevertheless, too much higher tensile strain gives rise to island-grain growth in the films, leading to the weak and even disappeared MIT.