LSCO Films Research Articles

Dielectric and ferroelectric photovoltaic properties of epitaxial BiFeO3 film with La0.5Sr0.5CoO3 bottom electrode

In order to deeply investigate the dielectric and ferroelectric photovoltaic properties of BiFeO3 (BFO) thin films, the Pt/BFO/La[Formula: see text]Sr[Formula: see text]CoO3 (LSCO) heterostructure ferroelectric capacitor grown on (001) SrTiO3 (STO) substrate is successfully fabricated by off-axis magnetron sputtering. The X-ray diffraction (XRD) and Phi scanning results show that the STO-based BFO and LSCO films are both (001) epitaxial structure. Although the BFO ferroelectric film exhibits an obvious dielectric dispersion phenomenon, a remarkable ferroelectric photovoltaic performance is obtained with open circuit voltage ([Formula: see text]) of 0.38[Formula: see text]V and short-circuit current ([Formula: see text]) of 0.23[Formula: see text]mA/cm2, respectively. With increasing test temperatures, [Formula: see text] decreases slowly and then rapidly, while [Formula: see text] increases rapidly and then decreases. At a critical temperature of 80∘C, the BFO ferroelectric film exhibits a faster photovoltaic response, in which the values of [Formula: see text] and [Formula: see text] are about 0.19[Formula: see text]V and 0.28[Formula: see text]mA/cm2, respectively. Moreover, the energy band analysis indicates that a large work function difference ([Formula: see text][Formula: see text]eV) between LSCO and Pt leads to a strong built-in electric field in the BFO film, which is beneficial to separate the photo-generated carriers and improve ferroelectric photovoltaic effect. In all, this study not only shows that BFO is an excellent and environmentally friendly photovoltaic candidate material, but also provides a practical strategy for improving the performance of BFO ferroelectric photovoltaic devices.

Film Thickness Dependence of the Critical Temperature of Superconductivity of LSCO Films: A Bipolaron Model Approach

Film Thickness Dependence of the Critical Temperature of Superconductivity of LSCO Films: A Bipolaron Model Approach

Optical Properties of Electrochemically Gated La1−xSrxCoO3−δ as a Topotactic Phase‐Change Material

AbstractMaterials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators for x < 0.18 but lossy metals for x > 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors with x > 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n > 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at high x. At x < 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn > 0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories.

Controlling Magnetization Vector Depth Profiles of La0.7Sr0.3CoO3/La0.7Sr0.3MnO3 Exchange Spring Bilayers via Interface Reconstruction.

The La0.7Sr0.3CoO3-δ/La0.7Sr0.3MnO3-δ (LSCO/LSMO) bilayer system is an ideal perovskite oxide platform for investigating interface reconstruction and its effect on their magnetic properties. Previous studies have shown that LSCO can separate into magnetic sublayers, which possess distinct trends as the total LSCO thickness increases. In this study, we used polarized neutron reflectometry to quantify changes in the magnetic and chemical depth profiles, and it confirms the formation of ∼12 Å-thick interfacial LSCO and LSMO layers, characterized by a decreased nuclear scattering length density compared to the bulk of the layers. This decrease is attributed to the combined effects of oxygen vacancy formation and interfacial charge transfer, which lead to magnetically active Co2+ ions with ionic radii larger than the Co3+/Co4+ ions typically found in bulk LSCO or single-layer films. The interfacial magnetization values, as well as Co2+ ion and oxygen vacancy concentrations, depend strongly on the LSCO layer thickness. These results highlight the sensitive interplay of the cation valence states, oxygen vacancy concentration, and magnetization at interfaces in perovskite oxide multilayers, demonstrating the potential to tune their functional properties via careful design of their structure.

Heater-assisted intense pulsed light irradiation for lanthanum strontium cobaltite thin film electrode fabrication

This study describes the bottom-heater-assisted flash light irradiation method for fabrication of lanthanum strontium cobaltite (La0.6Sr0.4CoO3−δ, LSCO) thin films. Unlike the conventional sintering process, flash light irradiation proceeds instantly under ambient conditions. Since the flash light irradiation process involves the photothermal effect, heat energy supplied by a bottom heater can compensate for decreased irradiation energy. The substrate temperature was varied from room temperature to 200 °C and 300 °C, and the radiation energy requirement for obtaining an optimized LSCO film was decreased with increasing temperature. In addition, the instantaneous temperature difference due to highly intense energy between the top surface and the substrate can be relieved. With the innovative sintering system, an electrolyte-supported solid oxide fuel cell could be fabricated by the flash light irradiation method, and electrochemical characterization of the fuel cells was conducted.

Structure and Electrical Properties of Na0.5Bi0.5TiO3 Epitaxial Films with (110) Orientation

Pt/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 (Pt/NBT/LSCO) ferroelectric capacitors were fabricated on (110) SrTiO3 substrate. Both NBT and LSCO films were epitaxially grown on the (110) SrTiO3 substrate. It was found that the leakage current density of the Pt/NBT/LSCO capacitor is favorable to ohmic conduction behavior when the applied electric fields are lower than 60 kV/cm, and bulk-limited space charge-limited conduction takes place when the applied electric fields are higher than 60 kV/cm. The Pt/NBT/LSCO capacitor possesses good fatigue resistance and retention, as well as ferroelectric properties with Pr = 35 μC/cm2. The ferroelectric properties of the Pt/NBT/LSCO capacitor can be modulated by ultraviolet light. The effective polarization, ΔP, was reduced and the maximum polarization Pmax was increased for the Pt/NBT/LSCO capacitor when under ultraviolet light, which can be attributed to the increased leakage current density and non-reversible polarization P^ caused by the photo-generated carriers.

Correlations between oxygen octahedral distortions and magnetic and transport properties in strained La0.5Sr0.5CoO3 thin films

We quantitatively evaluated oxygen octahedral distortions in epitaxial thin films of the itinerant ferromagnet, ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3}$ (LSCO), and investigated their impact on strucure-property relationships. The compressively strained film on a $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrate has a lower electrical resistivity and higher ferromagnetic transition temperature than the tensilely strained film on a ${\mathrm{La}}_{0.30}{\mathrm{Sr}}_{0.70}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_{3}$ substrate. The magnetic anisotropy is also found to depend on the type of strain, with perpendicular magnetic anisotropy induced in the compressively strained film and in-plane magnetization seen in the tensilely strained film. Our synchrotron x-ray-diffraction measurements and quantitative analysis reveal distinct oxygen octahedral distortions accommodated in these films and show that the out-of-plane and in-plane Co--O bond lengths of the compressively strained film are comparable to the in-plane and out-of-plane bond lengths of the tensilely strained film. These results indicate that the bond-length changes in LSCO modify hybridization between Co $3d$ and O2p orbitals, leading to the strain-dependent properties. These results highlight the significant role of octahedral distortions for structure-property relationships in the strained LSCO films.

Optical conductivity of overdoped cuprate superconductors: Application to La2−xSrxCuO4

We argue that recent measurements on both the superfluid density and the optical conductivity of high-quality LSCO films can be understood almost entirely within the theory of disordered BCS d-wave superconductors. The large scattering rates deduced from experiments are shown to arise predominantly from weak scatterers, probably the Sr dopants out of the CuO$_2$ plane, and correspond to significant suppression of $T_c$ relative to a pure reference state with the same doping. Our results confirm the "conventional" viewpoint that the overdoped side of the cuprate phase diagram can be viewed as approaching the BCS weak-coupling description of the superconducting state, with significant many-body renormalization of the plasma frequency. They suggest that, while some of the decrease in $T_c$ with overdoping may be due to weakening of the pairing, disorder plays an essential role.

Switching properties of epitaxial La0.5Sr0.5CoO3/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 ferroelectric capacitor

La0.5Sr0.5CoO3/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 (LSCO/NBT/LSCO) ferroelectric capacitors have been successfully fabricated on (001) SrTiO3 substrate, in which the LSCO film is prepared by magnetron sputtering and the NBT film by pulsed laser deposition.

Stretching Epitaxial La0.6Sr0.4CoO3−δ for Fast Oxygen Reduction

The slow kinetics of the oxygen reduction reaction (ORR) is one of the key challenges in developing high performance energy devices, such as solid oxide fuel cells. Straining a film by growing on a lattice-mismatched substrate has been a conventional approach to enhance the ORR activity. However, due to the limited choice of electrolyte substrates to alter the degree of strain, a systematic study in various materials has been a challenge. Here, we explore the strain modulation of the ORR kinetics by growing epitaxial La0.6Sr0.4CoO3-{\delta} (LSCO) films on yttria-stabilized zirconia substrates with the film thickness below and above the critical thickness for strain relaxation. Two orders of magnitude higher ORR kinetics is achieved in an ultra-thin film with ~0.8% tensile strain as compared to unstrained films. Time-of-flight secondary ion mass spectrometry depth profiling confirms that the Sr surface segregation is not responsible for the enhanced ORR in strained films. We attribute this enhancement of ORR kinetics to the increase in oxygen vacancy concentration in the tensile-strained LSCO film owing to the reduced activation barrier for oxygen surface exchange kinetics. Density functional theory calculations reveal an upshift of the oxygen 2p-band center relative to the Fermi level by tensile strain, indicating the origin of the enhanced ORR kinetics.

Glass‐Like Through‐Plane Thermal Conductivity Induced by Oxygen Vacancies in Nanoscale Epitaxial La0.5Sr0.5CoO3−δ

AbstractUltrafast time‐domain thermoreflectance (TDTR) is utilized to extract the through‐plane thermal conductivity (ΛLSCO) of epitaxial La0.5Sr0.5CoO3−δ (LSCO) of varying thickness (<20 nm) on LaAlO3 and SrTiO3 substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room‐temperature ΛLSCO of LSCO on both substrates (1.7 W m−1 K−1) are nearly a factor of four lower than that of bulk single‐crystal LSCO (6.2 W m−1 K−1). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m−1 K−1 for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass‐like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measured ΛLSCO is rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution to ΛLSCO along the through‐plane direction for these ultrathin LSCO films on insulating substrates.

Tuning the magnetic properties ofLa0.67Sr0.33CoO3−δfilms by oxygen pressure

In this paper, oxygen-deficient $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{S}{\mathrm{r}}_{0.33}\mathrm{Co}{\mathrm{O}}_{3}$ (LSCO) thin films are prepared by adjusting the oxygen pressure during the deposition. As oxygen vacancies are introduced into the LSCO films, the out-of-plane lattices are elongated, as revealed by x-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Modulated bright and dark stripes observed in the HRTEM images of the oxygen-deficient LSCO films are induced by cation ordering and oxygen vacancy ordering, respectively. Magnetic measurements and first-principles calculations show that the oxygen vacancies depress the magnetism of the LSCO films greatly. We also demonstrate that doping Sr into $\mathrm{LaCo}{\mathrm{O}}_{3}$ and removing oxygen from LSCO have a similar effect on the magnetic properties, which is correlated with a derived formula.

Anomalous magnetism in strained La(1-x)Sr(x)CoO3 epitaxial films (0 ≤ x ≤ 0.5).

Spin state controlling has always been a focus of intensive studies due to its importance for novel effect exploration and information technology. Complex oxides with competitive mechanisms are suitable objects of study for this purpose due to their susceptibility to external stimuli. Perovskite cobaltate La1-xSrxCoO3 is one of such oxides. Combined effects of lattice strains and hole-doping have been studied for the LSCO films with 0 ≤ x ≤ 0.5. It is found that the lattice strain, either tensile or compressive, destabilizes the ferromagnetic (FM) state of the epitaxial films, leading to a nonmagnetic state that extensively exists in a doping window embedding deep into the range of the FM phase in bulk counterparts. Density functional theory calculations reveal a distinct spin state transition due to the combined effects of lattice distortion and hole-doping, explaining the unique magnetic behaviors of LSCO.

Structural origin of enhanced critical temperature in ultrafine multilayers of cuprate superconducting films

The interface layers of ${\mathrm{La}}_{2\ensuremath{-}x}$${\mathrm{Sr}}_{x}$${\mathrm{CuO}}_{4}$ (LSCO) thin films epitaxially grown on ${\mathrm{LaSrAlO}}_{4}$ substrates by molecular beam epitaxy were investigated using transmission electron microscopy (TEM). In single-phase LSCO film, we observed an irregular layering sequence near the interface between the film and the substrate, as well as an abundance of oxygen vacancies in ${\mathrm{CuO}}_{2}$ layers. A multilayer LSCO film with a high critical temperature (${T}_{c}$ = 44.5 K) showed perfect interfaces between the sublayers. Furthermore, by combining scanning TEM, electron energy-loss spectroscopy, and electron holography, we show that there is little or no interdiffusion between the sublayers. The interfacial defects and oxygen vacancies reduce ${T}_{c}$, while the compressive strain in high-quality multilayers enhances ${T}_{c}$.

Backward-Like Rectifying Behavior of La1.85Sr0.15CuO4/Nb:SrTiO3 Heterojunctions

La1.85Sr0.15CuO4+δ(LSCO)/Nb:SrTiO3 (NSTO) heterojunction, in which the LSCO film was excessively annealed in pure oxygen, was created by a combined technique including pulsed electron deposition, photolithography, and Ar-ion milling. Subsequent characterizations via measuring the current–voltage (I–V) relations revealed great sensitivity of the rectifying properties to the annealing, where a backward-like diode behavior was observed, which probably can be interpreted in connection with the band diagram of the heterojunction, and more insulating interface resulted from over-annealing effect on both LSCO and NSTO was presumably the primary cause. This study will provide an opportunity to understand more about the complicated physics underlying the rich properties in such heterojunctions.

Preparation and characterization of La 0.5Sr 0.5CoO 3 /Al-doped ZnO thin film heterojunctions

In this study, p–n heterojunctions with La 0.5Sr 0.5CoO 3 (LSCO) and Al-doped ZnO (AZO) thin films were fabricated by the radio frequency (r.f.) magnetron sputtering technique. The LSCO/AZO heterojunction was obtained by stacking the p-type LSCO thin film on the annealed n-type AZO thin film under different Ar: O 2 sputter gas ratio atmosphere. The thickness of LSCO and AZO thin films are about 400 nm and 500 nm, respectively. Good crystalline match between LSCO and AZO films was observed from the SEM and XRD analysis. The heterojunction diode clearly demonstrated rectifying behavior in the range of −8 to +8 V in reverse shape. The turn-on voltage of the diodes is obtained around 1.5 V and is in agreement with the value obtained from the difference in the work functions of LSCO and AZO. The band structure of the heterojunction was proposed based on the results of analysis.

Conductive Perovskite-type Metal Oxide Thin Films Prepared by Chemical Solution Deposition Technique

Metal oxide electrode have been widely developed for high-performance electric device because they possess some attractive characteristic such as thermal/chemical stabilities and change compensation for oxygen vacancies in interconnected dielectric layers, etc., which is often hardly achieved by convention metal electrodes. As almost all metal oxide electrodes were usually fabricated by some vapour deposition techniques which require large-scale equipments, power, resources and costs, film deposition via solution technique would be worthy for familiarizing the metal oxide electrodes. In this research, thin films of conductive perovskite-type oxides, (La,Sr)CoO3 [LSCO], were fabricated by chemical solution deposition technique. The precursor solution for LSCO was prepared using metal nitrate, acetates, and iso-propoxide and 2-methoxyethanol. The solution was spin-coated on substrates, followed by drying, pyrolysis and RTA-treatment for crystallization at 500-750°C, for 5 min in air. These processes were repeated to obtain desired film thickness. (100)Si and (100)SrTiO3 were used as substrate. XRD analysis indicated that both of LSCO films fabricated on (100)SrTiO3 and (100)Si substrates were crystallized at and above 600°C. The films on (100)SrTiO3 had preferential crystal orientation of (100)LSCO normal to the substrate surface, while random crystal orientation was confirmed for the films on (100)Si. Electrical resistivity of the both films fabricated at 700°C were 6.09 × 10−5 Ω cm and 1.12 × 10−4 Ω cm, respectively, which is almost same as the LSCO films fabricated by conventional vapour deposition technique.

Growth, Microstructure, and Infrared-Ultraviolet Optical Conductivity of La0.5Sr0.5CoO3 Nanocrystalline Films on Silicon Substrates by Pulsed Laser Deposition

La(0.5)Sr(0.5)CoO(3) (LSCO) nanocrystalline (nc) films have been directly grown on silicon wafers under different substrate temperatures by pulsed laser deposition. The X-ray diffraction analysis indicate that the films are polycrystalline with the pure perovskite phase at higher substrate temperatures. The columnar growth formation with the nanocrystalline structure in the films has been confirmed by microscopy experiments. Infrared-ultraviolet optical properties of the LSCO films have been investigated with the aid of spectroscopic ellipsometry (SE). Dielectric function in the photon energy range of 1.1-3.1 eV (400-1100 nm) has been extracted by reproducing the experimental data with a Lorentz oscillator model. It is found that the real part is decreased from 4.7 to -0.7 at the near-infrared region with increasing substrate temperature. The optical conductivity shows a different variation trend for the lower and higher growth temperatures, respectively. Note that the films deposited above 650 degrees C exhibit the well-defined metallic phase behavior. The discrepancies could be mainly ascribed to different crystalline structure and surface morphology. The present results may be crucial for future applications of ferromagnetic-based optoelectronic and spin-electronic devices.

Switching Properties of the Pt/PST/LSCO/STO Ferroelectric Heterostructure

We have investigated the switching properties of ferroelectric lead strontium titanate capacitors using epitaxial La 0.5 Sr 0.5 CoO 3 electrodes to evaluate their potential for non-volatile memory applications. It is proposed that the good performance of the PST capacitors can be attributed to the high degree of orientation of the PST films in the ⟨001⟩ direction induced by the epitaxial LSCO film. Fatigue tests were also carried out systematically by varying the switching pulse width with a fixed duty cycle and with a fixed switching period. A gradual increase followed by an abrupt increase of the fatigue rate was observed with the increase of the switching pulse width. Field-induced defect diffusion was used to explain the results.

Structural and electrical properties of Pb(Zr 0.53Ti 0.47)O 3 films prepared on La 0.5Sr 0.5CoO 3 coated Si substrates

PbZr 0.53Ti 0.47O 3 (PZT) thin films with thickness of 0.9 μm were prepared on La 0.5Sr 0.5CoO 3 (LSCO) coated Si substrates. Both PZT and LSCO were prepared by the sol–gel method. The concentration of LSCO sol was varied from 0.3 to 0.1 mol/L, which could modify the preferential orientation of PZT thin films and consequently affect the dielectric and ferroelectric properties. The LSCO electrode layers derived from lower sol concentration of 0.1 mol/L have much more densified structure, which facilitates the formation of (1 0 0) textured PZT films with smooth and compact columnar grains. PZT thin films prepared on the optimized LSCO films exhibit the enhanced dielectric constant and remnant polarization of 980 and 20 μC/cm 2, respectively.