SrTiO3 Substrates Research Articles

Growth of epitaxial Bi0.5(Na1-xKx)0.5TiO3 films by hydrothermal reaction time and their characterization

The hydrothermally grown films can be used in various applications because of their relatively lower reaction temperature than the other growth methods. Bi0.5(Na1-xKx)0.5TiO3 films on SrTiO3 substrates are synthesized using the hydrothermal reaction to study its structural and piezoelectric response. The different morphological analysis of films by field emission scanning electron microscope shows the grains on films get through the dissolution-recrystallization process with Ostwald ripening. Also, the structural analyses are carried out using X-ray diffraction and transmission electron microscope which confirmed the formation of epitaxial films with their high crystallinity, and tetragonal symmetry under compressive strain. Through X-ray photoelectron spectroscopy, it was confirmed that the constituent elements and element ratios of BNKT vary according to the reaction time, and the piezoelectric and ferroelectric behaviors were confirmed through a piezoelectric force microscope and piezoelectric coefficient. A combination of the structural and piezoelectric measurements reveals that the reaction time affects the composition of our epitaxial film.

Atomic construction and spectroscopic characterization of FeSe-derived thin films on SrTiO3 substrates

Controllably fabricating low-dimensional systems and unraveling their exotic states at the atomic scale is a pivotal step for the construction of quantum functional materials with emergent states. Here, by utilizing the elaborated molecular beam epitaxy growth, we obtain various FexSey phases beyond the single-layer FeSe/SrTiO3 films. A synthetic strategy of lowering substrate temperature with superfluous Se annealing is implemented to achieve various stoichiometric FeSe-derived phases, ranging from 1:1 to 5:8. The phase transitions and electronic structure of these FexSey phases are systematically characterized by atomic resolution scanning tunneling microscopy measurements. We observe the long-ranged antiferromagnetic order of the Fe4Se5 phase by spin-polarized signals with striped patterns, which is also verified by their magnetic response of phase shift between adjacent domains. The electronic doping effect in insulating Fe4Se5 and the kagome effect in metallic Fe5Se8 are also discussed, where the kagome lattice is a promising structure to manifest both spin frustration of d electrons in a quantum-spin-liquid phase and correlated topological states with flat-band physics. Our study provides promising opportunities for constructing artificial superstructures with tunable building blocks, which is helpful for understanding the emergent quantum states and their correlation with competing orders in the FeSe-based family.

Electrical transport behavior of the oxygen vacancies-rich LaAlO3/SrTiO3 heterogeneous interface at high temperature

Oxygen vacancy is one of the original mechanisms of the two-dimensional electron gas (2DEG) at the LaAlO3 (LAO) and SrTiO3 (STO) heterogeneous interface, and it has an important impact on the electrical properties of LAO/STO heterojunction. In this work, the LAO thin films were grown on the STO substrates by pulsed laser deposition, and the electrical transport behavior of the LAO/STO interface at high temperature and high vacuum were systematically studied. It was found that at high temperature and high vacuum, the oxygen vacancies-rich LAO/STO heterojunction would undergo a metal–insulator transition, and return to metal conductivity when the temperature is further increased. At this time, the conduction mechanism of the sample is drift mode and the thermal activation energy is 0.87 eV. While during the temperature decreasing, the conduction mechanism would transfer to hopping conduction with the thermal activation energy of 0.014 eV and the resistance would increase dramatically and present a completely insulated state. However, when the oxygen vacancies-rich sample is exposed to air, the resistance would gradually decrease and recover.

Separated Electron-Phonon and Phonon-Phonon Scatterings Across Interface in Thin Film LaCoO3 /SrTiO3.

Electron-phonon coupling (EPC) and phonon-phonon scattering (PPS) are at the core of the microscopic physics mechanisms of vast quantum materials. However, to date, there are rarely reports that these two processes can be spatially separated, although they are usually temporally detached with different characteristic lifetimes. Here, by employing ultrafast spectroscopy to investigate the photo-carrier ultrafast dynamics in a LaCoO3 thin film on a (100) SrTiO3 substrate, intriguing evidence is found that the two interactions are indeed spatially separated. The EPC mainly occurs in the thin film, whereas PPS is largely in the substrate, especially at the several atomic layers near the interface. Across-interface penetration and decay of optical phonons into acoustic phonons thus naturally occur. An EPC strength λEg =0.30 is also obtained and an acoustic phonon mode at 45.3GHz is observed. The finding lays out a cornerstone for future quantum nano device designs.

Epitaxial-orientation-controlled magnetic anisotropy in the La0.67Sr0.33MnO3/SrTiO3 heterostructures

Magnetic anisotropy has significant importance in both designing high-efficiency magnetic devices and understanding noncollinear spin textures. Here, La0.67Sr0.33MnO3 layers are epitaxially fabricated on the (001)-, (110)-, and (111)-orientated SrTiO3 substrates to manipulate magnetic anisotropy. While the [11¯0] axis is the in-plane magnetic easy axis for the (001) La0.67Sr0.33MnO3 layer, it becomes the hard axis for the (110) and (111) samples. Interestingly, the (110) samples exhibit a great enhancement in magnetic anisotropy energy, reaching at 6.1 × 105 erg/cm3 that is 1–2 orders of magnitude higher than ones in (001) and (111) films. It is also noted that such improved magnetic anisotropy of (110) samples is maintained up to 30 nm. Those observations can be interpreted in terms of the orientation-modified orbital symmetry. While the energetically favored 3dx2−y2 orbital and degenerated eg orbitals drive the two-dimensional and bulk-like ferromagnetism in (001) and (111) La0.67Sr0.33MnO3 layers, respectively, the 3d3z2−r2 orbital provides the lowest orbital symmetry and results in the highest magnetic anisotropy energy in (110) samples. Meanwhile, this model can also explain the thickness-dependent Curie temperatures with different epitaxial orientations, deepening the understanding of the complex oxide heterostructures for magnetic modulations.

Leakage current characteristics affected by crystallinity and domain wall current of epitaxial Bi5Ti3FeO15 thin films

We investigated leakage current characteristics affected by crystallinity and domain wall currents of epitaxial Bi5Ti3FeO15 (BTFO) thin films on Nb-doped SrTiO3 substrates. Highly a-oriented BTFO thin films, highly c-oriented BTFO thin films, and BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were prepared by controlling the substrate temperature and the pulsed laser deposition deposition rate. Highly c-oriented BTFO thin films exhibited the best leakage current characteristics because the Bi2O2 layers were placed perpendicular to the c-axis to reduce leakage currents. The BTFO thin films with a mixture of a-oriented and c-oriented crystallinity showed larger leakage currents compared to highly c-oriented BTFO thin films. The current domains of the BTFO thin films corresponding to the ferroelectric domain structures were observed by a conducting atomic force microscope, and it was observed that leakage currents were formed around the domain walls. In particular, the largest leakage currents are formed at the boundaries of c-oriented domains and a-oriented domains, and these domain boundaries confirmed that the BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were responsible for the largest leakage currents.

Giant Ferroelectric and Piezoelectric Properties of Lead Free Ba0.85Ca0.15Ti0.90Zr0.10O3 Thin Film in 100 nm Thickness Range

Lead‐free Ba0.85Ca0.15Ti0.90Zr0.10O3/SrRuO3 (BCZT/SRO)‐heterostructured thin film grown on (001)‐SrTiO3 substrate is prepared by pulse laser deposition. The microstructure, ferroelectric, and piezoelectric of the thin film are investigated in detail. Giant ferroelectric properties of the BCZT thin films with thickness ≈100 nm and remanent polarization Pr of 57.1 μC cm−2 are obtained, which are attributed to the (00L)‐oriented epitaxial structure and lower substrate clamping. In addition, the BCZT/SRO heterostructure exhibits excellent piezoelectric properties with the d33 values of different grain size (40–140 nm) varied from 75 to 130 pm V−1, providing a potential candidate used as piezoelectric material for magnetoelectric composite films. The BCZT/SRO‐heterostructured thin film with excellent ferroelectric and piezoelectric properties may be a good alternative to Pb(ZrxTi1−x)O3 thin films for piezoelectric device applications.

Variation of the performance of YBa[formula omitted]Cu[formula omitted]O[formula omitted] radio frequency superconducting quantum interference device with bicrystal grain boundary angle

High transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) based on bicrystal grain boundary junctions have been utilized in a wide range of applications. To explore the effect of the bicrystal grain boundary angle on the property of the high-Tc SQUID, we fabricated and characterized YBa2Cu3O7−δ (YBCO) radio frequency (rf) SQUIDs using [001]-tilt grain boundary junctions on SrTiO3 (STO) bicrystal substrates with three different misorientation angles of θ=18∘, 24°, and 30°. Unlike the device of θ=18∘ showing only the triangular waveform of the voltage–flux (V−Φ) characteristics, for device of θ=24∘ and 30°, the shape of the V−Φ curve can be tuned from approximately triangular into nearly rectangular by adjusting the working point of the device. It suggests that, as θ increases from 18° to 30°, the operation of the device may change from the hysteretic mode into the nonhysteretic mode. This is backed by electrical transport measurements of YBCO bicrystal junctions on separate STO substrates, which help to reveal that lying behind the indicated changeover of the SQUID operation mode is the exponential decrease of the junction critical current density Jc as θ increases. The white noise of the device, which decreases with increasing flux-to-voltage transfer coefficient ∂V/∂Φ, shows a large drop as θ increases to 24°. For both 24° and 30° devices, the white field noise at 77 K is about 90 fT/Hz1/2 at respective optimum working points. For device of 30° with nearly rectangular V−Φ characteristics showing the largest ∂V/∂Φ among the fabricated samples, the white noise region extends below 10 Hz and a field noise of 145 fT/Hz1/2 at 1 Hz is shown, demonstrating the potential of obtaining high-performance bicrystal YBCO rf SQUIDs with this grain boundary angle.

A convenient and robust design for diamond-based scanning probe microscopes.

Nitrogen-vacancy centers in diamond have been developed as a sensitive magnetic sensor and broadly applied on condensed matter physics. We present a design of a scanning probe microscope based on a nitrogen-vacancy center that can operate under various experimental conditions, including a broad temperature range (20-500K) and a high-vacuum condition (1 × 10-7 mbar). The design of a compact and robust scanning head and vacuum chamber system is presented, which ensures system stability while enabling the convenience of equipment operations. By showcasing the temperature control performance and presenting confocal images of a single-layer graphene and a diamond probe, along with images of a ferromagnetic strip and an epitaxial BiFeO3 film on the SrTiO3 substrate, we demonstrate the reliability of the instrument. Our study proposes a method and a corresponding design for this microscope that extends its potential applications in nanomagnetism and spintronics.

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.

Effect of stiff substrates on enhancing the fracture resistance of Barium Titanate thin films

Damage tolerance of a thin film attached to a substrate is dependent on several parameters such as film thickness, film orientation, residual stresses, nature of interfaces, microstructure and defects present. Here we study the fracture resistance and crack growth trajectory in BaTiO3 thin films attached to elasticially stiff substrates using micromechanical experiments and finite element modeling. Microcantilever fracture tests are carried out on bi-layered systems of BaTiO3 film on Pt-Si and SrTiO3 substrates to study the effect of interfaces, texture and elastic mismatch on fracture toughness. The substrates in the bi-layers are chosen to force the crack to experience a shielding effect in terms of a decrease in the crack driving force. Experiments revealed an unexpectedly large increase in fracture toughness when the crack tip is closer to a stiffer substrate, qualitatively matching the predictions from the numerical model. The bi-layered films attached to the substrate showed 125% (on Pt-Si) and 160% (on SrTiO3) increase in fracture toughness compared to the free-standing films, for the first time, revealing the significant effect of elastic modulus of the substrate on improving the fracture resistance at such micrometer length scales.

Relaxor Ferroelectric AgNbO3 Film Fabricated on (110) SrTiO3 Substrates via Pulsed Laser Deposition

AgNbO3-based materials have attracted extensive attention in energy storage due to their double hysteresis loops, but they suffer from low breakdown strength (Eb). AgNbO3 films with few defects and small thickness exhibit high Eb, which helps to improve the energy storage performance. In this work, we successfully prepared AgNbO3 thin films on (110) SrTiO3 substrate using pulsed laser deposition technology. The AgNbO3 film shows good crystalline and relaxor ferroelectric behavior. A high Eb up to 1200 kV/cm is obtained in AgNbO3 film, which contributes to good recoverable energy storage density Wrec up to 10.9 J/cm3 and energy efficiency η of 75.3%. Furthermore, the Wrec remains above 2.9 J/cm3 and the η varies between 72.5% and 82.5% in a wide temperature range of 30–150 °C. This work reveals the great potential of relaxor ferroelectric AgNbO3 film for energy storage.

Effect of heat treatment on resistive switching memory characteristics of NiO nanodots of tens of nanometers shattered by AFM tips

The characteristics of resistive random access memory (RRAM) at the nanometer scale are increasingly important for RRAM cells with further miniaturization. We report the RRAM characteristics of NiO nanodots in the order of tens of nanometers. NiO nanodots of approximately 70 nm were created on a single-crystal Nb-doped SrTiO3 (Nb:STO) substrate by dip-pen nanolithography. For comparison, a NiO thin film was also deposited on the Nb:STO substrate by spin coating. To reduce the size of NiO nanodots, they were shattered by an atomic force microscope tip, obtaining nanodots of approximately 10 nm. Using a conductive atomic force microscope, we characterized the current according to the applied electric field of the NiO nanodots on the Nb:STO substrate with a capacitor configured of Pt/NiO/Nb:STO substrate. When compared with the NiO thin film, the forming voltage of the NiO nanodots decreased. In addition, the shattered NiO nanodots exhibited unipolar switching characteristics, and the forming and set/reset voltages decreased with the reduction in nanodot size. Furthermore, the NiO nanodots exhibited a substantially reduced forming and set/reset voltages after heat treatment in vacuum. We suggest that the oxygen-depleted layer formed on the surface of NiO nanodots by heat treatment plays a crucial role in the reduction of the forming and operation voltages.

Physical properties of LaBO3 (B = Mn, Fe, Co) thin films grown on SrTiO3 by pulsed laser deposition technique

Realizing an oxide thin film with proper stoichiometry is one of the most challenging objects in materials science. Owing to the growth dynamics as well as kinetics, the physical properties of thin films often differ from their bulk counterparts. Here, we report pulsed laser-deposited thin films of LaBO3 (B = Mn, Fe, Co) grown on a SrTiO3 (001) substrate under various thermodynamic conditions. Structural, magnetic, and optical studies have been carried out. The x-ray diffraction study confirms that an appropriate choice of growth thermodynamics may help one to realize epitaxially grown films on the SrTiO3 substrate with out-of-plane lattice parameters 3.976, 3.984, and 3.825 Å for LaMnO3 (LMO), LaFeO3 (LFO), and LaCoO3 (LCO), respectively. A mixed valence state of Mn2+, 3+, 4+ for LMO, a Fe3+ state for LFO, and a mixed state of Co2+, 3+ for LCO have been confirmed by x-ray photoelectron spectroscopy, which is in good agreement with the Ellingham diagram. The optical study showed a bandgap of 1.2, 2.5, and 1.5 eV for LMO, LFO, and LCO, respectively. Ultraviolet photoelectron spectroscopy (UPS) shows a glimpse of the valence band maximum and Fermi level position. UV, UPS, and photoconductive study simultaneously results in a type II band bending, i.e., staggered type bending is observed at these interfaces. Room temperature weak ferromagnetism along with the insulating nature and a sign of photovoltaic application of these thin films fascinate to carry forward rigorous study from fundamental as well as technological points of view.

Effect of surface reconstruction of SrTiO3(001) on the FeSe thin film growth

We report on a dramatic effect of the surface reconstruction of the substrate on thin film growth. We prepared two types of surface reconstruction, namely 2 × 1 and (√13 × √13)-R33.7° reconstructions, by annealing the SrTiO3(001) substrates in ultrahigh vacuum (UHV) and an oxygen environment, respectively. Subsequent co-depositions of Fe and Se atoms onto the 2 × 1 and (√13 × √13)-R33.7° reconstructed SrTiO3 surfaces lead to the growth of thin films of distinct six-fold and four-fold symmetry, respectively, under nominally the same growth conditions. Based on in-situ angle-resolved photoemission spectroscopy (ARPES) measurements of the electronic structure, we ascribe the aforementioned two types of thin film to the reported hexagonal and tetragonal phases of FeSe, respectively. The observation of the growth of hexagonal FeSe on the tetragonal SrTiO3 surface with a 2 × 1 reconstruction is unexpected, the underlying mechanism of which remains an outstanding puzzle. Our finding adds a new dimension to the material phase control via the choice of surface reconstruction of the substrate used for thin film growth.

Controlling surface reconstruction of SrTiO3(100) with adhesive outgassing

We report on an unexpected effect of adhesives on the surface reconstruction of SrTiO3. We used two types of commercially available high-temperature adhesives, the platinum (Pt) and carbon (C) pastes, to mount two identical SrTiO3(100) substrates onto the sample holders, respectively. Two different surface reconstructions, namely 2 × 1 and (√13 × √13)-R33.7°, were obtained using Pt and C paste under nominally identical annealing conditions in ultrahigh vacuum (UHV). We show that the same two surface reconstructions can be obtained separately with annealing in UHV and in an O2 environment from identical substrates mounted without adhesives. Residual gas analysis of the adhesive outgassing suggests that the silicate binder in the C paste releases O2 at high temperatures while the Pt paste does not. Due to the limited amount of O2 that can be released from the C paste, a change of the surface reconstruction from (√13 × √13)-R33.7° to 2 × 1 was observed on the same substrate mounted with the C paste upon increasing the duration of annealing in UHV. Our study indicates that slight outgassing of certain adhesives may have a profound impact on the surface reconstruction of SrTiO3. This finding also provides an interesting new knob to tune the surface reconstruction of SrTiO3 in the same annealing environment.

The influence of interface contact condition on resistive switching of Au/Nb:SrTiO3 heterojunctions

The influence of the interface contact condition on the resistive switching (RS) effects of metal/Nb:SrTiO3 heterojunctions was investigated. Two Au/Nb:SrTiO3 samples with or without in situ substrate cleaning treatment were fabricated. Through comparative analysis of I–V hysteresis and switching performance, it was observed that the RS effect in the heterojunction formed by the in situ cleaned Nb:SrTiO3 substrate was significantly weakened. In addition, the SEM image demonstrates intimate contact between metal and the in situ cleaned substrate. In contrast, the untreated Au/Nb:SrTiO3 interface exhibits poor contact. The poor contact, as confirmed by interface AFM and CAFM images, is responsible for the resistive switching observed. Our research provides an innovative approach to elucidating the mechanism of the resistive switching effect, which will pave the way for better application of the device.

Spin Hall Magnetoresistance in Pt/BiFeO3 Bilayer

Multiferroic materials are general antiferromagnets with negligibly small net magnetization, which strongly limits their magnetoelectric applications in spintronics. Spin Hall magnetoresistance (SMR) is sensitive to the orientation of the Néel vector, which can be applied for the detection of antiferromagnetic states. Here, we apply SMR on the unique room-temperature antiferromagnetic multiferroic material BiFeO3 (BFO). The angular dependence of SMR in a bilayer of epitaxial BFO (001) and heavy metal Pt is studied. By rotating the sample under a magnetic field of 80 kOe in the film plane, the resistance shows the maximum when the field is perpendicular to the current while it shows the minimum when the field is along the current. This can be well explained by the SMR in the bilayer of heavy metal/antiferromagnet with the relative orientation between the Néel vector and current direction. In contrast, the angular dependence of the resistance of Pt directly deposited on a SrTiO3 (001) substrate shows a 90° shift with the magnetic field rotating in the film plane, which originates from the Hanle magnetoresistance of Pt. The obtained spin mixing conductance at the Pt/BFO interface clearly confirms the efficient spin transmission. Our results provide a possible solution for applications of antiferromagnetic multiferroic materials in spintronics.

Visualizing symmetry-breaking electronic orders in epitaxial Kagome magnet FeSn films

Kagome lattice hosts a plethora of quantum states arising from the interplay of topology, spin-orbit coupling, and electron correlations. Here, we report symmetry-breaking electronic orders tunable by an applied magnetic field in a model Kagome magnet FeSn consisting of alternating stacks of two-dimensional Fe3Sn Kagome and Sn2 honeycomb layers. On the Fe3Sn layer terminated FeSn thin films epitaxially grown on SrTiO3(111) substrates, we observe trimerization of the Kagome lattice using scanning tunneling microscopy/spectroscopy, breaking its six-fold rotational symmetry while preserving the translational symmetry. Such a trimerized Kagome lattice shows an energy-dependent contrast reversal in dI/dV maps, which is significantly enhanced by bound states induced by Sn vacancy defects. This trimerized Kagome lattice also exhibits stripe modulations that are energy-dependent and tunable by an applied in-plane magnetic field, indicating symmetry-breaking nematicity from the entangled magnetic and charge degrees of freedom in antiferromagnet FeSn.

Sputter‐Deposited α‐MoO3 Interlayers for van der Waals Epitaxy and Film Transfer

AbstractIntegration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. A van der Waals epitaxy technology that accomplishes the transfer of oxide or metal thin films via exfoliation or dissolution of sacrificial α‐MoO3 layers produced by sputtering is presented. The α‐MoO3 thin films, consisting of weakly bonded 2D layers, grow epitaxially on SrTiO3 (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α‐MoO3 are transferred by mechanical exfoliation or by dissolving the α‐MoO3 in water at 45 °C. Spinel‐structured CoFe2O4 thin films grown on α‐MoO3 layers are easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe2O4 films is modulated by bending.