STO Substrates Research Articles

Uncovering an Interfacial Band Resulting from Orbital Hybridization in Nickelate Heterostructures.

The interaction of atomic orbitals at the interface of perovskite oxide heterostructures has been investigated for its profound impact on the band structures and electronic properties, giving rise to unique electronic states and a variety of tunable functionalities. In this study, we conducted an extensive investigation of the optical and electronic properties of epitaxial NdNiO3 synthesized on a series of single-crystal substrates. Unlike nanofilms synthesized on other substrates, NdNiO3 on SrTiO3 (NNO/STO) gives rise to a unique band structure featuring an additional unoccupied band situated above the Fermi level. Our comprehensive investigation, which incorporated a wide array of experimental techniques and density functional theory calculations, revealed that the emergence of the interfacial band structure is primarily driven by orbital hybridization between the Ti 3d orbitals of the STO substrate and the O 2p orbitals of the NNO thin film. Furthermore, exciton peaks have been detected in the optical spectra of the NNO/STO film, attributable to the pronounced electron-electron (e-e) and electron-hole (e-h) interactions propagating from the STO substrate into the NNO film. These findings underscore the substantial influence of interfacial orbital hybridization on the electronic structure of oxide thin films, thereby offering key insights into tuning their interfacial properties.

Enhanced ferroelectric and piezoelectric properties of SrBi2Ta2O9 thin films through crystalline orientation

This study investigates how variations in substrate temperatures and deposition rates during pulsed laser deposition on Nb-doped SrTiO3 substrates affect the crystallinity, and consequently, the ferroelectric and piezoelectric properties of SrBi2Ta2O9 (SBT) thin films. The SBT thin film developed at 850 °C with a deposition rate of 0.34 nm/pulse showed lower remanent polarization and piezoelectric coefficients, a result of its c-oriented crystallinity. In contrast, SBT thin films produced at 750 °C with deposition rates of 0.37 nm/pulse demonstrated enhanced ferroelectric and piezoelectric responses due to mixed a-oriented crystallinity. In particular, the SBT thin film, fabricated at 750 °C with a lower deposition rate of 0.04 nm/pulse, demonstrated the most enhanced ferroelectric and piezoelectric properties due to its highly refined a-oriented crystallinity. Our findings illuminate the relationship between the characteristics of the crystals and functional properties in SBT thin films, highlighting the integration of a-domains within c-domains as a key strategy for optimizing SBT film performance and opening avenues for designing specialized Bi-layered perovskite thin films.

Dual-component anomalous Hall effect in a helical spin-spiral metamagnet

We report a dual-component anomalous Hall effect (AHE) in polycrystalline Fe3Ga4 thin films grown on STO (001) and Al2O3 substrates. Systematic magnetic and magnetotransport measurements reveal an AHE consisting of positive and negative contributions that coexist across a wide range of temperatures and magnetic phases. We find that both magnitudes are nearly equal in the low-temperature ferromagnetic (FM) phase, but that their relative ratio is reduced upon heating through the antiferromagnetic helical spin-spiral state where they compete with metamagnetism and topological Hall effects, maintaining finite values at least up to the high-temperature FM phase.

Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.

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.

Resistive switching in tetragonal tungsten bronze Sr0.6Ba0.4Nb2O6 thin films and control of Schottky barrier by insertion of BiFeO3 layer

Tetragonal tungsten bronze Sr0.6Ba0.4Nb2O6 (SBNO) has exceptional electro-optical, piezoelectric, and thermoelectric properties. Its potential as an electronic device is, furthermore, also relay on the good insulation and multiple-channels structural. However, the transport behavior in SBNO films remains poorly researched. Here, a Pt/SBNO/LSMO capacitor was prepared using the magnetron sputtering method on STO (001) substrates, and its transport behavior was measured. Interestingly, an ultra-high resistive-switching ratio over 103 was obtained in SBNO film. The read voltage when the switch ratio is ∼103 drops from 2 V to 0.5 V by inserting a BFO-layer between the interface of SBNO and LSMO. The findings demonstrate that the interface barrier in SBNO/BFO/LSMO devices between the SBNO and BFO layers is essential for maintaining a high switching ratio at low voltage. As a result, the SBNO/BFO/LSMO demonstrates a method for reducing read voltage and offers an approach for future resistive-switching applications.

High-mobility spin-polarized two-dimensional electron gas at the interface of EuTiO3/SrTiO3 heterostructures

Spin polarization of two-dimensional electron gas (2DEG) at the interface of EuTiO3/SrTiO3(STO) heterostructures has been theoretical predicted and experimentally observed via x-ray magnetic circular dichroism and polarized x-ray absorption spectroscopy, which, however, is lack of magnetotransport evidence. Here, we report the fabrication of high-quality EuTiO3/STO heterostructures by depositing antiferromagnetic insulating EuTiO3 thin films onto STO substrates. Shubnikov-de Haas oscillation, Hall, and magnetoresistance (MR) measurements show that the interface is not only highly conducting, with electron mobility up to cm2V−1s−1 at 1.8 K, but also shows low-field hysteretic MR effects. MR of ∼9% is observed at 1.8 K and 20 Oe, which is one order of magnitude higher than those observed in other spin-polarized 2DEG oxide systems. Moreover, the heterostructures show ferromagnetic hysteresis loops. These results demonstrate that the high-mobility 2DEG is spin polarized, whose origin is attributed to the interfacial Ti3+-3d states due to oxygen deficiency and the exchange interactions between interfacial Eu spins and itinerant Ti-3d electrons.

Controlled BZO Nanorod Growth and Improved Flux Pinning in YBCO Films Grown on Vicinal STO Substrates

Controlled BZO Nanorod Growth and Improved Flux Pinning in YBCO Films Grown on Vicinal STO Substrates

Development of YBCO Patterned Multi-Filamentary Film Using Photolithography Method

High-temperature superconducting (HTS)-coated conductors (CCs) can be applied to magnets under low temperature and high magnetic field conditions, as well as in electric power equipment. Reducing the screening current and AC loss is an important topic for practical applications. In this study, we propose a multifilamentary YBCO thin film, fabricated by depositing YBCO on a substrate with elevated Zr stripes patterned by using photolithography techniques. While the thin film exhibited superconductivity, the crystal grain orientation was disturbed above the stripe, which locally inhibited the superconducting current flow. The results of this study indicate that a continuous YBCO film can be dividing into separate superconducting filaments by locally suppressing the good grain alignment required for a high intergrain <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> with Zr stripes between the STO substrate and the YBCO film.

Heteroepitaxial growth of anatase (0 0 1) films on SrTiO3 (0 0 1) by PLD and MBE

The epitaxial growth of anatase (001) films deposited by pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) on SrTiO3 (001) (STO) single crystals has been studied using X-ray diffraction and surface sensitivity UHV techniques. The evolution of the strain represented by the microstrain and the change of the in-plane and out-of-plane lattice parameters with film growth temperature, the effect of the annealing temperature and the influence of the oxygen content of the film have been investigated.The out-of-plane lattice strain shows a compressive (−0.2 %) or expansive (+0.3 %) behavior, in the range 600–900 °C, for temperatures below or above 700 °C, respectively. The in-plane lattice parameters, as well as the cell volume of the film, remain under compression over the entire temperature range explored.PLD films grow into square islands that align with the surface lattice directions of the STO substrate. The maximum size of these islands is reached at growth temperatures close to 875–925 °C. Film annealing at temperatures of 800 °C or higher melts the islands into flat terraces. Larger terraces are reached at high annealing temperatures of 925 °C for extended periods of 12 h. This procedure allows flat surface terrace sizes of up to 650 nm to be achieved.The crystalline quality achieved in anatase films prepared by PLD or MBE growth methods is similar. The two-step anatase growth process used during the synthesis of the films with both methods: film growth and post-annealing treatment in oxygen or air at ambient pressure, using temperature and time as key parameters, allows to control the surface terrace size and stoichiometry of the films, as well as the anatase/rutile intermixing rates at sufficiently high temperatures. This growth process could allow the substitution of their equivalent single crystals. The range of applicability of these films would include their use as structural and electronic model systems, or in harsh experimental conditions due to their low production cost.

Tunable resistive nature of LaMnO3 / Nd0.7Sr0.3MnO3 interfaces: Role of swift heavy ion irradiation

In the present communication, high crystalline quality LaMnO3/Nd0.7Sr0.3MnO3/SrTiO3 (LMO/NSMO/STO) structures were fabricated using low cost chemical solution deposition (CSD) technique to perform swift heavy ion (SHI) irradiation using 100 MeV O+7 ions. X–ray diffraction (XRD) θ–2θ, XRD φ–scan and XRD ω–scan results show a single phase nature having finite and ion fluence influenced strain state, four–fold lattice symmetry with epitaxial growth and an effective influence of ion fluence on the crystalline granular structure of LMO/NSMO/STO structure. Atomic force microscopy (AFM) images suggest an effect of ion fluence on the surface morphology through the creation of defects and local annealing effect. Zero field cooled (ZFC) protocol based LMO/NSMO interface resistivity suggests that transport nature of the LMO/NSMO interface of all studied LMO/NSMO/STO structures is strongly governed by lattice strain between LMO//NSMO layers and STO substrate, strain across LMO and NSMO manganite layers, crystalline granular structure, grain size, grain boundary density and grain boundary nature of the LMO and NSMO manganite thin layers. ZFC protocol followed by field cooled cooling (FCC) and field cooled warming (FCW) protocols for the realization of LMO/NSMO interface resistivity behaviors have been understood on the bases of freezing and trapping of tiny high temperature high resistive insulating clusters within the coexisting low temperature low resistive metallic phase fraction. Observed LMO/NSMO interface resistivity behaviors have been understood using the percolation model fits to the recorded experimental LMO/NSMO interface resistivity data in addition to irradiation influences. Obtained fitting parameters have been discussed in detail based on their variations with different employed resistivity measurement protocols for all pristine and irradiated LMO/NSMO/STO structures.

The Vertically Heteroepitaxial Structure for Lead-Free Piezoelectric K0.5Na0.5NbO3 Films

The effect of epitaxial strain on the electrical properties of ferroelectric films has been widely investigated. However, this kind of strain is generally attributed to the substrate clamping constraints and is easily relaxed when the thickness of films is over 100 nm. In this work, a vertically epitaxial strain was introduced into lead-free piezoelectric K0.5Na0.5NbO3 films to improve the electrical properties of ferroelectric films. Two-phase, vertically epitaxial composite KNN-ZnO thin films were grown on the (001) STO substrate using a pulsed laser deposition (PLD) method. The highly (001) preferentially oriented KNN phase and (112¯ 0)-oriented ZnO phase were orderly arranged. Two types of morphologies of “square-like” and “stripe-looking” grains were observed in the surface image. An asymmetric “square” out-of-plane phase hysteresis loop and a “butterfly” displacement loop were exhibited in the KNN phase, whereas the ZnO phase showed a closed phase hysteresis loop and a slim displacement-voltage loop. Two different kinds of polarization behaviors for domains were also observed under applied electric fields, in which the domain of the KNN phase is easily switched to the opposite state, whereas the ZnO phase keeps a stable domain state when applying a DC bias of ±50 V. the vertically epitaxial growth of the KNN-ZnO composited films in this work provides a new way to fabricate complex nanoscale materials.

Magnetic anisotropy of high-entropy oxides with negative Poisson's ratio

High-entropy oxide (HEO) materials have recently been extensively studied and attracted unprecedented attention. The HEO can be regulated by introducing different atomic species, resulting in excellent physical properties. Herein, we fabricated the spinel HEO (Cr,Mn,Fe,Co,Ni)3O4 (CMFCN) film with various of thicknesses on perovskite STO substrate by PLD technology. Interestingly, all the CMFCN films exhibit negative Poisson's ratio behavior and demonstrate ferrimagnetic behavior at room temperature. The strain-induced Poisson's ratio significantly affects the magnetism of the HEO film. The magnetic anisotropy decreases as the Poisson's ratio decreases, while coercivity increases from ∼270 to ∼1700 Oe. The axis of easy magnetization changes from in-plane to out-of-plane. These results indicate that the thickness-modulated magnetic of HEO epitaxial films has potential applications in integrating magnetic insulating oxides with spintronics.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Temperature dependence of the superconducting gap of single-layer FeSe/SrTiO3 : Direct comparison between transport and spectroscopic measurements

We studied the superconducting properties of the one-unit cell FeSe grown on high-doped ${\mathrm{SrTiO}}_{3}$(001) (STO) substrate by in situ transport and angle-resolved photoemission spectroscopy (ARPES) measurements. By comparing with previous data taken for the low-doped STO substrate [Phys. Rev. Lett. 124, 227002 (2020)], it was revealed that the onset of superconductivity occurs at $\ensuremath{\sim}40$ K irrespective of the STO doping level. Furthermore, from the temperature dependence of the critical current, we were able to deduce the superconducting gap that is relevant in the transport phenomena, which gradually increases by cooling the sample down from 40 K. In contrast, the coherent peak emerges at 60 K that likely corresponds to the Cooper pair formation, while the leading edge shift near the Fermi level is clearly observed at 40 K in the ARPES spectra. We speculate that the STO surface presumably acts as an effective ``disorder'' in this system and the Cooper pairs are preformed well above the onset of the resistance drop.

Enhancement of superconductivity in multilayer FeSe film by Nb coating

Recently, the superconductivity of FeSe has been greatly enhanced in one-unit-cell films and ultrathin surface doped films grown on STO substrates, which lead to extensive research on the interfacial effect and charge carriers doping in FeSe films. Herein, by using ion beam sputtering deposition, transition metal (Nb) is successfully coated on the multilayer FeSe films and the superconductivity of FeSe is significantly enhanced compared to pristine FeSe. The superconducting transition temperature (Tc) varies with the thickness of Nb coatings and the highest Tc up to 13.0 K is obtained in the FeSe film with Nb coating about 10 nm. X-ray diffraction analysis (XRD) demonstrates Nb incorporates in FeSe lattice. In particular, the existence of Nb5+ in FeSe films is confirmed by the X-ray photoelectron spectroscopy (XPS). The salient change of Nb binding energy measured by XPS demonstrate the binding and hole-carriers doping mechanisms between Nb5+ and FeSe layers. In particular, the positive Hall coefficients demonstrate the hole carriers dominant in Nb-doped FeSe films. This work demonstrates the doping characteristics of Fe site in FeSe, providing an avenue for exploring diverse superconductivity of FeSe by transition metal doping.

Tailoring interface epitaxy and magnetism in La1−xSrxMnO3/SrTiO3 heterostructures via temperature-driven defect engineering

Defect engineering of La1−xSrxMnO3 (LSMO)—a strongly correlated oxide displaying half metallicity and ferromagnetism above room temperature—has been the focus of a long-standing quest aimed at the exploitation of this material as a functional building block for memory storage and spintronic applications. Here, we discuss the correlation between structural defects and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures as a function of growth temperature and post-deposition annealing. Upon increasing the growth temperature from 500 to 700 °C at a fixed oxygen partial pressure of 0.007 mbar, the sputter-deposited epitaxial LSMO films experience a progressive increase in Curie temperature Tc from 110 to 270 K and saturation magnetization Ms from 1.4 to 3.3 μB/u.c. owing to a reduction in oxygen deficiencies. Concurrently, however, growth temperatures above 600 °C trigger the formation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface as a possible aftermath of temperature-driven diffusion of impurities from the STO substrate. Notably, although the interfacial spurious islands cause an increase in sample surface roughness, the heterostructure still preserves high-quality epitaxy. In general, the best compromise in terms of both structural and magnetic properties, comprising high-quality epitaxy, atomically flat surface, and robust ferromagnetism above room temperature, is obtained for LSMO films grown at a relatively low temperature of about 500–540 °C followed by a post-deposition annealing treatment at 900 °C for 1 h in air. Our study compares effective routes based on temperature-controlled defect engineering to finely tailor the complex interplay between microstructure and magnetism in LSMO thin films.

Fractional-unit-cell-doped spinel/perovskite oxide interfaces with switchable carrier conduction

The two-dimensional hole gas (2DHG) at the polar LaAlO3/SrTiO3 interface remains elusive. Different from isostructural perovskite-type interfaces, the spinel/perovskite heterointerface of γ-Al2O3/SrTiO3 (GAO/STO) enables us to control interfacial states with sub-unit-cell precision. Herein, we present the epitaxial growth of fractionally doped GAO/STO heterointerfaces, where GAO is precisely doped on the scale of 1/4-unit-cell (0.2 nm) by ferromagnetic Fe3O4 and nonmagnetic ZnO atomic layers. Notably, the conduction of the engineered interfaces depends critically on the position of the dopant, where a coexistence of electron and hole conduction is measured at even sublayer-doped GAO/STO interfaces. First-principles density functional theory calculations indicate that electron conductivity is from the interfacial TiO2 layers of the STO substrate, while the hole conductivity is from the Zn-doped GAO film. The presence of hole conduction can be explained from the alternating structural feature of a doped layer without oxygen vacancies. This work sheds additional insight on the emergence of 2DHG at oxide interfaces and provides opportunities for atomically engineered oxide interfaces with non-isostructural layers.

Defect-induced magnetism in homoepitaxial SrTiO3

Along with recent advancements in thin-film technologies, the engineering of complex transition metal oxide heterostructures offers the possibility of creating novel and tunable multifunctionalities. A representative complex oxide is the perovskite strontium titanate (STO), whose bulk form is nominally a centrosymmetric paraelectric band insulator. By tuning the electron doping, chemical stoichiometry, strain, and charge defects of STO, it is possible to control the electrical, magnetic, and thermal properties of such structures. Here, we demonstrate tunable magnetism in atomically engineered STO thin films grown on STO (001) substrates by controlling the atomic charge defects of titanium (VTi) and oxygen (VO) vacancies. Our results show that the magnetism can be tuned by altering the growth conditions. We provide deep insights into its association to the following defect types: (i) VTi, resulting in a charge rearrangement and local spin polarization, (ii) VO, leading to weak magnetization, and (iii) VTi–VO pairs, which lead to the appearance of a sizable magnetic signal. Our results suggest that controlling charged defects is critical for inducing a net magnetization in STO films. This work provides a crucial step for designing magnetic STO films via defect engineering for magnetic and spin-based electronic applications.

High-mobility two-dimensional electron gas in γ−Al2O3/SrTiO3 heterostructures

The origin of the two-dimensional electron gas (2DEG) in the interface between $\gamma$-Al$_2$O$_3$ (GAO) and SrTiO$_3$ (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principle calculations based on the density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure has the interface composed of Al and TiO$_2$ layers. For the heterostructure without oxygen vacancy on the GAO side, the 2DEG originates from the polar discontinuity near the interface, and there is a critical thickness for the GAO film, below which the 2DEG would not present and the heterostructure exhibits insulator characteristics. For the case that only the GAO film contains oxygen vacancies, the polar discontinuity near the interface disappears, but the 2DEG still exists. In this situation, the critical thickness of the GAO film for 2DEG formation does not exist either. When the GAO film and STO substrate both contain oxygen vacancies, it is found that the 2DEG retains as long as the oxygen vacancies on the STO side are not very close to the interface. The low-temperature mobilities of the 2DEGs in these GAO/STO heterostructures are considered to be governed by the ionized impurity scattering, and $\sim$3 to $\sim$11 times as large as that in LaAlO$_3$/SrTiO$_3$ heterojunction. The high mobility of the 2DEG is mainly due to the small electron effective mass in GAO/STO heterostructure.