LSAT Substrates Research Articles

Effect of substrate on structural and transport properties of La0.05Sr0.95SnO3 and La0.05Ba0.95SnO3 epitaxial films

Perovskite-structured stannate La0.05Sr0.95SnO3 and La0.05Ba0.95SnO3 films were deposited on SrTiO3, LaAlO3, LSAT, PMN-PT, and MgO substrates using pulsed laser deposition, respectively. The effects of substrates on the structure, transport properties, and optical properties of the films were investigated in detail. High-resolution X-ray diffraction results revealed the epitaxial growth of all films on each substrate without any impurity phase, and a high lattice quality indicated by the rocking curves. Atomic force microscopy images confirmed the low root-mean-square roughness observed in all films. Hall effect measurements showed that the maximum mobility was obtained for both La0.05Sr0.95SnO3 films and La0.05Ba0.95SnO3 films grown on SrTiO3 substrates, with values of 26.46 cm2/Vs and 43.09 cm2/Vs, respectively. The temperature dependence of resistivity at low temperatures was attributed to e−-e− interactions. Optical transmittance spectra showed high transparency of all films in both visible and near-infrared regions. Collectively, these results indicate that La-doped SrSnO3 and BaSnO3 films exhibit excellent performance across various substrates, providing a wide range of options for optoelectronic device applications.

Microstructural Investigations of VO2 Thermochromic Thin Films Grown by Pulsed Laser Deposition for Smart Windows Applications

The structural properties of VO2 thin films, grown on either LSAT or Si substrates by pulsed laser deposition (PLD), are elucidated by means of transmission electron microscopy (TEM) methods. The TEM observations confirmed the successful growth of VO2 by PLD in variable thicknesses, by optimizing the O2 partial pressure and growth temperature. The films adopt a columnar polycrystalline morphology with narrow columns, up to the film thickness height. Four VO2 polymorphs have been detected by electron diffraction and high-resolution TEM (HRTEM) analysis, with M1 being by far the most abundant phase. Post-experimental strain measurements in HRTEM images have revealed that the actual residual strain is minimized due to the columnar morphology of the VO2 grains, as well as intrinsic oxide layers in the VO2/Si epitaxy. The TEM outcomes confirmed the complementary electrical and magnetic measurements in the films, where a transition from a monoclinic M1 to a rutile VO2 R phase has been identified, influenced by the initial percentage of phases in thick VO2 films.

Ellipsometry study of the infrared-active phonon modes in strained SrMnO3 thin films

Abstract We performed infrared and time-domain terahertz spectroscopic ellipsometry measurements of thin films of the perovskite antiferromagnetic insulator SrMnO3 that were grown by pulsed laser deposition (PLD) on LaAlO3, SrLaGaO4, and LSAT substrates which yield an epitaxial strain ranging from −0.3 to 1.7%. Taking these thin films as a representative example, we discuss the strategies for analyzing the ellipsometry spectra and extracting the information about the thin film dielectric response that can be equally applied to a variety of oxide based thin films and heterostructures. In particular, for the room temperature spectra we show that the three infrared-active phonon modes of the cubic perovskite structure of SrMnO3 undergo the expected softening with increasing tensile strain. For the SrMnO3 film on SrLaGaO4, we find that the low-energy (TO1) phonon mode reveals anomalous temperature dependence in the vicinity of the Néel temperature of about 170 K that signifies a strong spin-phonon coupling. For the SrMnO3 film on LSAT, we identify some irreversible changes of the infrared ellipsometry spectra that occur as the sample is heated to elevated temperature up to 560 K. These changes of the ellipsometry spectra have been attributed to a partial oxygen loss of the SrMnO3 thin film since they can be reverted with a post annealing treatment under high oxygen pressure.

Statistical metal–insulator transition properties of electric domains in NdNiO3 nanowires

Neodymium nickelate, NdNiO3 (NNO), with a perovskite structure shows resistance change of 1–2 orders owing to insulator–metal-transition (IMT) and metal–insulator-transition (MIT) at around 200 K and its IMT/MIT properties are affected by strain effects (Ni–O–Ni angular distribution). Since the resistance changes in the NNO system are considered to be dominated by competing nanoscale electronic phases, the reduction in sample size down to the individual domain scale could realize the direct investigation of single electric domains. In this study, 100 nm wide NNO nanowire structures were produced on NdGaO3(110) and LSAT(100) substrates, and the statistical IMT/MIT properties of electric domains under different strained structures were investigated. The nanowires showed prominent step resistance changes reflecting intrinsic first-order transition properties with different transition temperature distributions. A statistical transition model unveils the quantitative relationship between the IMT properties of the NNO nano-electronic phase and the strain effect due to the Ni–O–Ni angular distribution in NNO.

Self-healing Growth of LaNiO3 on a Mixed-Terminated Perovskite Surface.

Developing atomic-scale synthesis control is a prerequisite for understanding and engineering the exotic physics inherent to transition-metal oxide heterostructures. Thus, far, however, the number of materials systems explored has been extremely limited, particularly with regard to the crystalline substrate, which is routinely SrTiO3. Here, we investigate the growth of a rare-earth nickelate─LaNiO3─on (LaAlO3)(Sr2AlTaO6) (LSAT) (001) by oxide molecular beam epitaxy (MBE). Whereas the LSAT substrates are smooth, they do not exhibit the single surface termination usually assumed necessary for control over the interface structure. Performing both nonresonant and resonant anomalous in situ synchrotron surface X-ray scattering during MBE growth, we show that reproducible heterostructures can be achieved regardless of both the mixed surface termination and the layer-by-layer deposition sequence. The rearrangement of the layers occurs dynamically during growth, resulting in the fabrication of high-quality LaNiO3/LSAT heterostructures with a sharp and consistent interfacial structure. This is due to the thermodynamics of the deposition window as well as the nature of the chemical species at interfaces─here, the flexible charge state of nickel at the oxide surface. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide synthesis.

Imaging of strain driven magnetic domains and strong spin-phonon coupling in epitaxial thin films of SrRuO3

SrRuO3 epitaxial thin films with large strain disorder have been grown using pulsed laser deposition on (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) and SrTiO3 (STO) substrates. The films demonstrate two distinct transition temperatures in magnetic measurements. We present a visual evolution of magnetic domains across the two transitions using magnetic force microscopy in these films. The study clearly shows that the magnetic anisotropy corresponding to the two transitions is different. The perpendicular magnetic anisotropy is found dominating in films which results in domain spin orientation preferably in out-of-plane direction. The Raman study shows that the magnetic order strongly influences the lattice. The analysis of the phonon spectra around the magnetic transitions reveals the existence of strong spin-phonon coupling and the calculations resulted in the spin-phonon coupling constant λ ∼ 0.65 cm−1 and 1.96 cm−1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively.

Optical properties of La1−xSrxVO3 (0 ≤ x ≤ 1) films grown on LSAT substrates using radio frequency sputtering deposition

La1−xSrxVO3 (0 ≤ x ≤ 1) films can be transparent conducting oxides such as n-type SrVO3 and p-type La2/3Sr1/3VO3 films. They also show a semiconductor-to-metal transition (SMT) near x = 0.2. The optical and electrical properties of La1−xSrxVO3 (0 ≤ x ≤ 1) films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using RF sputtering deposition were investigated. Resistivities of La1−xSrxVO3 showed SMT near x = 0.2. La1−xSrxVO3 films grown on LSAT substrates were found to have high crystallinity using θ−2θ scan and grazing incidence x-ray diffraction measurements. Dielectric functions of La1−xSrxVO3 films were measured using spectroscopic ellipsometry. The evolution of optical spectra of La1−xSrxVO3 films from LaVO3 to SrVO3 was compared to the electronic band structures as a function of Sr composition, which was either experimentally observed or theoretically calculated in the literature. With increasing Sr composition, the interband transition energies of O 2p to V 3d states near 4 eV changed linearly as a function of Sr content.

Strain modulated luminescence in (Ca,Sr)TiO3:Pr3+ perovskite thin films

We report in-plane strain effect on the photoluminescence property of perovskite phosphor thin films. Epitaxial perovskite thin films of Ca0.6Sr0.4TiO3:Pr3+ phosphor were prepared on perovskite oxide substrates with different lattice constant a, LaAlO3 (LAO, a = 3.815 Å), (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7(LSAT, 3.868 Å), and SrTiO3 (STO, 3.905 Å) by a pulsed laser deposition method. From X-ray diffraction, the lattice constants of thin films on LAO, LSAT, and STO substrates were (a, c) = (3.815 Å, 3.956 Å), (3.871 Å, 3.880 Å), and (3.905 Å, 3.865 Å), respectively. Therefore it was confirmed that compressive-strained (∼4%), nearly strain-free, and tensile-strained (∼1%) films were obtained on the LAO, LSAT and STO substrates, respectively. Under N2 laser excitation at 337 nm which is above bandgap energy of thin films, the CSTO film on STO (tensile-strained) and on LSAT (strain-free) substrate showed intense red photoluminescence peaking at 612 nm due to Pr3+:1D2→3H4 transition and no photoluminescence lines from the 3PJ revels were observed at 300 K. On the other hand, the phosphor film on LAO (compressive-strained) showed blue and red luminescence which ascribed to the transitions from the Pr3+:3P0 level below 200 K. These results showed that initial state of photoluminescence can be tuned from the 1D2 level to the 3P0 level by biaxial strain not by composition in the Pr3+ doped perovskite phosphor films. The temperature dependence of photoluminescence was also explained by simplified four-level-model with carrier-trapping-center probably related to Ti center.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Magnetic anisotropy and spin scattering in (La2/3Sr1/3)MnO3/CaRuO3 bilayers

Complex ferromagnetic oxides have been identified as possible candidate materials for sources of spin currents. Here we study bilayers of ferromagnetic (La2/3Sr1/3)MnO3 (LSMO) and metallic CaRuO3 (CRO) on LSAT substrates as a model system for spin pumping. Ferromagnetic resonance (FMR) measurements of these bilayers show evidence of spin pumping across the interface in the form of an increase in Gilbert damping with the addition of CRO. FMR indicates that the presence of CRO modifies the magnetic anisotropy of the LSMO. By increasing CRO thickness, we find a reduction of the out-of-plane anisotropy and simultaneous rotation of the easy axis within the plane, from the ⟨110⟩ to ⟨100⟩ axis. The evolution of magnetic anisotropy determined by FMR disagrees with that measured by bulk SQUID magnetometry and is accompanied by structural distortions in the LSMO layer as measured by x-ray diffraction, thus suggesting a change in magnetic anisotropy attributed to structural distortions imposed on LSMO by CRO. These results suggest that while LSMO and CRO remain promising candidates for efficient pure spin current generation and detection, respectively, epitaxial integration of perovskites will cause additional changes which must be accounted for in spintronics applications.

Growth and characterization of La0.67Sr0.33MnO3/YBa2Cu3O7−δ bilayers

Understanding the interplay of ferromagnets and superconductors requires high quality interfaces. To this end, we have fabricated heterostructures of the cuprate superconductor YBa2Cu3O7−δ (YBCO) with the metallic ferromagnet La0.67Sr0.33MnO3 (LSMO) via pulsed laser deposition on SrTiO3 (STO) (001) and (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) (001) substrates. By varying the YBCO thickness from 1.7-13.6 nm while keeping the LSMO thickness at 12 nm, we have studied the interplay between ferromagnetism and superconductivity in the in-plane transport and magnetic properties of the bilayers. X-ray reflectivity data indicate the presence of a 2-3 nm thick interfacial layer between the LSMO and YBCO layers in all heterostructures. In-plane transport measurements exhibit suppression of the superconducting transition temperature from bulk YBCO values. Magnetometry measurements indicate high LSMO saturation magnetization values for samples grown on STO substrates and even higher (bulk-like) magnetization for bilayers grown on LSAT substrates. Together these results indicate that the interdiffused layer is largely attributed to a modified YBCO layer.

The strain induced magnetic and anisotropic variations of LaCoO3 thin films

The emergence of ferromagnetism in tensile strained LaCoO3 (LCO) films is still being researched due to the lack of any uniform explanation. Herein, the magnetic anisotropy and the precise ratio of Co2+ ions in LCO films growth on different substrates are reported. The emergence of ferromagnetic state can be observed irrespective of the tensile or compressive strain on the LCO films. A small magnetization along the out-of-plane easy axis is exhibited in the LCO film under compressive strain. The maximum magnetization (1.11 μB/u.c.) is observed in the LCO film over tensile strained LSAT substrate along the in-plane easy axis, which is around four times compared to the magnetization of LCO film on compressive strain. In addition, the exact percentage of Co2+ ions in different LCO films deviated from 12.8% to 10.6% as the strain changed from tensile to compressive. This little variation has a relatively weak effect on the hugely different magnetization of LCO films. Subsequently, the magnetization of LCO film grown on STO buffer layer has a great increase compared to the film direct on LAO substrate. All these results support the proposition that the tensile strain should induce more IS (t2g5eg1) or HS (t2g4eg2) spin states of Co3+ ions, which plays the key role for enhancement of ferromagnetism.

Strain healing of spin–orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 μB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 μB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin–orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin–orbit coupling.

Perpendicular magnetic anisotropy in ferrimagnetic Mn4N films grown on (LaAlO3)0.3(Sr2TaAlO6)0.7(0 0 1) substrates by molecular beam epitaxy

Many studies have shown that Mn4N thin films possess perpendicular magnetic anisotropy (PMA) caused by in-plane tensile strain. Since cubic (LaAlO3)0.3(Sr2TaAlO6)0.7 [LSAT] (0 0 1) has a smaller lattice constant than Mn4N with a mismatch of 0.8%, a compressive strain is expected to be induced in Mn4N epitaxial films grown on an LSAT substrate. We grew Mn4N thin films on an LSAT(0 0 1) substrate by molecular beam epitaxy, and investigated the optimum growth temperature (TS) from the aspects of crystallinity, magnetic properties, and magneto-transport properties. Mn4N films were grown epitaxially at temperatures between TS = 700 and 800 °C with maintaining PMA, and the optimum TS was determined to be approximately 750 °C. In-plane and out-of-plane X-ray diffraction measurements showed that the Mn4N films were under tensile stress in contrast to our prediction. From the thickness dependence of magnetization, a dead layer of approximately 10 nm existed at the Mn4N/LSAT interface. Transmission electron microscopy observation revealed that Mn-N composites other than Mn4N existed in the early stage of MBE growth.

Strain relaxation in epitaxial γ′-Fe4N ultrathin films

To investigate substrate-induced strain effects, we grew epitaxial γ’-Fe4N ultrathin films on three different single crystal substrates, MgO, SrTiO3 (STO), and LaSrAlTa oxide. The lattice mismatch was −9.7%, −2.6%, and −1.5% between the Fe4N layer and the MgO, STO, and LSAT substrates, respectively. Therefore, the strain was varied depending on the substrates. Cross-sectional transmission electron microscopy revealed that the lattice strain of Fe4N layer on the STO and LSAT maintained from the interfaces up to ∼2 nm, note here that we achieved to induce strain for the entire Fe4N film thickness. On the other hand, rapid relaxation appeared through many misfit dislocations in the case on MgO. The strain effects in the ultrathin Fe4N films were confirmed as the different behaviors for both ferromagnetic resonance and anisotropic magnetoresistance. These results broaden the amount of modulation in Fe4N based device properties by strain when we reduce the thickness down to several nanometer, which is a dimension encountered in recent spintronic switching devices.

Effect of thickness on metal to semiconductor transition in La doped BaSnO3 films deposited on high mismatch LSAT substrates

1% La doped BaSnO3 thin films of different thicknesses, ranging from 15 to 300 nm, were obtained on single crystal Lanthanum Aluminate-Strontium Aluminate Tantalate [LSAT(001)] substrates via Pulsed Laser Deposition. The films grow epitaxially on these substrates (cube-on-cube epitaxy) and are almost relaxed with a strain of ≈0.51% for 300 nm films. All films show n-type conducting behavior with their conductivity varying from 65.36 S cm−1 to 465.11 S cm−1 as the thickness of the film is increased. Low temperature carrier concentration measurements indicate that the films are degenerate semiconductors. Films with a thickness ≥30 nm exhibit metal to semiconductor transition (MST) at low temperatures. Temperature dependent resistivity analysis of the films shows evidence of electron-electron interaction rather than weak localization as the governing transport mechanism below MST. The transition temperature shifts toward lower values at higher thicknesses, strengthening the metallic transport in such films.

Josephson Junctions and SQUIDs Created by Focused Helium-Ion-Beam Irradiation ofYBa2Cu3O7

By scanning with a $30\, \mathrm{keV}$ focused He ion beam (He-FIB) across YBa$_2$Cu$_3$O$_7$ (YBCO) thin film microbridges, we create Josephson barriers with critical current density $j_\mathrm{c}$ adjustable by irradiation dose $D$. The dependence $j_\mathrm{c} (D)$ yields an exponential decay. At $4.2\, \mathrm{K}$, a transition from flux-flow to Josephson behavior occurs when $j_\mathrm{c}$ decreases below $\approx 2\, \mathrm{MA/cm^2}$. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model, without excess current for characteristic voltages $V_\mathrm{c} \lesssim 1\, \mathrm{mV}$. Devices on MgO and LSAT substrates show non-hysteretic IVCs, while devices on SrTiO$_3$ show a small hysteresis. For all junctions an approximate scaling $V_\mathrm{c} \propto j_\mathrm{c}^{1/2}$ is found. He-FIB irradiation with high dose produces barriers with $j_\mathrm{c}=0$ and high resistances of $10\, \mathrm{k\Omega} \ldots 1\, \mathrm{G\Omega}$. This provides the possibility to write highly resistive walls or areas into YBCO using a He-FIB. Transmission electron microscopy reveals an amorphous phase within the walls, whereas for lower doses the YBCO stays crystalline. We have also ``drawn'' superconducting quantum interference devices (SQUIDs) by using a He-FIB for definition of the SQUID hole and the junctions. The SQUIDs show high performance, with flux noise $< 500\, \mathrm{n \Phi_0/Hz^{1/2}}$ in the thermal white noise limit for a device with $19\, \mathrm{pH}$ inductance.

Overlapping growth windows to build complex oxide superlattices

Perovskite oxide superlattices are of particular interest due to novel phenomena emerging at interfaces which are beyond the bulk properties of the constituent layers. However, building perovskite superlattices comprised of stoichiometric layers with sharp interfaces has proven challenging. Here, the synthesis of a series of high quality (SrTiO3)n/(CaTiO3)n superlattice structures grown on LSAT substrates is demonstrated by employing hybrid molecular beam epitaxy, where Ti was supplied using metal-organic titanium tetraisopropoxide (TTIP), and Sr and Ca were supplied using conventional effusion cells. By careful adjustment of the cation fluxes of Sr and Ca with respect to the TTIP flux, the growth windows of SrTiO3 and CaTiO3 were overlapped, allowing us to grow the individual superlattice layers with self-regulated stoichiometry. Stable and repeatable reflection high-energy electron diffraction oscillations during the entire ∼2.5 h growth period indicated good source flux stability. The structural quality of the superlattice films were determined by scanning transmission electron microscopy and synchrotron-based X-ray diffraction, revealing periodic, phase pure, homogenous superlattice structures with abrupt interfaces. Utilization of perovskite stoichiometric growth windows offers great potential for accessing and realizing interface driven phenomena in versatile perovskite superlattice materials with chemistries beyond titanates.

Epitaxial growth and electrical properties of VO2 on [LaAlO3]0.3[Sr2AlTaO6]0.7 (111) substrate

The authors report the epitaxial growth and the electrical properties, especially the metal-to-insulator transition, of vanadium dioxide (VO2) thin films synthesized on [LaAlO3]0.3[Sr2AlTaO6]0.7 (LSAT) (111) (LSAT) substrates by pulsed laser deposition. X-ray diffraction studies show that the epitaxial relationship between the VO2 thin films and LSAT substrate is given as VO2(020)||LSAT(111) and VO2[001]||LSAT[112¯]. The authors observed a sharp 4 orders of magnitude change in the longitudinal resistance for the VO2 thin films around the transition temperature. The authors also measured distinct Raman spectra below and above the transition point indicating a concomitant structural transition between the insulator and metallic phases, in agreement with past investigations.

Electronic properties of ultra-thin YCrO3 films

We report on the heteroepitaxial stabilization of YCrO3 ultra-thin films on LSAT (001) substrate. Using a combination of resonant X-ray absorption spectroscopy (XAS) and atomic multiplet cluster calculation, the electronic structure of YCrO3 thin film was investigated. Polarization dependent Cr L3,2 edge XAS measurements reveal the presence of x-ray linear dichroism spectrum uncharacteristic of a 3d3 electronic system in an octahedral crystal field. Atomic multiplet calculation demonstrates the critical importance of charge transfer energy, coulomb correlation strength, and hopping interaction in realizing this dichroism spectrum.