Epitaxial Thin-film Heterostructures Research Articles

Targeted Dehydration As a Route to Site-Selective Atomic Layer Deposition at TiO2 Defects

Crystallographic perfection in epitaxial thin film heterostructures can eliminate interface defects that dilute unique properties and reduce device performance. However, the requirement for epitaxial perfection greatly limits the selection of material candidates and deposition processes. Using selective interface reactions (SIRs), an atomic layer deposition (ALD)-based technique, we target transformation of undesirable defect sites at imperfect surfaces. Defects on the TiO2 surface affect the electronic properties, interfaces, and performance of optoelectronic devices that leverage TiO2 interfaces. We present first principles calculations to predict the difference in hydration/hydroxylation of pristine TiO2 terraces and minority atomic configurations (i.e. “defects”) including step edges and oxygen vacancies. We investigate hydroxylation differences through temperature dependent scanning tunneling microscopy (STM), and ultimately exploit these differences to selectively react ALD precursors at surface defect sites.

Emergent and Tunable Topological Surface States in Complementary Sb/Bi2Te3 and Bi2Te3/Sb Thin-Film Heterostructures.

Epitaxial thin-film heterostructures offer a versatile platform for realizing topological surface states (TSSs) that may be emergent and/or tunable by tailoring the atomic layering in the heterostructures. Here, as an experimental demonstration, Sb and Bi2Te3 thin films with closely matched in-plane lattice constants are chosen to form two complementary heterostructures: Sb overlayers on Bi2Te3 (Sb/Bi2Te3) and Bi2Te3 overlayers on Sb (Bi2Te3/Sb), with the overlayer thickness as a tuning parameter. In the bulk form, Sb (a semimetal) and Bi2Te3 (an insulator) both host TSSs with the same topological order but substantially different decay lengths and dispersions, whereas ultrathin Sb and Bi2Te3 films by themselves are fully gapped trivial insulators. Angle-resolved photoemission band mappings, aided by theoretical calculations, confirm the formation of emergent TSSs in both heterostructures. The energy position of the topological Dirac point varies as a function of overlayer thickness, but the variation is non-monotonic, indicating nontrivial effects in the formation of topological heterostructure systems. The results illustrate the rich physics of engineered composite topological systems that may be exploited for nanoscale spintronics applications.

Oxygen diffusion barriers for epitaxial thin-film heterostructures with highly conducting SrMoO3 electrodes

Transition metal perovskite oxide SrMoO3 with a Mo4+ 4d2 electronic configuration exhibits a room-temperature resistivity of 5.1 μΩcm in a single-crystal form and, therefore, is considered a prominent conducting electrode material for all-oxide microelectronic devices. Stabilization of the unfavorable Mo4+ valence state in SrMoO3 thin films necessitates reductive growth conditions that are often incompatible with a highly oxidative environment necessary to grow epitaxial heterostructures with fully oxygenated functional layers (e.g., tunable dielectric BaxSr1−xTiO3). Interestingly, only a few unit cells of the perovskite titanate capping layers SrTiO3, BaTiO3, and Ba0.5Sr0.5TiO3 act as an efficient oxygen barrier and minimize SrMoO3 oxidation into electrically insulating SrMoO4 in the broad range of the thin-film growth parameters. The Mo valence state in SrMoO3, determined by x-ray photoelectron spectroscopy, is used to analyze oxygen diffusion through the capping layers. The lowest level of oxygen diffusion is observed in Ba0.5Sr0.5TiO3. A Ba0.5Sr0.5TiO3 film with a thickness of only 6 unit cells preserves the Mo4+ oxidation state in the SrMoO3 underlayer up to the oxygen partial pressure of 8 mTorr at the temperature of 630 °C. Results, therefore, indicate that SrMoO3 films covered with atomically thin Ba0.5Sr0.5TiO3 remain conducting in an oxygen environment and can be integrated into all-oxide thin-film heterostructures with other functional materials.

An optimized sample preparation approach for atomic resolution in situ studies of thin films.

This work provides the details of a simple and reliable method with less damage to prepare electron transparent samples for in situ studies in scanning/transmission electron microscopy. In this study, we use epitaxial VO2 thin films grown on c-Al2 O3 by pulsed laser deposition, which have a monoclinic-rutile transition at ~68°C. We employ an approach combining conventional mechanical wedge-polishing and Focused Ion beam to prepare the electron transparent samples of epitaxial VO2 thin films. The samples are first mechanically wedge-polished and ion-milled to be electron transparent. Subsequently, the thin region of VO2 films are separated from the rest of the polished sample using a focused ion beam and transferred to the in situ electron microscopy test stage. As a critical step, carbon nanotubes are used as connectors to the manipulator needle for a soft transfer process. This is done to avoid shattering of the brittle substrate film on the in situ sample support stage during the transfer process. We finally present the atomically resolved structural transition in VO2 films using this technique. This approach significantly increases the success rate of high-quality sample preparation with less damage for in situ studies of thin films and reduces the cost and instrumental/user errors associated with other techniques. The present work highlights a novel, simple, reliable approach with reduced damage to make electron transparent samples for atomic-scale insights of temperature-dependent transitions in epitaxial thin film heterostructures using in situ TEM studies.

Preparation and characterization of the superlattice (Sm-doped ceria/yttria-stabilized zirconia)N electrolyte film

As is well known, a solid oxide fuel cell(SOFC) is a device that can convert chemical energy directly into electrical energy. The possibility to grow thin films of oxide materials is of great importance in the development of SOFC. Recently, it is expected that oxide heterostructures, with almost ideal interfaces, should lead to the interesting artificial materials with some novel properties. So we have prepared superlattice electrolyte thin films.On the MgO single-crystal substrates, multilayer epitaxial thin-film heterostructures of 20 mol% samarium-oxide-doped ceria (Ce0.8Sm0.2O2-, SDC) and 8 mol% yttria-stabilized zirconia (Y2O3: ZrO2YSZ) have been deposited in turn, using the pulsed laser deposition (PLD) technique. Five different superlattices (SDC/YSZ)N (N=3, 5, 10, 20, 30) films are fabricated, keeping the total thickness constant (300 nm), but with a different number of hetero-interfaces. The choice of coupling SDC and YSZ aims to have both layers in the superlattices made of oxygen-ion conductors with compatible crystallographic features. On one hand, we have to remove any potential contribution of the deposition substrate to the total conductivity, and the superlattices may be grown on 110-oriented MgO single-crystalline wafers. On the other hand, the YSZ electrolyte film requires that the temperature should be higher, and SDC as the electrolyte leakage; both should be combined with each other, so as to have complementary advantages. In this way, not only the temperature of SOFCs is reduced, but also the leakage avoided. Because YSZ and SDC both belong to cubic fluorite structure and have similar lattice parameters, the lattice constant of YSZ is 5.14 , and the lattice constant of SDC is 5.44 . Compatible crystal properties, oxygen ion conductivity, and great matching defects can lead to a semi-coherent interface between the ZrO2 and CeO2 layers. The interface effect is obvious. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and AC impedance mearurements are used to characterize the surface morphology, phase structure and electrical properties of the electrolyte film. Results show that the superlattice (SDC/YSZ)N electrolyte films will form obviously interface and better superlattice structure. This kind of thin films do not show crack, with no element diffusion at the interfaces, growing uniformly, densely and smoothly. Electrochemical measurements show a sizable increase in conductivity with increasing number of SDC/YSZ interfaces. So it is an ideal low-temperature fuel cell electrolyte materials.

Tensile Lattice Distortion Does Not Affect Oxygen Transport in Yttria-Stabilized Zirconia–CeO2 Heterointerfaces

Biaxially textured epitaxial thin-film heterostructures of ceria and 8 mol % yttria-stabilized zirconia (8YSZ) were grown using pulsed laser deposition (PLD) with the aim to unravel the effect of the interfacial conductivity on the charge transport properties. Five different samples were fabricated, keeping the total thickness constant (300 nm), but with a different number of heterointerfaces (between 4 and 60). To remove any potential contribution of the deposition substrate to the total conductivity, the heterostructures were grown on (001)-oriented MgO single-crystalline wafers. Layers free of high-angle grain boundaries and with low density of misfit dislocations were obtained, as revealed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) analysis. The crystallographic quality of these samples allowed the investigation of their conduction properties, suppressing any transport effects along grain boundaries and/or interfacial dislocation pathways. Electrochemical impedance spectroscopy (EIS) and secondary ion mass spectroscopy (SIMS) measurements showed that for these samples the interfacial conductivity has a negligible effect on the transport properties.

Localization of two-dimensional electron gas in LaAlO3/SrTiO3heterostructures

We report strong localization of two-dimensional electron gas in LaAlO${}_{3}$/SrTiO${}_{3}$ epitaxial thin-film heterostructures grown on (LaAlO${}_{3}$)${}_{0.3}$-(Sr${}_{2}$AlTaO${}_{3}$)${}_{0.7}$ substrates by using pulsed laser deposition with in situ reflection high-energy electron diffraction. Using longitudinal and transverse magnetotransport measurements, we have determined that disorder at the interface influences the conduction behavior, and that increasing the carrier concentration by growing at lower oxygen partial pressure changes the conduction from strongly localized at low carrier concentration to metallic at higher carrier concentration, with indications of weak localization. We interpret this behavior in terms of a changing occupation of Ti 3$d$ bands near the interface, each with a different spatial extent and susceptibility to localization by disorder, and differences in carrier confinement due to misfit strain and point defects.

Effect of PbZr0.52Ti0.48O3 thin layer on structure, electronic and magnetic properties of La0.65Sr0.35 MnO3 and La0.65Ca0.30MnO3 thin-films

Epitaxial thin film heterostructures of high dielectric PbZr1-xTixO3 (PZT) and La1-xAxMnO3 (A-divalent alkaline earth metals such as Sr (LSMO) and Ca (LCMO)) were grown on SrTiO3 substrates and their structure, temperature dependence of electrical resistivity, and magnetization were investigated as a function of the thickness of the LSMO(LCMO) layer. The microstructures of the samples were analyzed by TEM. By applying an electric field across the PZT layer, we applied a ferrodistortive pressure on the manganite layer and studied the correlations between lattice distortion and electric transport and magnetic properties of the CMR materials.

An approach to achieving a negative index of refraction using coincident resonances

A novel method of achieving a negative index of refraction in the far-infrared region using intrinsic material properties is proposed. SrTiO3 has a dielectric resonance at ∼100 cm−1 that can be lowered by lowering the temperature or with Ba doping. NiO has an antiferromagnetic resonance at ∼35 cm−1 that can be raised with an applied field or with Co doping. These resonance frequencies can therefore be engineered to occur at the same frequency to achieve a negative index of refraction over an interval just above the coincident resonance. We show that SrTiO3 and NiO are readily chemically compatible and non-reacting at temperatures up to 1550 °C and can be formed into polycrystalline composite bodies. Epitaxial thin film heterostructures of (SrTiO3/NiO)n/MgO with n = 1, 2 have also been fabricated with very high crystal quality.

Atomic Control of the Electronic Structure at Complex Oxide Heterointerfaces

AbstractThe following article is based on the Outstanding Young Investigator Award presentation given by Harold Y. Hwang of the University of Tokyo on March 29, 2005, at the Materials Research Society Spring Meeting in San Francisco. Hwang was cited for “innovative work on the physics of transition-metal oxides and the atomic-scale synthesis of complex oxide heterostructures.” Perovskite oxides range from insulators to superconductors and can incorporate magnetism as well as couple to phonon instabilities. The close lattice match between many perovskites raises the possibility of growing epitaxial thin-film heterostructures with different ground states that may compete or interact. The recent development of superconducting Josephson junctions, magnetic tunnel junctions, ferroelectric memory cells, and resistive switching can be considered examples within this new heteroepitaxial family. In this context, Hwang presents his studies of electronic structure at atomically abrupt interfaces grown by pulsed laser deposition. Some issues are generic to all heterointerfaces, such as the stability of dopant profiles and diffusion, interface states and depletion, and interface charge arising from polarity discontinuities. A more unusual issue is the charge structure associated with Mott insulator/band insulator interfaces. The question is, how should one consider the correlated equivalent of band bending? This semiconductor concept is based on the validity of rigid single-particle band diagrams, which are known to be an inadequate description for strongly correlated electrons. In addition to presenting an interesting scientific challenge, this question underlies the attempts to develop new applications of doped Mott insulators in device geometries.

Dynamic nonlinear dielectric response of relaxor ferroelectric(PbMg1/3Nb2/3O3)0.68−(PbTiO3)0.32thin films

The dynamic nonlinear dielectric response of relaxor ferroelectric $({\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}{)}_{0.68}\ensuremath{-}({\mathrm{PbTiO}}_{3}{)}_{0.32}$ thin films was qualitatively analyzed. The dielectric response of epitaxial thin-film heterostructures deposited on ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_{3}/(001)\mathrm{MgO}$ was measured as a function of the amplitude of the ac drive. In thin films, the temperature and frequency dependences of dynamic linear, third-order nonlinear and scaled third-order nonlinear, dielectric permittivities $({\ensuremath{\varepsilon}}_{1},{\ensuremath{\varepsilon}}_{3}{,a}_{3})$ were reconstructed from the measured dielectric response of the heterostructures. The behavior of ${\ensuremath{\varepsilon}}_{1},$ ${\ensuremath{\varepsilon}}_{3},$ and ${a}_{3}$ was in qualitative and quantitative agreement with that experimentally observed in single-crystal relaxor ferroelectrics. The peaks in ${\ensuremath{\varepsilon}}_{1}$ and ${\ensuremath{\varepsilon}}_{3},$ and the increase in ${a}_{3}$ with decreasing temperature below that of the dielectric maximum corresponded to the recent modeling and could indicate a glassy state in the films.

Dielectric nonlinearities in ferroelectric thin-film heterostructures

The influence of the film–electrode interface on the ac-electric field dependence of the dielectric permittivity in ferroelectric thin-film heterostructures has been studied. The dielectric nonlinearities in epitaxial thin-film heterostructures of ferroelectric PbZr0.65Ti0.35O3 and relaxor ferroelectric (PbMg1/3Nb2/3O3)0.68–(PbTiO3)0.32 were measured at subswitching fields. A dramatic difference between the dielectric nonlinearities possessed by ferroelectric films and those exhibited by the corresponding heterostructures was revealed both by model evaluations and experimental observations. In the heterostructures, due to the presence of an interface layer the dielectric nonlinearities can be considerably suppressed and their type can be changed compared to those in the films.

Structure and properties of epitaxial ferroelectric PbLu0.5Nb0.5O3 thin films

Epitaxial thin film heterostructures of PbLu0.5Nb0.5O3 (PLuN) and La0.5Sr0.5CoO3 (LSCO) were deposited by in situ pulsed laser ablation onto MgO and LaAlO3 (100) substrates. The formation of crystalline phases, epitaxy, film-electrode-substrate orientation relationships, crystal perfection and chemical order were studied by x-ray diffraction. The structural properties were found to depend on the deposition conditions and on the substrate. The perovskite PLuN films were pseudocubic and epitaxial, with (001) planes parallel to the substrate surface. At room temperature, Au/PLuN/LSCO capacitors exhibited ferroelectric behavior and zero-field dielectric permittivity about ε ≅ 210–260, approximately three times higher than in ceramics. A diffuse peak in ε(T) was observed around 100°C, about 150°C below the sharp PE-AFE transition in bulk ceramics. The lower degree of chemical order in the films was considered to be an important factor in the different behavior of thin films and bulk ceramics.

Strain modification of epitaxial perovskite oxide thin films using structural transitions of ferroelectric BaTiO3 substrate

Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of SrRuO3 and La0.67Sr0.33MnO3 by structural transitions of ferroelectric BaTiO3 substrates have been studied. Large jumps of electrical resistivity (∼5% in SrRuO3 and ∼12% in La0.67Sr0.33MnO3) and low field magnetization (∼70% in La0.67Sr0.33MnO3) have been observed in the films at the structural transition temperatures of BaTiO3 substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.

Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-polarized quasiparticles.