Epitaxial SrTiO3 Research Articles

Enhanced carrier density in Nb-doped SrTiO3 thermoelectrics

We study epitaxial SrTiO3 interfaced with Nb-doped SrTi1-xNbxO3 (x = 0, 0.125, 0.25, 0.375, and 0.5) by full-potential density functional theory. From the electronic band structures obtained by our ab-initio calculations we determine the dependence of the induced metallicity on the Nb concentration. We obtain a monotonous increase of the carrier density with the Nb concentration. The results are confirmed by experiments for SrTi0.88Nb0.12O3 and SrTi0.8Nb0.2O3, demonstrating the predictive power and limitations of our theoretical approach. We also show that the Seebeck coefficient decreases monotonously with increasing temperature.

Stress Generated Modifications of Structural and Morphologic Properties of Epitaxial SrTiO3 Films on Sapphire

Owing to the strong connection between strain and ferroelectricity, large shifts of the Curie temperature and polarization are observed in strained ferroelectric material. In this work, the effects of lattice-mismatch induced stress upon the crystallographic structure, strain, and generation of different types of defects in epitaxial SrTiO3 films on CeO2 buffered sapphire are examined and discussed in context with the resulting impact of strain on the polarization of the ferroelectric layers. Depending on the thickness of the SrTiO3 layer, characteristic changes in their structural perfection and crystallographic orientation with respect to the substrate system are observed. For thin films, misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The structural modifications and the formation of defects are explained in a model based on lattice misfit induced stress and energy considerations. It is demonstrated that intrinsic mismatch and thermal mismatch must be considered to explain strain dependent effects such as induced ferroelectricity and modifications of the permittivity of these complex heteroepitaxial layer systems.

Kinetics of formation and growth of epitaxial SrTiO3 films of single-crystal (001) SrTiO3 supports

The aim of this study was to quantitatively estimate the kinetics of the formation and growth of oxide SrTiO3 (STO) films using the method of the in situ reflection high-energy electron diffraction (RHEED) and compare the obtained results with the known growth models and theoretical estimates. The kinetics of the relaxation and crystallization of particles is studied under pulsed laser deposition (PLD) from oxide targets onto (001) STO supports or onto the surface of STO film growth at 650–800°C. Deposition frequencies of 0.1–10 Hz typical of PLD were used. The surface morphology and film structure was studied ex situ using the methods of AFM and X-ray-structural analysis. It was found that the time of relaxation of deposited particles is within the range of 2–20 s, which greatly exceeds or is comparable to the relative pulse duration. It was experimentally shown that structural distortions in epitaxial films for temperatures of ≤900°C are mainly due to the high rate of deposition and limited surface mobility of particles. The effect of structural relaxation in films is observed after the end of deposition; the time constant of bulk structural relaxation is ∼10 − ∼102 s or more. The obtained kinetic parameters of the formation of an oxide structure may be useful for the development of crystallization theory, as well as to optimize the conditions of epitaxial oxide film growth.

Suppression of vacancy defects in epitaxial La-doped SrTiO3 films

Variable energy positron annihilation lifetime spectroscopy of high-mobility La-doped SrTiO3 grown by molecular beam epitaxy found that the films contained sufficiently low concentrations of Sr vacancies and vacancy cluster defects to allow the observation of positron annihilation events from the perfect lattice. This enabled the concentrations of charged cation vacancies to be estimated, and these were found to be at least an order of magnitude below the La-dopant concentrations.

KTa0.65Nb0.35O3 thin films epitaxially grown by pulsed laser deposition on metallic and oxide epitaxial electrodes

Ferroelectric KTa0.65Nb0.35O3 (KTN) thin films were grown by pulsed laser deposition on Pt and LaNiO3 epitaxial electrodes, on (100) and (110) SrTiO3 substrates. The effect of the nature of the electrode on structural and microstructural quality of KTN films was investigated. While epitaxial KTN thin films were successfully obtained on both electrodes, two orientations compete on Pt, whatever the main orientation of Pt is (100) or (110). On LaNiO3 in contrast, pure (100) and (110) oriented KTN films were achieved with a high crystalline quality illustrated by narrow ω-scans (Δω=0.56° and Δω=0.80° for (100) and (110) KTN, to be compared to 0.048° and 0.22° for (100) and (110) LaNiO3, respectively). Electrical measurements performed in tunneling atomic force microscopy (TUNA mode) on a KTN/Pt heterostructure showed a high asymmetry of the conduction mechanisms when a positive or a negative bias is applied on the sample. In particular leakage currents appear even at very low positive applied voltage. TUNA imaging operated at a moderate negative applied voltage of −3V shows that some areas corresponding to grain boundaries seem to be more leaky than others.

Direct measurement of electron channeling in a crystal using scanning transmission electron microscopy

Self-focusing of an atomic-scale high-energy electron wave packet by channeling along a zone axis in crystalline silicon is directly measured by scanning transmission electron microscopy using thin epitaxial SrTiO${}_{3}$(100) islands grown on Si(100) as test objects. As the electron probe propagates down a silicon atom column, it is progressively focused onto the column, resulting in a fourfold increase in the scattered signal at the channeling maximum. This results in an enhancement of the visibility of the SrTiO${}_{3}$ islands, which is lost if the sample is flipped upside down and the channeling occurs only after the probe scatters off the SrTiO${}_{3}$ layer. The evolution of the probe wave function calculated by the multislice method accurately predicts the trends in the channeling signal on an absolute thickness scale. We find that while electron channeling enhances the visibility of on-column atoms, it suppresses the contribution from off-column atoms. It can therefore be used as a filter to selectively image the atoms that are most aligned with the atomic columns of the substrate. By using this technique, coherent islands can be distinguished from relaxed islands. For SrTiO${}_{3}$ films formed in a topotactic reaction on Si(100), we show that only $\ensuremath{\sim}$55$%$ of the SrTiO${}_{3}$ is aligned with the Si atom columns. The fraction of coherent SrTiO${}_{3}$ islands on Si(100) can be increased by choosing growth conditions away from equilibrium.

Coercivity enhancement driven by interfacial magnetic phase separation in SrTiO3(001)/Nd0.5Sr0.5CoO3

Thin-film perovskite cobaltites have been found to exhibit coercivity values enhanced by almost 2 orders of magnitude in comparison to bulk. In this paper, we have investigated this unexplained coercivity enhancement in detail, focusing on epitaxial SrTiO${}_{3}$(001)/Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_{3}$ [SrTiO${}_{3}=\mathrm{STO}$] films, which display coercivity values up to 40 kOe at low temperatures. Thickness-dependent (10--800 \AA{}) magnetometry and magnetotransport studies demonstrate that nanoscopic magnetic phase separation occurs in the interface region of Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_{3}$ [consistent with recent work on SrTiO${}_{3}$(001)/La${}_{1\ensuremath{-}x}$Sr${}_{x}$CoO${}_{3}$], which is responsible for the degradation in magnetic and electronic properties in the very-thin-film limit. The coercivity is shown to be intimately related to the existence of this (70-\AA{}-thick) interfacial phase-separated layer, leading us to advance an explanation for the coercivity enhancement in terms of the pinning of domain walls by interfacial nanoscopic ferromagnetic clusters and a crossover to single domain clusters at very low thickness. Simple estimates of the magnetocrystalline anisotropy (from the maximum coercivity), cluster dimensions (from the superparamagnetic blocking temperature), multidomain to single domain crossover point, and domain-wall width, provide quantitative support for this picture.

Interfacial and rectifying characteristic of epitaxial SrTiO 3-δ/GaAs p–n junctions

Oxygen-deficient SrTiO3-δ thin films were grown on p-GaAs substrates to form p–n heterojunctions. The interfacial and transport characteristics of the heterojunctions have been investigated. The sharp interface was found to be epitaxially crystallized. The junctions present good rectifying behaviors and thickness dependence of current–voltage properties. The electronic transport of 200 nm thick SrTiO3-δ/GaAs could be explained by a space charge limited model. When the thickness was reduced further, the diode ideality factors were almost temperature independent, which might be attributed to a strain-assisted tunneling mechanism.

Electronic structure of epitaxial Fe-doped SrTiO3 thin films

X-ray photoelectron spectroscopy has been applied to study the electronic structure of the epitaxial Fe-doped SrTiO3 thin films with an Fe concentration of 1 and 2 at%. The results were compared to undoped thin films and lightly Fe-doped single crystal of SrTiO3. Fe doping was found to increase the amount of the in-gap states which can be related to the increased ability of Fe-doped thin films to exhibit resistive switching behaviour. Core level analysis indicates the mixed-valence character of Fe atoms in the films, with a divalent contribution situated on the surface.

Local conductivity of epitaxial Fe-doped SrTiO3 thin films

Atomic Force Microscopy (AFM) measurements have been performed for Fe-doped SrTiO3 thin films with an Fe concentration of 1 and 2 at%. Thin films with a thickness of 15–50 nm were grown by pulsed laser deposition on single crystalline SrTi0.99Nb0.01O3 substrates. Low-energy electron diffraction examination showed that the films are single crystalline. The regions treated with the AFM tip (applied dc voltage up to 6 V) showed inhomogeneity of the electrical conductivity. Resistive switching was studied by the use of local conductive atomic force microscopy and was present in all samples. Fe doping was found to influence the character of switching.

Thermodynamic stability and growth kinetics of epitaxial SrTiO3on silicon

We determine the thermodynamic phase diagram of the epitaxial SrTiO${}_{3}$/Si heterostructure from first-principles density functional theory calculations. We demonstrate that the system is not thermodynamically stable with respect to formation of an interfacial SiO${}_{2}$ region under any experimentally attainable oxygen partial pressures. To understand the experimental observation of an atomically abrupt interface without a SiO${}_{2}$ layer, we construct a kinetic model of the growth process. We show that the observed stability of the nonequilibrium interface under both growth and ambient conditions is directly related to the interface chemistry, which gives rise to a large (2.2 eV) barrier for oxygen diffusion to the interface layer and kinetically traps an atomically abrupt interface structure. Our model leads to the prediction of an interface structure in good agreement with experiment, demonstrating that a combination of first-principles computations, thermodynamics, and simple kinetic modeling can be used to determine non $+$ -thermodynamic equilibrium atomic structures in complex heterosystems.

Strain-induced ferromagnetism in LaCoO 3: Theory and growth on Si (1 0 0)

We have grown epitaxial ferromagnetic LaCoO 3 on (1 0 0)-oriented silicon. The films are grown by molecular beam epitaxy using a relaxed epitaxial SrTiO 3 buffer layer. X-ray diffraction measurements show a tetragonal LaCoO 3 film that is strained to the SrTiO 3 buffer layer. XPS measurements are consistent with reported spectra for single crystal, powder, and thin film LaCoO 3. Density functional calculations of strained LaCoO 3 confirms that the formation of the ferromagnetic state is accompanied by a partial opening of the Co–O–Co bond angle along the c-axis and a reduction in the local symmetry of the CoO 6 octahedra from tetragonal to orthorhombic.

Growth-mode induced defects in epitaxial SrTiO3 thin films grown on single crystal LaAlO3 by a two-step PLD process

Abstract

Epitaxial integration of ferromagnetic correlated oxide LaCoO3 with Si (100)

We have grown epitaxial strained LaCoO3 on (100)-oriented silicon by molecular beam epitaxy using a relaxed epitaxial SrTiO3 buffer layer. Superconducting quantum interference device magnetization measurements show that, unlike the bulk material, the ground state of the strained LaCoO3 on silicon is ferromagnetic with a TC of 85 K. First principles calculations suggest that a ferromagnetic ground state can be stabilized in LaCoO3 by a sufficiently large biaxial tensile strain with the transition accompanied by a partial untilting of the CoO6 octahedra.

Infrared phonon spectroscopy of a compressively strained (001)SrTiO3 film grownon a (110) NdGaO3 substrate

Polarized infrared reflectivity was measured between 5 and 300 Kon a 17 nm thick, 1.1% compressively strained epitaxial (001)SrTiO3 film and theorthorhombic (110) NdGaO3 substrate upon which it was grown. A strong in-plane infrared anisotropy of theNdGaO3 substrate was observed and polar modes withB1u-and amixture of B2u + B3u-symmetry were seen. At low temperatures three new modes arose in the90–130 cm − 1 range, which we assigned to 4f Nd electronic transitions. The in-planeSrTiO3 film phonons showed strong stiffening compared to the phonon frequencies of bulk unstrainedSrTiO3, particularly the soft mode, and the in-plane phonon peaks were found tosplit. No anomalies were detected as a function of temperature in eitherthe infrared response or lattice parameters of the compressively strainedSrTiO3 film, providing an absence of evidence for the out-of-plane ferroelectric phase transitionpredicted by theory.

The atomic structure and polarization of strained SrTiO3/Si

For thin film devices based on coupling ferroelectric polarization to charge carriers in semiconductors, the role of the interface is critical. To elucidate this role, we use synchrotron x-ray diffraction to determine the interface structure of epitaxial SrTiO3 grown on the (001) surface of Si. The average displacement of the O octahedral sublattice relative to the Sr sublattice determines the film polarization and is measured to be about 0.05 nm toward the Si, with Ti off-center displacements 0.009 nm away from the substrate. Measurements of films with different boundary conditions on the top of the SrTiO3 show that the polarization at the SrTiO3/Si interface is dominated by oxide-Si chemical interactions.

Antiferrodistortive Structural Phase Transition in Compressively-Strained Epitaxial SrTiO3 Film Grown on (La, Sr)(Al, Ta)O3 Substrate

(001)-epitaxial SrTiO3 films were grown on (La, Sr)(Al, Ta)O3 substrates for investigating the impact of biaxial strain on the antiferrodistortive (AFD) structural phase transition in SrTiO3. The films were fully constrained by the substrates, which resulted in the large in-plane compressive strain of -0.9%. The AFD transition temperature estimated using temperature controlled x-ray diffraction and transmission electron microscope was over 150K higher than the theoretical prediction, but rather supported the experimental results reported on SrTiO3 films on LaAlO3 substrates.

Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical function and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO3 films deposited on KTaO3 (100) single-crystal substrates. Optical functions in the 200–1200 nm spectral range were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO3 films and for homoepitaxial strain-free SrTiO3: Cr (0.01 at %) films deposited on SrTiO3 (100) single crystalline substrates showed that this “blue” shift of the band gap could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of the polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO3 films.

Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $\Delta_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $\Delta_A$ gap results to be well determined ($\Delta_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $\Delta_B$ is more uncertain ($\Delta_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\Delta/kT_c$ values close to 2 and 7.

Thermal stability of SrTiO3/SiO2/Si Interfaces at Intermediate Oxygen Pressures

The thermal stability of epitaxial SrTiO3 thin films grown by molecular-beam epitaxy on Si (001) has been studied using x-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and thermodynamic calculations. Our studies focus on the stability of the SrTiO3/Si structures under the conditions typically employed in the pulse laser deposition (PLD) growth of complex metal oxide heteroepitaxy on Si. We observe additional Bragg peaks in thermally treated SrTiO3 buffered Si structures, corresponding to possibly TiSi2 and/or SrSiO3, reaction products which are consistent with the reaction schemes we propose. In addition, OM and SEM reveal microstructures that are not readily accounted for solely by the solid state reactions as put forth by previous workers but can be reasonably explained by our proposed reaction schemes. Using our observations and thermodynamic analysis, we argue that reactions involving the gaseous species SiO(g), the reactivity of which has not been previously considered in this system, are important. We attribute the onset of degradation of the SrTiO3 film at high temperatures, to the circular void forming reaction Si(s)+SiO2(s)→2SiO(g) at the interface and suggest that the reactions considered by previous workers involving all solid state reactants occurs only at the conclusion of degradation. Our results points to the need for keeping the PLD temperature as low as possible to minimize the production of reactive SiO(g) in avoiding the deleterious reactions.