Epitaxial SrTiO3 Thin Films Research Articles

Effect of misfit strain on the electrocaloric effect in epitaxial SrTiO3 thin films

Based on a phenomenological Landau-Devonshire theory, the effect of misfit strain on the electrocaloric effect in epitaxial SrTiO 3 thin films is investigated. Theoretical analysis indicates that both the electrocaloric coefficient and the adiabatic temperature change strongly depend on the misfit strain. Moreover, the maximum of electrocaloric coefficient almost does not change with increasing the compressive or tensile misfit strain. However, the enhancement of misfit strain enlarges both the adiabatic temperature change and the temperature at which it is maximized. Most importantly, the largest room-temperature electrocaloric effect can be attained at a critical misfit strain. Therefore, it may open more opportunities for practical application in refrigeration devices.

Ferromagnetism in Co-doped (La,Sr)TiO3

The origin of ferromagnetism in Co-doped (La,Sr)TiO3 epitaxial thin films is discussed. While the as-grown samples are not ferromagnetic at room temperature or at 10 K, ferromagnetism at room temperature appears after annealing the films in reducing conditions and disappears after annealing in oxidizing conditions. Magnetic measurements, x-ray absorption spectroscopy, x-ray photoemission spectroscopy and transmission electron microscopy experiments indicate that within the resolution of the instruments the activation of the ferromagnetism is not due to the presence of pure Co.

Realization of regular arrays of nanoscale resistive switching blocks in thin films of Nb-doped SrTiO3

We report on the realization of short-range-ordered arrays of nanoscale resistive switching blocks in epitaxial Nb-doped SrTiO3 thin films. These blocks can be individually addressed by the tip of a conductive tip atomic force microscope and reversibly switched between a high and a low resistance state reaching an Roff to Ron ratio of up to 50. Scanning micrometer-scale areas with an appropriately biased tip, all blocks within the scanned area can be switched between the two resistive states. We suggest a connection between these nanoscale switching blocks and defect-rich nanoclusters which were detected with high resolution transmission electron microscopy.

Polarization rotation transitions in anisotropically strained SrTiO3 thin films

Commensurately strained epitaxial SrTiO3 thin films on a GdScO3 substrate with anisotropic in-plane tensile strains of 1.46% and 1.59% were grown. By using optical second harmonic generation and polarization measurements, a ferroelectric transition from a tetragonal 4∕mmm phase to a ferroelectric mm2 phase at ∼400K and an antiferrodistortive transition to a multiferroic phase at 150–175K are observed, which are in agreement with thermodynamic calculations. In addition, a series of polarization rotation transitions between ⟨100⟩p to ⟨l00⟩p (0<l<1) is observed in the 4–400K temperature range.

Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser deposition, of thicknesses down to 40 nm, were studied using infrared transmission and reflection spectroscopy. The complex dielectric responses of polar phonon modes, particularly ferroelectric soft mode, in the films were determined quantitatively. The compressed epitaxial STO films on (100) La0.18Sr0.82Al0.59Ta0.41O3 substrates (strain 0.9%) show strongly stiffened phonon responses, whereas the soft mode in polycrystalline film on (0001) sapphire substrate shows a strong broadening due to grain boundaries and/or other inhomogeneities and defects. The stiffened soft mode is responsible for a much lower static permittivity in the plane of the compressed film than in the bulk samples.

Critical thickness control by deposition rate for epitaxial BaTiO3 thin films grown on SrTiO3(001)

Ba Ti O 3 ∕ Sr Ti O 3 ( 001 ) epitaxial thin films were prepared at various growth rates by pulsed laser deposition, and their heterostructures were evaluated by synchrotron x-ray diffraction measurements and cross-sectional scanning transmission electron microscopy observations. In a film grown at a low deposition rate (0.01nm∕s), misfit dislocations are found near the interface and a fully relaxed BaTiO3 thin film grows epitaxially on the substrate. On the other hand, a film grown at a high deposition rate (0.04nm∕s) consists of strained and relaxed BaTiO3 lattices. Our results showed that the critical thickness of BaTiO3∕SrTiO3(001) epitaxial thin films can be controlled by the deposition rate and that the critical thickness increases with increasing deposition rate, and by adjusting the deposition rate, we were able to prepare epitaxial thin films consisting of fully strained BaTiO3, partially strained BaTiO3, or fully relaxed BaTiO3.

Growth and defects of epitaxial SrTiO3 thin films on LaAlO3 substrate

Growth and defects of epitaxial SrTiO3 thin films on LaAlO3 substrate

Annealing effect on dislocations in SrTiO3∕LaAlO3 heterostructures

Epitaxial SrTiO3 thin films were deposited on (001) LaAlO3 single crystals by pulsed laser deposition. The postannealing effect was studied by annealing the deposited films at 1100°C in O2 atmosphere for 4h. It was found that the postannealing treatment enhances the in-plane permittivity of the film. An analysis of the film lattice parameters from selected area electron diffraction and high-resolution electron microscopy shows a relaxation of the compressive strain by annealing. In addition, the density of threading dislocations is significantly reduced, and stacking faults with translation vector R=1∕2a⟨110⟩ resulting from the dissociation of threading dislocations are formed. Both structural and dielectric characterizations indicate that the enhancement of permittivity is mainly due to the relaxation of the misfit strain between the film and substrate. The reduction of the threading dislocation density by annealing may be beneficial to the reduction of the extrinsic dielectric loss in the film.

Strain distribution in epitaxial SrTiO3 thin films

The lattice strain distributions of epitaxial SrTiO3 films deposited on LaAlO3 were investigated by in situ x-ray diffraction at the temperature range of 25–300K, grazing incident x-ray diffraction, and high resolution x-ray diffraction. The nearly linear temperature dependence of the out-of-plane lattice constant of SrTiO3 was observed in the measured temperature range. The depth distribution of the lattice strain at room temperature for SrTiO3 films includes the surface layer, strained layer, and interface layer.

Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927Å) of the STO layer as compared to its bulk value (3.905Å). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of the STO layer.

Effects of substrate on the dielectric and tunable properties of epitaxial SrTiO3 thin films

Tunable dielectric thin films of SrTiO3 (STO) were prepared on different single-crystalline substrates, including insulating LaAlO3, conductive Nb-doped STO (NSTO), and superconducting YBa2Cu3O7−δ. Substrate effects including morphology, orientation, and lattice mismatch induced strains were investigated. We found that a change of substrate used for STO thin films can strongly affect the dielectric properties of STO thin films in terms of dielectric constant, loss tangent, and tunability. Effects of substrate properties on the temperature dependence of dielectric constant and loss tangent were investigated. At low temperatures, STO thin films under minimal strain yield high dielectric constant and low loss tangent while the thin films under either tensile or compressive strain exhibit the reduced dielectric constant and high loss. The tunability of about 77% in STO∕NSTO system, close to the value found in STO single crystal, was observed at 10K. Physical origin of observed phenomena was discussed.

Relaxor ferroelectricity in strained epitaxial SrTiO3 thin films on DyScO3 substrates

The ferroelectric properties of 500Å thick strained, epitaxial SrTiO3 films grown on DyScO3 substrates by reactive molecular-beam epitaxy are reported. Despite the near 1% biaxial tensile strain, the x-ray rocking curve full widths at half maximum in ω are as narrow as 7arcsec (0.002°). The films show a frequency-dependent permittivity maximum near 250K that is well fit by the Vogel-Fulcher equation. A clear polarization hysteresis is observed below the permittivity maximum, with an in-plane remanent polarization of 10μC∕cm2 at 77K. The high Tmax is consistent with the biaxial tensile strain state, while the superimposed relaxor behavior is likely due to defects.

Sulfide and Oxide Heterostructures For the SrTiO3 Thin Film Growth on Si and Their Structural and Interfacial Stabilities

Epitaxial SrTiO3 (STO) thin films with high electrical properties were grown on Si using ZnS single- and SrS/MnS hetero-buffer layers. STO films on both ZnS-buffered and SrS/MnS-buffered Si showed two growth orientations, (100) and (110). The temperature dependence of the growth orientation for STO films was different for the ZnS single-buffer layer in comparison with the SrS/MnS heterobuffer layers. (100) growth of STO films on SrS/MnS-buffered Si became dominant at high temperatures about 700 °C, while (100) growth of STO films on ZnS-buffered Si became dominant at a relatively low growth temperature of 550 °C. STO(100) films on ZnS-buffered and SrS/MnS-buffered Si showed lattice and domain matches for epitaxial relationships with [001]ZnS∥[011]STO and SrS[001]∥[011]STO, respectively via 45° in-plane rotation of STO films relative to both ZnS and SrS layers. The ZnS buffer layer contained many stacking faults because of the mismatch between ZnS and Si, however, those defects were terminated at the ZnS/STO interface. In contrast, the MnS buffer was very stable against stacking defect formation. Transmission electron microscopy measurements revealed the presence of a disordered region at the ZnS/Si and MnS/Si interfaces. Auger electron spectroscopy and transmission electron microscopy results showed that a good MnS/Si interface at the initial growth stage degraded to a SiS2-x-rich phase during MnS deposition and again into a SiO2-x-rich phase during STO deposition at the high growth temperature of 700 °C. It was also observed that STO on SrS/MnS-buffered Si showed a markedly high dielectric constant compared with that of STO on ZnS-buffered Si.

INFLUENCE OF PROCESSING CONDITIONS ON THE STRUCTURE OF STRONTIUM TITANATE THIN FILMS GROWN ON SI BY LASER MBE

ABSTRACT Epitaxial SrTiO3 thin films have been deposited on Si (100) substrates by laser molecular-beam epitaxy. The experiments have confirmed that the use of Sr buffer layer is essential for making high quality SrTiO3 film. Apart from this, processing parameters including the deposition temperature and time for each layer also have significant influence on the crystallinity and roughness of SrTiO3. The thin films prepared under optimized conditions were found to have a pure perovskite phase, well crystallized and epitaxially aligned. The electrical measurements indicate that the SrTiO3 film has a capacitance effective thickness of less than 16 Å.

Ferroelectric (Pb,Sr)TiO3 epitaxial thin films on (001) MgO for room temperature high-frequency tunable microwave elements

Ferroelectric Pb0.35Sr0.65TiO3 (PSTO) thin films were grown on (001) MgO by using pulsed laser deposition. Microstructure studies from x-ray diffraction and electron microscopy indicate that the as-grown PSTO films have excellent single crystal quality and good epitaxial behavior with their c-axis oriented perpendicular to the plane of the films. The interface relationships between the PSTO films and MgO were determined to be [100]PSTO∕∕[100]MgO and (001)PSTO∕∕(001)MgO. The high frequency dielectric property measurements (up to 20 GHz) reveal that the as-grown films have a high dielectric constant value above 1420 and very large dielectric tunability above 34% at room temperature. These results suggest that the as-grown PSTO thin films on MgO are a good candidate for developing room-temperature high-frequency tunable microwave elements.

Atomic structure of epitaxial SrTiO3–GaAs(001) heterojunctions

We have examined the atomic and electronic structures of epitaxial SrTiO3 thin films on GaAs (001) deposited under different growth conditions in order to understand the interfacial structure-property relationships. High-resolution Z-contrast images show an atomically sharp heterointerface with SrTiO3[110] in perfect registry with GaAs [100] and the interfacial structure remains unchanged if a submonolayer of Ti was deposited prior to the SrTiO3 film growth. X-ray photoelectron spectroscopy shows that the Fermi level was pinned during the initial stage of growth when a submonolayer of Ti was deposited on As-terminated GaAs(001); subsequent SrTiO3 growth alleviated this pinning. These results indicate a self-driven interfacial atomic structure formation, independent of the initial stage of growth.

Structural and dielectric properties of strain-controlled epitaxial SrTiO3 thin films by two-step growth technique

Structural and dielectric properties of epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates fabricated by a two-step growth technique using pulsed laser deposition were investigated by in situ and ex situ observations in terms of strain relaxation from the lattice mismatch between the film and the substrate. In a first step, a very thin STO layer of less than 10nm was deposited at low temperature. The main part of the film was deposited in a second step at high temperature, as commonly used for epitaxial growth. In situ reflection high-energy electron diffraction observations showed that almost full strain relaxation was realized before the deposition of the second layer, whereas the normally grown film was not fully relaxed even when the film thickness exceeded 300nm. The relaxation process of the two-step-grown film took place through the crystallization of the first layer with substantial misfit dislocation formation, which is therefore fundamentally different from that of the normally grown film. Since the normally grown film possesses compressive strain due to less strain relaxation, the two-step-grown film showed larger permittivity and larger tuning compared to the normally grown film. The estimation of the extrinsic loss contribution from the measured loss tangent implied that the two-step growth technique could suppress the extrinsic loss due to a redistribution of defects in the film. The first layer leads to two effects: (i) strain relaxation and (ii) lower loss of the film.

Interfacial reaction in the growth of epitaxial SrTiO3 thin films on (001) Si substrates

The SrTiO3∕Si interface was investigated by transmission electron microscopy for SrTiO3 films grown on (001) Si by molecular-beam epitaxy with different native oxide (SiO2) removal treatments, and Sr∕Ti flux ratios. The interface and film microstructure were independent of the process used to remove the native oxide, but the interface reactivity was dependent on the Sr∕Ti flux ratio. A low Sr∕Ti flux ratio (∼0.8) resulted not only in a layer of amorphous material at the film∕substrate interface but also in the formation of crystalline C49 TiSi2 precipitates at that interface. These results are consistent with thermodynamic expectations in which it is paramount to maintain separation between TiO2 and the underlying silicon.

Strain relaxation of epitaxial SrTiO3 thin films on LaAlO3 by two-step growth technique

Using pulsed-laser deposition, a two-step growth technique was applied to epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates providing a way to obtain an effective strain relaxation in these films otherwise strained due to lattice mismatch between film and substrate. By changing the thickness of a first layer, deposited at a temperature as low as 100°C before the deposition of the main layer at 750°C, different strain relaxation states of the films could be systematically realized. With a 10-nm-thick first layer, an almost full strain relaxation at the deposition temperature of the main layer was achieved, suggesting a strong impact of this method on strain relaxation. The in-plane dielectric measurements displayed that the ferroelectric transition temperature increases with strain relaxation during the growth. This trend is correct and compatible with the theoretical prediction of the behavior of strained STO derived from Landau theory.

Effect of misfit strain and external electric field on dielectric behavior of epitaxial SrTiO3 thin films

Based on a Landau-Ginsburg-Devonshire (LD)-type phenomenological model, the effect of misfit strain and externally applied electric field on dielectric properties of epitaxially single-domain SrTiO3 thin films is investigated by taking into account the existence of two coupled instabilities (ferroelectric and antiferrodistortive). The epitaxy-induced strain may enable the phase of films transform to ferroelectric phase that is impossible for bulk “quantum paraelectric.” At low temperature, the critical tensile strain is in favor of relative high dielectric constant along [100] direction, while the critical compressive strain is useful to that of [001] direction. A high tunability may be obtained by adjusting misfit strain, especially in the vicinity of a structural phase transition. It is also shown that the electric field is facile to induce ferroelectricity in the case of purely structural orthorhombic (SO) phase or tetragonal (ST) phase under reasonable misfit strains when the external electric field is parallel with the order parameter qi. The structural order parameter induced may explain the “weak peak anomaly” phenomenon. Furthermore, the electric field dependence of dielectric responses of different structure phases is also discussed with various misfit strains. Our theoretical results are in good agreement with the experimental ones reported in the literature.