Epitaxial growth of perovskite oxide films facilitated by oxygen vacancies

The authors would like to thank P. Yudin for valuable discussions, N. Nepomniashchaia for VASE studies, and S. Cichon for XPS analysis. The authors acknowledge support from the Czech Science Foundation (Grant No. 19-09671S), the European Structural and Investment Funds and the Ministry of Education, Youth and Sports of the Czech Republic through Programme ‘‘Research, Development and Education’’ (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16-019/0000760), and ERA NET project Sun2Chem (E. K. and L. R.). Calculations have been done on the LASC Cluster in the ISSP UL.Single-crystal epitaxial films of technologically important and scientifically intriguing multifunctional ABO3 perovskite-type metal oxides are essential for advanced applications and understanding of these materials. In such films, a film-substrate misfit strain enables unprecedented crystal phases and unique properties that are not available in their bulk counterparts. However, the prerequisite growth of strained epitaxial films is fundamentally restricted by misfit relaxation. Here we demonstrate that introduction of a small oxygen deficiency concurrently stabilizes epitaxy and increases lattice strain in thin films of archetypal perovskite oxide SrTiO3. By combining experimental and theoretical methods, we found that lattice distortions around oxygen vacancies lead to anisotropic local stresses, which interact with the misfit strain in epitaxial films. Consequently, specific crystallographic alignments of the stresses are energetically favorable and can facilitate epitaxial growth of strained films. Because anisotropic oxygen-vacancy stresses are inherent to perovskite-type and many other oxides, we anticipate that the disclosed phenomenon of epitaxial stabilization by oxygen vacancies is relevant for a very broad range of functional oxides.This work is licensed under CC BY, CC BY-NC licenses.Czech Science Foundation (Grant No. 19-09671S); European Structural and Investment Funds and the Ministry of Education, Youth and Sports of the Czech Republic through Programme ‘‘Research, Development and Education’’ (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16-019/0000760), and ERA NET project Sun2Chem; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Effects of doping and epitaxy on optical behavior of NaNbO3 films

Cube-on-cube epitaxy of perovskite sub-cell of Pr-doped and undoped NaNbO3 is obtained in 130-nm-thick films on top of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrates. Experimental studies show that the edge of optical absorption red-shifts and some interband transitions change in the films compared to crystals. Bright red luminescence is achieved at room-temperature under ultraviolet excitation in the Pr-doped film. An interband mechanism of luminescence excitation is detected in the film, which is in contrast to the intervalence charge transfer mechanism in the crystal. The results are discussed in terms of epitaxially induced changes of crystal symmetry and ferroelectric polarization in the films. It is suggested that the band structure and interband transitions in NaNbO3and the transition probabilities in the Pr ions can be significantly modified by these changes.Peer reviewe

Control of Mooij correlations at the nanoscale in the disordered metallic Ta - nanoisland FeNi multilayers

Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10^3-10^5 Bohr magnetons (\mu_B). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer lead to a decrease in the Drude contribution of free charge carriers and the appearance of the low-energy electronic excitations in the 1-2 eV spectral range characteristic of electronic correlations, which may accompany the formation of electronic inhomogeneities. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated localised electronic states. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc conductivity falls down below the MIR limit by about 60%. The discovered effect leading to the dc conductivity fall below the MIR limit can be associated with non-ergodicity and purely quantum (many-body) localisation phenomena, which need to be challenged further.Comment: 29 pages, 8 figures. This is a post-peer-review, precopyedit version of an article published in Scientific Reports. The final authenticated version is available online at http://dx.doi.org/10.1038/s41598-020-78185-

Strong spin-phonon coupling in infrared and Raman spectra of SrMnO3

Infrared reflectivity spectra of cubic SrMnO3 ceramics reveal 18% stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T-N = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first-principles calculations. Moreover, polar phonons become Raman active below T-N, although their activation is forbidden by symmetry in the Pm (3) over barm space group. This gives evidence that the cubic Pm (3) over barm symmetry is locally broken below T-N due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small nonstoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers n(e) = 3.6 x 10(20) cm(-3) and the sample composition Sr2+Mn0.984+Mn0.023+O2.992-. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundariesclose

Small-polaron transport in perovskite nickelates

Abstract Knowledge of the explicit mechanisms of charge transport is preeminent for a fundamental understanding of the metal-to-insulator transition in ABO₃-type perovskite rare-earth nickelates and for potential applications of these technologically promising materials. Here we suggest that owing to intrinsic Jahn–Teller-driven carrier localization, small-polaron transport is innate in nickelates. We demonstrate experimental evidence for such transport by investigating AC conductivity over a broad range of temperatures and frequencies in epitaxial SmNiO₃ films. We reveal the hopping mechanism of conductivity, Holstein-type activation energy for hopping, nonclassical relaxation behavior, and nonclassical consistency between activation and relaxation. By analyzing these observations, we validate small-polaron transport. We anticipate that our findings can lead to precise tailoring of the DC and AC conductivity in nickelates as requested for fruitful employment of these materials. We also believe that further investigations of self-trapped small polarons are essential for a comprehensive understanding of nickelates

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

Abstract In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions

Strain enhancement due to oxygen vacancies in perovskite oxide films

Abstract Control of lattice strain in epitaxial films of ABO3 perovskite oxides is crucial for modern understanding and applications of these scientifically and technologically important materials. Here, we show that oxygen vacancies have unique impacts on lattice strain in such films. We suggest that in the presence of substrate-imposed misfit in epitaxial or highly oriented films, the crystallographic alignment of anisotropic elastic dipole tensors of oxygen vacancies is energetically favorable. The dipolar alignment leads to an enhanced above-misfit magnitude of maximal lattice strain and to increased inhomogeneous strain or strain gradients. The vacancy-induced remarkably strong elastic effects are experimentally validated by varying the misfit strain and oxygen content in thin films of perovskite niobate (ANbO3) and titanate (ATiO3) ferroelectrics. It is anticipated that elastic effects of oxygen vacancies are relevant for controlling strain in epitaxial films of a broad range of functional oxides

Mn diffusion in Ga1-xMnxAs/GaAs superlattices

Ga1-xMnxAs/GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1-xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1-xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1-xMnxAs/GaAs superlattices

Optical Properties of Ferroelectric Epitaxial K0.5Na0.5NbO3 Films in Visible to Ultraviolet Range.

The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K0.5Na0.5NbO3 films. The 20-nm-thick cube-on-cube-type epitaxial films are grown on SrTiO3(001) and DyScO3(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15-4.30 eV and differ by 0.2-0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films

Strain fluctuations in BaTiO₃/SrTiO₃ heterostructures

Abstract Epitaxy of perovskite oxide ferroelectric heterostructures with large lattice misfit is crucial for numerous emerging applications. Here we demonstrate cube-on-cube-type epitaxial growth of BaTiO₃ films on strongly mismatched (001)SrTiO₃ single-crystal substrates for the films with thicknesses significantly larger than that of misfit relaxation. The films experience strain originating from the film-substrate thermal expansion mismatch. Using a dedicated digital analysis of electron microscopy images, we show that the films contain random nanoregions of substantial strain fluctuations: the local strains vary in the range of ∼(0.5–2)% compared with the average strain magnitude of ∼1%. Because of strong strain-polarization coupling in ferroelectrics, fluctuations of strain produce fluctuations of polarization, which are suggested to cause relaxor-like properties in many mismatched heterostructures