22 research outputs found

    Characterization of crystalline structure and morphology of Ga2O3 thin film grown by MOCVD technique

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    Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H2 and N2. Obtained film is polycrystalline and predominantly consisted of (201) oriented ÎČ-Ga2O3. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H2 gas proved to have beneficial effect on film quality and overall growth process.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 CAMART

    Permanent photodoping of plasmonic gallium-ZnO nanocrystals

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    This work was supported by the Latvian Council of Science in the framework of FLPP (Plasmonic oxide quantum dots for energy saving smart windows, lzp-2018/1-0187). Tanel KÀÀmbre acknowledges financial support for the XPS instrumentation maintenance from the Estonian Centre of Excellence in Research project “Advanced materials and high- technology devices for sustainable energetics, sensorics and nanoelectronics” (TK141).Donor dopants in oxide semiconductors are compensated not only by valuable electrons but also by other point defects, leading to a decrease in electric conductivity and infrared absorption. We demonstrate that the electron compensation mechanism in Ga doped ZnO nanocrystals can be promoted by photodoping. Unexpectedly, the electrons from photodoping are stable in the open air for months.Latvian Council of Science lzp-2018/1-0187; Estonian Centre of Excellence in Research TK141; 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 CAMART

    Polarization and acoustic properties of barium-modified lead-free potassium–sodium niobate ceramics

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    The publication costs of this article were covered by the Estonian Academy of Sciences and the University of Tartu.Results of polarization and polarization switching in samples prepared by solid phase synthesis of the (1-х)(K0.5Na0.5)(Nb0.93Sb0.07)O3–xBaTiO3 + 0.5mol%MnO2 system at x = 0.01, 0.02, and 0.04 (KNNS7–xBT) studied over a wide range of temperatures are reported. The observed features of the dielectric nonlinearity at the ferroelectric and structural phase transitions in the materials are described. The dielectric response data of the samples of different contents of the admixture in the KNNS7–xВT ceramics are compared with the behaviour of their elastic properties.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 CAMART

    Behaviour of thermal expansion of (1-x)Pb(Ni1/3Nb2/3)O3–xPbTiO3 solid solutions

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    The publication costs of this article were covered by the Estonian Academy of Sciences and the University of Tartu.The features of the dependence of the bulk coefficient of thermal expansion of the (1-x)Pb(Ni1/3Nb2/3)O3-xPbTiO3 solid solution studied in the series of x = 0-0.8 in the temperature range from 100 to 750 K are reported. The obtained data and observed anomalies of thermal expansion are compared with dielectric properties, heat capacity, and the phase diagram of concentration. The anomalous and lattice constituents of deformation and of the coefficient of thermal expansion are analysed and the data on deformation are used to estimate the Burns temperature and the mean square polarization of the samples found to decrease with the decrease of the PbTiO3 concentration.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 CAMART

    Local structural investigation of hafnia-zirconia polymorphs in powders and thin films by X-ray absorption spectroscopy

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    Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphism in hafnia- and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and to separate structural from chemical effects. As a first step toward the analysis of thin films, we start with the analysis of bulk references. After that, we successfully demonstrate an approach that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance

    Local structural investigation of hafnia-zirconia polymorphs in powders and thin films by X-ray absorption spectroscopy

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    Björn Matthey (Fraunhofer IKTS, Dresden) is acknowledged for providing HfO2 and ZrO2 powders on short notice after DESY’s renowned customs office punished us. Parts of this research were carried out at Petra III at DESY, a member of the Helmholtz Association (HGF). The experiments on single Si:HfO2 thin film samples were performed at the CLAESS beamline at ALBA Synchrotron with the collaboration of ALBA staff. We would like to thank Edmund Welter for assistance (in using beamline P65) and DESY for enabling this research for proposal no. 20160591 and for travel support. T.S. acknowledges the German Research Foundation (DFG) for funding this work in the frame of the project “Inferox” (project no. MI 1247/11-2). B.J., J.L.J., and U.S. acknowledge funding from the Army Research Office through contract number W911NF-15-1-0593. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the U.S. National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphism in hafnia- and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and to separate structural from chemical effects. As a first step toward the analysis of thin films, we start with the analysis of bulk references. After that, we successfully demonstrate an approach that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance.DESY proposal no. 20160591; German Research Foundation MI 1247/11-2; Army Research Office W911NF-15-1-0593; State of North Carolina and the U.S. National Science Foundation (award number ECCS-1542015); 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 CAMART

    Ferroelectric Effects Probed by in situ Transmission Electron Microscopy

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    Ferroelectric perovskite oxides are widely used in sensors, actuators and optical modulators and, at the same time, they show promise for implementation in future applications such as energy storage, memory and cooling devices. At the infancy of the discovery of polycrystalline ferroelectrics, BaTiO3 was considered as a candidate for various applications, but the breakthrough for the commercialization of ferroelectrics came in 1952 when the ferroelectric PbZrO3 - PbTiO3 (PZT) solid solution system was discovered. PZT and other lead containing perovskite oxides remained on the forefront of scientific and industrial interest. However, concerns over the environmental and health hazards posed by these toxic materials in the electronic and electrical equipment created a legislation change. Nowadays, all applicable products in the EU must pass RoHS (Restriction of Hazardous Substances also known as Directive 2002/95/EC) compliance. This change in the industrial standards fueled research in lead-free ferroelectric materials once again. Attention is currently placed on materials based on BaTiO3, BaZrO3, (Na/K)NbO3, Na1/2Bi1/2TiO3. Their widespread application is yet to be realized as their physical properties pale in comparison to PZT. Substantial challenges also lie in the miniaturization of polar perovskites where effects at nano and microscopic scales become dominant, which inhibit their performance. In this thesis one of the most prominent lead-free ferroelectric perovskites BaTiO3 is studied by transmission electron microscopy (TEM) techniques in combination with in situ temperature and electric field control. In detail, a reliable focused ion beam sample preparation for in situ TEM electrical biasing for ferroelectrics is established. The method is tested on BaTiO3 at room temperature, where the electrical response is observed at the expected field values, further confirmed by finite element calculations. Domain area (polarization) - applied bias loops are directly measured revealing strong pinning at lower fields, while higher fields depin domain walls allowing for more free movement following Rayleigh's law. Upon increasing the temperature, BaTiO3 exhibits morphological transformations in its domain structure from ferroelectric 180° (close to room temperature) to ferroelastic 90° state (below the Curie temperature). Electrical biasing experiments of ferroelastic needle domains show two different forward growth mechanisms. In one case, the needle domains are shown to move freely, influenced mainly by Peierls-like potentials, while when perpendicular domains meet, their movement is hindered by strong domain domain pinning mediated by electromechanical fields. The latter one produces square shaped P-E like loops with distinct steps, indicative of Barkhausen pulses. Finally, the effect of ferroelectric phase transition on the electronic structure is studied with in situ electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculations. Core-loss EELS measurements show that changes in the Ti 3d states qualitatively agree with DFT calculations. Off-axis low-loss EELS allows precise bandgap measurements in both phases. Custom experimental and data treatment methodologies are developed to retrieve momentum resolved dielectric functions, which show excellent fit with DFT calculated ones at small momentum transfer. A pathway to study oxygen deficient perovskites with EELS is theoretically demonstrated with DFT

    Individual Barkhausen pulses of ferroelastic nanodomains

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    Ferroelectric materials, upon electric field biasing, display polarization discontinuities known as Barkhausen jumps, a subclass of a more general phenomenon known as crackling noise. Herein, we follow at the nanoscale the motion of 90 degree needle domains induced by an electric field applied in the polarization direction of the prototypical ferroelectric BaTiO3, inside a transmission electron microscope. The nature of motion and periodicity of Barkhausen pulses leads to real-time visualization of distinctive interaction mechanisms of the domains with each other but without coming into contact, a mechanism that has not been observed before, or/and with the lattice where the domain walls appear to be moving through the dielectric medium relatively freely, experiencing weak Peierls-like potentials. Control over the kinetics of ferroelastic domain wall motion can lead to novel nanoelectronic devices pertinent to computing and storage applications
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