A Novel Concept of an Oxide Thin Film Sensor for Integrated Filtering and Local Detection of Biosignals

Engineering of frequency dependence of ferroelectric via diffusion process has been done. A series of planar capacitors based on Ba0.5Sr0.5TiO3 applicable for filtering and signal detection was made. The properties of the capacitors were investigated as a function of deposition parameters and as a result of crystal structure of ferroelectric. Structures were obtained via RF-magnetron sputtering. Cryogenic and X-ray measurements showed that changing of substrate temperature of deposition directly influenced on formation of BSTO films. Analysis of permittivity and tangent loss as a function of frequency shows that received samples conform of the Maxwell-Wagner behavior. A comparison of received data suggests that changing of substrate temperature leads to different diffusion of platinum into BSTO films, which in turn has an influence on frequency characteristics

Characterization and Interpretation of the Critical Properties of 2nd Generation HTS Coated Conductors

Characterization and Interpretation of the Critical Properties of 2nd Generation HTS Coated ConductorsR. Wördenweber, E. Hollmann, and R. KutznerPeter Grünberg Institute (PGI-8), Forschungszentrum Jülich, 52425 Jülich, Germanye-mail: [email protected] Due to low irreversible fields, low current-carrying capabilities and, last not least, the production costs of the 1st generation HTS wires and cables, nowadays much effort is focused on development of the 2nd generation HTS conductors, i.e. the coated conductors that are based on the deposition of textured YBCO films on inexpensive tapes in long lengths. The biaxial alignment of the typical micrometer thick YBCO layer is achieved either by ion beam assisted deposition, inclined substrate deposition, or via rolling and recrystallisation of the metal tape (typically NiW) known as rolling assisted biaxially textured substrates (RABITS). In all cases a granular structure of the substrate is obtained which leads to a granular YBCO film. Nevertheless zero-field critical current densities are acchieved that are comparable to that of epitaxial grown YBCO films (e.g. Jc(77K, 0T) = 1-2MA/cm2). However, with the application of magnetic fields the critical current decreases strongly which hampers the use of the tapes for a number of applications. We develop a setup to examine the field and temperature dependence of YBCO coated conductors by resistive measurements with currents up to 500A. Chemically deposited as well as physically deposited tapes show the typical behavior, large critical currents in self-field that decrease strongly with the application of a magnetic field. Hysteretic effects in field sweep measurements indicate that the strong decrease is caused by the granular structure of the YBCO layer. Simulation of the field penetration using a critical state model verify this assumption and sketch possible routes to improvements of the critical properties of the tape in elevated fields by modifying the grain boundaries or the interconnectivity of the grains

Critical Properties of Large-Scale Deposited, All-Solution Coated Conductors

Due to low irreversible fields, low current-carrying capabilities and, last not least, the production costs of the 1st generation HTS wires and cables, nowadays much effort is focused on development of the 2nd generation HTS conductors, i.e. the coated conductors that are based on the deposition of textured YBCO films on inexpensive tapes in long lengths. The biaxial alignment of the typical micrometer thick YBCO layer is achieved either by ion beam assisted deposition, inclined substrate deposition, or via rolling and recrystallisation of the metal tape (typically NiW) known as rolling assisted biaxially textured substrates (RABITS). In all cases a granular structure of the substrate is obtained which leads to a granular YBCO film. Nevertheless zero-field critical current densities are acchieved that are comparable to that of epitaxial grown YBCO films (e.g. Jc(77K, 0T) = 1-2MA/cm2). However, with the application of magnetic fields the critical current decreases strongly which hampers the use of the tapes for a number of applications. We develop a setup to examine the field and temperature dependence of YBCO coated conductors by resistive measurements with currents up to 500A. For comparison magnetization data are recorded as function of the magnetic field and temperature using a standard PPMS system. Especially all-solution chemically deposited (CSD) and, for comparison, physically deposited tapes are characterized with these setups. Critical current densities in the range of 2.5-3.5MA/cm2 (equivalent to Ic/w up to 350A/cm) are obtained at 77K and selffield for the CSD samples, the typical lift factor is of the order of 2-2.5 for 30K and 1T. The field dependence shows the expected flux-line sheer dependence, however with a field-dependence that is steeper than predicted by the classical model. The field dependence is strongly affected by the morphology of the sample, optimization of the morphology leads to larger critical currents and a shallower field dependence. Finally, the role of local variations of the critical properties and their impact on the performance of the tapes are discussed

Continuous Large-Scale Deposition and Critical Properties of All-Solution Coated Conductors

Due to low irreversible fields, low current-carrying capabilities and, last not least, the production costs of the 1st generation HTS wires and cables, nowadays much effort is focused on development of the 2nd generation HTS conductors, i.e. the coated conductors that are based on the deposition of textured YBCO films on inexpensive tapes in long lengths. The biaxial alignment of the typical micrometer thick YBCO layer is achieved either by ion beam assisted deposition, inclined substrate deposition, or via rolling and recrystallisation of the metal tape (typically NiW) known as rolling assisted biaxially textured substrates (RABITS). In all cases a granular structure of the substrate is obtained which leads to a granular YBCO film. Nevertheless zero-field critical current densities are acchieved that are comparable to that of epitaxial grown YBCO films (e.g. Jc(77K, 0T) = 2-3MA/cm2). However, with the application of magnetic fields the critical current decreases strongly which hampers the use of the tapes for a number of applications. We develop a setup to examine the field and temperature dependence of YBCO coated conductors by resistive measurements with currents up to 500A. For comparison magnetization data are recorded as function of the magnetic field and temperature using a standard PPMS system. Especially all-solution chemically deposited (CSD) and, for comparison, physically deposited tapes are characterized with these setups. Critical current densities in the range of 2.5-3.5MA/cm2 (equivalent to Ic/w up to 350A/cm for 1m thick YBCO films) are obtained at 77K and selffield for the CSD samples, the typical lift factor is of the order of 2-2.5 for 30K and 1T. The field dependence shows the expected flux-line sheer dependence, however with a field-dependence that is steeper than predicted by the classical model. The field dependence is strongly affected by the morphology of the sample, optimization of the morphology leads to larger critical currents and a shallower field dependence. Finally, the role of local variations of the critical properties, weak-links, and hot-spots, and their impact on the performance of the tapes are discussed

Relaxor ferro- and paraelectricity in anisotropically strained SrTiO3 films

The ferroelectric properties of anisotropically strained SrTiO3 films are analyzed by detailed measurements of the complex dielectric constant as function of temperature, frequency, bias voltage and electric field direction. At low temperatures, strain induces a relaxor-ferroelectric phase that persists up to room temperature. The transition temperature and characteristic parameters (e.g. Curie temperature, static freezing temperature, degree of diffuseness of the phase transition, activation energy) of the relaxor phase depend strongly on the orientation of the electric field and therefore on the amount of structural strain in the given electric field direction. Also above the ferroelectric transition temperature a relaxation of the permittivity is visible, i.e. the strain causes a relaxor-paraelectric behavior. Only at high enough temperatures the relaxation time constant tends to zero and the ‘classical’ dielectric state is obtained. Frequency and time dependent relaxation experiments demonstrate an extremely large distribution of the relaxation rates in both relaxor states (ferroelectric and paraelectric) which is indicative for the large distribution in the mobility of polar SrTiO3 regions with randomly distributed directions of dipole moments in the film. The large distribution might be taken as an indication for a large distribution in size and orientation of nanosize domains in the anisotropically strained SrTiO3 film

Engineering the ferroelectric and resistivity Properties of Oxide Films via Compressive and Tensile Strain

Strain can strongly modify the electronic characteristics of oxide materials. For instance the phase transition from the ferroelectric to the dielectric state can be shifted by up to 300 K in either directions. As a result, room temperature permittivity can be enhanced significantly, e.g. for SrTiO3 from εRT≈600 to εRT≈25000. Moreover the resulting ferroelectrics are highly anisotropic and show a number of properties that are extremely interesting for various applications. In this work we try to perform a systematic study of the impact of strain on the system BaxSr(1-x)TiO3. Films with different stoichiometric and thickness are epitaxially grown on DyScO3, TbScO3 and GdScO3 substrates. The lattice mismatch between substrate and film leads to different in-plane compressive and tensile strain within -1.5% to 1.5% in these systems. Tensile strain causes an increase of the in-plane ferroelectric dielectric phase transition temperature, while compressive strain decreases the transition temperature. The films show a metal-insulator transition and an extremely large tunability, they represent relaxor-type ferroelectrics and the ferroelectric properties are highly anisotropic. The data are discussed in terms of existing model for relaxor-type ferroelectrics. The potential of these films for sensors (e.g. surface or bulk acoustic wave devices) is examined

Nonlinear Dielectric Response in Anisotropically Strained Epitaxial Ferroelectric Films

Strain can not only strongly modify the electronic characteristics of ferroelectric material, it can also induce interesting partially novel properties in their systems. In this work, the impact of ac and dc electric field and field direction on the dielectric properties of anisotropically strained epitaxial SrTiO3 films grown on DyScO3 are examined. The anisotropic lattice mismatch between the SrTiO3 film and DyScO3 leads to different in-plane tensile strain in the different crystalline direction of 0.95% and 1.05%, respectively. As a result, (i) the tensile strain leads to an increase of the ferroelectric-dielectric phase transition temperature to Tmax=288 K and Tmax=258 K under large and small tensile strain, respectively. (ii) With increasing amplitude of ac electric field, the extrinsic contribution to the dielectric permittivity increases nonlinearly, which indicates the dynamic of domain wall is activated by the ac field. (iii) The dielectric permittivity is strongly suppressed by an additional dc bias electric field for the temperature ranging from 180 K to 320 K. The different dielectric responses are discussed in the term of domain wall dynamic and pinning induced relaxor type model. Keywords: anisotropic strain, thin films, ferroelectrics, domain wall

Ferroelectric properties of anisotropically strained epitaxial NaNbO3 films grown on NdGaO3

Due to the lattice mismatch between the film and substrate, an anisotropically in-plain strain can be imposed to an epitaxially grown film, which (i) depends on a number of parameters and (ii) can strongly affect the electronic properties of this thin layer. NaNbO3 films are epitaxially deposited on (110) NdGaO3 via Metal Organic Chemical Vapor Deposition (MOCVD). X-ray analysis shows that up to a thickness of approximately 15nm, the film is completely strained, while above this thickness, relaxation of the compressive strain takes place. The analysis of the complex permittivity of three films of different thickness (27 – 80nm) as a function of temperature, electric field direction, AC and DC electric field reveals that (i) the compressive strain shifts the temperature of maximum (Tmax) from about 628K of a bulk NaNbO3 to close to room temperature of thin films, (ii) the room temperature permittivity of these strained films is enhanced by a factor of 1.06 – 2.74 compared to that of bulk material, (iii) there is a strong anisotropy in all ferroelectric characteristics for electric field orientation along the [110] and [001] orientations of NdGaO3, and (iv) a strong dependence of the permittivity on the ac amplitude of the electric field as well as the dc component of the electric field. The experimental results are discussed in terms of theories on domain wall mobility and pinning as well as effect of relaxor ferroelectrics. These strained films represent ideal candidates for the applications of sensors, such as surface acoustic wave sensor