E. Hollmann

Ferroelectric Tuning of Planar and Bulk Microwave Devices

Benefits of ferroelectric component applications at microwaves is discussed. Experience recently gained in the high-temperature film-production technology has been used for obtaining high-quality ferroelectric tunable components. The disk made from bulk SrTiO3 single crystal covered with double-sided YBa2Cu3Y7 films was used as a high-quality TM010 mode tunable resonator. Planar structures containing thin film ferroelectric layers: planar capacitor, sandwich capacitor, coplanar line, and fin line have been studied. Modeling dielectric response of low-temperature incipient ferroelectrics (SrTiO3, KTaO3) has been applied for simulation of tunable planar structures. The quality factor of a tunable component (QFCT) is suggested to characterize the validity of the component for practical applications. The high-quality planar capacitors are pioneered for the applications. The wide frequency band fin line phase shifter has been studied and simulated. The prospects for applications of ferroelectric planar structures at room temperature is discussed.

Induced ferroelectricity in strained epitaxial SrTiO3 films on various substrates

The impact of strain on structure and ferroelectric properties of epitaxial SrTiO3 films on various substrate materials—substrates with larger (DyScO3) and smaller (NdGaO3 and CeO2/Al2O3) in-plane lattice constant, respectively—was analyzed. In all cases, (001)-oriented strained epitaxial SrTiO3 was obtained. It is demonstrated that the mismatch of the lattices or, alternatively, the mismatch of the thermal expansion coefficients of films and substrate, imposes biaxial strain on the SrTiO3 films. The strain leads to a small tetragonal distortion of the SrTiO3 lattice and has a large impact on the ferroelectric properties of the films. With decreasing film thickness and at low temperatures the permittivity deviates from the “classical” Curie-Weiss behavior. Furthermore, strain-induced ferroelectricity is observed, which agrees with theoretical predictions. For electric fields parallel to the film, surface-induced ferroelectricity is observed for SrTiO3 that is exposed to in-plane tensile strain, i.e., SrTi...

Effect of structural and morphological imperfections on the microwave surface resistance of YBCO thin films

Abstract The average grain size, non-uniform lattice distortion and the orthorhombic lattice parameters have been measured by means of X-ray diffraction analysis for a series of YBCO thin films. These films exhibited a microwave surface resistance ranging from less than 0.05 to 0.8 Ω (measured at 60 GHz and 77 K) but similarly high dc critical parameters ( T c = 89–90 K, J c (77 K,0 T) > 2×10 6 A/cm 2 ). It has been found for the examined films that the decrease of the orthorhombic splitting, which results from the lattice distortion in the a - b plane, correlates with the increase of the surface resistance. On the other hand, the perfection of the YBCO lattice along the c axis, i.e. the average grain size and the lattice distortion, does not affect the microwave losses in these films. The microwave losses in the YBCO films were also correlated to the film surface morphology.

Operation of hybrid current limiter based on high-Tc superconducting thin film

A high-Tc superconducting (HTSC) thin film with an inductive shunt was investigated as the model of a hybrid current limiter based on the superconducting-normal state transition. In addition to features previously observed in operation of other current limiters, sharp peaks of rather high frequency were found to appear on the background of a basic low-frequency voltage signal. This is attributed to self-oscillations of temperature and current in the superconductor. The influence of the frequency and magnitude of the circuit current on the character of self-oscillations was experimentally investigated. Qualitative analysis of the conditions for appearance of the self-oscillations under dc and ac conditions was carried out.

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 demonstrat...

Stress generated modifications of epitaxial ferroelectric SrTiO3 films on sapphire

The effect of lattice-mismatch induced stress upon the crystallographic structure, strain, strain relaxation, and the generation of different types of defects in heteroepitaxially grown SrTiO3 films on CeO2 buffered sapphire is examined. Depending on the thickness of the SrTiO3 layer, characteristic changes in the structural perfection of the layers, their crystallographic orientation with respect to the substrate system, and their strain is observed. For thin films misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The cracks are orientated along two predominant crystallographic orientations of the sapphire. The structural modifications and the formation of misfit defects and cracks are explained in a model based on lattice misfit induced stress, on the one hand, and energy considerations taking into account the stress release due to crack formation and the energy necessary for the formation of new surfaces at the crack, on the other h...

Pattern induced phase transition of vortex motion in high-Tc films

A micropattern induced transition in the mechanism of vortex motion and vortex mobility is demonstrated for high-Tc films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) leads to a sudden change in the direction of vortex motion that is accompanied by a change of the critical current density and microwave losses. The latter demonstrates the difference in vortex mobility in the different phases of vortex motion in between and within the rows of antidots.

Laser scanning microscopy of guided vortex flow in microstructured high-Tc films

We report the visualization of guidance of vortices by artificial microholes (antidots) in superconducting thin films using a low-temperature laser scanning microscope. Previously, guided motion of vortices via tilted rows of antidots in YBa2Cu3O7 films was detected indirectly by using resistive Hall-type measurements. Here we prove that vortices are steered between antidots into a priori chosen direction by imaging of resistive photoresponse with a spatial resolution down to about 1μm. We observe predominant paths for vortex motion. Vortices are nucleated and annihilated at antidots, i.e., antidots define starting and ending points of predominant vortex paths. Depending on the misorientation angle between rows of antidots and the current-driven direction of vortex motion, different channels dominate in antidot-guided vortex motion. Our experimental results can be explained by the n-channel model. Finally, we present direct measurements of the local critical currents. This technique can be used as a quant...

Impact of compressive in-plane strain on the ferroelectric properties of epitaxial NaNbO3 films on (110) NdGaO3

Epitaxial a-axis oriented NaNbO3 films are grown on (110) oriented NdGaO3 substrates. The lattice mismatch between substrate and film leads to compressive strain of ∼0.7% in the a-c plane. As a consequence, the in-plane permittivity and tunability are strongly enhanced compared to bulk NaNbO3, and a pronounced maximum in the temperature dependence of the permittivity occurs. Below the maximum at Tmax ≈ 250 K, ferroelectric behavior is observed that seems to vanish above Tmax. The pristine phase of the film at T < Tmax is antiferroelectric and is easily suppressed by small applied electric fields. The ferroelectric phase shows a relaxor type behavior.

Quantitative analysis of the guidance of vortices in superconducting films with magnetic dots

The guidance of vortices in superconducting Nb films with periodic arrangements of magnetic Ni dots is studied via simultaneous four-probe measurements in standard and Hall configuration. Ni dots are fabricated via e-beam lithography and lift-off technology. Arrays with periodicity down to 400 nm and dot size of 200 nm are obtained. Subsequently, the arrays are covered with a thin superconducting Nb layer. Simultaneous Hall and standard resistive measurements of the flux dynamic in Nb bridges with and without magnetic dots demonstrate and quantify the guidance of flux via the magnetic dots. Guidance of vortices is only possible in a limited current regime; guidance improves with decreasing temperature. Within the identified current regime most of the vortices (>40% for zero field) and in the optimum case all of the vortices are guided by the magnetic dots.