John U. Trefny

Fabrication and characterization of Ba1−xSr1−xTiO3 tunable thin film capacitors

Abstract Polycrystalline Ba 1− x Sr x TiC 3 ( x = 0.9) thin films have been successfully prepared from sintered stoichiometric ceramic targets by means of the 90 °C off-axis r.f. sputtering technique at substrate temperatures from 500 to 600 °C. Films grown on lanthanum aluminate(100) at 550 °C are predominantly c -axis oriented. Those deposited on sapphire(⊼1102) did not reveal any preferred growth orientation. Dielectric measurements were performed using planar capacitor geometries. The dielectric constant of these films showed a broad ferroelectric-to-paraelectric transition at 50 to 70 K, depending on the thickness of the films. The dielectric constant of these films also showed a small electric field dependence, changing by 5% at 77 K and at a field strength of 20kVcm −1 . The results are compared with those obtained from bulk samples, and are interpreted in the framework of phenomenological theory.

TUNABILITY OF THE DIELECTRIC CONSTANT OF BA0.1SR0.9TIO3 CERAMICS IN THE PARAELECTRIC STATE

Ba1−xSrxTiO3 (x = 0.9) ferroelectric ceramics were prepared successfully using a new wet technique and their structure and dielectric properties compared with those synthesized by the solid-state reaction method. The voltage dependence of the dielectric constant in the paraelectric phase was examined. It was found that the dielectric constants of these materials, prepared by both methods, exhibit large changes with applied voltage in the paraelectric phase. Tunability (the percentage change of the dielectric constant from its zero-bias value in the presence of a de-biasing electric field) was observed to exceed 30% at only 1.7 kV/cm at 77 K in the samples prepared by the solid-state reaction method. The tunability was found to decrease dramatically as the operating temperatures increased above the Curie point. These observations are interpreted in light of an existing phenomenological theory. The dependence of the dielectric constant in the paraelectric state upon a de-biasing electric field is also demonstrated as a potential method for the characterization of dielectric nonuniformities in ferroelectric ceramics.

Effects of Postdeposition Heat‐Treatment on Morphology and Microstructure of CdTe Grown by Electrodeposition

The effects of postdeposition heat-treatment on the morphology and microstructure of thin-film CdTe grown by electrodeposition on CdS/SnO 2 /glass substrates have been investigated. Scanning electron microscopy analysis indicates that the grains of as-deposited CdTe thin films have a faceted morphology with an average grain size of ∼0.4 μm. After annealing, the facets become less well-defined and the tops of the grains become domed or rounded. Grain growth studies indicate that the grain growth exponent n is smaller than the usual value of 0.5. X-ray diffraction (XRD) measurements confirm that as-grown CdTe thin films have a zinc-blende structure with a strongly preferred orientation such that the cubic directions are perpendicular to the film plane while annealed CdTe thin films show near-random orientations, indicating that recrystallization occurs during the heat-treatment. The dynamics of recrystallization was investigated by using the ratio of integrated intensities of XRD peaks as an indication of the degree of recrystallization. It was observed that the orientation first tends toward randomness upon annealing but for longer time anneals tends to a preferred orientation of the cubic axes again. The recrystallization activation energy was determined to be 2.5 ± 0.3 eV The presence of CdCl 2 in the postdeposition annealing process was observed to accelerate the recrystallization. The residual stress in CdTe films as a function of annealing temperature and time was determined and the type of residual stress changed from compressive to tensile during the annealing.

Thermal transport properties of single crystal lanthanum aluminate

The thermal diffusivity and thermal conductivity of single crystal (100) lanthanum aluminate (LaAlO3) have been determined in the temperature range 77–353 K. The thermal diffusivity was measured using a transient heat‐pulse technique and the thermal conductivity by a steady state dc technique. The specific heat capacity was calculated from the thermal diffusivity and thermal conductivity data obtained. The temperature dependence of the thermal diffusivity was found to be of the form [T exp(TD/bdT)] and that of the thermal conductivity of the form [T3 exp(TD/bcT)]. Both results are consistent with the expected behavior of the thermal transport, limited by phonon‐phonon collision (‘‘Umklapp’’) processes, in high purity dielectric single crystals. The Debye temperature (TD) was calculated as 720±22 K and the parameters bd and bc were found to be 1.4±0.1 and 0.7±0.1, respectively. The average sound velocity was calculated to be (5.4±0.6)×105 cm/s. The phonon mean‐free‐path at 303 K was found to be (2.2±0.2)×1...

The Art of Experimental Physics

Physics - A Human Endeavour Objectives of the Physics Laboratory Laboratory Notebook Error Analysis Significant Figures Graphical Analysis Curve Fitting Journals Of Physics Responsibility of the Experimentalist Spectroscopy: An Important Class of Experiments Laboratory Instrumentation Transmission of Radiation: Coaxial Cable Waveguide Fibre Optics Lasers: Helium, Neon and Helium-Neon Laser Spectra Laser Cavity Modes Microcomputers Computer Assisted Experimentation Fundamental Experiments Thermodynamics of Photons and Electrons: Electron Physics Blackbody Radiation Waves and Particles: Photoelectron Spectroscopy Bragg Spectroscopy Atomic and Molecular Physics: Frank-Hertz Experiment Isotope Structure and Fine Structure of Hydrogen Molecular Structure and Properties of N142 Effect Zeeman Condensed Matter Physics: Nuclear Magnetic Resonance Electron Spin Resonance Electrical Conductivity and the Hall Effect Nuclear Physics: Interactions of Gamma Rays with Matter Compton Scattering Ionization of Gases by Alpha Particles Rutherford Scattering Physical Optics Faraday Effect.

Experiments using a tunnel diode oscillator

A description is given of an apparatus suitable for measuring magnetic susceptibility in the undergraduate laboratory. Several applications are considered in detail, and extension of the technique to nonmagnetic studies is briefly discussed.

Fabrication of Ba1−xSrxTiO3 tunable capacitors with Tl2Ba2Ca1Cu2Ox electrodes

Abstract Tunable coplanar Ba 1− x Sr x TiO 3 (BSTO) thin-film capacitors with superconducting TlBaCaCuO (2212) electrodes have been successfully prepared using RF sputtering onto LaAlO 3 (100) substrates. The capacitors were fabricated in a five-step process consisting of: deposition of a Ba 2 CaCu 2 O x precursor films on LaAlO 3 substrates; patterning of the precursor film into a planar capacitor with a gap of 5 to 40 μm; formation of the superconducting phase by vapor-phase reaction with thallium; deposition of an amorphous BSTO overlayer onto the gap; and finally post-deposition annealing of the BSTO layer in flowing oxygen. The details of each fabrication step are presented. In addition, the structure, microstructure and composition of the films are anaylzed. The temperature- and the voltage-dependence of the low frequency (10 kHz–100 kHz) capacitance and loss tangent are presented. A tunability of 10% at 77 K and 100 kHz was achieved. The capacitance-tunability results are compared with those obtained from YBCO-BSTO-YBCO capacitors.

Thermoelectric power of YBa2Cu3O7−δ superconductive thin films as a function of oxygen contents

Experimental correlations of thermoelectric power (TEP), oxygen deficiency (δ), and c -lattice constant in YBa 2 Cu 3 O 7−δ (YBCO) c -oriented, polycrystalline, thin-film, high-temperature oxide superconductors are presented. The TEP values in the normal state were observed to be positive for samples with oxygen deficiencies between 0.08 ≤ δ ≤ 1.00 and negative for 0.00 ≤ δ ≤ 0.08. Furthermore, the TEP magnitudes were highly sensitive to the amount of oxygen and were found to increase with increasing oxygen deficiency, ranging from values of −2 μ V/K to more than two orders of magnitude higher. In addition, relatively temperature-independent TEP values were observed for hole-conductive samples with low oxygen deficiencies (δ ≤ 0.30). The lattice constants (c) of the same samples were measured by XRD. These were found to increase with oxygen deficiency and to correlate systematically with the TEP values. We conclude that TEP is a reliable indicator of the oxygen content in YBCO materials.

Thermoelectric power of indium tin oxide films

Abstract The thermoelectric powers S of several indium tin oxide films of known carrier densities were measured in the temperature range from 120 to 300 K. Each film was studied in three states: as deposited, after annealing in forming gas at 475 K and after annealing at 575 K. All the data exhibited linear behavior when plotted as 1 S vs. 1 T . These plots yielded values of the reduced Fermi energy and its temperature coefficient for each sample. The data show an interesting correlation between these two material parameters which is not explained.

Absorption of sound in helium vapor

We have studied the absorption of sound in helium vapor using a broad band heat pulse technique. The results are in good accord with classical hydrodynamics and with experimental values of the viscosity and thermal conductivity.