P. M. Suherman

Comparison of Techniques for Microwave Characterization of BST Thin Films

Various techniques involving coplanar waveguide transmission lines, coplanar resonators, and interdigital capacitors, have been employed to characterize the dielectric properties of Ba_0.05Sr_0.95TiO₃ thin films at cryogenic conditions. The devices were patterned from a high-temperature superconductor deposited on top of a single Ba_0.05Sr_0.95TiO₃ thin film. The measurements from these various techniques show a good agreement

Effect of growth defects on microwave properties in epitaxial Ba0.5Sr0.5TiO3 thin films grown on (001) MgO by pulsed laser deposition

Ba0.5Sr0.5TiO3 (BSTO) films have been grown heteroepitaxially on (001) MgO substrates by pulsed laser deposition (PLD) to fabricate microwave phase shifters for the wide frequency range 45 MHz–50 GHz. Both as-grown and ex situ annealed films have a cube on cube epitaxial relationship with ⟨100⟩BSTO//⟨100⟩MgO. Threading dislocations are the dominant defects, mostly with Burgers vectors b = ⟨101⟩. Growth at 10−1 mbar oxygen pressure, compared to 10−4 mbar, resulted in significantly better properties. Ex situ annealing of the film grown at 0.1 mbar resulted in a reduction of 40% in threading dislocation density and a 40% increase in dielectric tunability.

Novel Tunable Bandpass Filter Realized Using Barium–Strontium–Titanate Thin Films

A novel three-pole electrically tunable narrow bandpass filter has been fabricated from bi-layer YBCO/Ba0.05Sr0.95TiO3 thin films. At 77 K, and without bias, the filter has a center frequency of 3.50 GHz and 5.8% bandwidth. Independent application of dc bias to each of the resonators of 120, 182, and 100 V shifted the center frequency to 3.68 GHz. The insertion loss was less than 1.4 dB and return loss was better than 20 dB. A similar filter was made using bi-layer Ag/Ba0.5Sr 0.5TiO3 thin films and measured at room temperature. The insertion loss of the filter was, as expected, much higher, although it showed improved tunability as compared with the YBCO version.

Structural, vibrational, and dielectric properties of a Ba0.5Sr0.5TiO3 thin film: Temperature and electric field dependence from Raman spectroscopy, x-ray diffraction, and microwave measurements

We present a Raman spectroscopy study of the phase transition occurring in a Ba0.5Sr0.5TiO3 thin film deposited by ablation laser on a MgO substrate. This Raman investigation is compared to x-ray diffraction studies of the temperature dependence of the structure and of the dielectric properties at microwave frequencies. These different probes evidence a diffuse phase transition in the range of 243–283K from the low temperature ferroelectric-tetragonal phase to the high temperature paraelectric cubic phase. Stabilization of the tetragonal phase in the thin film down to 83K was observed instead of the expected transition to an orthorhombic phase below 190K. This stabilization may be attributed to the tensile stress induced by the substrate. A careful analysis of the frequency dependence of the dielectric response suggested that the observed tunability and dielectric loss were determined by extrinsic effects such as charged defects and/or depletion layers.

Temperature Dependent Dielectric Properties of Coplanar Phase Shifters Fabricated from Ba0.5Sr0.5TiO3 Thin Films

This paper presents an analysis of the temperature dependent properties and performance of coplanar waveguide transmission lines on Ba 0.5 Sr 0.5 TiO 3 (BST) thin films. Comparison of the temperature dependent permittivity at microwave frequencies with models of heterogeneous ferroelectrics reveals significant physical properties, such as the Curie temperature, Curie-Weiss constant, and statistical defects in the films. The maximum phase shift and tunability are obtained at the temperature where the permittivity is highest.

Comparison of structural, microstructural, and electrical analyses of barium strontium titanate thin films

The results of structural and electrical characterizations of a barium strontium titanate (Ba0.5Sr0.5TiO3) film, including low temperature x-ray diffraction, low temperature, field dependent Raman spectroscopy, transmission electron microscopy, and low temperature, field dependent microwave measurements, are compared and contrasted. The structural characterization showed the film to be a good single crystal, epitaxial with the (001) MgO substrate. In the ferroelectric state, the tetragonal axis lies in plane leading to a 90° domain structure. A high density of misfit and threading dislocations was observed in the film. The transition to the paraelectric state on warming was shown to be diffuse and appeared to be strongly influenced by local variations in strain which were attributed to the defective microstructure. The dielectric response was also dominated by such effects.

BaxSr1-xTiO3 Thin Films for Tunable Microwave Devices in Cryogenic and Room Temperature Application

The microwave properties of YBCO/Ba 0.05 Sr 0.95 TiO 3 (BST5) and Ag/Ba 0.5 Sr 0.5 TiO 3 (BST50) films were characterized at 80 K and 290 K. At zero bias, the BST5 had a permittivity of 400 and a loss tangent of 0.02; the BST50 had a permittivity of 900 and a loss tangent of 0.1. The respective tunabilities at 3.6 V μ m− 1 were 15% and 44%. Analysis of the performance of phase shifters and tunable filters based on these films showed that both conductor and dielectric losses need to be considered in the selection of materials for devices. A high tunability alone does not guarantee good performance.

On-Wafer Measurements of Tuneability in Ba0.5Sr0.5TiO3 Thin Films

On-wafer dielectric measurements of Ba0.5Sr0.5TiO3 thin films have been established over broad frequency ranges (40 Hz–50 GHz), using metallic coplanar transmission lines patterned onto the films. The films show tuneability of 27% and 18% at 1.6 kV cm−1 for low and microwave frequencies, respectively.

Uncertainties in the permittivity of thin films extracted from measurements with near field microwave microscopy calibrated by an image charge model

The microwave microscope is a device which utilises near fields to characterise material properties of samples on length scales smaller than the operating wavelength. The errors associated with extracting the permittivity of a high permittivity thin film on a low permittivity substrate from measurements using such a device are found to be of the order of 25% when using a widely used image charge model of the tip-sample interaction. The uncertainties arise from the model-based extraction of the permittivity from the raw frequency shift data, and in the current case are shown to come from the assumption in the model that the tip of the microwave probe can be modelled as a sphere.The raw data from the microscope contain random uncertainties of the order of 1% and reveal variations in the properties of the thin film with sub-wavelength resolution demonstrating the microwave microscope as a sub-wavelength characterisation technique for thin films.

Superconducting Tunable Composite Right/Left-Handed Transmission Lines Using Ferroelectric Thin Films with a Resistive Bias Network

Ferroelectric/superconductor thin films (Ba_0.05Sr_0.95TiO₃/YBa₂Cu₃O_7-delta) are used to realize an electrically tunable, low-loss composite right/left-handed transmission line. A resistive line is implemented as both DC bias path and RF choke. The whole device maintains an all-planar configuration. The composite right/left-handed transmission lines are well matched showing a wide pass band. As a phase shifter, the figure-of-merit of the device is evaluated and compared with that of a room-temperature device based on Ba_0.5Sr_0.5TiO₃ and silver. The effect of wire bonding on the performance is discussed.