Jeffrey M. Pond

Microwave properties of tetragonally distorted (Ba0.5Sr0.5)TiO3 thin films

A strong correlation is observed between the structure and the microwave dielectric properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films deposited onto (001) MgO by pulsed laser deposition. Films were deposited at 750 °C in an oxygen pressure that was varied from 3 to 1000 mTorr. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D=a/c) of the films depends on the oxygen deposition pressure. D varied from 0.996 at 3 mTorr to 1.003 at 800 mTorr. At microwave frequencies (1–20 GHz), BST films with low distortion have higher dielectric constants (e∼500), and lower dielectric loss (tan δ∼0.02) compared to films with higher distortion. The correlation of the microwave properties with the film structure can be attributed to stresses and polarizability in the film. The BST film grown at the oxygen deposition pressure of 50 mTorr exhibits a large dielectric constant change and a low dielectric loss at the same time, which corresponds to the film in low stress (D=1.0004). For tu...

The effect of annealing on the microwave properties of Ba0.5Sr0.5TiO3 thin films

Oriented, single phase thin films (∼5000 A thick) of Ba0.5Sr0.5TiO3 (BST) have been deposited onto (100) MgO and (100) LaAlO3 (LAO) substrates using pulsed laser deposition. The capacitance and dielectric Q (1/tan δ) of as-deposited and annealed films have been measured from 1 to 20 GHz as a function of electric field (0–80 kV/cm) at room temperature using interdigitated Ag electrodes deposited on top of the film. For films deposited onto MgO, it is observed that, after a postdeposition anneal (1000–1200 °C), the dielectric constant decreases and the dielectric Q increases. For films deposited onto LAO, a postdeposition anneal (⩽ 1000 °C) resulted in a significant increase in the dielectric constant and a decrease in Q. The observed dielectric properties of the BST films are attributed to the changes in film stress, which affects the extent of ionic polarization.

THE EFFECT OF ANNEALING ON THE STRUCTURE AND DIELECTRIC PROPERTIES OF BAXSR1-XTIO3 FERROELECTRIC THIN FILMS

The effect of a postdeposition anneal on the structure and dielectric properties of epitaxial BaxSr1−xTiO3 (BST) thin films (x=0.35–0.65) have been measured. The films were grown by pulsed laser deposition on LaAlO3 (001) substrates. The films were single phase and (001) oriented with a lattice parameter larger than the bulk. The dielectric properties of the x=0.35 film exhibited a broad temperature dependence and a peak at 168 K, which is 36 K below the peak observed in bulk BST (x=0.35). Annealing films for 8 h in flowing oxygen at 900 °C caused the lattice parameter to decrease and dielectric properties to become more like the bulk. Annealing also resulted in an increased electric field dependent dielectric tuning without increased dielectric loss.

Influence of strain on microwave dielectric properties of (Ba,Sr)TiO3 thin films

Epitaxial Ba1−xSrxTiO3 (BST) thin films have been deposited onto (100)MgO and LaAlO3 substrates using pulsed-laser deposition. Thick (>1 μm) Ag interdigitated capacitors capped with a thin protective layer of Au have been deposited on top of the BST films using electron-beam deposition. The capacitance (C) and dielectric quality factor (Q=1/tan δ) of the structure has been measured at microwave frequencies (1–20 GHz) as a function of electric field (E⩽67 kV/cm) at room temperature. In epitaxial BST films, either high dielectric tuning (4:1), which is defined as {[C(0)−C(E)]/C(0)}×100, or high dielectric Q (∼100–250) was observed but not both at the same time. Film strain was observed by x-ray diffraction and is closely related to the dielectric properties as limiting the ability to obtain both high tuning and high dielectric Q in epitaxial BST thin films. A thin BST buffer layer was used to relieve the strain in the films. In strain-relieved films, both dielectric tuning and dielectric Q were increased af...

On the application of complex resistive boundary conditions to model transmission lines consisting of very thin superconductors

A resistive boundary condition for the case where the resistivity is assumed to be a complex quantity is shown to be an accurate model for a superconducting film which is thin compared to the super-conducting penetration depth. The imaginary part of the conductivity is the dominant terms and is a measure of the inductive energy stored in the superconductor. Numerical solutions of superconducting microstrip are obtained and compared to experimental results and to analytic solutions for superconducting parallel-plate waveguides. Excellent agreement is found between experimental, analytical, and numerical results. >

Microwave measurement of the dielectric constant of Sr0.5Ba0.5TiO3 ferroelectric thin films

Measurement of the relative dielectric constant of a Sr0.5Ba0.5TiO3 (SBT) thin film is presented as a function of electric field strength and temperature over a broad frequency range using a microstrip transmission line. The transmission line was fabricated from a trilayer structure where the SBT film, grown by pulsed laser deposition, was bounded by silver and platinum metallization layers. Such structures involving ferroelectric films could be useful for microwave applications because of the substantially smaller bias voltages (≊1–10 V) compared to those required for bulk material. The SBT film was found to exhibit a dielectric constant of ≊120–250 and a large electric field modulation of ≊50% at 200 kV/cm. These properties of the material as well as the Curie temperature are compared to those of bulk SBT.

Penetration depth and microwave loss measurements with a YBa2Cu3O7−δ/LaAlO3/YBa2Cu3O7−δ trilayer transmission line

A trilayer YBa2Cu3O7−δ/LaAlO3/YBa2Cu3O7−δ transmission line was fabricated to measure the penetration depth of the high‐temperature superconducting layers, to determine the microwave losses, and to demonstrate the potential for practical high‐temperature superconductor multilayer microwave circuits. All layers were deposited onto a (100) MgO substrate by pulsed laser deposition. The superconducting films are c‐axis oriented with critical temperatures of 89 K. From the phase velocity measurements, the penetration depth was found to be consistent with weak‐coupled BCS theory in the local limit with a zero‐temperature value of 135 nm. A simple phenomenological temperature dependence for the penetration depth is also presented. This successful trilayer fabrication process, besides allowing the determination of the penetration depth and microwave loss, also expands the possible applications of the high‐temperature superconductors.A trilayer YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}/LaAlO{sub 3}/YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} transmission line was fabricated to measure the penetration depth of the high-temperature superconducting layers, to determine the microwave losses, and to demonstrate the potential for practical high-temperature superconductor multilayer microwave circuits. All layers were deposited onto a (100) MgO substrate by pulsed laser deposition. The superconducting films are {ital c}-axis oriented with critical temperatures of 89 K. From the phase velocity measurements, the penetration depth was found to be consistent with weak-coupled BCS theory in the local limit with a zero-temperature value of 135 nm. A simple phenomenological temperature dependence for the penetration depth is also presented. This successful trilayer fabrication process, besides allowing the determination of the penetration depth and microwave loss, also expands the possible applications of the high-temperature superconductors.

Microwave dielectric properties of strained (Ba0.4Sr0.6)TiO3 thin films

Ferroelectric Ba0.4Sr0.6TiO3 (BST) thin films grown on (001) MgO by pulsed laser deposition show a strong correlation between their structure and their microwave dielectric properties. Epitaxially grown BST films are observed by x-ray diffraction to be tetragonally distorted. The oxygen deposition pressure affects the magnitude of the tetragonal distortion (the ratio of in-plane and surface normal lattice parameters, D=a/c) of the deposited BST films. D varied from 0.996 to 1.004 at oxygen deposition pressure of 10–800 mTorr. The dielectric properties of BST films measured at microwave frequencies (1–20 GHz) exhibit an oxygen deposition pressure dependent dielectric constant (e=100–600), and quality factor Q (1/tan δ=10–60). The BST film grown at the oxygen deposition pressure of 200 mTorr exhibits the highest figure of merit [% tuning in e×Q0V, where % tuning is 100×(e0−eb)/e0, and e0 and eb are dielectric constant at 0 and 80 kV/cm]. This corresponds to the film with the lowest distortion (D=1.001). The observed microwave properties of the films are explained by a phenomenological thermodynamic theory based on the strain along in-plane direction of the films.Ferroelectric Ba0.4Sr0.6TiO3 (BST) thin films grown on (001) MgO by pulsed laser deposition show a strong correlation between their structure and their microwave dielectric properties. Epitaxially grown BST films are observed by x-ray diffraction to be tetragonally distorted. The oxygen deposition pressure affects the magnitude of the tetragonal distortion (the ratio of in-plane and surface normal lattice parameters, D=a/c) of the deposited BST films. D varied from 0.996 to 1.004 at oxygen deposition pressure of 10–800 mTorr. The dielectric properties of BST films measured at microwave frequencies (1–20 GHz) exhibit an oxygen deposition pressure dependent dielectric constant (e=100–600), and quality factor Q (1/tan δ=10–60). The BST film grown at the oxygen deposition pressure of 200 mTorr exhibits the highest figure of merit [% tuning in e×Q0V, where % tuning is 100×(e0−eb)/e0, and e0 and eb are dielectric constant at 0 and 80 kV/cm]. This corresponds to the film with the lowest distortion (D=1.001). The...

Structure/property relationships in ferroelectric thin films for frequency agile microwave electronics

Abstract Ferroelectric thin films, deposited by pulsed laser deposition (PLD), are currently being used to develop a new class of tunable microwave circuits based on the electric field dependence of the dielectric constant. Single phase, (100) oriented Ba0.5Sr0.5TiO3 (BST) films have been deposited onto (100) LaAlO3, SrTiO3, and MgO substrates. Interdigitated capacitors patterned on top of the ferroelectric film have been used to measure the dielectric constant and dissipation factor of these films as a function of DC bias and temperature at 1 MHz and as a function of DC bias and frequency (1 to 20 GHz) at room temperature. The dielectric properties of the ferroelectric film is sensitive to both the deposition and post processing conditions. Optical imaging of the ferroelectric films using confocal scanning optical microscopy (CSOM) shows reproducible polarization fluctuations over sub-micrometer length scales for BST films deposited onto SrTiO3 which are not observed for films deposited onto MgO. Dielect...

Strain-relieved Ba0.6Sr0.4TiO3 thin films for tunable microwave applications

This article discusses tunable dielectric thin films, with particularly emphasis on strain-relieved and defect-reduced tunable dielectric thin films that significantly reduce dielectric loss at microwave frequencies. Ba0.6Sr0.4TiO3 (BST) thin films (∼0.3 μm thick) were deposited onto (100) MgO single crystalline substrates by pulsed laser deposition at 750 °C and 200 mTorr O2 with a nominal 600-A-thick BST buffer layer. These films were observed to be strain relieved and show better dielectric properties by exhibiting a significantly high dielectric Q(=1/tan δ>100) while retaining a useful dielectric tuning (=[C(0)–C(23 V/μm)]/C(0)>20%, where C is the film capacitance) at 8 GHz compared to strained BST thin films. The BST buffer layer could be composed of any porous BST phase with randomly oriented grains between a nearly amorphous phase and a fully crystalline phase. A further increase in the dielectric Q was observed in strain-relieved BST thin films mixed with MgO and strain-relieved BST thin films dop...