Carl H. Mueller

Electrospun polyaniline/polyethylene oxide nanofiber field-effect transistor

We report on the observation of field-effect transistor (FET) behavior in electrospun camphorsulfonic-acid-doped polyaniline/polyethylene oxide (PEO) nanofibers. Saturation channel currents are observed at surprisingly low source–drain voltages. The hole mobility in the depletion regime is 1.4×10−4 cm2/V s, while the one-dimensional (1-D) charge density (at zero gate bias) is calculated to be approximately 1 hole per 50 two-ring repeat units of polyaniline, consistent with the rather high channel conductivity (∼10−3 S/cm). Reducing or eliminating the PEO content in the fiber is expected to enhance device parameters. Electrospinning is thus proposed as a simple method of fabricating one-dimensional polymer FETs.

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.

Nonlinear behavior of thin film SrTiO3 capacitors at microwave frequencies

The voltage-dependent dielectric constant (e) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capac...

Optimization of SrTiO/sub 3/ for applications in tunable resonant circuits

A series of SrTiO/sub 3/ (STO) films have been grown at a wide range of deposition conditions in order to determine the optimal growth parameters to maximize ferroelectric tuning while maintaining the lowest dielectric losses. The deposition pressure of the ambient O/sub 2/ (40 mTorr<P/sub dep/<800 mTorr) and substrate temperature (750/spl deg/C<T/sub sub/<850/spl deg/C) were varied independently while the fluence was held constant at 2.0 J/cm/sup 2/. The deposited films were characterized by X-ray diffraction (XRD) and low frequency dielectric measurements (1 MHz). The out-of-plane STO lattice parameter (a/sub z/(film)) of the deposited films was found by XRD to be a strong function of the deposition pressure: a/sub z/(film)<a(bulk) at low pressures (P/sub dep/<65 mTorr) and a/sub z/(film)>a(bulk) at higher pressures (P(dep)>65 mTorr). The dielectric constant (/spl epsiv//sub r/) and loss tangent (tan/spl delta/) were determined as a function of applied field at room temperature (300 K) and at liquid nitrogen temperature (77 K). The low frequency dielectric properties of the STO films were found to be a weak function of the strain of the ferroelectric film.

Thin film multilayer conductor/ferroelectric tunable microwave components for communication applications

Abstract A study of Au/SrTiO3/YBa2Cu3O7-δ/LaAlO3 and(Au,YBa2Cu3O7-δ)/BaxSr1-xTiO3/LaAlO3 (x=0, 0.50, and 0.40) multilayered structures is presented. At 1.0 MHz, the largest tuning of Au/SrTiO3/YBa2Cu3O7-δ parallel plate capacitors corresponded to single-phased, epitaxial 300–500 nm thick SrTiO3 thin films deposited at 800 °C. For SrTiO3/LaAlO3 structures having SrTiO3 films of similar quality, we observed that at 1.0 MHz and 77 K, interdigital capacitors exhibit higher tunabilities and lower losses than parallel plate configurations, but required higher dc voltage. For a 300 nm thick SrTiO3 film, a 25 Ω YBa2Cu3O7-δ/SrTiO3/LaAlO3 phase shifter exhibited a phase shift ∼2.6 times larger than its Au/SrTiO3/LaAlO3 counterpart. At 19 GHz and 32 V, a 360°phase shift could in principle be achieved with coupled microstripline sections only 7.0 mm long. At 14 GHz, 77 K and 260 V, for 1.0 μm and 300 nm thick SrTiO3 films, 25 Ω 360° Au/SrTiO3/LaAlO3 phase shifters would be nominally 4.0 mm and 12 mm long, respectivel...

Ferroelectric thin film and broadband satellite systems

There are several microwave applications where tunable ferroelectric devices could play a key role in improving system performance, or they could provide the key technology that enables a system to be deployed. This article focuses on the Ka-band phased array antennas, but tunable filters, oscillators, and switches could also utilize these materials.

Response time and power handling capability of tunable microwave devices using ferroelectric films

Abstract The microwave (MW) performance of tunable stripline resonators with ferroelectric (SrTiO3 or (Ba, Sr)TiO3) planar capacitors under exposure to high voltage video pulses and MW (10 GHz) signals are studied. The resonator switching times were less than 30 nanoseconds, and these values were limited by the measurement apparatus. The dependence of film capacitance and MW loss in ferroelectric films as a function of MW signal level, and their variation during the pulse, were measured. The large signal MW dielectric constant and loss tangent varied by less than 10% with increasing MW voltage amplitude for MW field strengths up to 1 MV/m in SrTiO3 films, and 0.5 MV/m in (Ba, Sr)TiO3 films.

Tunable SrTiO/sub 3/ varactors using parallel plate and interdigital structures

The dielectric properties of SrTiO/sub 3/ (STO)/YBCO and STO films deposited on LaAlO/sub 3/ substrates were evaluated, for potential varactor applications wherein the capacitance is altered via a dc voltage applied across the STO film. The optimal growth temperature for the STO films in the STO/YBCO structures was 800/spl deg/C; higher temperatures resulted in interfacial degradation and poor film quality, and lower temperatures resulted in films with lower dielectric constants (/spl epsi//sub r/) and tunabilities. Interdigital varactors comprised of STO films deposited directly on LaAlO/sub 3/ displayed higher tunabilities and significantly lower losses.

Tunable Dielectric Materials and Devices for Broadband Wireless Communications

At present, wireless phone use is concentrated in highly developed countries, where over 15% of the population may own mobile phones. Conversely, the wireless penetration rate in underdeveloped countries is generally less than 1%. Next-generation service providers have established several goals designed to improve the capabilities of wireless communication systems and increase the attractiveness and affordability of this technology to a wider range of people. The first goal is to expand coverage to more areas via the use of satellite systems. Many people in underdeveloped countries have no access to phones of any type, and wireless systems are usually a more cost-effective means of providing service than wired networks. A reliable, easily accessible telecommunications infrastructure is essential to a solid economy, and most emerging nations are interested in using wireless systems as a means of building this infrastructure. There are a number of microwave applications where tunable dielectric devices could play a key role in improving system performance, or perhaps provide the key technology that enables a system to be deployed. Traditional tuning methods have not demonstrated that they are capable of meeting the performance and cost requirements for these applications, whereas prototype tunable dielectric devices have shown considerable promise. Although, there are still a number of materials and device issues that must be resolved before serious commercialization of tunable dielectric devices will take place, the foreseen impact of this technology is extraordinary.

Effect of SrTiO3 deposition temperature on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 structures

Abstract We report on the effect of the deposition temperature of SrTiO3 on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 thin film multilayer structures. In these structures, the YBa2Cu3O7-δ (YBCO) films were deposited at 800°C by laser ablation, followed by the in-situ deposition of the SrTiO3 (STO) layer at one of the following temperatures: 750°C, 650°C, 550°C, 450°C, 350°C, and 250°C. Gold (Au) films were deposited and patterned on top of the STO layer to form planar Au/STO/YBCO capacitor structures. The electrical response was studied by measuring the dielectric constant (er) and loss tangent (tanδ) of the ferroelectric film from 300–40 K, at 1.0 MHz, and at electric fields up to 100 kV/cm. Our results show 750°C to be a deposition temperature which allows for large variations of er with applied field, and with limited enhancement of tanδ. Lower deposition temperatures cause a reduction of the induced change in er and an increase in tanδ with applied field.