George J. Valco

Nanoengineered device for drug delivery application

A high precision nanoengineered device was developed to yield long term zero-order release of drugs for therapeutic applications. The device contains nanochannels that were fabricated in between two directly bonded silicon wafers and therefore poses high mechanical strength. The fabrication is based upon selectively growing oxide and then removing it, and thus defining nanochannels by consuming a specified layer of silicon during oxide growth. Diffusion through the nanochannels is the rate limiting step for the release of drugs. A measurement of glucose released through such nanochannels validates the zero-order release profile. Device design, fabrication details and the glucose release profile through 60 nm channels are presented.

Hybrid high temperature superconductor/GaAs 10 GHz microwave oscillator: temperature and bias effects

Hybrid YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconductor/GaAs microwave oscillators have been designed, fabricated and characterized. The planar oscillators were built on a single 10 mm*10 mm LaAlO/sub 3/ substrate. The active elements in the hybrid oscillators were GaAs MESFETs. A ring resonator was used to select and stabilize the frequency. A superconducting ring resonator had a loaded Q at 77 Kg which was 8 times larger than the loaded Q of a ring resonator fabricated out of copper. S-parameters of the GaAs FET were measured at cryogenic temperatures and used to design the oscillator which had a reflection mode configuration. The transmission lines, RF chokes and bias lines were all fabricated from YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconducting thin films. The performance of the oscillators was measured as a function of temperature. The rate of change of the frequency as a function of temperature was smaller for an oscillator patterned from a pulsed laser deposited film than for an oscillator patterned from a sputtered film. As a function of bias at 77 K, the best circuit had an output power of 11.5 dBm and a maximum efficiency of 11.7% The power of the second harmonic was 25 dB to 35 dB below that of the fundamental, for every circuit. At 77 K, the best phase noise of the superconducting oscillators was 68 dBc/Hz at an offset frequency of 10 kHz and less than -93 dBc/Hz at an offset frequency of 100 kHz. At an offset frequency of 10 kHz, the superconducting oscillator had 12 dB less phase noise than the copper oscillator at 77 K. >

10 GHz YBa2Cu3O7-delta Superconducting Ring Resonators on NdGaO3 Substrates

YBa2Cu3O7- delta thin films were formed on NdGaO3 substrates by laser ablation. Critical temperatures greater than 89 K and critical current densities exceeding 2*106 A cm-2 at 77 K were obtained. The microwave performance of films patterned into microstrip ring resonators with gold ground planes was measured. An unloaded quality factor six times larger than of that of a gold resonator of identical geometry was achieved. The unloaded quality factor decreased below 70 K for both the superconducting and gold resonators due to increasing dielectric losses in the substrate. The temperature dependence of the loss tangent of NdGaO3 was extracted from the measurements.

A 10 GHz Y-Ba-Cu-O/GaAs hybrid oscillator proximity coupled to a circular microstrip patch antenna

A 10-GHz hybrid Y-Ba-Cu-O/GaAs microwave oscillator proximity coupled to a circular microstrip antenna has been designed, fabricated, and characterized. The oscillator was a reflection mode type using a GaAs MESFET as the active element. The feedline, transmission lines, RF chokes, and bias lines were all fabricated from YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconducting thin films on a 1-cm*1-cm lanthanum aluminate substrate. The output feedline of the oscillator was wire bonded to a superconducting feedline on a second 1 cm*1 cm lanthanum aluminate substrate, which was in turn proximity coupled to a circular microstrip patch antenna. Antenna patterns from this active patch antenna and the performance of the oscillator measured at 77 K are reported. The oscillator had a maximum output power of 11.5 dBm at 77 K, which corresponded to an efficiency of 10%. In addition, the efficiency of the microstrip patch antenna together with its high temperature superconducting feedline was measured from 85 K to 30 K and was found to be 71% at 77 K, increasing to a maximum of 87.4% at 30 K.<<ETX>>

Sequentially Evaporated Thin Film YBa2Cu3O7−x Superconducting Microwave Ring Resonator

There is great interest in the application of thin film high temperature superconductors in high frequency electronic circuits. A ring resonator provides a good test vehicle for assessing the microwave losses in the superconductor and for comparing films made by different techniques. Ring resonators made of YBa2Cu3O7−X have been investigated on LaA1O3 substrates. The superconducting thin films were deposited by sequential electron beam evaporation of Cu, Y, and BaF2 with a post anneal. Patterning of the superconducting film was done using negative photolithography. A ring resonator was also fabricated from a thin gold film as a control. Both resonators had a gold ground plane on the backside of the substrate. The ring resonators’ reflection coefficients were measured as a function of frequency from 33 to 37 GHz at temperatures ranging from 20 K to 68 K. The resonator exhibited two resonances which were at 34.5 and 35.7 GHz at 68 K. The resonant frequencies increased with decreasing temperature. The magnitude of the reflection coefficients is used in the calculation of the unloaded Q-values. The performance of the evaporated and gold resonator are compared with the performance of a laser ablated YBa2Cu3O7−X resonator. The causes of the double resonance are discussed.

Electrical characteristics of diamond films synthesized from methane/hydrogen and acetone/hydrogen mixtures

We present the results of detailed electrical measurements on diamond films grown by hot‐filament chemical‐vapor deposition. Two different mixtures of reaction species, hydrogen/methane and hydrogen/acetone, were utilized to grow diamond films. The latter is useful for introducing dopants during growth in a relatively safe manner. For the diamond films grown using hydrogen and methane, a high‐temperature anneal increased the resistivity of the films by seven orders of magnitude to about 1012 Ω cm while the I–V characteristics retained the same qualitative shape. Further annealing was found to change the I–V characteristics of the film itself, not the contacts. Spatial variation of the electrical characteristics is also reported. In addition, for the diamond films grown using the hydrogen and acetone, a variety of different results was obtained. Electrical measurements and Raman spectroscopy suggest that some areas of these films were high‐resistivity diamond while other areas may contain nondiamond carbon...

Photoresponse of YBa2Cu3O(7-delta) granular and epitaxial superconducting thin films

The response is reported of thin films of YBa2Cu3O(7-delta) with either a very grainy or a smooth epitaxial morphology to visible radiation. SrTiO3 substrates were employed for both types of films. The grainy films were formed by sequential multi-layer electron beam evaporation while the epitaxial films were formed by laser ablation. Both films were patterned into H shaped detectors via a negative photolithographic process employing a Br/ethanol etchant. The bridge region of the H was 50 microns wide. The patterned films formed by laser ablation and sequential evaporation had critical temperatures of 74 K and 72 K respectively. The bridge was current biased and illuminated with chopped He-Ne laser radiation and the voltage developed in response to the illumination was measured. A signal was detected only above the critical temperature and the peak of the response coincided with the resistive transition for both types of films although the correspondence was less exact for the grainy film. The details of the responses and their analysis are presented.

Sequentially evaporated thin Y-Ba-Cu-O superconductor films: Composition and processing effects

Thin films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} have been grown by sequential evaporation of Cu, Y, and BaF{sub 2} on SrTiO{sub 3} and MgO substrates. The onset temperatures were as high as 93 K, while T{sub c} was 85 K. The Ba/Y ratio was varied from 1.9 to 4.0. The Cu/Y ratio was varied from 2.8 to 3.4. The films were then annealed at various times and temperatures. The times ranged from 15 min to 3 hours, while the annealing temperatures used ranged from 850 {degree}C to 900 {degree}C. There was found a good correlation between transition temperature (T{sub c}) and the annealing conditions; the films annealed at 900 {degree}C on SrTiO{sub 3} had the best T{sub c}s. There was a weaker correlation between composition and T{sub c}. Barium poor films exhibited semiconducting normal state resistance behavior while barium rich films were metallic. The films were analyzed by resistance versus temperature measurements and scanning electron microscopy. The analysis is of the films and the correlations are reported.

Characterization of ZrO2 buffer layers for sequentially evaporated Y-Ba-CuO on Si and Al2O3 substrates

A large variety of techniques have been employed for the formation of thin films of the high temperature superconducting oxides on many different substrates. These efforts have been driven by the desire to investigate and develop electronic applications of these materials. Several of the substrates that have been used possess undesirable electronic properties, such as the large dielectric constant of SrTiO3. For many applications it is desirable to obtain the superconducting films on silicon or, for high frequency applications, on substrates such as Al2O3 or GaAs. Unfortunately, the deleterious interactions between the films and these substrates or common dielectrics1, such as silicon dioxide, result in unacceptable degradation of both the superconductor and the substrate. This has led to the investigation of the use of thin films of materials such as ZrO2 2 or silver3 as buffer layers between the superconductor and the substrate.

Proton Implanted Gallium Arsenide Optical Waveguides

Results of a study of proton implanted gallium arsenide (GaAs) waveguides are presented. The waveguides were made using standard implant equipment available to the microelectronics industry. Protons at energies of 190 Kev were implanted into heavily doped, n-type gallium arsenide. Implant doses were usually at 5E15 per cm2., with water used as the ion source. The implants were performed at room temperature, although some heating of the samples occurred. The main anneals were done at 350°C for 30 minutes. The waveguides were tested at 1.06μm using a YAG II laser. Loss measurements were done as a function of the substrate doping and etch pit density of the samples. Good guides were made using a doping of 3.5E18 cm-3 and etch pit density of 35000 cm-2. Low etch pit densities did not yield acceptable guides using the above fabrication procedures.