Kristin H. Gilchrist

A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc.) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties. Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices. The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges. In particular, we present an ultrafast surface-illuminated PD with 11.4-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication. Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application.

On-chip electron-impact ion source using carbon nanotube field emitters

A lateral on-chip electron-impact ion source utilizing a carbon nanotube field emission electron source was fabricated and characterized. The device consists of a cathode with aligned carbon nanotubes, a control grid, and an ion collector electrode. The electron-impact ionization of He, Ar, and Xe was studied as a function of field emission current and pressure. The ion current was linear with respect to gas pressure from 10−4to10−1Torr. The device can operate as a vacuum ion gauge with a sensitivity of approximately 1Torr−1. Ion currents in excess of 1μA were generated.

Piezoelectric scanning mirrors for endoscopic optical coherence tomography

A novel piezoelectric scanning mirror design for endoscopic optical coherence tomography (OCT) is presented. OCT is an interferometric technique providing microscopic tomographic sectioning of biological samples with mm-range penetration capability in tissue and is suited for integration with endoscopes using fiber-based light delivery to the sample. The piezoelectric scanning mirror was designed to provide wide-range rapid forwarding-looking scanning of the optical beam at the distal end of a compact catheter. The optical scanner provides a large ratio of mirror aperture to device size with rectangular mirror sizes ranging from 600 µm × 840 µm to 840 µm × 1600 µm. Static angular displacements up to ±7° (mechanical angle) were demonstrated and resonance frequencies of hundreds of Hz (and up to 1–2 kHz) were measured, depending on the mirror size. The imaging capability of the piezoelectric scanner was demonstrated using a bench-top spectrometer-based Fourier-domain OCT system.

High-throughput cardiac safety evaluation and multi-parameter arrhythmia profiling of cardiomyocytes using microelectrode arrays.

Microelectrode arrays (MEAs) recording extracellular field potentials of human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CM) provide a rich data set for functional assessment of drug response. The aim of this work is the development of a method for a systematic analysis of arrhythmia using MEAs, with emphasis on the development of six parameters accounting for different types of cardiomyocyte signal irregularities. We describe a software approach to carry out such analysis automatically including generation of a heat map that enables quick visualization of arrhythmic liability of compounds. We also implemented signal processing techniques for reliable extraction of the repolarization peak for field potential duration (FPD) measurement even from recordings with low signal to noise ratios. We measured hiPS-CMs on a 48 well MEA system with 5minute recordings at multiple time points (0.5, 1, 2 and 4h) after drug exposure. We evaluated concentration responses for seven compounds with a combination of hERG, QT and clinical proarrhythmia properties: Verapamil, Ranolazine, Flecainide, Amiodarone, Ouabain, Cisapride, and Terfenadine. The predictive utility of MEA parameters as surrogates of these clinical effects were examined. The beat rate and FPD results exhibited good correlations with previous MEA studies in stem cell derived cardiomyocytes and clinical data. The six-parameter arrhythmia assessment exhibited excellent predictive agreement with the known arrhythmogenic potential of the tested compounds, and holds promise as a new method to predict arrhythmic liability.

In Vivo Real-Time 3-D Intracardiac Echo Using PMUT Arrays

Piezoelectric micromachined ultrasound transducer (PMUT) matrix arrays were fabricated containing novel through-silicon interconnects and integrated into intracardiac catheters for in vivo real-time 3-D imaging. PMUT arrays with rectangular apertures containing 256 and 512 active elements were fabricated and operated at 5 MHz. The arrays were bulk micromachined in silicon-on-insulator substrates, and contained flexural unimorph membranes comprising the device silicon, lead zirconate titanate (PZT), and electrode layers. Through-silicon interconnects were fabricated by depositing a thin-film conformal copper layer in the bulk micromachined via under each PMUT membrane and photolithographically patterning this copper layer on the back of the substrate to facilitate contact with the individually addressable matrix array elements. Cable assemblies containing insulated 45-AWG copper wires and a termination silicon substrate were thermocompression bonded to the PMUT substrate for signal wire interconnection to the PMUT array. Side-viewing 14-Fr catheters were fabricated and introduced through the femoral vein in an adult porcine model. Real-time 3-D images were acquired from the right atrium using a prototype ultrasound scanner. Full 60° × 60° volume sectors were obtained with penetration depth of 8 to 10 cm at frame rates of 26 to 31 volumes per second.

Improved pulse-echo imaging performance for flexure-mode pMUT arrays

Piezoelectric micromachined ultrasound transducers (pMUTs) are potential candidates for catheter-based ultrasound phased arrays. pMUTs consist of lead zirconate titanate (PZT) thin film membranes formed on silicon substrates and are operated in flexure mode by driving the PZT film above its coercive field to induce flextensional motion. The fundamental operation of pMUT devices has been demonstrated; however, pulse-echo imaging has been limited to date. The objective of this work was to optimize transducer design for improved pulse-echo imaging performance. Flexure mode operation was optimized by (1) increasing transmit voltage above the PZT coercive field to induce ferroelectric domain switching, and (2) using partial cycle transmit pulses to increase the polarization in the PZT thin film and increase receive signal. As a result, pulse-echo images of tissue were obtained. 1-D arrays operating at 5 MHz were capable of resolving targets in a commercial tissue phantom as well as human anatomy. Real-time 3-D imaging was also demonstrated using 2-D arrays at 5 and 12.5 MHz. These results suggest that pMUTs have sufficient performance for application in ultrasound imaging with frequency range suitable for catheter-based phased-array transducers.

Applying microfabrication to helical vacuum electron devices for THz applications

A new class of helical THz vacuum electron devices is under development using unconventional applications of microfabrication technology, modern computer modeling, and novel materials. The resulting slow wave circuits consist of a coil of gold wire, smaller in outside diameter than a human hair, supported by a thin diamond sheet and suspended within a diamond box. This configuration will extend the operating range of the helical slow wave circuit into the THz frequency band. Previously, the advantages of the wide bandwidth and high efficiency of the helical slow wave circuit have been available only for operation at frequencies below 50 or 60 GHz because of the difficulty of winding small coils of wire and because it is impossible to transmit a significant beam current through the small aperture offered by the center of the helix. These obstacles are overcome by fabricating the helices lithographically and by passing the electron beam around the outside of the helix. The design and fabrication of a 650 GHz backward wave oscillator (BWO) will be described as well as proposed applications of this technology to traveling wave tubes (TWTs) operating at frequencies as high as 1.0 THz. A THz amplifier, possibly with multioctave bandwidth, would have a wide range of important applications.

95 GHz helical TWT design

The helical slow-wave circuit is an attractive choice for traveling wave tube amplifiers (TWTAs) because of its inherently large bandwidth and relatively high RF efficiency. Unfortunately, as the operational frequency increases beyond Q-or V-band, its use has been limited by conventional fabrication techniques, and by the difficulty of passing enough current through the center of such a small structure. This paper describes the design and fabrication status of a 95 GHz TWT using microfabrication technology to create and assemble the helix. The electron beam propagates as two kidney shaped beamlets between the helix outer diameter and barrel.

Microfabrication of diamond-based slow-wave circuits for mm-wave and THz vacuum electronic sources

Planar and helical slow-wave circuits for THz radiation sources have been made using novel microfabrication and assembly methods. A biplanar slow-wave circuit for a 650 GHz backward wave oscillator (BWO) was fabricated through the growth of diamond into high aspect ratio silicon molds and the selective metallization of the tops and sidewalls of 90 μm tall diamond features using lithographically created shadow masks. Helical slow-wave circuits for a 650 GHz BWO and a 95 GHz traveling wave tube were created through the patterning of trenches in thin film diamond, electroplating of gold half-helices, and high accuracy bonding of helix halves. The development of new techniques for the microfabrication of vacuum electronic components will help to facilitate compact and high-power sources for terahertz range radiation.

High voltage microelectromechanical systems platform for fully integrated, on-chip, vacuum electronic devices

We demonstrate a fully integrated, on-chip, vacuum microtriode capable of handling voltages up to 800V. The ability to operate at such high voltages is achieved by the addition of a 10μm thick silicon dioxide layer to the device. The device is fabricated using microelectromechanical systems fabrication principles and utilizes carbon nanotubes as field emitters. A dc amplification factor of 600 was obtained. This is the highest value reported for carbon nanotube-enabled microtriode devices. The high voltage capability of these microscale devices will enable their use in a wider variety of applications.