Thomas E. Dillon

Room temperature operation of epitaxially grown Si/Si0.5Ge0.5/Si resonant interband tunneling diodes

Resonant interband tunneling diodes on silicon substrates are demonstrated using a Si/Si0.5Ge0.5/Si heterostructure grown by low temperature molecular beam epitaxy which utilized both a central intrinsic spacer and δ-doped injectors. A low substrate temperature of 370 °C was used during growth to ensure a high level of dopant incorporation. A B δ-doping spike lowered the barrier for holes to populate the quantum well at the valence band discontinuity, and an Sb δ-doping reduces the doping requirement of the n-type bulk Si by producing a deep n+ well. Samples studied from the as-grown wafers showed no evidence of negative differential resistance (NDR). The effect of postgrowth rapid thermal annealing temperature was studied on tunnel diode properties. Samples which underwent heat treatment at 700 and 800 °C for 1 min, in contrast, exhibited NDR behavior. The peak-to-valley current ratio (PVCR) and peak current density of the tunnel diodes were found to depend strongly on δ-doping placement and on the annea...

Fabrication and characterization of three-dimensional silicon tapers

We present the fabrication of 3D adiabatically tapered structures, for efficient coupling from an optical fiber, or free-space, to a chip. These structures are fabricated integrally with optical waveguides in a silicon-on-insulator wafer. Fabrication involves writing a single grayscale mask in HEBS glass with a high-energy electron beam, ultra-violet grayscale lithography, and inductively coupled plasma etching. We also present the experimentally determined coupling efficiencies of the fabricated tapers using end-fire coupling. The design parameters of the tapered structures are based on electromagnetic simulations and are discussed in this paper.

Multiscale free-space optical interconnects for intrachip global communication: motivation, analysis, and experimental validation.

The use of optical interconnects for communication between points on a microchip is motivated by system-level interconnect modeling showing the saturation of metal wire capacity at the global layer. Free-space optical solutions are analyzed for intrachip communication at the global layer. A multiscale solution comprising microlenses, etched compound slope microprisms, and a curved mirror is shown to outperform a single-scale alternative. Microprisms are designed and fabricated and inserted into an optical setup apparatus to experimentally validate the concept. The multiscale free-space system is shown to have the potential to provide the bandwidth density and configuration flexibility required for global communication in future generations of microchips.

Epitaxially grown Si resonant interband tunnel diodes exhibiting high current densities

This study presents the room-temperature operation of /spl delta/-doped Si resonant interband tunneling diodes which were fabricated by low-temperature molecular beam epitaxy. Post growth rapid thermal annealing of the samples was found to improve the current-voltage (I-V) characteristics. Optimal performance was observed for a 600/spl deg/C 1 min anneal, yielding a peak-to-valley current ratio (PVCR) as high as 1.38 with a peak current density (J/sub p/) as high as 1.42 kA/cm/sup 2/ for a device with a 4-nm intrinsic Si tunnel barrier. When the tunnel barrier was reduced to 2 nm, a PVCR of 1.41 with a J/sub p/ as high as 10.8 kA/cm/sup 2/ was observed. The devices withstood a series of burn-in measurements without noticeable degradation in either the J/sub p/ or PVCR. The structures presented are strain-free, and are compatible with a standard CMOS or HBT process.

Si resonant interband tunnel diodes grown by low-temperature molecular-beam epitaxy

Si resonant interband tunnel diodes that demonstrate negative differential resistance at room temperature, with peak-to-valley current ratios greater than 2, are presented. The structures were grown using low-temperature (320 °C) molecular-beam epitaxy followed by a postgrowth anneal. After a 650 °C, 1 min rapid thermal anneal, the average peak-to-valley current ratio was 2.05 for a set of seven adjacent diodes. The atomic distribution profiles of the as-grown and annealed structures were obtained by secondary ion mass spectrometry. Based on these measurements, the band structure was modeled and current–voltage trends were predicted. These diodes are compatible with transistor integration.

Design and characterization of thin multiple aperture infrared cameras

We describe a multiple-aperture long-wave infrared camera built on an uncooled microbolometer array with the objective of decreasing camera thickness. The 5 mm thick optical system is an f/1.2 design with a 6.15 mm effective focal length. An integrated image is formed from the subapertures using correlation-based registration and a least gradient reconstruction algorithm. We measure a 131 mK NETD. The systems spatial frequency is analyzed with 4 bar targets. With proper calibration, our multichannel interpolation results recover contrast for targets at frequencies beyond the aliasing limit of the individual subimages.

Design and performance of a distributed aperture millimeter-wave imaging system using optical upconversion

Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is used in which the effective aperture size can be increased without the associated volumetric increase in imager size. However, such systems typically require high frequency (~ 30 - 300 GHz) signal routing and down conversion as well as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system operating at 35 GHz.

''p-on-n'' Si interband tunnel diode grown by molecular beam epitaxy

Si interband tunnel diodes have been successfully fabricated by molecular beam epitaxy and room temperature peak-to-valley current ratios of 1.7 have been achieved. The diodes consist of opposing n- and p-type δ-doped injectors separated by an intrinsic Si spacer. A “p-on-n” configuration was achieved for the first time using a novel low temperature growth technique that exploits the strong surface segregation behavior of Sb, the n-type dopant, to produce sharp delta-doped profiles adjacent to the intrinsic Si spacer.

Fiber-to-waveguide coupler based on the parabolic reflector

We present a fiber-to-waveguide coupling structure, the so-called vertical J coupler, based on the parabolic reflector. The device addresses the multiple objectives of high coupling efficiency, large bandwidth operation, polarization insensitivity, and compact footprint. The optical mode emanating from a fiber arranged normal to the plane of the substrate is incident underneath the parabolic reflector, turned through 90 degrees and focused into a dielectric waveguide. The viability of the coupler is demonstrated by finite-difference time-domain electromagnetic simulation as well as preliminary fabrication and optical testing of the device.

Passive 77 GHz millimeter-wave sensor based on optical upconversion

A passive millimeter-wave (mmW) sensor operating at a frequency of 77 GHz is built and characterized. The sensor is a single pixel sensor that raster scans to create an image. Optical upconversion is used to convert the incident mmW signal into an optical signal for detection. Components were picked to be representative of a single element in a distributed aperture system. The performance of the system is analyzed, and the noise equivalent temperature difference is found to be 0.5 K (for a 1 s integration time) with a diffraction limited resolution of ~8 mrad. Representative images are shown that demonstrate the phenomenology associated with this spectrum.