Publications

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Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

We developed new detector technologies to identify the presence of radioactive materials for nuclear forensics applications. First, we investigated an optical radiation detection technique based on imaging nitrogen fluorescence excited by ionizing radiation. We demonstrated optical detection in air under indoor and outdoor conditions for alpha particles and gamma radiation at distances up to 75 meters. We also contributed to the development of next generation systems and concepts that could enable remote detection at distances greater than 1 km, and originated a concept that could enable daytime operation of the technique. A second area of research was the development of room-temperature graphene-based sensors for radiation detection and measurement. In this project, we observed tunable optical and charged particle detection, and developed improved devices. With further development, the advancements described in this report could enable new capabilities for nuclear forensics applications.

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Conversion of plane waves to surface waves prior to detection allows key advantages in changes to the architecture of the detector pixels in a focal plane array. We have integrated subwavelength patterned metal nanoantennas with various detector materials to incorporate these advantages: midwave infrared indium gallium arsenide antimonide detectors and longwave infrared graphene detectors. Nanoantennas offer a means to make infrared detectors much thinner by converting incoming plane waves to more tightly bound and concentrated surface waves. Thinner architectures reduce both dark current and crosstalk for improved performance. For graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass plane wave absorption is insufficient. Using III-V detector material, we reduced thickness by over an order of magnitude compared to traditional devices. We will discuss Sandia's motivation for these devices, which go beyond simple improvement in traditional performance metrics. The simulation methodology and design rules will be discussed in detail. We will also offer an overview of the fabrication processes required to make these subwavelength structures on at times complex underlying devices based on III-V detector material or graphene on silicon or silicon carbide. Finally, we will present our latest infrared detector characterization results for both III-V and graphene structures.

Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (MWNTs) with Ohmic back contacts were grown in nanopore templates on arbitrary substrates. The templates were prepared by sputter depositing Nd-doped Al films onto W-coated substrates, followed by anodization to form an aluminum oxide nanopore array. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer. The WO3 can be selectively etched to enable electrodeposition of Co catalysts with control over the Co site density. This led to control of the site density of MWNTs grown by thermal chemical vapor deposition, with W also serving as a back electrical contact. Ohmic contact to MWNTs was confirmed, even following ultrasonic cutting of the entire array to a uniform height.

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Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors. Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design. Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector. © 2014 SPIE.

Graphene, a planar, atomically thin form of carbon, has unique electrical and material properties that could enable new high performance semiconductor devices. Graphene could be of specific interest in the development of room-temperature, high-resolution semiconductor radiation spectrometers. Incorporating graphene into a field-effect transistor architecture could provide an extremely high sensitivity readout mechanism for sensing charge carriers in a semiconductor detector, thus enabling the fabrication of a sensitive radiation sensor. In addition, the field effect transistor architecture allows us to sense only a single charge carrier type, such as electrons. This is an advantage for room-temperature semiconductor radiation detectors, which often suffer from significant hole trapping. Here we report on initial efforts towards device fabrication and proof-of-concept testing. This work investigates the use of graphene transferred onto silicon and silicon carbide, and the response of these fabricated graphene field effect transistor devices to stimuli such as light and alpha radiation.

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