Todd L. Williamson

Fabrication of high density, high aspect-ratio polyimide nanofilters.

A novel fabrication process produces high porosity polymer nanofilters with smooth, uniform, and straight pores with high aspect ratios. The process utilizes electron beam lithography and energetic neutral atom beam lithography and epitaxy techniques. The method has the potential to produce a new generation of high-precision, very-high-porosity, biocompatible filters with pore sizes down to 100nm.

In-rich InGaN thin films: Progress on growth, compositional uniformity, and doping for device applications

A number of In-rich InGaN films with In contents in the 20–40% range have been grown at moderately low temperatures on sapphire and silicon substrates at high growth rates using a versatile molecular beam epitaxy-type technology that utilizes an energetic beam of N atoms called energetic neutral atom beam lithography and epitaxy to overcome reaction barriers in the group III-nitride system. Extensive characterization results on the crystalline, optical, and electrical properties of the In-rich InGaN materials are reported. It was found that N-rich growth conditions are required to produce materials that have excellent crystallinity, uniform compositions, and bright band edge photoluminescence. For In-rich InGaN growth on sapphire, electrical transport measurements show reasonably low carrier concentrations and high mobilities. Successful p-type doping of In-rich InGaN with ∼20% and ∼40% In contents is demonstrated, and preliminary results on the formation of a p–n junction are reported. For In-rich InGaN ...

Extended hot carrier lifetimes observed in bulk In0.265±0.02Ga0.735N under high-density photoexcitation

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In0.265Ga0.735N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 1016 cm−3. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 1018 cm−3. This is the longest carrier thermalization time observed in bulk InGaN alloys to date.

InGaN/Si heterojunction tandem solar cells

Tandem solar cells using InxGa1-xN heterojunctions with silicon as the active junction were fabricated using gas-source molecular beam epitaxy (MBE) and by a novel deposition method incorporating an energetic nitrogen atom source. N-type InGaN layers were grown on p-Si(111) to evaluate predicted low-resistance tunnel junction properties. Ohmic behavior was observed, showing that these junctions can be used to connect the two pn subcells of an InGaN/Si tandem without the requirement of the heavily doped layers used in current multijunction cells. Undoped and Mg-doped films were grown by MBE on n-Si(111) using a AlN buffer layer. Depletion is observed on the Si side of the junction and efficiencies approaching 5% were measured for this “hybrid” cell design. Conditions for achieving depletion on the p-InGaN and producing a “single-junction” tandem cell are discussed.

InGaN Thin Films Grown by ENABLE and MBE Techniques on Silicon Substrates

The prospect of developing electronic and optoelectronic devices, including solar cells, that utilize the wide range of energy gaps of InGaN has led to a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys. Recently, significant progress has been achieved in the growth and doping of InGaN over the entire composition range. In this paper we present structural, optical, and electrical characterization results from InGaN films grown on Si (111) wafers. The films were grown over a large composition range by both molecular beam epitaxy (MBE) and the newly developed “energetic neutral atomic-beam lithography & epitaxy” (ENABLE) techniques. ENABLE utilizes a collimated beam of ∼2 eV nitrogen atoms as the active species which are reacted with thermally evaporated Ga and In metals. The technique provides a larger N atom flux compared to MBE and reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible. Electrical characteristics of the junctions between n- and p-type InGaN films and n- and p-type Si substrates were measured and compared with theoretical predictions based on the band edge alignment between those two materials. The predicted existence of a low resistance tunnel junction between p-type Si and n-type InGaN was experimentally confirmed.

Characterization of the thin-film NbN superconductor for single-photon detection by transport measurements

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA(Dated: May 21, 2013)The fabrication of high-quality thin superconducting films is essential for single-photon detectors. Theirdevice performance is crucially a ected by their material parameters, thus requiring reliable and nondestructivecharacterization methods after the fabrication and patterning processes. Important material parameters to knoware the resistivity, superconducting transition temperature, relaxation time of quasiparticles, and uniformity ofpatterned wires. In this work, we characterize micro-patterned thin NbN films by using transport measurementsin magnetic fields. We show that from the instability of vortex motion at high currents in the flux-flow state ofthe IV characteristic, the inelastic life time of quasiparticles can be determined to be about 2 ns. Additionally,from the depinning transition of vortices at low currents, as a function of magnetic field, the size distribution ofgrains can be extracted. This size distribution is found to be in agreement with the film morphology obtainedfrom scanning electron microscopy and high-resolution transmission electron microscopy images.

Gamma-ray waveguides

We have developed an approach for gamma-ray optics using layered structures acting as planar waveguides. Experiments demonstrating channeling of 122keV gamma rays in two prototype waveguides validate the feasibility of this technology. Gamma-ray waveguides allow one to control the direction of radiation up to a few MeV. The waveguides are conceptually similar to polycapillary optics, but can function at higher gamma-ray energies. Optics comprised of these waveguides will be able to collect radiation from small solid angles or concentrate radiation into small area detectors. Gamma-ray waveguides may find applications in medical imaging and treatment, astrophysics, and homeland security.

High In content InxGa1−xN grown by energetic neutral atom beam lithography and epitaxy under slightly N-rich conditionsa)

Two series of In-rich InGaN films with compositions of ∼25% and ∼35% In, grown over a substrate temperature range from 490 to 620 °C, show how the film properties improve as the growth temperature is lowered below the InN decomposition temperature of ∼550 °C in vacuum. These InGaN films have been grown using a novel growth technique utilizing energetic N atoms as the active growth species. Under N-rich growth conditions, these InGaN films show how compositional uniformity, crystallinity, band edge photoluminescence, and surface morphology are improved as growth temperatures are reduced. The results emphasize the importance of energetic N atoms and lower substrate temperatures for overcoming difficulties associated with growing high-quality In-rich InxGa1−xN thin film materials. Utilizing energetic N atoms allows for the growth of high-quality, thick (>500 nm) InxGa1−xN films at temperatures below 500 °C.

Structural and optical investigations of GaN-Si interface for a heterojunction solar cell

In recent years the development of heterojunction silicon based solar cells has gained much attention, lead largely by the efforts of Panasonics HIT cell. The success of the HIT cell prompts the scientific exploration of other thin film layers, besides the industrially accepted amorphous silicon. The band gap, mobilities, and electron affinity of GaN make it an interesting candidate to solve problems of parasitic absorption while selectively extracting electrons. Using a novel MBE based growth technique, thin films of GaN have been deposited at temperature significantly lower than industry standards. Crystalline measurements and absorption data of GaN are presented. Additionally, effects of deposition on the silicon wafer lifetimes are presented.

Development of sensors and sensing technology for hydrogen fuel cell vehicle applications

One related area of hydrogen fuel cell vehicle (FCV) development that cannot be overlooked is the anticipated requirement for new sensors for both the monitoring and control of the fuel cells systems and for those devices that will be required for safety. Present day automobiles have dozens of sensors on-board including those for IC engine management/control, sensors for state-of-health monitoring/control of emissions systems, sensors for control of active safety systems, sensors for triggering passive safety systems, and sensors for more mundane tasks such as fluids level monitoring to name the more obvious. The number of sensors continues to grow every few years as a result of safety mandates but also in response to consumer demands for new conveniences and safety features.