Fred A. Herrero

Fabrication of high aspect ratio Si nanogratings with smooth sidewalls for a deep UV-blocking particle filter

To measure space plasmas and neutral particles one must filter out high-energy ultraviolet photons that would increase background count or damage sensors. To enable sensitive neutral particle measurements, a photon-to-particle rejection rate of 1014 is desired, far exceeding the requirements of prior filters. The authors propose a high-aspect ratio Si grating with densely packed, sub-100 nm slits. In this article, the authors report the development of a new technique for fabricating sturdy, self-supported transmission gratings in silicon using nanoimprint lithography and deep reactive ion etching, resulting in grating slits with scalloping under 7 nm and high (8.5:1) aspect ratios.

Comparison of 4H-SiC impact ionization models using experiments and self-consistent simulations

We report comparisons of measured photocurrent versus voltage curves of avalanche photodiodes (APDs) with those calculated using different 4H-SiC hole and electron impact ionization coefficients. As the published impact ionization coefficients result in ionization rates that differ greatly in magnitude, the predicted breakdown voltages using these models vary by many volts. To this end, we investigate the breakdown voltage prediction capability of three prevailing impact ionization models in conjunction with several experiments. To obtain APD performance numerically, we developed a device simulator, which shows that the inclusion of proper electric field-dependent impact ionization rates can accurately predict a variety of measured current-voltage curves, breakdown voltages, and current multiplication rates.

A micromachined flat plasma spectrometer (FlaPS)

Through the application of a new approach to energy analysis to microelectromechanical systems (MEMS), the Flat Plasma Spectrometer (FlaPS) presented here provides a solution to the investigation of plasma distributions in space. It is capable of measuring the kinetic energy and angular distributions of ions/electrons in the space environment for energies ranging from a few eV to 50keV. A single pixel of a FlaPS instrument has been designed, built and tested to occupy a volume of approximately one cubic centimeter, and is characterized by a high throughput-to-volume ratio, making it an ideal component for small-scale satellites. The focus of this paper is on the design, fabrication, simulation, and testing of the instrument front end that consists of a collimator, parallel plate energy analyzer, and energy selector mask. Advanced micro-fabrication techniques enable fabrication of the miniature plasma spectrometer with geometric factor 4.9x10-5 cm2-sr per pixel and an entrance aperture area of 0.01cm2. Arrays of narrow collimator channels with 4° angular divergence and high transmission allow energy analysis of ions/electrons without the need for focusing, the key feature that enables large mass reduction. It is also shown that the large plate factors achievable with this approach to energy analysis offers definite advantages in reducing the need for excessively high voltages.

Development of ultra-high sensitivity wide-band gap UV-EUV detectors at NASA Goddard Space Flight Center

Rapid progress in the AlGaN (Eg=3.4-6.2eV), 4H-SiC (Eg=3.2eV) and ZnMgO (Eg=2.8-7.9eV) material systems over the last five years has led to the demonstration of a number of opto-electronic devices. These wide energy band gap devices offer several key advantages for space applications, over conventional Si (Eg=1.1eV) based devices, such as visible-blind detection, high thermal stability, better radiation hardness, high breakdown electric field, high chemical inertness and greater mechanical strength. Furthermore, the shorter cut-off wavelength of these material systems eliminates the need for bulky and expensive optical filtering components mitigating risk and allowing for simpler optical design of instrumentation. In this paper, we report on the development at NASA/Goddard of ultra-sensitive, high quantum efficiency AlGaN and 4H-SiC Schottky barrier UV-EUV photodiodes, 4H-SiC UV single photon avalanche diodes, large format 256x256 AlGaN UV p-i-n photodiode arrays and recent progress in elemental substitution for p-type and enhanced n-type doping of ZnO.

Wind Ion-drift Neutral Composition Suite cathode activation procedure and current–voltage characteristics

The Wind Ion-drift Neutral Composition Suite (WINCS) uses three BaO thermionic cathodes in three ion sources for its neutral air measurements. The cathode activation procedure, obtained in laboratory measurements on a series of stock WINCS cathodes, ensures optimum cathode emission and life. The procedure begins by heating the cathode to 300–500°C to evolve CO2 and other gaseous products of the binder and the BaCO3; then the cathode temperature is raised to above 900°C for breakdown to BaO and sintering some of the Ba into the tungsten substrate; finally, activation begins by applying a small extraction voltage to the anode in front of the cathode. After activation, the cathode is ready to operate with any selected anode voltage. Electron emission of the WINCS cathodes easily exceeds 1 mA, and the fraction transmitted through the WINCS anodes exceeds 10% as required for WINCS. A maximum electron kinetic energy of about 90 eV was established as safe, also providing optimal ionization efficiency.

WINCS on-orbit performance results

The Winds-Ions-Neutral Composition Suite (WINCS) instrument, also known as the Small Wind and Temperature Spectrometer (SWATS), was designed and developed jointly by the Naval Research Laboratory (NRL) and NASA/Goddard Space Flight Center (GSFC) for ionosphere-thermosphere investigations in orbit between 120 and 550 km altitude. The WINCS instrument houses four spectrometers in a single package with size, weight, and power compatible with a CubeSat. These spectrometers provide the following measurements: neutral winds, neutral temperature, neutral density, neutral composition, ion drifts, ion temperature, ion density and ion composition. The instrument is currently operating on the International Space Station and on the STP-Sat3 spacecraft. Data from the Ion-Drift Temperature-Spectrometer (IDTS) are used to compute the ion drift, temperature, and density in the presence of large changes in spacecraft potential. A summary is given of future flight manifests.

A gas kinetic method for investigations of the ionospheric plasma and the thermosphere

The gas kinetic method (GKM) obtains the Maxwellian velocity distributions of ionospheric plasma and thermospheric species (e.g. O +, O +2, O and N 2) from in situ measurements of energy–angle distributions of the air stream entering an orbiting spectrometer. Referenced to the spectrometer ram axis, measurements based on the GKM yield the ion-drift or neutral wind vector , the ion or neutral temperature T, and ion or neutral number density n with sensitivities that may be implemented with present technologies for energy–angle spectrometers, i.e. the CubeSat WINCS spectrometers, designed to measure ion-drift and wind vectors together with ion and neutral temperatures, and composition.

Numerical modeling and design of single photon counter 4H-SiC avalanche photodiodes

We report device performance investigation of 4H-SiC avalanche photodiodes (APDs) with or without absorbing AlGaN cap layers, as 4H-SiCspsila potential use in single photon counter APDs have drawn interest. Wide bandgap 4H-SiC photodiodes have low dark current levels at high temperatures and under intense radiation compared to silicon, and the 4H-SiC APDs are ldquosolar blindrdquo - transparent to the sunpsilas visible spectrum. Additionally, they offer avalanche multiplications approaching a million, making single photon count possible. However, design and optimization of single photon counter APDs require a thorough understanding of impact ionization and optical absorption at the material level, and steady-state and transient APD operation at the device level. Here we address both of these levels.

Graphene Chemical Sensor for Heliophysics Applications

Graphene is a single layer of carbon atoms that offer a unique set of advantages as a chemical sensor due to a number of its inherent properties. Graphene has been explored as a gas sensor for a variety of gases, and molecular sensitivity has been demonstrated by measuring the change in electrical properties due to the adsorption of target species (Schedin, F.; Geim, A.K.; Morozov, S.V.; Hill, E.W.; Blake, P.; Katsnelson, M.I.; Novoselov, K.S. Nat. Mater 2007, 6, 652–655. doi:10.1038/nmat1967). In this paper, we discuss the development of an array of chemical sensors based on graphene and its relevance to plasma physics due to its sensitivity to radical species such as O+, H+ and the corresponding neutrals. We briefly discuss the great impact such sensors will have on a number of heliophysics applications such as ground-based manifestations of space weather.

A numerical study of the 30° parallel plate analyzer for ionospheric electron spectroscopy

A charged-particle spectrometer is described that uses a configuration of the 30° parallel plate analyzer that differs from the previous one in that the entrance slit sits at the base plate while the exit slit is, as before, displaced to a point beneath the base plate. This approach preserves the second-order focusing previously found, yielding energy resolution of 0.01−0.3 eV for electrons in the energy range 1−30 eV. This is sufficient to address two identified problems in ionospheric electron energy distributions. In addition, the analyzer provides focusing of electron energies along a flat plane to operate as an energy spectrograph with a geometric factor of 10−3 cm2-sr at an energy resolution Δ K/K=0.01; the performance required for very high resolution photoelectron spectroscopy in the ionospheric plasma.