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All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Atomically Precise Surface Engineering for Producing Imagers

High-quality surface coatings, and techniques combining the atomic precision of molecular beam epitaxy and atomic layer deposition, to fabricate such high-quality surface coatings are provided. The coatings made in accordance with the techniques set forth by the invention are shown to be capable of forming silicon CCD detectors that demonstrate world record detector quantum efficiency (>50%) in the near and far ultraviolet (155 nm-300 nm). The surface engineering approaches used demonstrate the robustness of detector performance that is obtained by achieving atomic level precision at all steps in the coating fabrication process. As proof of concept, the characterization, materials, and exemplary devices produced are presented along with a comparison to other approaches.

Greer, Frank↗
NASA NTRS2013

High Throughput, High Yield Fabrication of High Quantum Efficiency Back-Illuminated Photon Counting, Far UV, UV, and Visible Detector Arrays

In this paper we discuss the high throughput end-to-end post fabrication processing of high performance delta-doped and superlattice-doped silicon imagers for UV, visible, and NIR applications. As an example, we present our results on far ultraviolet and ultraviolet quantum efficiency (QE) in a photon counting, detector array. We have improved the QE by nearly an order of magnitude over microchannel plates (MCPs) that are the state-of-the-art UV detectors for many NASA space missions as well as defense applications. These achievements are made possible by precision interface band engineering of Molecular Beam Epitaxy (MBE) and Atomic Layer Deposition (ALD).

spectroscopy↗
NASA NTRS1997

Optical and Atomic Force Microscopy Characterization of PbI2 Quantum Dots

Lead iodide (PbI2) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO3)2 in H2O, D2O, CH3OH, and C3H7OH media. The observation of the absorption features above 350 nm with the help of integrating sphere accessory strongly suggests the quantum dot formation of PbI2 in solution. Spectral comparison between the synthesized PbI2 clusters in solution and PbI2 nanophase by impregnation of PbI2 in four different pore-sized porous silica indicates that the PbI2 cluster size in solution is less than 2.5 nm in lateral dimension. Atomic force microscopy (AFM) measurements show that the PbL clusters deposited onto three different molecularly flat surfaces are single-layered. The measured height is 1.0 - 0.1 nm. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI2 molecules, which is supported by ab initio calculation. Raman scattering measurement of LO and TO modes of PbI2 in bulk and in the confined state were also conducted in 50-150 cu cm region. The observed three bands at 74, %, 106 1/cm are assigned to TO2, LO2, and LO, mode, respectively. The relatively small red-shift in LO modes may be caused by the surface phonon polaritons of PbI2 nanophase in the porous silica.

Mu, R.↗
NASA NTRS1989

Pulsed Molecular Beams For Growth Of InAs On GaAs

Pauses for annealing reduce number of defects. Deposition process that includes pulsed molecular beams produces high-quality epitaxial layers of indium arsenide on gallium arsenide substrates. Layers made as much as 30 atoms thick without introducing excessive numbers of dislocations, despite 7.4-percent mismatch between InAs and GaAs crystal lattices. Layers offer superior electrical properties in such devices as optically addressed light modulators, infrared sensors, semiconductor lasers, and high-electron-mobility transistors. Technique applicable to other epitaxial systems in which lattices highly mismatched.

Grunthaner, Frank J.↗
NASA NTRS2005

N-Type delta Doping of High-Purity Silicon Imaging Arrays

A process for n-type (electron-donor) delta doping has shown promise as a means of modifying back-illuminated image detectors made from n-doped high-purity silicon to enable them to detect high-energy photons (ultraviolet and x-rays) and low-energy charged particles (electrons and ions). This process is applicable to imaging detectors of several types, including charge-coupled devices, hybrid devices, and complementary metal oxide/semiconductor detector arrays. Delta doping is so named because its density-vs.-depth characteristic is reminiscent of the Dirac delta function (impulse function): the dopant is highly concentrated in a very thin layer. Preferably, the dopant is concentrated in one or at most two atomic layers in a crystal plane and, therefore, delta doping is also known as atomic-plane doping. The use of doping to enable detection of high-energy photons and low-energy particles was reported in several prior NASA Tech Briefs articles. As described in more detail in those articles, the main benefit afforded by delta doping of a back-illuminated silicon detector is to eliminate a "dead" layer at the back surface of the silicon wherein high-energy photons and low-energy particles are absorbed without detection. An additional benefit is that the delta-doped layer can serve as a back-side electrical contact. Delta doping of p-type silicon detectors is well established. The development of the present process addresses concerns specific to the delta doping of high-purity silicon detectors, which are typically n-type. The present process involves relatively low temperatures, is fully compatible with other processes used to fabricate the detectors, and does not entail interruption of those processes. Indeed, this process can be the last stage in the fabrication of an imaging detector that has, in all other respects, already been fully processed, including metallized. This process includes molecular-beam epitaxy (MBE) for deposition of three layers, including metallization. The success of the process depends on accurate temperature control, surface treatment, growth of high-quality crystalline silicon, and precise control of thicknesses of layers. MBE affords the necessary nanometer- scale control of the placement of atoms for delta doping. More specifically, the process consists of MBE deposition of a thin silicon buffer layer, the n-type delta doping layer, and a thin silicon cap layer. The n dopant selected for initial experiments was antimony, but other n dopants as (phosphorus or arsenic) could be used. All n-type dopants in silicon tend to surface-segregate during growth, leading to a broadened dopant-concentration- versus-depth profile. In order to keep the profile as narrow as possible, the substrate temperature is held below 300 C during deposition of the silicon cap layer onto the antimony delta layer. The deposition of silicon includes a silicon- surface-preparation step, involving H-termination, that enables the growth of high-quality crystalline silicon at the relatively low temperature with close to full electrical activation of donors in the surface layer.

Blacksberg, Jordana↗
NASA NTRS1991

Molecular dynamics and quasidynamics simulations of the annealing of bulk and near-surface interstitials formed in molecular-beam epitaxial Si due to low-energy particle bombardment during deposition

The relaxation, diffusion, and annihilation of split and hexagonal interstitials resulting from 10 eV Si irradiation of (2x1)-terminated Si(100) are investigated. Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential are used in the investigation. The interstitials are created in layers two through six, and stable atomic configurations and total potential energies are derived as a function of site symmetry and layer depth. The interstitial Si atoms are allowed to diffuse, and the total potential energy changes are calculated. Lattice configurations along each path, as well as the starting configurations, are relaxed, and minimum energy diffusion paths are derived. The results show that the minimum energy paths are toward the surface and generally involved tetrahedral sites. The calculated interstitial migration activation energies are always less than 1.4 eV and are much lower in the near-surface region than in the bulk.

Kitabatake, M.↗
NASA NTRS1985

Understanding the high pressure properties of molecular solids and molecular surfaces deposited on hetrogeneous substrates

Work directed toward understanding the high pressure properties of molecular solids and molecular surfaces deposited on hetrogeneous substrates is reported. The motivation, apart from expanding our basic knowledge about these systems, was to understand and predict the properties of new materials synthesized at high pressure, including pressure induced metallic and superconducting states. As a consequence, information about the states of matter of the Jovian planets and their satellites, which are natural high pressure laboratories was also provided. The work on molecular surfaces and finite two and three dimensional clusters of atoms and molecules was connected with the composition and behavior of planetary atmospheres and on the processes involved in forming surface layers, which is vital to the development of composite materials and microcircuitry.

Etters, R. D.↗
NASA NTRS2001

Meteoric Ions in Planetary Ionospheres

Solar system debris, in the form of meteoroids, impacts every planet. The flux, relative composition and speed of the debris at each planet depends on the planet's size and location in the solar system. Ablation in the atmosphere evaporates the meteoric material and leaves behind metal atoms. During the ablation process metallic ions are formed by impact ionization. For small inner solar system planets, including Earth, this source of ionization is typically small compared to either photoionization or charge exchange with ambient molecular ions. For Earth, the atmosphere above the main deposition region absorbs the spectral lines capable of ionizing the major metallic atoms (Fe and Mg) so that charge exchange with ambient ions is the dominant source. Within the carbon dioxide atmosphere of Mars (and possibly Venus), photoionization is important in determining the ion density. For a heavy planet like Jupiter, far from the sun, impact ionization of ablated neutral atoms by impacts with molecules becomes a prominent source of ionization due to the gravitational acceleration to high incident speeds. We will describe the processes and location and extent of metal ion layers for Mars, Earth and Jupiter, concentrating on flagging the uncertainties in the models at the present time. This is an important problem, because low altitude ionosphere layers for the planets, particularly at night, probably consist predominantly of metallic ions. Comparisons with Earth will be used to illustrate the differing processes in the three planetary atmospheres.

Pesnell, W. D.↗
NASA NTRS2001

Inexpensive Method for Growing Unidirectional Oriented Materials Using Friction-Transferred Poly(tetrafluoroethylene) Substrates

In recent years, there has been an increasing interest in properties of unidirectional materials grown on highly oriented polymer surfaces such as the poly(tetrafluoroethylene) (PTFE) films coated onto smooth glass surfaces. In this technique, a thin PTFE film is deposited by sliding or rubbing a Teflon bar over a heated glass surface at a specific temperature. Depending on the pressure, temperature and sliding rate the thickness of deposited layers can be varied from 30 A to 150 A. The PTFE-coated substrate is simple yet surprising versatile for oriented growth of many organic, inorganic and polymer. The significant aspect of PTFE films is their unusual ability to molecularly align a wide variety of materials grown onto them from solution, melt or vapor phase deposition, at temperature below the melting point of PTFE (approx. 340 C). The PTFE structure may be derived, in part, from the multitude of ridges forming in the film and these ridges can presumably provide effective nucleation sites to induce the unidirectional growth of variety of materials. Electron diffraction studies of these films revealed that the PTFE macromolecules are oriented parallel to each other along the sliding direction. Direct observation of the film thickness, molecular structure and individual fluorine atoms from these highly oriented films were achieved using atomic force microscopy (AFM). In this paper, we also report the first direct observation of individual fluorine atoms, and the measurement of the fluorine-helix and carbon-helix radii to be 1.70 A and 0.54 A, respectively.

Lee, Jonathan A.↗
NASA NTRS2000

Elemental, Isotopic, and Organic Analysis on Mars with Laser TOF-MS

The in-depth landed exploration of Mars will require increasingly sophisticated robotic analytical tools for both in situ composition science [1] and reconnaissance for sample return [2]. Beyond dust, rock surfaces, and topsoil, samples must be accessed within rocks and ice, well below surface soil, and possibly in elevated deposit layers. A range of spatial scales will be studied, and for the most information-rich microscopic analyses, samples must be acquired, prepared, and positioned with high precision. In some cases samples must also be brought into a vacuum chamber. After expending such resources, it will be important to apply techniques that provide a wide range of information about the samples. Microscopy, mineralogy, and molecular/organic, elemental, and isotopic analyses are all needed, at a minimum, to begin to address the in situ goals at Mars. These techniques must work as an efficient suite to provide layers of data, each layer helping to determine if further analysis on a given sample is desired. In the spirit of broad-band and efficient data collection, we are developing miniature laser time-of-flight mass spectrometers (TOF-MS) for elemental, isotopic, and molecular/organic microanalysis of unprepared solid samples. Laser TOF-MS uses a pulsed laser to volatilize and ionize material from a small region on the sample. The laser energy and focus can be adjusted for atomic and molecular content, sampling area, and depth. Ions travel through the instrument and are detected at a sequence of times proportional to the square root of their mass-to- charge ratios. Thus, each laser pulse produces a complete mass spectrum (in less than 50 microseconds). These instruments can now be significantly miniaturized (potentially to the size of a soda can) without a loss in performance. This effort is reviewed here with an emphasis on applications to Mars exploration.

Brinckerhoff, W. B.↗
NASA NTRS1993

Optical characterization of LDEF contaminant film

Dark brown molecular film deposits were found at numerous locations on the Long Duration Exposure Facility (LDEF) and have been documented in great detail by several investigators. The exact deposition mechanism for these deposits is as yet unknown, although direct and scattered atomic oxygen, and solar radiation interacting with materials outgassing products have all been implicated in the formation process. Specimens of the brown molecular film were taken from below the flange of the experimental tray located at position D10 on the LDEF. The tray was one of two, comprising the same experiment, the other being located on the wake facing side of the LDEF satellite at position B4. Having access to both trays, we were able to directly compare the effect that orientation with respect to the atomic oxygen flux vector had on the formation of the brown molecular film deposits. The film is thickest on surfaces facing toward the exterior, i.e. the tray corner, as can be seen by comparing the lee and wake aspects of the rivets. The patterns appear to be aligned not with the velocity vector but with the corner of the tray suggesting that flux to the surface is due to scattered atomic oxygen rather than direct ram impingement. The role of scattered flux is further supported by more faint plume patterns on the sides of the tray. The angle of these plumes is strongly aligned with the ram direction but the outline of the deposit implies that incident atoms are scattered by collisions with the edges of the opening resulting in a directed, but diffuse, flux of atomic oxygen to the surface. Spectral reflectance measurements in the 2 to 10 micron (4000 to 1000 wavenumbers) spectral range are presented for the film in the 'as deposited' condition and for the free standing film. The material was analyzed by FTIR (Fourier Transform Infrared) microspectroscopy using gold as the reference standard. The 'as deposited' specimen was on an aluminum rivet taken from beneath the tray flange while the free film was obtained by chipping some of the material from the rivet. The transmission spectrum over the 2 to 10 micron range for the free film is presented. This spectrum appears to be essentially the same as that presented by Crutcher et.al. for films formed at vent sites which faced into the ram direction and suggested to originate from urethanes and silicones used on the LDEF. Banks et. al. state that silicones, when exposed to atomic oxygen, release polymeric scission fragments which deposit on surfaces and form a glassy, dark contaminant layer upon further atomic oxygen exposure and solar irradiation.

Blakkolb, Brian K.↗
NASA NTRS1992

Surface contamination on LDEF exposed materials

X-ray photoelectron spectroscopy (XPS) has been used to study the surface composition and chemistry of Long Duration Exposure Facility (LDEF) exposed materials including silvered Teflon (Ag/FEP), Kapton, S13GLO paint, quartz crystal monitors (QCM's), carbon fiber/organic matrix composites, and carbon fiber/Al Alloy composites. In each set of samples, silicones were the major contributors to the molecular film accumulated on the LDEF exposed surfaces. All surfaces analyzed have been contaminated with Si, O, and C; most have low levels (less than 1 atom percent) of N, S, and F. Occasionally observed contaminants included Cl, Na, K, P, and various metals. Orange/brown discoloration observed near vent slots in some Ag/FEP blankets were higher in carbon, sulfur, and nitrogen relative to other contamination types. The source of contamination has not been identified, but amine/amide functionalities were detected. It is probable that this same source of contamination account for the low levels of sulfur and nitrogen observed on most LDEF exposed surfaces. XPS, which probes 50 to 100 A in depth, detected the major sample components underneath the contaminant film in every analysis. This probably indicates that the contaminant overlayer is patchy, with significant areas covered by less that 100 A of molecular film. Energy dispersive x-ray spectroscopy (EDS) of LDEF exposed surfaces during secondary electron microscopy (SEM) of the samples confirmed contamination of the surfaces with Si and O. In general, particulates were not observed to develop from the contaminant overlayer on the exposed LDEF material surfaces. However, many SiO2 submicron particles were seen on a masked edge of an Ag/FEP blanket. In some cases such as the carbon fiber/organic matrix composites, interpretation of the contamination data was hindered by the lack of good laboratory controls. Examination of laboratory controls for the carbon fiber/Al alloy composites showed that preflight contamination was the most significant factor for all the contaminants generally detected at less than 1 atom percent, or detected only occasionally (i.e., all but Si, O, and C). Flight control surfaces, including sample backsides not exposed to space radiation or atomic oxygen flux, have accumulated some contamination on flight (compared to laboratory controls), but experimentally, the LDEF exposed surface contamination levels are generally higher for the contaminants Si and O. For most materials analyzed, Si contamination levels were higher on the leading edge surfaces than on the trailing edge surfaces. This was true even for the composite samples where considerable atomic oxygen erosion of the leading edge surfaces was observed by SEM. It is probable that the return flux associated with atmospheric backscatter resulted in enhanced deposition of silicones and other contaminants on the leading edge flight surfaces relative to the trailing edge. Although the Si concentration data suggested greater on-flight deposition of contaminants on the leading edge surfaces, the XPS analyses did not conclusively show different relative total thicknesses of flight deposited contamination for leading and trailing edge surfaces. It is possible that atomic oxygen reactions on the leading edge resulted in greater volatilization of the carbon component of the deposited silicones, effectively 'thinning' the leading edge deposited overlayer. Unlike other materials, exposed polymers such as Kapton and FEP-type Teflon had very low contamination on the leading edge surfaces. SEM evidence showed that undercutting of the contaminant overlayer and damaged polymer layers occurred during atomic oxygen erosion, which would enhance loss of material from the exposed surface.

Hemminger, Carol S.↗
NASA NTRS1997

The Solar-Stellar Connection (NAG5-6124: SOHO Guest Investigator Program)

The following is a final report from the SOHO Guest Investigator program to use the SUMER far-UV spectrometer to obtain imaging spectroscopy in support of the goals of the so-called "solar-stellar connection." In particular, a major emphasis was utilization of the long-slit time-resolved maps of the solar surface in bright far-UV emission lines to deduce how particular aspects of the temporally and spatially averaged line profiles trace back to individual structural features of the magnetically disturbed outer atmosphere; to help interpret the unresolved line profiles from high quality stellar observations (say, with the Space Telescope Imaging Spectrograph). The researchers served two tours of duty in the SOHO Operations Center as SUMER planners, during which time we conducted an extensive series of observing programs. These can be divided into three general categories: surface mapping, translimb spectroscopy, and active region diagnostics. We have analyzed some of the large volumes of data to the point where we have presented them in poster papers, and in invited papers at national and international meetings. Listed below are the titles of the preliminary publications we have written, including brief abstracts to indicate the main results. (1) Chromospheric structure and Dynamics-- Observations -- The chromosphere is a highly structured dynamic 'layer' of the solar outer atmosphere. Here, not only are the effects of mechanical heating first evident (moving upward in altitude from the deep photosphere), but also the amount of nonradiative energy deposited is far greater than in the albeit much hotter overlying transition region and corona. Further, the chromosphere is by far the thickest zone of the solar atmosphere with respect to the pressure scale height. A major goal of stellar astrophysics is to understand how the chromosphere is heated and why it adopts its peculiar structure. A cursory examination of solar filtergrams and high-resolution movies demonstrates that much of the chromospheric "action" must be occurring on fine spatial scales and short times; particularly in the cell interior transient brightenings, but also in the longer-lived network fragments. That regime of investigation is far removed from what one usually associates with "synoptic" measurements. Nevertheless, synoptic observations of chromospheric indices, filtergrams, and globally-averaged profile parameters not only can provide important insight concerning the crucial role of the cycle-variable part of the solar magnetic field; but they also can forge a key link with analogous measurements of the stars, where often the phenomena can be significantly exaggerated from the solar case, but high spatial resolution reconnaissance is not even a remote possibility. In addition to discussing the synoptic aspects of chromospheric structure and dynamics, I summarized new insights into the general problem of the solar chromosphere that have been obtained recently with the SUMER far-ultraviolet spectrometer on SOHO. (2) CO and the Temperature Structure of the Solar Atmosphere -- The surface layers of the Sun provide a crucial boundary condition for many of the processes that occur in the deep interior. The stratification of the outer solar atmosphere once was thought to be well understood. However, studies of thermally sensitive molecular absorptions in the infrared revealed puzzling anomalies. Strong lines of the CO fundamental vibration-rotation bands near 5 microns showed very cool temperatures at the extreme limb, and remarkable off-limb emissions extending well into the supposedly hot chromosphere. The conflicting pictures of the photosphere/chromosphere interface, from the widely separated wavelength regimes, has raised suspicions that those "layers" of the atmosphere are much more inhomogeneous than previously suspected. One proposal is that the low chromosphere is dominated by cool gas, the "COmosphere," which is threaded by a network of persistent small-scale hot magnetic filaments and occasionally disrupted by localized acoustic disturbances. The COmosphere is capped by the merged fields of the network elements in the chromospheric "canopy." (3) Translimb Spectroscopy with SOHO/SUMER -- We have used SUMER to obtain deep exposures of the 1300-1400 A spectrum, at the extreme limb and off-limb. Previous "translimb" studies in the thermal infrared had revealed remarkable extensions of cold material (T is approximately equal to 3000 K)-traced by carbon monoxide emission lines-into the heart of the hot chromosphere. A main objective of our program was to search for corresponding far-UV signatures of the "thermally-bifurcated" low chromosphere; for example, radiatively fluoresced emissions of the CO A-X 4th-positive system (collisional excitation would be negligible in cold gas). We conducted two separate observing programs with SUMER. Both made use of the 1 inch-diameter circular aperture, translated across the limb in the minimum motor step increments of 0."375, along the central meridian in the Northern polar coronal hole. The first program executed for nine hours beginning 19UT 25 Oct 1996. The full wavelength range was 1340-1400 A. It was recorded in two overlapping segments, placing key regions of the spectrum alternately on the KBr and bare parts of the detector, to help isolate 2nd-order features. Each segment was integrated for 500 s, and 32 pairs were obtained to span a 12" swath centered on the optical limb. The second program was conducted 00-09UT 01 Dec 1996. It consisted of a single wavelength setting (1300-1340 A) with exposure time 500 s, but twice the spatial coverage of the earlier series: 64 steps, for a total displacement of 24". The strong chromospheric resonance lines of atomic oxygen (1302-1305 A) and ionized carbon (1334-1335 A) were observed on the bare part of the MCP camera.

Ayres, Thomas R.↗