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NASA NTRS1990

Study of temperature-dependent ultrathin oxide growth on Si(111) using variable-angle spectroscopic ellipsometry

The monolayer-sensitive variable-angle spectroscopic ellipsometry technique was used to study the temperature-dependent growth mechanisms of an ultrathin oxide layer on top of silicon. The oxidation was done in atomic oxygen produced in a pure oxygen plasma and driven by an RF power source. The results have been compared with the recently proposed model of Murali and Murarka for ultrathin oxide growth on top of silicon. The activation energies of different growth parameters associated with the oxide growth have also been determined.

De, Bhola N.↗

NASA NTRS2014

Organics Exposure in Orbit (OREOcube): A Next-Generation Space Exposure Platform

The OREOcube (ORganics Exposure in Orbit cube) experiment on the International Space Station (ISS) will investigate the effects of solar and cosmic radiation on organic thin films supported on inorganic substrates. Probing the kinetics of structural changes and photomodulated organic− inorganic interactions with real-time in situ UV−visible spectroscopy, this experiment will investigate the role played by solid mineral surfaces in the (photo)chemical evolution, transport, and distribution of organics in our solar system and beyond. In preparation for the OREOcube ISS experiment, we report here laboratory measurements of the photostability of thin films of the 9,10-anthraquinone derivative anthrarufin (51 nm thick) layered upon ultrathin films of iron oxides magnetite and hematite (4 nm thick), as well as supported directly on fused silica. During irradiation with UV and visible light simulating the photon flux and spectral distribution on the surface of Mars, anthrarufin/iron oxide bilayer thin films were exposed to CO2 (800 Pa), the main constituent (and pressure) of the martian atmosphere. The time-dependent photodegradation of anthrarufin thin films revealed the inhibition of degradation by both types of underlying iron oxides relative to anthrarufin on bare fused silica. Interactions between the organic and inorganic thin films, apparent in spectral shifts of the anthrarufin bands, are consistent with presumed free-electron quenching of semiquinone anion radicals by the iron oxide layers, electively protecting the organic compound from photodegradation. Combining such in situ real-time kinetic measurements of thin films in future space exposure experiments on the ISS with postflight sample return and analysis will provide time-course studies complemented by in-depth chemical analysis. This will facilitate the characterization and modeling of the chemistry of organic species associated with mineral surfaces in astrobiological contexts.

Organics↗

NASA NTRS1980

Effect of W and WC on the oxidation resistance of yttria-doped silicon nitride

The effect of tungsten and tungsten carbide contamination on the oxidation and cracking in air of yttria-doped silicon nitride ceramics is investigated. Silicon nitride powder containing 8 wt % Y2O3 was doped with 2 wt % W, 4 wt % W, 2 wt % WC or left undoped, and sintered in order to simulate contamination during milling, and specimens were exposed in air to 500, 750 and 1350 C for various lengths of time. Scanning electron and optical microscopy and X-ray diffraction of the specimens in the as-sintered state reveals that the addition of W or WC does not affect the phase relationships in the system, composed of alpha and beta Si3N4, melilite and an amorphous phase. Catastrophic oxidation is observed at 750 C in specimens containing 2 and 4 wt % W, accompanied by the disappearance of alpha Si3N4 and melilite from the structure. At 1350 C, the formation of a protective glassy oxide layer was observed on all specimens without catastrophic oxidation, and it is found that pre-oxidation at 1350 C also improved the oxidation resistance at 750 C of bars doped with 4 wt % W. It is suggested that tungsten contamination from WC grinding balls may be the major cause of the intermediate-temperature cracking and instability frequently observed in Si3N4-8Y2O3.

Schuon, S.↗

NASA NTRS1987

Response of a metal-oxide-semiconductor field-effect transistor to a cosmic-ray ion track

A cosmic-ray ion track passing perpendicularly through the oxide layer of an enhancement-mode metal-oxide-semiconductor field-effect transistor (MOSFET) forms a conducting path, the resistance of which is proportional to the stopping power of the cosmic ion and independent of the cross-sectional area of the ion track. The voltage across the oxide capacitance may drop below the threshold voltage if the gate bias is sufficiently low or if the external resistance in the gate-source circuit is sufficiently high. The first of a pair of MOSFETs forming a flip-flop circuit may thus be turned off, and the second transitor may turn on, providing it has a sufficiently short delay time, thereby completing a single-event upset.

Benumof, Reuben↗

NASA NTRS1988

Variable angle spectroscopic ellipsometry - Application to GaAs-AlGaAs multilayer homogeneity characterization

Variable angle spectroscopic ellipsometry has been applied to a GaAs-AlGaAs multilayer structure to obtain a three-dimensional characterization, using repetitive measurements at several spots on the same sample. The reproducibility of the layer thickness measurements is of order 10 A, while the lateral dimension is limited by beam diameter, presently of order 1 mm. Thus, the three-dimensional result mainly gives the sample homogeneity. In the present case three spots were used to scan the homogeneity over 1 in of a wafer which had molecular-beam epitaxially grown layers. The thickness of the AlGaAs, GaAs, and oxide layers and the Al concentration varied by 1 percent or less from edge to edge. This result was confirmed by two methods of data analysis. No evidence of an interfacial layer was observed on top of the AlGaAs.

Alterovitz, Samuel A.↗

NASA NTRS1983

Interfacial layers in high-temperature-oxidized NiCrAl

The utility of Auger electron spectroscopy combined with ball cratering for depth analysis of oxide and diffusion layers produced in a Ni-14Cr-24Al alloy by oxidation in air at 1180 C for 25 hr is demonstrated. During postoxidation cooling, the oxide layers formed by this alloy spalled profusely. The remaining very thin oxide was primarily Cr2O3 with a trace of Ni. The underlying metal substrate exhibited gamma/gamma-prime and beta phases with a metallic interfacial layer which was similar to the bulk gamma/gamma-prime phase but slightly enriched in Cr and Al. These data are compared to electron microprobe results from a nominally identical alloy. The diffusion layer thickness is modelled with a simple mass balance equation and compared to recent results on the diffusion process in NiCrAl alloys.

Larson, L. A.↗

NASA NTRS2005

Nanostructured and layered lithium manganese oxide and method of manufacturing the same

Nanostructured and layered lithium manganese oxide powders and methods of producing same. The powders are represented by the chemical formula, LixMn1-yMyO2, where 0.5<x<1.33, 0?y?0.5 and have an average primary particle diameter from 5 nm to 300 nm, preferably between 5 and 100 nm, and M is at least one cation dopant. The powders can be formed into active cathode materials in Li-ion and Li rechargeable batteries.

Singhal, Amit↗

NASA NTRS1978

Measuring oxide trapping parameters in MOS structure

System for controlled injection of electrons or holes into oxide layer of MOS capacitor can be used to measure oxide trapping parameters. Since trapping mechanisms can cause degradation and ultimate failure of MOS elements exposed to ionizing radiation, system can be helpful in predicting device tolerance.

Maserjian, J.↗

NASA NTRS2014

Behavior of Particle Depots in Molten Silicon During Float-Zone Growth in Strong Static Magnetic Fields

In the photovoltaics industry, the largest market share is represented by solar cells made from multicrystalline silicon, which is grown by directional solidification. During the growth process, the silicon melt is in contact with the silicon nitride coated crucible walls and the furnace atmosphere which contains carbon monoxide. The dissolution of the crucible coating, the carbon bearing gas, and the carbon already present in the feedstock, lead to the precipitation of silicon carbide, and silicon nitride, at later stages of the growth process. The precipitation of Si3N4 and SiC particles of up to several hundred micrometers in diameter leads to severe problems during the wire sawing process for wafering the ingots. Furthermore the growth of the silicon grains can be negatively influenced by the presence of particles, which act as nucleation sources and lead to a grit structure of small grains and are sources for dislocations. If doped with Nitrogen from the dissolved crucible coating, SiC is a semi conductive material, and can act as a shunt, short circuiting parts of the solar cell. For these reasons, the incorporation of such particles needs to be avoided. In this contribution we performed model experiments in which the transport of intentionally added SiC particles and their interaction with the solid-liquid interface during float zone growth of silicon in strong steady magnetic fields was investigated. SiC particles of 7μm and 60μm size are placed in single crystal silicon [100] and [111] rods of 8mm diameter. This is achieved by drilling a hole of 2mm diameter, filling in the particles and closing the hole by melting the surface of the rod until a film of silicon covers the hole. The samples are processed under a vacuum of 1x10(exp -5) mbar or better, to prevent gas inclusions. An oxide layer to suppress Marangoni convection is applied by wet oxidation. Experiments without and with static magnetic field are carried out to investigate the influence of melt convection on the distribution of particles and their incorporation into the crystal. The field strengths applied by a superconducting magnet are 1T, 3T, 4.5T, and 5T. The increase in field strength dampens the melt flow, and so this study provides comparative data to the crystal growth experiment to be carried out onboard the sounding rocket mission TEXUS 51, where purely diffusive growth condition will be achieved under microgravity conditions.

Jauss, T.↗

NASA NTRS1991

An empirical approach to predicting long term behavior of metal particle based recording media

Alpha iron particles used for magnetic recording are prepared through a series of dehydration and reduction steps of alpha-Fe2O3-H2O resulting in acicular, polycrystalline, body centered cubic (bcc) alpha-Fe particles that are single magnetic domains. Since fine iron particles are pyrophoric by nature, stabilization processes had to be developed in order for iron particles to be considered as a viable recording medium for long term archival (i.e., 25+ years) information storage. The primary means of establishing stability is through passivation or controlled oxidation of the iron particle's surface. Since iron particles used for magnetic recording are small, additional oxidation has a direct impact on performance especially where archival storage of recorded information for long periods of time is important. Further stabilization chemistry/processes had to be developed to guarantee that iron particles could be considered as a viable long term recording medium. In an effort to retard the diffusion of iron ions through the oxide layer, other elements such as silicon, aluminum, and chromium have been added to the base iron to promote more dense scale formation or to alleviate some of the non-stoichiometric behavior of the oxide or both. The presence of water vapor has been shown to disrupt the passive layer, subsequently increasing the oxidation rate of the iron. A study was undertaken to examine the degradation in magnetic properties as a function of both temperature and humidity on silicon-containing iron particles between 50-120 deg C and 3-89 percent relative humidity. The methodology to which experimental data was collected and analyzed leading to predictive capability is discussed.

Hadad, Allan S.↗

NASA NTRS1986

Etching Silicon Films With Xenon Difluoride

Microscopic circuit structures prepared for probing. Xenon difluoride removes relatively large amounts of silicon from integratedcircuit or solar-cell structures while leaving SiO2, Si3N4, Al2O3, and other compounds intact. In Etching Apparatus, solid XeF2 sublimated in vacuum, then allowed to flow over sample at controlled rate and pressure. Wafer etched from back to expose SiO2 and Al layers for spectroscopic analysis of SiO2/Al interface. Using XeF2 technique, silicon wafer with oxide layer reduced in thickness from standard 300 micrometer to as little as 10 nanometer without adversely affecting oxide.

Hecht, M. H.↗

NASA NTRS1989

Reflection/suppression coatings for 900 - 1200 A radiation

The design and performance of multiple-layer, selective-reflection, selective-suppression coatings for the 900 - 1200 A band are described. These coatings are designed to optimize both high reflectivity at a desirable wavelength and low reflectivity at an undesirable wavelength. The minimum structure for a selective coating consists of a thin metal or metal oxide layer (50 - 150 A thickness) over an aluminum substrate protected with a semi-transparent dielectric (100 - 1000 A thickness). Predicted coating performance is strongly effected by varying the layer combination and thickness. A graphical method of optimizing the coating layer structure is developed. Aluminum, silicon, their oxides, and gold have been investigated as coating layer materials. A very simple coating with a 1026 to 1216 A reflectivity ratio greater than 100 was fabricated. Such reflection/suppression coatings may be of great utility to spaceborne EUV spectrographs.

Edelstein, Jerry↗

NASA NTRS1976

Particulate and aerosol detector

A device is described for counting aerosols and sorting them according to either size, mass or energy. The component parts are an accelerator, a capacitor sensor and a readout. The accelerator is a means for accelerating the aerosols toward the face of the capacitor sensor with such force that they partially penetrate the capacitor sensor, momentarily discharging it. The readout device is a means for counting the number of discharges of the capacitor sensor and measuring the amplitudes of these different discharges. The aerosols are accelerated by the accelerator in the direction of the metal layer with such force that they penetrate the metal and damage the oxide layers, thereby allowing the electrical charge on the capacitor to discharge through the damaged region. Each incident aerosol initiates a discharge path through the capacitor in such a fashion as to vaporize the conducting path. Once the discharge action is complete, the low resistance path no longer exists between the two capacitor plates and the capacitor is again able to accept a charge. The active area of the capacitor is reduced in size by the damaged area each time a discharge occurs.

Wortman, J. J.↗

NASA NTRS2011

CMOS Imager Has Better Cross-Talk and Full-Well Performance

A complementary metal oxide/semiconductor (CMOS) image detector now undergoing development is designed to exhibit less cross-talk and greater full-well capacity than do prior CMOS image detectors of the same type. Imagers of the type in question are designed to operate from low-voltage power supplies and are fabricated by processes that yield device features having dimensions in the deep submicron range. Because of the use of low supply potentials, maximum internal electric fields and depletion widths are correspondingly limited. In turn, these limitations are responsible for increases in cross-talk and decreases in charge-handling capacities. Moreover, for small pixels, lateral depletion cannot be extended. These adverse effects are even more accentuated in a back-illuminated CMOS imager, in which photogenerated charge carriers must travel across the entire thickness of the device. The figure shows a partial cross section of the structure in the device layer of the present developmental CMOS imager. (In a practical imager, the device layer would sit atop either a heavily doped silicon substrate or a thin silicon oxide layer on a silicon substrate, not shown here.) The imager chip is divided into two areas: area C, which contains readout circuits and other electronic circuits; and area I, which contains the imaging (photodetector and photogenerated-charge-collecting) pixel structures. Areas C and I are electrically isolated from each other by means of a trench filled with silicon oxide. The electrical isolation between areas C and I makes it possible to apply different supply potentials to these areas, thereby enabling optimization of the supply potential and associated design features for each area. More specifically, metal oxide semiconductor field-effect transistors (MOSFETs) that are typically included in CMOS imagers now reside in area C and can remain unchanged from established designs and operated at supply potentials prescribed for those designs, while the dopings and the lower supply potentials in area I can be tailored to optimize imager performance. In area I, the device layer includes an n+ -doped silicon layer on which is grown an n-doped silicon layer. A p-doped silicon layer is grown on top of the n -doped layer. The total imaging device thickness is the sum of the thickness of the n+, n, and p layers. A pixel photodiode is formed between a surface n+ implant, a p implant underneath it, the aforementioned p layer, and the n and n+ layers. Adjacent to the diode is a gate for transferring photogenerated charges out of the photodiode and into a floating diffusion formed by an implanted p+ layer on an implanted n-doped region. Metal contact pads are added to the back-side for providing back-side bias.

Pain, Bedabrata↗

NASA NTRS2006

Molecular Substrate Alteration by Solar Wind Radiation Documented on Flown Genesis Mission Array Materials

The Genesis spacecraft sampling arrays were exposed to various regimes of solar wind during flight that included: 313.01 days of high-speed wind from coronal holes, 335.19 days of low-speed inter-stream wind, 191.79 days of coronal mass ejections, and 852.83 days of bulk solar wind at Lagrange 1 orbit. Ellipsometry measurements taken at NASA s Johnson Space Center show that all nine flown array materials from the four Genesis regimes have been altered by solar wind exposure during flight. These measurements show significant changes in the optical constant for all nine ultra-pure materials that flew on Genesis when compared with their non-flight material standard. This change in the optical constant (n and k) of the material suggests that the molecular structure of the all nine ultra-pure materials have been altered by solar radiation. In addition, 50 samples of float-zone and czochralski silicon bulk array ellipsometry results were modeled with an effective medium approximation layer (EMA substrate layer) revealing a solar radiation molecular damage zone depth below the SiO2 native oxide layer ranging from 392 to 613 . This bulk solar wind radiation penetration depth is comparable to the depth of solar wind implantation depth of Mg measured by SIMS and SARISA.

Calaway, Michael J.↗

NASA NTRS1984

Ceramic-coated metals can survive contact with hot working fluid

Thermal barrier coatings (TBCs) have been developed as a means of protecting turbine blades and other engine hot section components whose surfaces are exposed to the most extreme operating conditions. By adding a thin, insulating ceramic oxide layer to an air-cooled turbine blade, the difference between the gas temperature and the metal temperature is further increased as a function of ceramic coating thickness, heat flux, and oxide thermal conductivity. An 0.04-cm thick ceramic layer can typically yield a 100-300 C temperature drop. Of the various techniques available for the deposition of thermal barrier coatings, the most common is that of plasma spraying. Significant improvements have been made in TBC durability through the use of bond coat compositions with increased oxidation resistance.

Levine, S. R.↗

NASA NTRS1980

Effect of W and WC on the oxidation resistance of yttria-doped silicon nitride

The effect of W and WC contamination on the oxidation and cracking in air of sintered Si3N4 - 8 w/o Y2O3 ceramics at 500, 750, and 1350 C is examined. A mixture of Si3N4 - 8Y2O3, milled with alumina balls, was divided into four portions. Three portions were doped with 2 w/o WC W, and 4 w/o W respectively, in order to simulate contamination during milling. The fourth portion was undoped and used on a control. The addition of W or WC did not affect the phase relationships in the system, as all bars with or without additions contained melilite as the major Si-Y-O-N phase after sintering. At 750 C, instability (rapid oxidation and cracking) of W-doped bars appears to have occurred as a result of oxidation of the tungsten containing melilite phase. No intermediate temperature instability was observed in bars containing 2 w/o WC or in bars with no additive. Specimens exposed at 1350 C had good oxidation resistance due to the formation of a protective siliceous oxide layer. A specimen containing 4 w/o W which was preoxidized at 1350 C had improved oxidation resistance at 750 C. The tendency towards oxidation and cracking of Si3N4 - 8 Y2O3 at 750 C is concluded to be related to tungsten content of the sintered bars.

Schuon, S.↗

NASA NTRS1985

Preparing InSb Substrates

Molecular-beam epitaxial substrates passivated by chloride film growth. Chloride layer produced by direct chlorination, by chloride radicals in solutions, or by formation of preliminary oxide layer followed by chloride substitution.

Vasquez, R. P.↗