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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Growth experiment of narrow band-gap semiconductor PbSnTe single crystals in space (M-1)

An experiment on crystal growth of Pb(1-x)Sn(x)Te in microgravity is planned. This material is an alloy of the compound semiconductors PbTe and SnTe. It is a promising material for infrared diode lasers and detectors in the wavelength region between 6 and 30 micron. Since the electrical properties of Pb(1-x)Sn(x)Te depend greatly on the Pb/Sn ratio and crystalline defects as well as impurity concentration, homogeneous, defect-free, high-quality crystals are anticipated. Although many growth methods, such as the pulling method, the Bridgman method, the vapor growth method, etc., have been applied to the growth of Pb(1-x)Sn(x)Te, large, homogeneous, low-defect-density crystals have not yet been grown on Earth. The unsuccessful results were caused by buoyancy-driven convection in the fluids induced by the specific gravity difference between heated and cooled fluids on Earth. A crystal is grown by cooling the melt from one end of the ampoule. In crystal growth from the melt, about 30 percent of the SnTe in the melt is rejected at the solid-liquid interface during solidification. On Earth, the rejected SnTe is completely mixed with the remaining melt by convection in the melt. Therefore, SnTe concentration in the melt, and accordingly in the crystal, increases as the crystal grows. In the microgravity environment, buoyancy-driven convection is suppressed because the specific gravity difference is negligible. In that case, the rejected SnTe remains at the solid-liquid interface and its concentration increases only at the interface. If the growth rate is higher than the PbTe-SnTe interdiffusion rate, the amount of SnTe which diffuses from the interface into the melt increases as SnTe piles up at the interface, and finally it balances the amount of rejected SnTe during solidification, resulting in steady-state SnTe transportation at the interface. By using this principle, compositionally homogeneous crystals can be grown. Furthermore, low-defect-density crystals will be grown in microgravity, because convection causes crystalline defects by mising hot and cold fluids and generating temperature fluctuations in them.

Yamada, Tomoaki↗
NASA NTRS1985

Structural defects in crystalline silicon

The basic photovoltaic properties of a given crystalline silicon specimen seem to be governed by density and nature of two to three dimensional lattice defects. These are mainly generated by primary growth conditions as grain boundaries of more or less intrinsic character or second phase precipitates from supersaturated solutions of carbon or oxygen. Considerably high values of both solubility and diffusivity in connection with their abundance in common refractory material systems account for the predominance of the two particular elements. Unsaturated dislocations of different types very often can be seen as a consequence of the existence of more dimensional defects as described initially. The final performance of a solar cell is dependent of the concentration and distribution of recombination active centers in the different regions of this device. Typical representatives are fast diffusing transition metals in form of either single atoms or simple complexes. Their avoidance, annihilation, or removal is of great concern in different fields of electronic materials development.

Sirtl, E.↗
NASA NTRS1988

Temperature Fluctuations During Crystal Growth

Technique developed to deconvolve period and relative amplitude of fluctuations of heat flow inBridgman crystal growth. Temperature-measuring device with enough sensitivity and frequency response to make desired measurements inserted close as possible to substance monitored. Time-dependent temperature response recorded, and time domain response converted to frequency domain for further analysis. Fast fourier transform (FFT) of data on temperature oscillations shows particular behavior at some specific frequencies corresponding with striations, or defects observed on crystal. Useful procedure to help determine sources of some growth-induced crystalline defects. Other processes sensitive to small temperature fluctuations, such as diffusion, precipitation, and corrsion, benefit from technique.

Fripp, Archibald L., Jr.↗
NASA NTRS1991

Mapping crystal defects with a digital scanning ultramicroscope

A computer controlled scanning ultramicroscope has been built to assist in the characterization of transparent crystals. The device measures the scattering of a focused He-Ne laser beam by crystalline defects. As an XYZ translation table moves the crystal under the ultramicroscope, the scattered light is measured by a photodetector whose output is digitized and recorded. From this data, contour maps or three dimensional perspective plots of the scattering regions of the crystal can be generated to assist in finding patterns of defects which might be correlated with perturbations in the growth process. The verified resolution of the present instrument is about 1 micrometer, which is limited by the minimum step of the stepper-motor driven translation stages, optical diffraction effects and the sensitivity of the detector at the laser light frequency. The instrument was used to build a database of defects patterns in commercial laboratory grown triglycine sulphate (TGS) crystals, and to map defects in a TGS crystal grown from aqueous solution during the flight of Spacelab 3. This crystal shows indications of a reduction both in the generation of defects at the seed-new growth interface and in their propagation into the new crystal.

Springer, J. M.↗
NASA NTRSDate not supplied

Vapor Growth of ZnSe-based Compound Semiconductors in Low Gravity environment on International Space Station

Six low-gravity experiments of crystal growth of ZnSe-based compound semiconductors by physical vapor transport (PVT) are scheduled to be processed inside Low Gradient Furnace (LGF) on Materials Science Research Rack (MSRR) in Material Science Laboratory (MSL) on International Space Station (ISS) starting from February 27th, 2023. The flight samples consist of three unseeded growths of ZnSe, Cr-doped ZnSe and ZnSeTe as well as growth of ZnSe on top of three single crystal ZnSe seeds with orientation of (100), (110) and (111). The main scientific objective of the flight experiments is to establish the effects of gravity-driven fluid flows on Earth to (i) the non-uniform incorporation of impurities and defects and (ii) the deviation from stoichiometry observed in the grown crystals caused by growth interface fluctuations and evolution as the results of buoyancy-driven convection and irregular fluid-flows. The investigation will also evaluate the effects of gravity on the PVT process by examining (i) the growth kinetics on various seed orientations, (ii) the dopant segregation and distribution in the Cr doped ZnSe, and (iii) the compositional segregation and distribution in the ternary ZnSeTe crystal grown by PVT. Additionally, from X-ray diffraction/topography and by measuring the density and distribution of various crystalline defects it will assess the self-induced strain effects developed during processing at elevated temperatures caused by the weight of the crystals. During the ground-based preparations, the following tasks have been accomplished: • Establish the thermodynamic properties of the ZnSe binary and related ternary systems by measuring the partial pressures of the vapor phase coexisting with the condensed phases and perform phase diagram calculations for comparison. • Evaluate the effects of gravity-driven convection in the growth process by performing experiments under various vapor transport orientations relative to gravity vector, i.e., horizontal, vertically stabilized and destabilized configurations. • Establish a quantitative correlation between growth parameters and the characteristics and properties of the grown crystals by characterizing the crystals using a variety of techniques. • Evaluate the fundamentals of the current theories on vapor transport and crystal growth kinetics by performing in-situ and real-time monitoring techniques during growth. • Develop a one-dimensional diffusion limited theory of mass transport and two- and three-dimensional numerical simulations of both mass transport and heat transfer and thus establish the microgravity requirements for the flight experiments and to predict the characteristics of the grown crystals such as the solid-vapor interface shapes… etc. and establish a fundamental understanding of the crystal growth process from the comparison with experimental results.

ZnSe↗
NASA NTRS1974

Low-temperature thermostatics of face-centered-cubic metallic hydrogen

The thermostatic properties of a high-symmetry phase of metallic hydrogen with atomic sphere radius between 0.1 and 1.5 bohr are studied, with special emphasis accorded to electronic screening and quantum proton motion. The electron-proton and proton-proton interactions receive a perturbation treatment based on the Singwi dielectric function, while the proton motion is handled by self-consistent harmonic approximation. Quantum behavior is found to be less pronounced than expected, and nuclear magnetism is absent. The phonon spectrum is, however, affected by screening and large proton motion. The zero-point vibrational energy and the superconducting critical temperature are below previous estimates. The crystalline-defect formation energies are a few times the Debye energy, which implies that defects contribute significantly to melting at the lower particle densities.

Caron, L. G.↗
NASA NTRS1985

Crystal Growth Research in Space

The objectives of this work are to develop growth techniques and theory leading to improved bulk growth of semiconductor single crystals. Ground based experiments will be complemented by experiments carried out in the low-g environment provided by the space shuttle. Analytical studies and laboratory investigations are being conducted to better define the causes of crystalline defects and inhomogeneities. The compound semiconductor lead-tin-telluride is being used as the modeling material. Theoretical techniques are being developed to predict the thermal and solutal fields which are present during bulk growth from a melt. Techniques for measuring the thermophysical properties of semiconductors at high temperatures have been developed. During the past year electrochemical etching techniques have been developed for delineation of inhomogeneous regions in crystals. Thermal diffusivity measurements have been completed for the solid and liquid phases of PbTe and PbSnTe. Preliminary results have been obtained on the effects on crystal morphology of gravity, interface shape and interaction between the melt and the container.

Crouch, R. K.↗
NASA NTRS1988

Electronic materials processing and the microgravity environment

The nature and origin of deficiencies in bulk electronic materials for device fabrication are analyzed. It is found that gravity generated perturbations during their formation account largely for the introduction of critical chemical and crystalline defects and, moreover, are responsible for the still existing gap between theory and experiment and thus for excessive reliance on proprietary empiricism in processing technology. Exploration of the potential of reduced gravity environment for electronic materials processing is found to be not only desirable but mandatory.

Witt, A. F.↗
NASA NTRS1991

Real time quantitative imaging for semiconductor crystal growth, control and characterization

A quantitative real time image processing system has been developed which can be software-reconfigured for semiconductor processing and characterization tasks. In thermal imager mode, 2D temperature distributions of semiconductor melt surfaces (900-1600 C) can be obtained with temperature and spatial resolutions better than 0.5 C and 0.5 mm, respectively, as demonstrated by analysis of melt surface thermal distributions. Temporal and spatial image processing techniques and multitasking computational capabilities convert such thermal imaging into a multimode sensor for crystal growth control. A second configuration of the image processing engine in conjunction with bright and dark field transmission optics is used to nonintrusively determine the microdistribution of free charge carriers and submicron sized crystalline defects in semiconductors. The IR absorption characteristics of wafers are determined with 10-micron spatial resolution and, after calibration, are converted into charge carrier density.

Wargo, Michael J.↗
NASA NTRS1993

Spectroscopic studies of model polar stratospheric cloud films

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric-acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid: nitric-acid monohydrate (NAM), dihydrate (NAD), and trihydrate (NAT). We also observe two distinct crystalline forms of the trihydrate, which we denote alpha- and beta-NAT. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to study the interaction of HCl with model PSC films. In this work we find that for HCl pressures in the range 10 exp -5 to 10 exp -7 Torr, HCl is taken up by ice at 155 K to form a thin layer of HCl.6H2O. At 193 K, the uptake of HCl by ice was consistent with less than or equal to monolayer coverage. Uptake of HCl by alpha and beta-NAT at 175 K was also consistent with less than or equal to monolayer coverage.

Tolbert, Margaret A.↗
NASA NTRS1992

Study of natural convection and interface shape in directional solidification of succinonitrile

Flow visualization experiments in a Bridgman furnace at zero growth velocity have been performed. The experiments were intended to investigate if the photochromic dye activation method could be used in a crystal growth study. The results from this work have confirmed that with a carefully designed experimental setup, the photochromic-dye method permits qualitative and quantitative evaluation of the flow field near the interface in crystal growth experiments. For the horizontal orientation of a 6 mm x 6 mm ampoule and an axial temperature gradient of 26.5 C/cm, velocity profiles have been obtained accurately at various positions near the interface. The maximum velocity of 1.29 mm/sec was measured in the central vertical plane and the flow was symmetrical about that plane. A flow inversion point was also noted, above which the flow was towards the interface and below it, away from the interface. The results obtained are useful for validating 3-dimensional numerical models and establishing a link between the macroscopic processing conditions and the formation of crystalline defects.

Kawaji, M.↗
NASA NTRS2003

Morphological Evidence for an Exclusively Inorganic Origin for Magnetite in Martian Meteorite ALH84001

The origin of magnetite crystals in Martian Meteorite ALH84001 is the focus of a debate about the possibility of past (and present) life on Mars. McKay et al. originally suggested that some of the magnetite crystals associated with carbonate globules in Martian Meteorite ALH84001 are biogenic in ori-gin, because they are single magnetic domain, free of crystalline defects, chemically pure, and coexist with other metastable phases in apparent disequilibrium. Thomas-Keprta et al. reported that a subpopulation of magnetite crystals (approx. 25%) associated with carbonate globules in ALH84001 and magnetite crystals produced by magnetotactic bacterial strain MV-1 have similar morphologies with crystal elongation along the [111] crystallographic axis that they describe as "truncated hexa-octahedral" ([111-THO]) magnetite. Along with several other properties, the [111]-THO morphology has been proposed to constitute a biomarker (i.e., formed only in biogenic processes), so that the presence of [111]-THO magnetite in ALH84001 may be evidence for past life on Mars.

Golden, D. C.↗
NASA NTRS2001

Silicon Carbide Solar Cells Investigated

The semiconductor silicon carbide (SiC) has long been known for its outstanding resistance to harsh environments (e.g., thermal stability, radiation resistance, and dielectric strength). However, the ability to produce device-quality material is severely limited by the inherent crystalline defects associated with this material and their associated electronic effects. Much progress has been made recently in the understanding and control of these defects and in the improved processing of this material. Because of this work, it may be possible to produce SiC-based solar cells for environments with high temperatures, light intensities, and radiation, such as those experienced by solar probes. Electronics and sensors based on SiC can operate in hostile environments where conventional silicon-based electronics (limited to 350 C) cannot function. Development of this material will enable large performance enhancements and size reductions for a wide variety of systems--such as high-frequency devices, high-power devices, microwave switching devices, and high-temperature electronics. These applications would supply more energy-efficient public electric power distribution and electric vehicles, more powerful microwave electronics for radar and communications, and better sensors and controls for cleaner-burning, more fuel-efficient jet aircraft and automobile engines. The 6H-SiC polytype is a promising wide-bandgap (Eg = 3.0 eV) semiconductor for photovoltaic applications in harsh solar environments that involve high-temperature and high-radiation conditions. The advantages of this material for this application lie in its extremely large breakdown field strength, high thermal conductivity, good electron saturation drift velocity, and stable electrical performance at temperatures as high as 600 C. This behavior makes it an attractive photovoltaic solar cell material for devices that can operate within three solar radii of the Sun.

Bailey, Sheila G.↗
NASA NTRS1986

Method for growing low defect, high purity crystalline layers utilizing lateral overgrowth of a patterned mask

A method for growing a high purity, low defect layer of semiconductor is described. This method involves depositing a patterned mask of a material impervious to impurities of the semiconductor on a surface of a blank. When a layer of semiconductor is grown on the mask, the semiconductor will first grow from the surface portions exposed by the openings in the mask and will bridge the connecting portions of the mask to form a continuous layer having improved purity, since only the portions overlying the openings are exposed to defects and impurities. The process can be iterated and the mask translated to further improve the quality of grown layers.

Morrison, Andrew D.↗
NASA NTRS1985

Low defect, high purity crystalline layers grown by selective deposition

The purity and perfection of a semiconductor is improved by depositing a patterned mask of a material impervious to impurities of the semiconductor on a surface of a blank. When a layer of semiconductor is grown on the mask, the semiconductor will first grow from the surface portions exposed by the openings in the mask and will bridge the connecting portions of the mask to form a continuous layer having improved purity, since only the portions overlying the openings are exposed to defects and impurities.

Morrison, A. D.↗
NASA NTRS1985

Surface Passivation and Junction Formation Using Low Energy Hydrogen Implants

New applications for high current, low energy hydrogen ion implants on single crystal and polycrystal silicon grain boundaries are discussed. The effects of low energy hydrogen ion beams on crystalline Si surfaces are considered. The effect of these beams on bulk defects in crystalline Si is addressed. Specific applications of H+ implants to crystalline Si processing are discussed. In all of the situations reported on, the hydrogen beams were produced using a high current Kaufman ion source.

Fonash, S. J.↗
NASA NTRS1996

The Influence of Reduced Gravity on the Crystal Growth of Electronic Materials

The imperfections in the grown crystals of electronic materials, such as compositional nonuniformity, dopant segregation and crystalline structural defects, are detrimental to the performance of the opto-electronic devices. Some of these imperfections can be attributed to effects caused by Earth gravity during crystal growth process and four areas have been identified as the uniqueness of material processing in reduced gravity environment. The significant results of early flight experiments, i.e. prior to space shuttle era, are briefly reviewed followed by an elaborated review on the recent flight experiments conducted on shuttle missions. The results are presented for two major growth methods of electronic materials: melt and vapor growth. The use of an applied magnetic field in the melt growth of electrically conductive melts on Earth to simulate the conditions of reduced gravity has been investigated and it is believed that the superimposed effect of moderate magnetic fields and the reduced gravity environment of space can result in reduction of convective intensities to the extent unreachable by the exclusive use of magnet on Earth or space processing. In the Discussions section each of the significant results of the flight experiments is attributed to one of the four effects of reduced gravity and the unresolved problems on the measured mass fluxes in some of the vapor transport flight experiments are discussed.

Su, Ching-Hua↗