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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Advanced methods for preparation and characterization of infrared detector materials

Crystal growth of Hg sub 1-x Cd sub x Te and density measurements of ingot slices are discussed. Radial compositional variations are evaluated from the results of infrared transmission edge mapping. The pseudo-binary HgTe-CdTe phase diagram is examined with reference to differential thermal analysis measurements. The phase equilibria calculations, based on the 'regular association solution' theory (R.A.S.) are explained and, using the obtained R.A.S. parameters, the activities of Hg, Cd, and Te vapors and their partial pressures over the pseudo-binary melt are calculated.

Lehoczy, S. L.↗
NASA NTRS1980

Hydrogen production by photoelectrolytic decomposition of H2O using solar energy

Photoelectrochemical systems for the efficient decomposition of water are discussed. Semiconducting d band oxides which would yield the combination of stability, low electron affinity, and moderate band gap essential for an efficient photoanode are sought. The materials PdO and Fe-xRhxO3 appear most likely. Oxygen evolution yields may also be improved by mediation of high energy oxidizing agents, such as CO3(-). Examination of several p type semiconductors as photocathodes revealed remarkable stability for p-GaAs, and also indicated p-CdTe as a stable H2 photoelectrode. Several potentially economical schemes for photoelectrochemical decomposition of water were examined, including photoelectrochemical diodes and two stage, four photon processes.

Rauh, R. D.↗
NASA NTRS1980

Advanced methods for preparation and characterization of infrared detector materials

Mercury cadmium telluride crystals were prepared by the Bridgman method with a wide range of crystal growth rates and temperature gradients adequate to prevent constitutional supercooling under diffusion-limited, steady state, growth conditions. The longitudinal compositional gradients for different growth conditions and alloy compositions were calculated and compared with experimental data to develop a quantitative model of the crystal growth kinetics for the Hg(i-x)CdxTe alloys, and measurements were performed to ascertain the effect of growth conditions on radial compositional gradients. The pseudobinary HgTe-CdTe constitutional phase diagram was determined by precision differential thermal analysis measurements and used to calculate the segregation coefficient of Cd as a function of x and interface temperature. Computer algorithms specific to Hg(1-x)CdxTe were developed for calculations of the charge carrier concentrations, charge carrier mobilities, Hall coefficient, optical absorptance, and Fermi energy as functions of x, temperature, ionized donor and acceptor concentrations, and neutral defect concentrations.

Lehoczky, S. L.↗
NASA NTRS1982

III-V semiconductor solid solution single crystal growth

The feasibility and desirability of space growth of bulk IR semiconductor crystals for use as substrates for epitaxial IR detector material were researched. A III-V ternary compound (GaInSb) and a II-VI binary compound were considered. Vapor epitaxy and quaternary epitaxy techniques were found to be sufficient to permit the use of ground based binary III-V crystals for all major device applications. Float zoning of CdTe was found to be a potentially successful approach to obtaining high quality substrate material, but further experiments were required.

Gertner, E. R.↗
NASA NTRS1982

Space processing of electronic materials

The bulk growth of solid solution alloys of mercury telluride and cadmium telluride is discussed. These alloys are usually described by the formula Hg1-xCdxTe, and are useful for the construction of infrared detectors. The electronic energy band gap can be controlled between zero and 1.6 electron volts by adjusting the composition x. The most useful materials are at x approximately 20%, suitable for detection wavelengths of about 10 micrometers. The problems of growing large crystals are rooted in the wide phase diagram of the HgTe-CdTe pseudobinary system which leads to exaggerate segregation in freezing, constitutional supercooling, and other difficulties, and in the high vapor pressure of mercury at the growth temperatures, which leads to loss of stoichiometry and to the necessity of working in strong, pressure resistant sealed containers.

Holland, L. R.↗
NASA NTRS1983

Advanced methods for preparation and characterization of infrared detector materials

Crystals were prepared by the Bridgman-Stockbarger method with a wide range of crystal growth rates and temperature gradients adequate to prevent constitutional supercooling under diffusion-limited, steady-state, growth conditions. The longitudinal compositional gradients for different growth conditions and alloy compositions were calculated and compared with experimental data to develop a quantitative model of solute redistribution during the crystal growth of the alloys. Measurements were performed to ascertain the effect of growth conditions on radial compositional gradients. The pseudobinary HgTe-CdTe constitutional phase diagram was determined by precision differential-thermal-analysis measurements and used to calculate the segregation coefficient of Cd as a function of x and interface temperature. Experiments were conducted to determine the ternary phase equilibria in selected regions of the Hg-Cd-Te constitutional phase diagram. Electron and hole mobilities as functions of temperature were analyzed to establish charge-carrier scattering probabilities. Computer algorithms specific to Hg(1-x)CdxTe were developed for calculations of the charge-carrier concentration, charge-carrier mobilities, Hall coefficient, and Dermi Fermi energy as functions of x, temperature, ionized donor and acceptor concentrations, and neutral defect concentrations.

Broerman, J. G.↗
NASA NTRS1983

Epitaxial growth of Hg/1-x/Cd/x/Te by chemical vapor transport

The chemical vapor transport technique was applied for the first time to the growth of Hg(1-x)Cd(x)Te epitaxial layers on CdTe substrates using a preannealed source material of Hg(0.8)Cd(0.2)Te. The experiment took place in a pretreated ampoule loaded with source material, HgI2 transport agent, and substrate and sealed under a residual pressure of about 10 to the -5th torr. The temperature of the source region was 590 C and that of the deposition zone 535 C with a nearly sigmoidal shaped profile between the end temperatures. The change in appearance of the substrate surface during the experiment is described, and the structures of the surface of the Hg(1-x)Cd(x)Te layer and of the layer itself as revealed by Nomarski contrast illumination and Laue X-ray diffraction, respectively, are discussed.

Wiedemeier, H.↗
NASA NTRS1982

Directional solidification and characterization of Hg(1-x)Cd(x)Te alloys

A series of Hg(1-x)Cd(x)Te alloy crystals was grown by high temperature gradient directional solidification at furnace translation rates ranging from 0.068 to 1.12 microns/s. For several ingots, the measured longitudinal compositional profiles were fitted to theoretical profiles to estimate the magnitude of D, the liquid HgTe-CdTe interdiffusion coefficient. The best-fit value of D was about 550,000 sq cm/s. The majority of the ingots showed significant radial compositional variations along the growth axis. These variations are attributed, at least in part, to fluid flows ahead of the growth interface. The results are discussed in terms of the heat transfer characteristics of the alloy/ampule/furnace system, and on the effects of these characteristics on the shape and stability of the growth interface in a 1-g environment.

Lehoczky, S. L.↗
NASA NTRS1982

Gravity-driven convection studies in compound semiconductor crystal growth by physical vapor transport

Experimental results are summarized, and it is pointed out that gravity-driven convection can alter the diffusive-advective mass transport behavior in the growth of crystals by physical vapor transport. Specially designed and constructed transparent furnaces are described which are being used to study the effects of gravity in the crystal growth of the compound semiconductors PbTe and CdTe. The theory underlying vapor transport behavior is reviewed, with attention given to the vapor-solid behavior of compound materials, to one-dimensional mass transport, and to gravity-induced (natural) convection. In the transparent furnaces, the quartz capillary tube mounted along the axis of the main quartz ampoule is used to measure the temperature at the growth surface (vapor-solid crystal interface) and the source, as well as the complete temperature profile along the axis of the tube. The light-pipe works to remove heat from the growth end of the ampoule by radiative heat transfer. The ampoules are sealed after being evacuated to the low 10 to the -8th torr range with a cryogenic vacuum pump.

Zoutendyk, J. A.↗
NASA NTRS1983

Density of liquid Hg(1-x)Cd(x)Te

Negative thermal expansion has been established in liquid Hg(1-x)Cd(x)Te for x less than 0.2 employing a pycnometric method. Pure HgTe increases in density from its melting point at 670 C to a maximum value at 750 C, where normal thermal expansion progressively resumes. The dependence of density on temperature for liquid Hg(1-x)Cd(x)Te arises almost exclusively from the HgTe portion of the melt, while CdTe acts as a diluent. The temperature corresponding to the maximum density changes slightly with composition, increasing by about 5 C for x = 0.1.

Chandra, D.↗
NASA NTRS1984

Second All-Union Seminar on Hydromechanics and Heat-Mass Transfer in Weightlessness. Abstracts of reports: Table of contents

Abstracts of reports are given which were presented at the Second All Union Seminar on Hydromechanics and Heat-Mass Transfer in Weightlessness. Topics inlcude: (1) features of crystallization of semiconductor materials under conditions of microacceleration; (2) experimental results of crystallization of solid solutions of CDTE-HGTE under conditions of weightlessness; (3) impurities in crystals cultivated under conditions of weightlessness; and (4) a numerical investigation of the distribution of impurities during guided crystallization of a melt.

Gershuni, G. Z.↗
NASA NTRS1984

Second All-Union Seminar on Hydromechanics and Heat and Mass Exchange in Weightlessness, summaries of reports

Abstracts of reports are given which were presented at the Second All Union Seminar on Hydromechanics and Heat-Mass Transfer in Weightlessness. Topics include: (1) features of crystallization of semiconductor materials under conditions of microacceleration; (2) experimental results of crystallization of solid solutions of CDTE-HGTE under conditions of weightlessness; (3) impurities in crystals cultivated under conditions of weightlessness; and (4) a numerical investigation of the distribution of impurities during guided crystallization of a melt.

Gershuni, G. Z.↗
NASA NTRS1983

Short wavelength infrared hybrid focal plane arrays

The employment of area focal plane arrays (FPA) has made it possible to obtain second generation infrared imaging systems with high resolution and sensitivity. The Short Wavelength Infrared (SWIR) region (1-2.5 microns) is of importance for imaging objects at high temperature and under conditions of reflected sunlight. The present investigation is concerned with electrooptical characterization results for 32 x 32 SWIR detector arrays and FPAs which are suitable for use in a prototype imaging spectrometer. The employed detector material is Hg(1-x)Cd(x)Te grown by liquid phase epitaxy on a CdTe transparent substrate. Attention is given to details of processing, the design of the detector array, the multiplexer, the fabrication of the hybrid FPA, and aspects of performance.

Vural, K.↗
NASA NTRS1985

Processing of Complex Compound Semiconductors in Space

The purpose of this first year ground based research effort was to develop growth and characterization facilities for bulk single crystals of compound semiconductor alloys. As a system of 2-6 alloy crystal growth, C sub x Mn sub (1-x)Te was chosen since it has a relatively narrow solidus-liquidus separation serving as a suitable first model for zone melting and solution growth. Also, it allows, in principle, perfect lattice matching to Cd sub x Hg sub(1-x)Te that needs to be studied as a model for the investigation of heteroepitaxial growth from Te-solutions. A vertical zone melting furnace was built and used in the synthesis of Cd sub x Mn sub(1-x)Te from the elements and crystal growth in carbon coated fused silica crucibles of Bridgman growth geometry. Photoluminescence spectra of the crystals reveal band to band recombination at energies that agree with the reported literature data. In addition, deep luminescence is present at energies corresponding to proportional shifts from the 1.4 eV deep emission observed for bulk single crystals of CdTe.

Bachmann, K.↗
NASA NTRS1985

The Influence of Low Frequency Mechanical Vibrations on the Growth of Single Crystals

The optimum conditions for crystal growth are usually achieved either by suppressing convective fluid flows (e.g., by the use of a low-gravity environment) or by over-riding thermal and solutal convection by the use of a strong stirring action. A novel stirring technique has been developed which involves subjecting a vertical crucible to a circle in a horizontal plane (without rotation). Use of an amplitude of 3 mm at a frequency of approx 6 Hz produced complete mixing of a non-uniform aqueous liquid in a few seconds. The mixing action involved the downward flow of liquid in the outer annulus of the liquid, driven by surface waves. When the downward flowing liquid reaches the bottom of the crucible, it is reflected in a central, upward flowing spiral. This flow pattern should be beneficial for crystal growth by the Bridgman method since it will sweep impurities away from the walls and produce a more convex solid-liquid interface. Initial attempts to apply the new stirring technique to CdTe crystal growth did not show significant improvement in the number of crystals nucleated, but the interface shape appeared to be close to that predicted.

Feigelson, R. S.↗
NASA NTRS1985

Potential of thin-film solar cell module technology

During the past five years, thin-film cell technology has made remarkable progress as a potential alternative to crystalline silicon cell technology. The efficiency of a single-junction thin-film cell, which is the most promising for use in flat-plate modules, is now in the range of 11 percent with 1-sq cm cells consisting of amorphous silicon, CuInSe2 or CdTe materials. Cell efficiencies higher than 18 percent, suitable for 15 percent-efficient flat plate modules, would require a multijunction configuration such as the CdTe/CuInSe2 and tandem amorphous-silicon (a-Si) alloy cells. Assessments are presented of the technology status of thin-film-cell module research and the potential of achieving the higher efficiencies required for large-scale penetration into the photovoltaic (PV) energy market.

Shimada, K.↗
NASA NTRS1987

Vacancy formation energies in II-VI semiconductors

Cation and anion vacancy formation energies are calculated for HgTe, ZnTe, CdTe, and their dilute alloys. Harrison's tight-bonding theory is extended to a cluster embedded in an extended crystal. Only neutral vacancies have been considered, and two final states for the removed atom have been addressed: a free atom in vacuum and an atom on an ideal (111) surface.

Berding, M. A.↗