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Hermann, A. M.

Publications and source records attributed to Hermann, A. M..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Chemical spray pyrolysis of Tl-Ba-Ca-Cu-O high-T(sub c) superconductors for high-field bitter magnets

The deposition of Tl-Ba-Ca-Cu-O thick films by spray pyrolyzing a Ba-Ca-Cu-O precursor film and diffusing thallium into the film to form the superconducting phase is examined. This approach was taken to reduce exposure to thallium and its health and safety hazards. The Tl-Ba-Ca-Cu-O system was selected because it has very attractive features which make it appealing to device and manufacturing engineering. Tl-Ba-Ca-Cu-O will accommodate a number of superconducting phases. This attribute makes it very forgiving to stoichiometric fluctuations in the bulk and film. It has excellent thermal and chemical stability, and appears to be relatively insensitive to chemical impurities. Oxygen is tightly bound into the systems, consequently there is no orthorhombic (conductor) to tetragonal (insulator) transition which would affect a component's lifetime. More significantly, the thallium based superconductors appear to have harder magnetic properties than the other high-Tc oxide ceramics. Estimates using magnetoresistance measurements indicate that at 77 K Tl2Ba2CaCu2O10 will have an upper critical field, H(sub c2) fo 26 Tesla for applied fields parallel to the c-axis and approximately 1000 Tesla for fields oriented in the a-b plane. Results to date have shown that superconducting films can be reproducibly deposited on 100 oriented MgO substrates. One film had a zero resistance temperature of 111.5 K. Furthermore, x ray diffraction analysis of the films showed preferential c-axis orientation parallel to the plane of the substrate. These results have now made it possible to consider the manufacture of a superconducting tape wire which can be configured into a topology useful for high-field magnet designs. The research which leads to the preparation of these films and plans for further development are reviewed.

Derochemont, L. Pierre↗
NASA NTRS1987

In situ process diagnostics of silane plasma for device-quality a-Si:H deposition

Coherent anti-Stokes Raman spectroscopy (CARS) and mass spectrometry (MS) have been applied to in situ process diagnostics of a silane plasma for device-quality a-Si:H film deposition. Silane depletion was directly measured by CARS and is linearly dependent on RF power in the region of 4-12 W with a slope of 0.5 percent/mW-sq cm. The depletion is also dependent on SiH4 flow rate starting with a 50 percent depletion at a low flow rate of 5.6 sccm and asymptotically approaching an 8 percent depletion at a flow rate of 80 sccm. The mass spectral line signal intensity of disilane increases with RF power and shows an apparent transition at 6 W. Disilane formation in silane plasma, film deposition rate, and silane depletion ratio as a function of the RF power indicate that the film growth mechanism in the low-power region of 3.5-6.5 W is substantially different from that in the high-power region of 6.5-12 W.

Shing, Y. H.↗
NASA NTRS1982

Minority carrier diffusion length measurements - A review and comparison of techniques

A review of the major techniques for measuring minority carrier diffusion lengths in solar cells is given. Limits of applicability are indicated for each method, as relevant to indirect bandgap materials such as silicon, and direct bandgap materials like gallium arsenide. A discussion and bibliography is presented for the following techniques: uniform generation, electron beam induced current, Schottky barrier (and other steady-state) photocurrent, surface photovoltage, transient decay, and luminescence.

Khan, A. A.↗
NASA NTRS1981

Performance of high resistivity n+pp+ silicon solar cells under 1 MeV electron irradiation

High resistivity (1250 and 84 ohm-cm) n(+)pp(+) silicon solar cells were irradiated and their performance evaluated as a function of fluence. The greatest degradation in power occurred for the higher resistivity cell. The data were analyzed under open circuit conditions, and the components of V sub oc determined as a function of fluence. It was found that the voltage contributions from the front and back junctions decreased while the base component (V sub B) increased with fluence. The anomalous behavior of V sub B was attributed to an increase in the base minority carrier gradient with fluence. An argument that the increased power degradation in the 1250 ohm-cm cells was attributable to an increased voltage drop in the base is presented. Diffusion lengths calculated under high injection conditions were significantly greater than those determined under low injection. This was attributed to a saturation of recombination centers under high injection conditions.

Weinberg, I.↗
NASA NTRS1980

Radiation damage in lithium-counterdoped n/p silicon solar cells

Lithium counterdoped n+/p silicon solar cells were irradiated with 1 MV electrons and their post irradiation performance and low temperature annealing properties were compared to that of the 0.35 ohm cm control cells. Cells fabricated from float zone and Czochralski grown silicon were investigated. It was found that the float zone cells exhibited superior radiation resistance compared to the control cells, while no improvement was noted for the Czochralski grown cells. Room temperature and 60 C annealing studies were conducted. The annealing was found to be a combination of first and second order kinetics for short times. It was suggested that the principal annealing mechanism was migration of lithium to a radiation induced defect with subsequent neutralization of the defect by combination with lithium. The effects of base lithium gradient were investigated. It was found that cells with negative base lithium gradients exhibited poor radiation resistance and performance compared to those with positive or no lithium gradients; the latter being preferred for overall performance and radiation resistance.

Hermann, A. M.↗
NASA NTRS1980

Radiation damage in silicon NIP solar cells

The performance parameters of n(+) p p(+) silicon solar cells of varying thicknesses with boron doped p base resistivities of 1250 and 84 ohm centimeters were determined. High injection theory was used to analyze the experimental data. Results from an analysis of open circuit voltages show a much greater contribution to V(oc) from the back junction than is the case for the lower resistivity in common use. The base minority carrier distribution is seen to be significant in determining the contribution of V(B), the base contribution to V(oc). Although V(B) is small, its value increases with increasing radiation fluence. In this connection it is noted that, with illumination from the p(+) side, the sign of V(B) becomes positive, and V(B) itself becomes an additive term to V(oc). Diffusion lengths determined under high injection conditions are significantly greater than those obtained under low injection, while damage coefficients under low injection are higher than those obtained under high injection conditions.

Weinberg, I.↗
NASA NTRS1980

Nordheim-Gorter like behavior and one carrier conduction in a quasi-one-dimensional system

A linear correlation has been observed between the thermopower and the conductivity in the high-temperature state (T greater than 220 K) of the TTF halide and pseudohalide salts. It is suggested that this correlation indicates conduction by a single type of carrier under two types of scattering and represents the first reported extension of the Nordheim-Gorter behavior to any quasi-one-dimensional conductor.

Datta, T.↗
NASA NTRS1980

Radiation damage in lithium-counterdoped N/P silicon solar cells

The radiation resistance and low-temperature annealing properties of lithium-counterdoped n(+)-p silicon solar cells are investigated. Cells fabricated from float zone and Czochralski grown silicon were irradiated with 1 MeV electrons and their performance compared to that of 0.35 ohm-cm control cells. The float zone cells demonstrated superior radiation resistance compared to the control cells, while no improvement was noted for the Czochralski grown cells. Annealing kinetics were found to lie between first and second order for relatively short times, and the most likely annealing mechanism was found to be the diffusion of lithium to defects with the subsequent neutralization of defects by combination with lithium. Cells with zero lithium gradients exhibited the best radiation resistance.

Hermann, A. M.↗
NASA NTRS1978

Optical, spin-resonance, and magnetoresistance studies of /tetrathiatetracene/2/iodide/3 - The nature of the ground state

A recent investigation in which Isett and Perez-Albuerne (1977) found that (tetrathiatetracene)2(iodine)3 is a stable organic metal down to 3.3 K is of considerable interest. In view of the diversity of suggestions made regarding this compound, measurements, were made of its electrical, magnetic, and optical properties. The results obtained indicate a metallic state at high temperatures, but also support a nonmetallic state at temperatures below 30 K. At 20-30 K, a metal-to-insulator phase transition occurs. This is indicated by the onset of a large and positive magnetoresistance, the leveling off in the temperature dependence of the ESR linewidth, and by recent derivative analysis of the electrical conductivity in this region. The investigated compound represents an interesting one-dimensional electronic system in which it may be possible to study the combined effects of disorder and interchain coupling on charge transport processes.

Somoano, R. B.↗
NASA NTRS1977

Electrical, magnetic, and optical properties of the tetrathiafulvalene /TTF/ pseudohalides, /TTF/12/SCN/7 and /TTF/12/SeCN/7

The electrical, magnetic, and optical properties of charge-transfer salts containing tetrathiafulvalene (TTF) and the pseudohalides, thiocyanate (SCN) and selenocyanate (SeCN), have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to a cation-radical-anion-radical system, such as tetrathiafulvalene tetracyanoquinodimethane (TTF) (TCNQ). Measurements of electrical conductivity, thermoelectric power, and optical reflectivity of single crystals of the nonstoichiometric salts (TTF)12(SCN)7 and (TTF)12(SeCN)7 show metal-like characteristics above 200 K (high-temperature region). The conductivities at room temperature are approximately 750 mho/cm comparable to those found in (TTF) (TCNQ), and increase with decreasing temperature down to approximately 200 K. The thermoelectric power at room temperature is small and positive (approximately 9 microvolts/deg K), and decreases linearly with decreasing temperature in this region (as expected for metal-like hole conduction along the TTF chains). The ESR intensity, however, decreases with decreasing temperature above 200 K. At 170 K a metal-nonmetal transition occurs, and the transport and magnetic properties below this temperature are characteristic of a semiconducting state.

Somoano, R. B.↗
NASA NTRS1977

Electrical properties of /DEPE/ /TCNQ/sub 4

The electrical conductivity of two phases of (DEPE) (TCNQ)sub 4 is evaluated using four probe and Montgomery methods. The highest values (approximately 50 Ohms/cm) were found along the b sub 0 axis of the monoclinic phase (I) crystals at room temperature. The resistivities are described as semiconducting. Thermopower is observed to saturate at high temperatures to 60 microvolts/deg K, characteristic of a quarter-filled correlated band. It is pointed out that a stable metallic state for (DEPE) (TCNQ)sub 4 is theoretically possible, but none has thus far been found.

Somoano, R. B.↗
NASA NTRS1977

On the crystal phases of /DEPE/ /TCNQ/4

X-ray powder diffraction studies of two semiconducting crystal phases in the organic charge transfer salt (DEPE)(TCNQ) sub 4 has revealed almost identical electrical properties to those of metallic crystals. It is hypothesized that the similarity arises from the nature of the measurements themselves, i.e., electrode geometry. The possibility that a separate metallic phase for (DEPE)(TCNQ) sub 4 exists, is not, however, ruled out.

Coleman, L. B.↗
NASA NTRS1975

The electrical and magnetic properties of /TTF//I/-sub 0.71

The synthesis and transport properties of the salts (TTF) (iodine)-sub x have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to the cation radical-anion radical systems such as (TTF)(TCNQ). The phase diagram of (TTF)(I)-sub x is discussed, and found to consist of two nonconducting, stoichiometric phases, x = 2.0 and 3.0, and two conducting, nonstoichiometric phases, x = 0.71 and 2.3. The transport properties of (TTF)(I)-sub 0.71 are discussed in detail. This phase is highly conducting, undergoes a sharp transition to a less conducting state at 210 K, and exhibits an unusual hysteresis in some of the transport properties upon thermal cycling. The nature of the high-temperature ground state is examined.

Somoano, R. B.↗
NASA NTRS1974

Drift mobility of holes in phenanthrene single crystals

The temperature dependence of drift mobilities of holes in single crystals of phenanthrene was measured in the range from 203 to 353 K in three crystallographic directions. Below the anomaly temperature of 72 C, the mobility temperature dependences are consistent with the Munn and Siebrand slow-phonon hopping process in the b direction and the Munn and Siebrand slow-phonon coherent mode in the a and c prime directions. The drift mobility temperature dependences in crystals that have been cooled through the anomaly temperature in the presence of illumination and an electric field are consistent with the model of Spielberg et al. (1971), in which the hindered vibration of the 4,5 hydrogens introduces a new degree of freedom above 72 C.

Sonnonstine, T. J.↗