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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

HgCdTe for NASA EOS missions and detector uniformity benchmarks

Important NASA Earth Observing System (EOS) missions, Atmospheric Infrared Sounder (AIRS) and Moderate Resolution Imaging Spectrometer (MODIS-N), which require detector spectral response in the range of 14 to 17 microns at medium background flux levels and operation in the range of temperatures between 65 to 95 K, will be flown beginning in the next few years. Currently, a prime candidate detector technology for these missions is trapping-mode photoconductive HgCdTe devices. These devices can be tailored to the exact cutoff wavelengths required by those missions, and thus offer the performance advantages of an intrinsic detector which is ideally matched to the mission wavelength. Under the long wavelength-background-temperature conditions of these EOS missions, any detector will at best be thermal generation-recombination noise limited. Photoconductive devices are generally preferred under these circumstances, since at elevated temperatures their performance degrades with n(sub i) while for photovoltaic detectors performance degrades as n sub i(exp 2) n sub i is the intrinsic carrier concentration which is a function of alloy composition and temperature, but not doping. Very high performance trapping-mode photoconductive HgCdTe detectors have been developed which can be reproducibly fabricated. Detectivity (D asterisk) at 80K and 16 micron cutoff wavelength in excess of 10(exp 11) Jones has been measured for these devices. Power dissipation is at least two orders of magnitude less than conventional HgCdTe photoconductors - on the order of 0.12 W/cm(exp 2) compared with 12 W/cm(exp 2). EOS missions define thermal noise limited conditions for the long wavelength operating bands. Trapping-mode photoconductive HgCdTe detectors are linear under such conditions and responsivity is independent of background flux. At lower temperatures or high flux conditions in which background flux limits detector performance, trapping-mode detectors have a responsivity which varies with background flux. Internal calibration must be provided for radiometric measurements under the latter conditions (not an EOS mission concern). Liquid phase epitaxy is used to grow these HgCdTe device structures. This technique has been shown to give control of the cutoff wavelength on the order of 16 plus or minus 1 micron or less, both from run to run and across wafer dimensions of several centimeters on a side.

Norton, Paul R.↗
NASA NTRS1978

Interpretation of scanning electron microscope measurements of minority carrier diffusion lengths in semiconductors

In scanning electron microscope (SEM) injection measurements of minority carrier diffusion lengths some uncertainties of interpretation exist when the response current is nonlinear with distance. This is significant in epitaxial layers where the layer thickness is not large in relation to the diffusion length, and where there are large surface recombination velocities on the incident and contact surfaces. An image method of analysis is presented for such specimens. A method of using the results to correct the observed response in a simple convenient way is presented. The technique is illustrated with reference to measurements in epitaxial layers of GaAs. Average beam penetration depth may also be estimated from the curve shape.

Flat, A.↗
NASA NTRS1998

Near-infrared spectroscopy of the proto-planetary nebula CRL 618 and the origin of the hydrocarbon dust component in the interstellar medium

A new 2.8-3.8 micrometers spectrum of the carbon-rich protoplanetary nebula CRL 618 confirms the previous detection of a circumstellar 3.4 micrometers absorption feature in this object (Lequeux & Jourdain de Muizon). The high resolution and high signal-to-noise ratio of our spectrum allow us to derive the detailed profile of this absorption feature, which is very similar to that observed in the spectrum of the Galactic center and also resembles the strong 3.4 micrometers emission feature in some post-asymptotic giant branch stars. A weak 3.3 micrometers unidentified infrared band, marginally detected in the CRL 618 spectrum of Lequeux & Jourdain de Muizon, is present in our spectrum. The existence of the 3.4 micrometers feature implies the presence of relatively short-chained, aliphatic hydrocarbon materials (-CH2-/-CH3 approximately = 2-2.5) in the circumstellar environment around CRL 618. It also implies that the carriers of the interstellar 3.4 micrometers feature are produced at least in part in circumstellar material, and it calls into question whether any are produced by the processing of interstellar ices in dense interstellar clouds, as has been previously proposed. Other features in the spectrum are recombination lines of hydrogen, rotational and vibration-rotation lines of molecular hydrogen, and a broad absorption probably due to a blend of HCN and C2H2 bands.

Spectroscopy, Near-Infrared↗
NASA NTRS1986

Heavy doping effects in high efficiency silicon solar cells

The temperature dependence of the emitter saturation current for bipolar devices was studied by varying the surface recombination velocity at the emitter surface. From this dependence, the value was derived for bandgap narrowing that is in better agreement with other determinations that were obtained from the temperature dependence measure on devices with ohmic contacts. Results of the first direct measurement of the minority-carrier transit time in a transparent heavily doped emitter layer were reported. The value was obtained by a high-frequency conductance method recently developed and used for doped Si. Experimental evidence is presented for significantly greater charge storage in highly excited silicon near room temperature than conventional theory would predict. These data are compared with various data for delta E sub G in heavily doped silicon.

Lindholm, F. A.↗
NASA NTRS1981

Surface recombination velocity measurement for silicon solar cells

For the design and fabrication of silicon solar cells approaching theoretical ultimate conversion efficiencies, surface recombination velocity plays a crucial role. A technique using a scanning electron microscope with pulsed electron beam has been developed for thy measurement of this important parameter for silicon surfaces. It is shown that the surface recombination velocity(s) increases by an order of magnitude when a freshly etched sample is left out for a few hours, presumably due to generation of surface states. A textured front surface field (FSF) cell with a high-low junction near the surface shows the effect of minority carrier reflection and an apparent reduction of s, whereas a tandem junction (TJ) cell with n+-p junction near the surface gives larger s value.

Daud, T.↗
NASA NTRS1990

Mechanistic studies of the CO-oxidation reaction on catalysts for use in long-life CO2 lasers

The catalytic recombination of carbon monoxide and oxygen was studied under conditions expected to be present in a sealed E-beam CO2 laser system. These conditions are typically a gas inlet temperature of 60 C, a substoichiometric CO/O2 ratio of ca. 2.5/1 with an oxygen feed rate of ca. 5 micromoles/s, a carrier gas comprising He, N2 and CO2 in the ratio of 3:2:1, near atmospheric pressure and a gas velocity of 4 m/s. Heterogeneous catalysts, based on precious metal supported on tin oxide, have been coated onto ceramic monoliths and tested for catalytic activity and stability after a reduction/passivation step. Two catalyst systems have been chosen. These are Pt/Pd/SnO2 and Pt/Ru/SnO2. Under the conditions described above, a characteristic decline in catalytic activity is apparent for both systems, and exit gas temperature has been recognized as a sensitive parameter by which to monitor the activity changes. A semilogarithmic plot of exit temperature as a function of time has revealed two distinct processes connected with the decline in activity: one process is associated with reduction of the oxidized precious metal (at Site A), whilst the other is related to the formation and approach to steady-state of an active site at the metal/support interface (Site B).

Dawood, Talat↗
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 NTRS1995

A theoretical analysis of steady-state photocurrents in simple silicon diodes

A theoretical analysis solves for the steady-state photocurrents produced by a given photo-generation rate function with negligible recombination in simple silicon diodes, consisting of a uniformly doped quasi-neutral region (called 'substrate' below) adjacent to a p-n junction depletion region (DR). Special attention is given to conditions that produce 'funneling' (a term used by the single-eventeffects community) under steady-state conditions. Funneling occurs when carriers are generated so fast that the DR becomes flooded and partially or completely collapses. Some or nearly all of the applied voltage, plus built-in potential normally across the DR, is now across the substrate. This substrate voltage drop affects substrate currents. The steady-state problem can provide some qualitative insights into the more difficult transient problem. First, it was found that funneling can be induced from a distance, i.e., from carriers generated at locations outside of the DR. Secondly, it was found that the substrate can divide into two subregions, with one controlling substrate resistance and the other characterized by ambipolar diffusion. Finally, funneling was found to be more difficult to induce in the p(sup +)/n diode than in the n(sup +)/p diode. The carrier density exceeding the doping density in the substrate and at the DR boundary is not a sufficient condition to collapse a DR.

Edmonds, L.↗
NASA NTRS1995

A theoretical analysis of steady-state photocurrents in simple silicon diodes.

A theoretical analysis solves for the steady-state photocurrents produced by a given photogeneration rate function with negligible recombination in simple silicon diodes, consisting of a uniformly doped quasi-neutral region (called "substrate" below) between a p-n junction depletion region (DR) and an ohmic contact (electrode). Special attention is given to conditions that produce "funneling" (a term used by the single-event-effects community) under steady-state conditions. Funneling occurs when carriers are generated so fast that the DR becomes flooded and partially or completely collapses. Some or nearly all of the applied voltage plus built-in potential normally across the DR is now across the substrate. This substrate voltage drop affects substrate currents. The steady-state problem can provide some qualitative insights into the more difficult transient problem. Chapter 6 discusses some similarities between the steady-state and transient problems...

Edmonds, Larry↗
NASA NTRS1990

Dissociation and recombination of positive holes in minerals

The formation mechanisms are described of positive holes - electronic defects in the O2 sublattice - with attention given to detecting the positive surface charge of minerals with these holes. Charge distribution analysis (CDA) is presented which measures dielectric polarization in an inhomogeneous field. CDA can be applied to the detection of the peroxide/superoxide functionality caused by positive holes on the surface. It is demonstrated with obsidian that the measurements provide data on O(-) mobility as a function of surface-charge carrier density and on O(-) generation as a function of temperature.

Freund, Friedemann↗
NASA NTRS1979

Heavily doped transparent-emitter regions in junction solar cells, diodes, and transistors

The paper presents an analytical treatment of transparent-emitter devices, particularly solar cells, that is more complete than previously available treatments. The proposed approach includes the effects of bandgap narrowing, Fermi-Dirac statistics, built-in field due to impurity profile, and a finite surface recombination velocity at the emitter surface. It is demonstrated that the transparent-emitter model can predict experimental values of Voc observed on n(plus)-p thin diffused junction silicon solar cells made on low-resistivity (0.1 ohm-cm) substrates. A test is included for the self-consistent validity of the transparent-emitter model. This test compares the calculated transit time of minority carriers across the emitter with the Auger-impact minority-carrier lifetime within the emitter region.

Shibib, M. A.↗
NASA NTRS1977

Ionization induced damage in crystalline silicon

Close examination of the interaction of the energetic knock-on atoms with the local lattice environment reveals a damage mechanism which does satisfy the experimental data on proton irradiation of silicon. A proton-atom interaction with high energy transfer is considered where the proton path is delineated by a trail of ionization, and the silicon ion path is characterized by much heavier ionization terminating in a dense displacement cluster. At collision, many of the silicon electrons are stripped off, and the resulting energetic ion subsequently loses energy rapidly by Coulomb interaction with bound electrons. The rate of energy loss depends on the charge state and velocity of the knock-on ion. For ion energies in excess of 1 MeV, the intensity of ionization is sufficient to permit lattice atoms, stripped of their binding electrons, to reorient randomly before having an opportunity to recombine with electrons and re-establish the lattice. The path of a knock-on ion thus becomes a thin cylinder of amorphous material within the crystal. Amorphous silicon has a Fermi level closer to mid-band than does single crystal silicon, and a strong field therefore, results around this damaged region. The field produces a large depletion region, representing a very large capture cross section for minority carriers.

Meulenberg, A., Jr.↗
NASA NTRS1984

Mechanisms limiting the performance of large grain polycrystalline silicon solar cells

The open-circuit voltage and short-circuit current of large-grain (1 to 10 mm grain diameter) polycrystalline silicon solar cells is determined by the minority-carrier diffusion length within the bulk of the grains. This was demonstrated by irradiating polycrystalline and single-crystal (Czochralski) silicon solar cells with 1 MeV electrons to reduce their bulk lifetime. The variation of short-circuit current with minority-carrier diffusion length for the polycrystalline solar cells is identical to that of the single-crystal solar cells. The open-circuit voltage versus short-circuit current characteristic of the polycrystalline solar cells for reduced diffusion lengths is also identical to that of the single-crystal solar cells. The open-circuit voltage of the polycrystalline solar cells is a strong function of quasi-neutral (bulk) recombination, and is reduced only slightly, if at all, by grain-boundary recombination.

Culik, J. S.↗
NASA NTRS1985

Generalized reciprocity theorem for semiconductor devices

A reciprocity theorem is presented that relates the short-circuit current of a device, induced by a carrier generation source, to the minority-carrier Fermi level in the dark. The basic relation is general under low injection. It holds for three-dimensional devices with position dependent parameters (energy gap, electron affinity, mobility, etc.), and for transient or steady-state conditions. This theorem allows calculation of the internal quantum efficiency of a solar cell by using the analysis of the device in the dark. Other applications could involve measurements of various device parameters, interfacial surface recombination velocity at a polcrystalline silicon emitter contact, for rexample, by using steady-state or transient photon or mass-particle radiation.

Misiakos, K.↗
NASA NTRS1980

The determination of transport parameters of minority carriers in n-p junctions by means of an electron microscope - Critique of recent developments

It has recently been shown that amplitude modulated electron beams provide a novel means for the determination of minority carrier lifetimes, diffusion lengths, etc., in n-p junctions. In this paper it is shown that: (1) a recently published analysis based on a cylindrically symmetric configuration is incorrect, (2) the correct approach leads to a system of dual integral equations for which the formal solution is given, (3) in general, the short circuit current can only be determined by means of extensive computer calculations except in the case of large front surface recombination velocities, and (4) the difficulties encountered with cylindrically symmetric configurations (circular ohmic contacts and the like) are completely avoided with a choice of a planar geometry since simple closed form expressions for the short circuit current are readily available in this case.

Von Roos, O.↗
NASA NTRS1990

Properties of insulator interfaces with p-HgCdTe

Heat treatment at 70 C of low carrier concentration p-type HgCdTe samples (p sub o = 8 x 10 exp 14/cm) generates an inverted surface layer. A two day anneal process below 95 C did not affect the Hall coefficient, whereas an almost complete recovery was obtained by annealing at 120 C. While bulk electron mobility, obtained from PEM data, remained high (about 9 x 10 exp 4 sq cm/V s at 77 K), surface mobility is lower by more than an order of magnitude. Surface recombination velocity indicates a continuous improvement with increased temperature, and the activation energy remains equal to the vacancies energy level. The proposed mechanism is that of positive changes in the sulfide migrating towards the interface and generating an image inversion layer.

Schacham, Samuel E.↗
NASA NTRS1980

An alternative approach to charge transport in semiconducting electrodes

The excess-carrier charge transport through the space-charge region of a semiconducting electrode is analyzed by a technique known as the flux method. In this approach reflection and transmission coefficients appropriate for a sheet of uniform semiconducting material describe its transport properties. A review is presented of the flux method showing that the results for a semiconductor electrode reduce in a limiting case to those previously found by Gaertner if the depletion layer is treated as a perfectly transmitting medium in which scattering and recombination are ignored. Then, in the framework of the flux method the depletion layer is considered more realistically by explicitly taking into account scattering and recombination processes which occur in this region.

Thomchick, J.↗