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At least 127 records · Page 7

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NASA NTRS1979

Outdiffusion of recombination centers from the substrate into LPE layers - GaAs

Experimental results are presented showing that outdiffusion of recombination centers from the GaAs substrate into the epitaxial layer takes place during growth. Such outdiffusion decreases the carrier lifetime in the epitaxial layer to much lower values than the radiative recombination limit. Furthermore, it introduces a lifetime gradient across the epitaxial layer which depends critically on the growth velocity and thermal treatment. High rates of growth (such as those attainable in electroepitaxy) and high cooling rates can minimize the adverse effects of normally available substrates on the epitaxial layers; however, good quality substrates are essential for the consistent growth of device quality layers.

Jastrzebski, L.↗

NASA NTRS1979

The OI/1S/ state - Its quenching by O2 and formation by the dissociative recombination of vibrationally excited O2/+/ ions

The rate coefficient for the quenching of metastable O(1S) atoms by O2 was measured as a function of temperature from 250 to 550 K. The resulting Arrhenius expression correlates well with previous laboratory work. It is suggested that the much larger value of the rate coefficient inferred from an analysis of artificial auroral experiment, Precede, may be explained by overestimation of the contribution of O(1S) production from O2(+) dissociative recombination. The possibility that O(1S) atoms are produced only by the dissociative recombination of vibrationally excited O2(+) ions is examined; such excited ions would not exist in the Precede experiment because of the rapid cooling of the ions by resonant charge transfer processes.

Zipf, E. C.↗

NASA NTRS1981

Determination of lifetimes and recombination currents in p-n junction solar cells, diodes, and transistors

New methods are presented and illustrated that enable the accurate determination of the diffusion length of minority carriers in the narrow regions of a solar cell or a diode. Other methods now available are inaccurate for the desired case in which the width of the region is less than the diffusion length. Once the diffusion length is determined by the new methods, this result can be combined with measured dark I-V characteristics and with small-signal admittance characteristics to enable determination of the recombination currents in each quasi-neutral region of the cell - for example, in the emitter, low-doped base, and high-doped base regions of the BSF (back-surface-field) cell. This approach leads to values for the effective surface recombination velocity of the high-low junction forming the back-surface field of BSF cells or the high-low emitter junction of HLE cells. These methods are also applicable for measuring the minority-carrier lifetime in thin epitaxial layers grown on substrates with opposite conductivity type.

Neugroschel, A.↗

NASA NTRS1980

Deep-level defects and recombination parameters in proton irradiated AlGaAs-GaAs solar cells

The deep-level defects produced in high efficiency AlGaAs-GaAs solar cells by proton energies of 50, 100, 200, and 290 keV are investigated. Proton fluences were from 10 to the 10th to 10 to the 13th p/sq cm. The results show that the diode ideality factor varies between 1.9 and 2.1, indicating that the dominant current component is due to the recombination of electron-hole pairs via deep-level traps in the junction space charge region of these diodes. The recombination current increases with increasing proton fluence and proton energy. The reduction of open circuit voltage, short circuit current, and conversion efficiency in the irradiated cells relates directly to the density of the deep-level defects induced by proton irradiation.

Li, S. S.↗

NASA NTRS1981

Electron-temperature dependence of dissociative recombination of electrons with CO/+/./CO/n-series ions

The dependence on electron temperature of the coefficients for electron recombination with molecular cluster ions of the carbon monoxide series, CO(+).(CO)n, is determined. A microwave discharge lasting approximately 0.1 msec was applied in 5-20 Torr neon containing a few tenths percent CO in an afterglow mass spectrometer apparatus, and the time histories of the various afterglow ions were measured. Expressions for the dependence of the recombination coefficients of the dimer and trimer ions CO(+).CO and CO(+).(CO)2 are obtained which are found to be significantly different from those previously obtained for hydronium and ammonium series polar cluster ions, but similar to those of simple diatomic ions.

Whitaker, M.↗

NASA NTRS1982

Viscous-shock-layer heating analysis for the shuttle windward-symmetry plane with surface finite catalytic recombination rates

The paper demonstrates the capability of a reacting, two-dimensional viscous-shock-layer solution using the equivalent axisymmetric body concept to predict heating rates on the shuttle windward centerline for a wide range of altitudes. Results indicate that the nonequilibrium effects persist through most of the STS-2 entry heating pulse down to an altitude of about 50 km. When results are compared to those of the inviscid flowfield plus boundary layer solution of Scott (1980), agreement is fair to poor for nonequilibrium calculations, although very good for equilibrium calculations. A parametric study to demonstrate the effect of uncertainties in oxygen surface recombination rate for RCG coated HRSI on heating, indicates favorable results when a wall recombination rate of 100 cm/sec is used in the temperature range 1400-900 K.

Shinn, J. L.↗

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 NTRS1983

Dielectronic recombination rates, ionization equilibrium, and radiative emission rates for Mn ions in low-density high-temperature plasmas

The analysis of optically-thin far-ultraviolet and X-ray emission lines of multiply-charged ions is one of the basic methods for determining the temperatures and densities of laboratory and astrophysical plasmas. In addition, the energy balance in these plasmas can be significantly influenced by the emission of radiation from relatively low concentrations of multiple-charged atomic ions. Because the populations of the excited levels are expected to depart substantially from their local thermodynamic equilibrium values a detailed treatment of the elementary collisional and radiative processes must be employed in order to predict the emission line intensities. In this investigation the authors present the results of calculations based on a corona equilibrium model in which a detailed evaluation is made of the dielectronic recombination rate coefficients. The ionization and autoionization following inner-shell electron excitation from each ground state are balanced by direct radiative and dielectronic recombination. The spectral line intensities emitted by the low-lying excited states, which are assumed to undergo spontaneous radiative decay in times that are short compared with the collision time, are evaluated in terms of the corona ionization equilibrium distributions of the ground states and their electron-impact excitation states.

Jacobs, V. L.↗

NASA NTRS1983

The dissociative recombination of O2(+) - The quantum yield of O(1S) and O(1D)

Data from the visible airglow experiment on the Atmosphere Explorer-E satellite have been used to determine the quantum yield of O(1S) and O(1D) from the dissociative recombination of O2(+). A range of values between 0.09 and 0.23 has been obtained for the quantum yield of O(1S). It is shown that the quantum yield of O(1S) depends on the ratio of electron density to atomic oxygen density. This suggests that the quantum yield of O(1S) may depend on the degree of vibrational excitation of the recombining O2(+). The quantum yield of O(1D) has been measured to be 1.23 + or - 0.42, with no dependence on the electron-oxygen ratio.

Abreu, V. J.↗

NASA NTRS1983

Termolecular ionic recombination at high ambient gas density

It is shown that the importance of diffusion relative to drift in ion-ion recombination is a decreasing function of the separation between the ions (contrary to a common supposition). Langevin's formula for the recombination coefficient is obtained very simply using a physical model which allows for both diffusion and drift.

Bates, D. R.↗

NASA NTRS1984

Recombination of electrons with H3(+) and H5(+) ions

A microwave afterglow-mass spectrometer apparatus is used to determine the dissociative recombination coefficients for capture of electrons by H3(+) and H5(+) ions as a function of electron temperature. At an ion and neutral temperature of 240 K, the coefficient for H3(+) is found to vary slowly with the electron temperature at first, decreasing from 1.6 x 10 to the -7th cu cm/s at an electron temperature of 240 K to 1.2 x 10 to the -7th cu cm/s at an electron temperature of 500 K; at electron temperatures ranging from 500 to 3000 K, the coefficient falls as the reciprocal of the electron temperature. It is noted that the results, which have an uncertainty of + or - 20 percent, agree satisfactorily over the common energy range (0.03-0.36 eV) with the recombination cross sections determined in merged beam measurements by Auerbach et al. (1977). Attention is also given to the implication of the results for modeling planetary atmospheres and interstellar clouds.

Macdonald, J. A.↗

NASA NTRS1985

Measuring Recombination Velocities in Solar Cells

Electron-beam scanning method suggested for determining back-surface recombination velocity, SB, of back-surface-field (BSF) solar photovoltaic cell. Recombination velocity important because smaller value correlated with smaller saturation current and therefore higher cell efficiency. Method involves analyzing short-circuit current measured while electron beam scanned along edge of cell.

Vonroos, O.↗

NASA NTRS1984

The energetics of hydrogen atom recombination - Analysis, experiments, and modeling

A theoretical investigation of the effect of the microscopic energetics of the recombination reaction on the performance of a microwave-plasma electrothermal propulsion system is described, and the results of the analysis are presented. A series of experiments to test the concept is described and analyzed by comparison with a computer model of the recombination reaction. It is concluded that internal energy considerations are not likely to significantly affect the design of a microwave-plasma electrothermal rocket. The experimental results indicate that the microwave power is far higher than the capacity of the gas to absorb it; the cooling needed to control the energy dominates the experimental results.

Filpus, J. W.↗

NASA NTRS1985

Quantification of the effects of generation volume, surface recombination velocity, and diffusion length on the electron-beam-induced current and its derivative Determination of diffusion lengths in the low micron and submicron ranges

A systematic and quantitative analysis is carried out to investigate the effects of the shape (point, cube, Gaussian) and size of the generation volume, the surface recombination velocity, and the diffusion length on the electron-beam-induced current (EBIC) and its derivative (DEIC). Thick homogeneously doped samples exhibiting diffusion lengths in the low micron and submicron range are considered. The results are presented in computed EBIC curves as a function of scanning distance and of the ratio true diffusion length/effective diffusion length. Shown using these curves are: (1) a simple and yet rigorous method for the determination of the true diffusion length, taking into consideration all of the factors cited above, (2) a method for the rapid determination of the surface recombination velocity, (3) the condition under which the source shape becomes insignificant, and (4) a new value for the lower limit of the diffusion length which can be determined by the EBOC technique.

Luke, K. L.↗

NASA NTRS1985

Effect of recombination on the open-circuit voltage of a silicon solar cell

A theoretical study of the influence of band-band Auger, band-trap Auger, and the ordinary Shockley-Read-Hall mechanism for carrier recombination on the open-circuit voltage VOC of a solar cell is presented. Under reasonable assumptions for the magnitude of rate constants and realistic values for trap densities, surface recombination velocities and band-gap narrowing, the maximum VOC for typical back surface field solar cells is found to lie in the range between 0.61 and 0.72 V independent of base width.

Von Roos, O.↗

NASA NTRS1986

Circuit for Lifetime and Surface-Recombination Measurements

Test circuit for silicon solar cells suppresses spurious effects. New circuit increases accuracy of measurements of recombination lifetime and effective surface recombination velocity in silicon solar cell. Fast electronic switch, circuit grounds forward-biased cell so rapidly transient voltage to be measured not affected significantly.

Lindholm, F. A.↗

NASA NTRS1985

Activated recombinative desorption: A potential component in mechanisms of spacecraft glow

The concept of activated recombination of atomic species on surfaces can explain the production of vibrationally and translationally excited desorbed molecular species. Equilibrium statistical mechanics predicts that the molecular quantum state distributions of desorbing molecules is a function of surface temperature only when the adsorption probability is unity and independent of initial collision conditions. In most cases, the adsorption probability is dependent upon initial conditions such as collision energy or internal quantum state distribution of impinging molecules. From detailed balance, such dynamical behavior is reflected in the internal quantum state distribution of the desorbing molecule. This concept, activated recombinative desorption, may offer a common thread in proposed mechanisms of spacecraft glow. Using molecular beam techniques and equipment available at Los Alamos, which includes a high translational energy 0-atom beam source, mass spectrometric detection of desorbed species, chemiluminescence/laser induced fluorescence detection of electronic and vibrationally excited reaction products, and Auger detection of surface adsorbed reaction products, a fundamental study of the gas surface chemistry underlying the glow process is proposed.

Cross, J. B.↗

NASA NTRS1985

Depletion layer recombination effects on the radiation damage hardness of gallium arsenide cells

The significant effect of junction depletion layer recombination on the efficiency of windowed GaAs cells was demonstrated. The effect becomes more pronounced as radiation damage occurs. The depletion is considered for 1 MeV electron fluences up to 10 to the 16th power e/sq m. The cell modeling separates damage in emitter and base or buffer layers using different damage coefficients is reported. The lower coefficient for the emitter predicts less loss of performance at fluences greater than 10 to the 15th power e/sq cm. A method for obtaining information on junction recombination effects as damage proceeds is described; this enables a more complete diagnosis of damage to be made.

Garlick, G. F. J.↗