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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 NTRS1976

Method and apparatus for measurement of trap density and energy distribution in dielectric films

Trap densities in dielectric films are determined by tunnel injection measurements when the film is incorporated in an insulated-gate field effect transistor. Under applied bias to the transistor gate, carriers (electrons or holes) tunnel into traps in the dielectric film. The resulting space charge tends to change channel conductance. By feeding back a signal from the source contact to the gate electrode, channel conductance is held constant, and by recording the gate voltage as a function of time, trap density can be determined as a function of distance from the dielectric-semiconductor interface. The process is repeated with the gate bias voltage at different levels in order to determine the energy distribution of traps as a function of distance from the interface.

Maserjian, J.↗
NASA NTRS1975

Measurement of trap density in dielectric film

Method uses basic circuit to examine quality and trap density of film used in insulated gate field effect transistors. Data are measured as function of performance and life expectancy.

Guisinger, J. E.↗
NASA NTRS1979

Application of the trap-plus-precipitation hard X-ray burst model to the flare of August 4, 1972

The theory of Melrose and Brown (1976) concerning the X-ray spectrum resulting from combined precipitation and in-trap acceleration of nonthermal electrons is applied to a large solar flare on the basis of high-time-resolution hard X-ray data. The time behavior of the electron source function is inferred as a function of energy and mean ambient trap density. The results are interpreted in the context of a purely thick-target model, a static trap with continuous injection of fresh electrons, and a dynamic trap with continuous redistribution of electron energies. It is concluded that: (1) a minimum trap density exists below which the static-trap description cannot be valid; (2) a mean ambient trap density less than the cited minimum is possible if a dynamic trap is invoked; and (3) the electron source function at a given instant in such low-density dynamic traps may bear little or no resemblance to a power-law spectrum despite its production, via nonthermal bremsstrahlung, of a power-law X-ray spectrum.

Emslie, A. G.↗
NASA NTRS1991

A new technique to study transient conductivity under pulsed monochromatic light in Cr-doped GaAs using acoustoelectric voltage measurement

The transient conductivity of high-resistivity Bridgman-grown Cr-doped GaAs under pulsed monochromatic light is monitored using transverse acoustoelectric voltage (TAV) at 83 K. Keeping the photon flux constant, the height and transient time constant at the TAV are used to calculate the energy dependence of the trap density and its cross section, respectively. Two prominent trap profiles with peak trap densities of approximately 10 to the 17th/cu cm eV near the valence and the conduction bands are detected. These traps have very small capture cross sections in the range of 10 to the -23 to 10 to the -21st cm sq. A phenomenon similar to the persistent photoconductivity with transient time constants in excess of a few seconds in high-resistivity GaAs at T = 83 K is also detected using this technique. These long relaxation times are readily explained by the spatial separation of the photo-excited electron-hole pairs and the small capture cross section and large density of trap distribution near the conduction band.

Tabib-Azar, Massood↗
NASA NTRS1975

Differential spectra and phase space densities of trapped electrons at Jupiter

Using Pioneer 10 data, differential spectra and phase-space densities have been constructed for trapped electrons at Jupiter. These quantities should assist in calculating synchrotron radiation from these particles and in evaluating the diffusion mechanisms that accelerate the particles. Absorption by the moons Io and Europa is evident, and injection by Io is demonstrated by a density peak in phase space, which demands a local source. There is also a rapid decrease in density between the moons, which could call for either a local loss mechanism or nonlocal losses fed by diffusion.

Mcilwain, C. E.↗
NASA NTRS1984

Defect studies in one MeV electron irradiated GaAs and in Al/sub x Ga/sub l-x As P-N junction solar cells

Deep level transient spectroscopy reveals that the main electron traps for one-MeV electron irradiated GaAs cells are E9c)-0.31, E(c)-0.90 eV, and the main hole trap is due to the level. Electron trap density was found to vary from 3/tens-trillion ccm for 2/one quadrillion cm 3/3.7 quadrillion cm for 21 sextillion cm electron fluence for electron fluence; a similar result was also obtained for the hole trap density. As for the grown-in defects in the Al(x)Ga(1-x)As p-n junciton cells, only two electron traps with energies of E(c)-0.20 and E(c)-0.34 eV were observed in samples with x = 0.17, and none was found for x 0.05. Auger analysis on the Al(x)Ga(1-x) As window layer of the GaAs solar cell showed a large amount of oxygen and carbon contaminants near the surface of the AlGaAs epilayer. Thermal annealing experiment performed at 250 C for up to 100 min. showed a reduction in the density of both electron traps.

Li, S. S.↗
NASA NTRS2013

The DUV Stability of Superlattice-Doped CMOS Detector Arrays

In this paper, we present experimental results and band structure calculations that illuminate the unique properties of superlattice-doped detectors. Numerical band structure calculations are presented to analyze the dependencies of surface passivation on dopant profiles and interface trap densities (Figure 3). Experiments and calculations show that quantum-engineered surfaces, grown at JPL by low temperature molecular beam epitaxy, achieve a qualitative as well as quantitative uniqueness in their near-immunity to high densities of surface and interface traps.

doped silicon surfaces↗
NASA NTRS1979

Analysis of the interaction of an electron beam with a solar cell. III - The effect of spacial variations of the number density of recombination centers on SEM measurements

By means of an exactly soluble model the short circuit current generated by a scanning electron microscope in a P-N junction has been determined in cases where the trap density is inhomogeneous. The diffusion length for minority carriers becomes then dependent on the spacial coordinates. It is shown that in this case the dependence of the Isc on characteristic parameters as cell thickness, distance of the beam excitation spot from ohmic contacts, etc., becomes very intricate. This fact precludes the determination of the local diffusion length in the usual manner. Although the model is somewhat simplified in order to make it amenable to exact solutions, it is nevertheless realistic enough to lead to the conclusion that SEM measurements of bulk transport parameters in inhomogeneous semiconductor material are impractical since they may lead to serious errors in the interpretation of the data by customary means.

Von Roos, O.↗
NASA NTRS1978

Study of electronic properties in proton- and electron-irradiated GaAlAs and GaAs solar cell materials

Diagnostical measurement techniques such as dark I-V, C-V, the thermally insulated capacitance, and the deep level transient spectroscopy methods were employed to study defect properties in the proton-irradiated n-GaAs materials. Defect energy levels, thermal emission rates, and capture cross sections of electrons as well as trap densities were deduced from these measurements and the results are presented. Correlations between the measured defect parameters and the dark I-V characteristics of the n-GaAs Schottky barrier diodes are also discussed. Defect energy levels (i.e., electron traps) determined are also compared with published data in order to identify their physical origins.

Li, S. S.↗
NASA NTRS2021

Probing the Origin of the Open Circuit Voltage in Perovskite Quantum Dot Photovoltaics

Perovskite quantum dots (PQDs) have many properties that make them attractive for optoelectronic applications, including expanded compositional tunability and crystallographic stabilization. While they have not achieved the same photovoltaic (PV) efficiencies of top-performing perovskite thin films, they do reproducibly show high open circuit voltage (VOC) in comparison. Further understanding of the VOC attainable in PQDs as a function of surface passivation, contact layers, and PQD composition will further progress the field and may lend useful lessons for non-QD perovskite solar cells. Here, we use photoluminescence-based spectroscopic techniques to understand and identify the governing physics of the VOC in CsPbI3 PQDs. In particular, we probe the effect of the ligand exchange and contact interfaces on the VOC and free charge carrier concentration. The free charge carrier concentration is orders of magnitude higher than in typical perovskite thin films and could be tunable through ligand chemistry. Tuning the PQD A-site cation composition via replacement of Cs+ with FA+ maintains the background carrier concentration but reduces the trap density by up to a factor of 40, reducing the VOC deficit. These results dictate how to improve PQD optoelectronic properties and PV device performance and explain the reduced interfacial recombination observed by coupling PQDs with thin-film perovskites for a hybrid absorber layer.

perovskite quantum dot↗
NASA NTRS1982

Detection of interstellar NH sub 3 in the far-warm and dense gas in Orion-KL

The detection of the (J,K) = a(4,3) yields s(3,3) rotation inversion transition of ammonia at 124.6 microns toward the center of the Orion-KL region is reported. The line is in emission and has a FWHM or = to 30 km s 0.15. The far IR ammonia line emission probably comes mainly from the 'hot core', a compact region of warm, very dense gas previously identified by the radio inversion lines of NH3. The a(4,3) yields s(3,3) line is very optically thick, and since it is seen in emission, radiative excitation of the (4,3) NH3 level by far IR emission from dust within the source can be ruled out. Radiative excitation via the 10 microns of vibrational transitions of NH3 also seems unlikely. Hence, the (4,3) level is probably collisionally excited and the gas in the hot core region is warmer than the dust. Since the far IR line emission is highly trapped, densities of approximately 10 to the 7th power cu cm are high enough to explain the observations. Shock heating by the mass outflow from IRc2 may account for the high gas temperatures in the hot core region.

Townes, C. H.↗
NASA NTRS1984

Studies of an artificially shock-loaded H group chondrite

SEM and TEM, together with thermoluminescence (TL), are used to study five samples of the naturally unshocked Kernouve (H6) meteorite that were shock-loaded to pressures of 70, 165, 270, and 390 kbar. Attention is given to olivine and orthopyroxene deformation mechanisms at these pressure levels. The microhardness of the kamacite in the samples increases with shock pressure, and it is noted that annealed kamacite displays incipient crystallinity, while alpha-martensite and taenite sometimes contain slip lines. At pressures over 200 kbar, there was a systematic decrease in both natural TL and TL sensitivity. Changes in the ratio of these two values for various regions of the TL glow curve suggest that two processes were effective during shock: thermal drainage of electron traps and a reduction in the effective trap density. Thermal effects with widespread annealing are noted in the case of a sample subjected to shock pulse.

Sears, D. W.↗
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

Behavior of inversion layers in 3C silicon carbide

A study on the field-induced surface-charge region in 3C silicon carbide (SiC) using 1 MHz capacitance-voltage (C-V) measurements at room temperature is here reported. A double column mercury probe was used on oxidized SiC substrates to form metal-oxide-semiconductor (MOS) structures. These structures were characterized in terms of the substrate doping profile, effective fixed oxide charge, and interface trap density. A distinctive feature of the MOS C-V curves from accumulation to inversion is that after going into deep depletion the capacitance rises to its equilibrium inversion level during the voltage sweep. Capacitance transient measurements indicate that the minority-carrier generation occurs at the SiO2/SiC interface.

Avila, R. E.↗
NASA NTRS1987

The growth and in situ characterization of chemical vapor deposited SiO2

This paper reports the results of studies of the kinetics of remote (indirect) plasma enhanced low pressure CVD growth of SiO2 on Si and InP and of the in situ characterization of the electrical surface properties of InP during CVD processing. In the latter case photoluminescence was employed as a convenient and sensitive noninvasive method for characterizing surface trap densities. It was determined that, provided certain precautions are taken, the growth of SiO2 occurs in a reproducible and systematic fashion that can be expressed in an analytic form useful for growth rate prediction. Moreover, the in situ photoluminescence studies have yielded information on sample degradation resulting from heating and chemical exposure during the CVD growth.

Iyer, R.↗
NASA NTRS1990

High frequency capacitance-voltage characteristics of thermally grown SiO2 films on beta-SiC

Silicon dioxide films grown under dry and wet oxidation environment on beta-SiC films have been studied. The beta-SiC films had been heteroepitaxially grown on both on-axis and 2-deg off-axis (001) Si substrates. Capacitance-voltage and conductance-voltage characteristics of metal-oxide-semiconductor structures were measured in a frequency range of 10 kHz to 1 MHz. From these measurements, the interface trap density and the effective fixed oxide charge density were observed to be generally lower for off-axis samples.

Tang, S. M.↗
NASA NTRS1991

Frequency-dependent characteristics and trap studies of lattice-matched (x = 0.53) and strained (x greater than 0.53) In(0.52)Al(0.48)As/In(x)Ga(1-x)As HEMT's

The low-frequency characteristics of InAlAs/In(x)Ga(1-x)As high-electron-mobility transistors (HEMTs) are studied, and trap densities are evaluated. The HEMTs' transconductance g(m) and output resistance R(ds) dispersion are smallest for 60 percent indium (In) content and largest for 53 percent In. The maximum dispersion for the 53 percent In sample is about 6 percent for g(m) and about 13.3 percent for R(ds), corresponding to lower values than observed in MESFETs. The R(ds) dispersion characteristics are weaker than in AlGaAs/GaAs HEMTs and manifest themselves primarily up to 100 kHz. An analysis of the dispersion results indicates that, unlike in the case of MESFETs, the channel region under the gate rather than the access regions is responsible for the dispersion. Interface state densities were extracted by the AC conductance method and were found to follow the same trend as the g(m) and R(ds) dispersion.

Ng, Geok-Ing↗