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At least 127 records · Page 7

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NASA NTRS1971

Recombination luminescence in irradiated silicon - Effects of thermal annealing and lithium impurity.

Use of luminescence in irradiated silicon to determine the thermal stability of the defects responsible for the recombination. It is found that the defect responsible for the zero-phonon line at 0.97 eV has an annealing behavior similar to that of the divacancy and that the zero-phonon line at 0.79 eV anneals in a manner similar to the G-15 or K-center. Annealing at temperatures up to 500 C generates other defects whose luminescence is distinct from that seen previously. Addition of lithium to the material produces defects with new characteristic luminescence. Of particular importance is a defect with a level at E sub g -1.045 eV.

Johnson, E. S.↗

NASA NTRS1973

Fine structure in the optical-absorption edge of silicon.

Investigation of the dependence of the photocurrent generated in silicon p-n junctions on the energy of the incident photons to obtain details concerning the fine structure in the indirect optical-absorption edge of silicon. It is found that at room temperature there are three prominent thresholds in the absorption tail. The first, which is not too well defined, occurs at 0.91 eV, the second at 0.99 eV, and the third at 1.026 eV. The dependence of the absorption coefficient on the photon energy following the latter two thresholds is found to differ from that found by Mcfarlane et al. (1958). The threshold energies and temperature dependence of the absorption coefficient suggest that two- and three-phonon processes give rise to the absorption tail. In the transverse-optical phonon-absorption region more structure is found in the absorption coefficient than is accounted for by the theory.

Anagnostopoulos, C.↗

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.↗

NASA NTRS1975

Thermal diffuse X-ray scattering in simple metals

Calculations are reported for the ionic structure factor and X-ray scattering cross section of sodium (at T=0 K and 90 K) and lithium (both isotopes at T=0 K) within the harmonic approximation. An evaluation of the appropriate displacement- displacement correlation function by the special point method circumvents the need for a multiphonon expansion. In the case of sodium, the structure in the one-phonon scattering was straightforwardly accounted for, and an approximate expansion was obtained for all multiphonon scattering. By treating core and conduction electrons on an equal footing, it is shown that information on the conduction electron system is present in the forward scattering component. In lithium the one-phonon cross section at small angles aids in the determination of the effective electron-ion interaction.

Straus, D. M.↗

NASA NTRS1976

The anomalous thermal properties of glasses at low temperatures

While experimentally there is great regularity below 1 deg K in the behavior of a particular thermal property for all amorphous dielectrics it is not understood why these properties should differ from those of crystalline dielectrics, since it would seem that at low temperatures long-wavelength elastic waves, similar in both cases, would determine the thermal properties. A model involving systems having very few levels is used in the present study, although the relation between the model's systems and the nature of the glassy state is not known. It is shown, among other effects, that: specific heat measurements above 0.1 K indicate a distribution of local modes independent of energy; ultrasonic velocity measurements give information about phonon-local mode coupling parameters; and thermal expansion and far infrared experiments indicate a phonon-assisted tunneling model.

Pohl, R. O.↗

NASA NTRS1978

Infrared photoresponse of silicon devices at 1.15 microns

The photoconductive absorption of silicon devices at 1.15 microns is through band-to-band transitions with the simultaneous absorption of a 57.7 meV phonon. Its temperature dependence arises both from the changing phonon population and, more significantly, from the change in silicon bandgap with temperature. The temperature dependence of the photoresponse of quasi-intrinsic silicon samples is in good agreement with the optical absorption data of Macfarlane et al. More heavily doped silicon devices show a photoresponse extending to a lower temperature and having a smaller temperature dependence than intrinsic silicon. This is explained in terms of bandgap narrowing. Comparison with theoretical curves enables the resolution of bandgap reductions at the meV level.

Lanyon, H. P. D.↗

NASA NTRS1984

Thermal detectors as X-ray spectrometers

Sensitive thermal detectors should be useful for measuring very small energy pulses, such as those produced by the absorption of X-ray photons. The measurement uncertainty can be very small, making the technique promising for high resolution nondispersive X-ray spectroscopy. The limits to the energy resolution of such thermal detectors are derived and used to find the resolution to be expected for a detector suitable for X-ray spectroscopy in the 100 eV to 10,000 eV range. If there is no noise in the thermalization of the X-ray, resolution better than 1 eV full width at half maximum is possible for detectors operating at 0.1 K. Energy loss in the conversion of the photon energy to heat is a potential problem. The loss mechanisms may include emission of photons or electrons, or the trapping of energy in long lived metastable states. Fluctuations in the phonon spectrum could also limit the resolution if phonon relaxation times are very long. Conceptual solutions are given for each of these possible problems.

Moseley, S. H.↗

NASA NTRS1984

Interactions between volume and surface EM waves in layered structures

Using light scattering spectroscopy, the interaction of Surface Plasmon Polariton SPP in silver films on a glass substrate is with the incident radiation and with optical phonons of an external medium in contact with the film. The mean free path and the field strength of SPP are determined. The same SPP plays an important role in light emitting tunnel junctions (LETJ) in which an electrical current is converted into Volume Electromagnetic Wave VEW. The efficiency of light emission from LETJ through a prism coupler rather than through surface roughness is discussed. The coupling between phonon surface polaritons (PhSP) and optical guided waves (OGW) in thin films of GaP is also examined.

Ushioda, S.↗

NASA NTRS1984

Thermal detectors as X-ray spectrometers

Moseley, S. H.↗

NASA NTRS1985

Analysis of vibronic transitions in titanium doped sapphire using the temperature of the fluorescence spectra

An analysis of the fluorescence spectra of titanium-doped sapphire crystals for temperatures ranging from 13 to 578 K is made. Zero- and multiphoton assisted electronic transitions are observed at temperatures below 125 K. Zero-phonon transition energies agree with previously reported measured and predicted values. Photon-assisted transitions suggest that two principle modes of lattice vibrations with energies of 220 and 260/cm are coupled to the electronic 2T2g state. A single phonon energy of 173/cm is observed to couple to the 2Eg electronic state. Crossover energy is estimated to be about 11,800/cm using a simple harmonic approximation. Lineshape analysis is performed. The fluorescence lifetime as measured to be a constant 3.9 microsec below 150 k decreasing to 2.9 microsec at 300 K.

Byvik, C. E.↗

NASA NTRS1988

Low-temperature photoluminescence studies of chemical-vapor-deposition-grown 3C-SiC on Si

Low-temperature photoluminescence studies of 26 cubic SiC films, ranging in thickness from 600 A to 25 microns, grown by CVD on (100)Si are presented. It is suggested that the G band near 1.90-1.92 eV and its phonon side bands G1 and G2 are related to dislocations and extended defects. Formulas for the band-gap shift due to an axial stress have been obtained and applied to the CVD 3C-SiC/Si system. The results indicate that a 1-3-micron transition layer greatly reduces the interface misfit strain, and that biaxial stress in the SiC/Si system depresses the intensity of the no-phonon line.

Choyke, W. J.↗

NASA NTRS1988

Raman scattering studies of chemical-vapor-deposited cubic SiC films of (100)Si

Raman scattering studies for a series of CVD-grown cubic SiC single-crystal films with film thickness from 600 A to 17 microns are discussed. The results suggest that the crystalline orientations of the Si substrate and the 3C-SiC film are the same. It is found that the Si 522/cm phonon from a Si wafer is enhanced in intensity by a factor of 2-3 due to a CVD overlayer of cubic SiC, and that the 3C-SiC longitudinal optical phonon at the Gamma point from SiC/Si samples is enhanced by a factor of two or three following the removal of the Si substrate. The variation of the Raman spectrum with incident power is investigated, and a method for determining the Raman cross section for 3 C-SiC is proposed.

Feng, Z. C.↗

NASA NTRS1990

The relation between ferroelasticity and superconductivity

The high-temperature superconductivity is explained widely by the layered crystal structure. The one- and two-dimensional subsystems and their interaction are investigated here. It is assumed that the high-T(sub c) superconductivity takes place in the two-dimensional subsystem and the increase of the phase transition temperature from 60 K up to 90 K is the consequence of turning on the influence of one-dimensional chains. The interaction between the two subsystems is transferred along the c axis by the phonons of breathing mode, which causes the hybridization of the electronic bonds between these subsystems. The experimental works indicate that the existence of both the chains Cu(1)-O and their interaction with the superconducting plane of Cu(2)-O modify the temperature of the transition to the superconducting state. It is seen from the neutron scattering data that the rates of the interatomic distance dependencies on temperature are changed around 140 K and 90 K. The 'zig-zag' order in Cu(1)-O chains has been postulated but, on the other hand, the vibrations with a large amplitude only were reported. The bi-stabilized situation of the oxygen ions can be caused by the change of distance between these ions and the Ba ions. It leads to the appearance of a two-well potential. Its parameters depend on temperature and the dynamics of the oxygen ions' movement. They can induce the antipolar order, which can be, however, more or less chaotic. The investigation of the ferroelastic properties of Y-Ba-Cu-O samples lead to the conclusion that they are related to jumps of ions inside the given chain and not to a diffusion between different sites in the ab plane. Researchers deduce thus that the fluctuating oxygen ions from these chains create dipoles in the ab plane. They can be described with the pseudo-spin formalism/ - Pauli matrices/. The system can be described with the Ising model. The pseudo-spins interact with phonons and influence the superconductivity in the second subsystem.

Molak, A.↗

NASA NTRS1991

The relation between ferroelasticity and superconductivity

The high-temperature superconductivity is explained widely by the layered crystal structure. The one- and two-dimensional subsystems and their interaction are investigated here. It is assumed that the high-T(sub c) superconductivity takes place in the two-dimensional subsystem and the increase of the phase transition temperature from 60 K up to 90 K is the consequence of turning on the influence of one-dimensional chains. The interaction between the two subsystems is transferred along the c axis by the phonons of breathing mode, which causes the hybridization of the electronic bonds between these subsystems. The experimental works indicate that the existence of both the chains Cu(1)-O and their interaction with the superconducting plane of Cu(2)-O modify the temperature of the transition to the superconducting state. It is seen from the neutron scattering data that the rates of the interatomic distance dependencies on temperature are changed around 240 K and 90 K. The 'zig-zag' order in Cu(1)-O chains has been postulated but, on the other hand, the vibrations with a large amplitude only were reported. The bi-stabilized situation of the oxygen ions can be caused by the change of distance between these ions and the Ba ions. It leads to the appearance of a two-well potential. Its parameters depend on temperature and the dynamics of the oxygen ions' movement. They can induce the antipolar order, which can be, however, more or less chaotic. The investigation of the ferroelastic properties of Y-Ba-Cu-O samples lead to the conclusion that they are related to jumps of ions inside the given chain and not to a diffusion between different sites in the ab plane. Researchers deduce, thus, that the fluctuating oxygen ions from these chains create dipoles in the ab plane. They can be described with the pseudo-spin formalism (- Pauli matrices). The system can be described with the Ising model. The pseudo-spins interact with phonons and influence the superconductivity in the second subsystem.

Molak, A.↗

NASA NTRS1992

Thermal conductivity of thermoelectric Si(0.8)-Ge(0.2) alloys

The thermal conductivity of heavily doped, n-type Si-Ge alloys has been studied from 300 to 1200 K. The scattering rate of several phonon scattering mechanisms has been calculated, including intrinsic scattering, mass defect and distortion scattering, phonon-electron scattering, and scattering by inclusions. These rates were then used to calculate the lattice thermal conductivity. The electronic component of the thermal conductivity was calculated from the calculated Lorenz ratio and measured values of the electrical conductivity. The total thermal conductivity was then compared to measured values for a specimen studied by Vining et al. (1991).

White, D. P.↗

NASA NTRS1991

Effect of small scattering centers on the thermoelectric properties of p-type SiGe alloys

Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity. To investigate this prediction, ultrafine particulates (20 to 120 A) of silicon nitride have been added to boron-doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultrafine powder have lower thermal conductivities than standard SiGe, but high-temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25 percent in thermal conductivity has been achieved in these samples. The magnitude of the reduction agrees with theoretical predictions.

Beaty, John S.↗

NASA NTRS1992

Reduced thermal conductivity due to scattering centers in p-type SiGe alloys

Spark erosion was used to produce ultra-fine particles of SiGe thermoelectric material and boron nitride, an inert phonon-scattering material. A homogeneous powder was made by mixing the two powders. The mixture was hot pressed to produce a thermoelectric material with uniformity dispersed, ultra-fine, inert, phonon-scattering centers. It is shown that, in samples with inert boron nitride or silicon nitride, thermal conductivity of a SiGe alloy can be reduced by about 25 percent while maintaining the electrical properties of the samples. Annealing of all the samples at 1525 K caused grain growth to over a micron, eliminating the detrimental effect attributable to small grains. Only in the sample with boron nitride the thermal conductivity did remain well below that for standard p-type SiGe (about 25 percent), while the electrical resistivity and Seebeck coefficient were very close to the values for standard p-type 80/20 SiGe.

Beaty, John S.↗

NASA NTRS1991

High quality flame deposited diamond films for infrared optical windows

The IR absorption in polycrystalline diamond films deposited in oxygen-acetylene flames is characterized using FTIR. The one-phonon absorption coefficient in the region from 7 to 12 microns that is related to extrinsic defects in the diamond films shows a strong dependence on the flame conditions as well as the substrate temperature. A high degree of diamond crystalline perfection, as judged from the undetectable one-phonon absorption, is achieved under the optimized deposition conditions for our flame setup. This is further supported by the sharp Raman peak at 1332/cm as well as the high purity in crystal orientation according to the X-ray diffraction pattern measured for the high quality diamond films.

Tzeng, Y.↗