Showing papers by "Venkatesh Narayanamurti published in 1980"

Propagation of Large-Wave-Vector Acoustic Phonons in Semiconductors

Abstract: The spatial and temporal characteristics of phonon pulses generated in the interband energy relaxation process of photoexcited e-h pairs in GaAs and InP are reported. With use of a superconducting Pb tunnel junction as a phonon threshold detector, it is shown that the energy transport occurs via near-zone-edge transverse acoustic phonons and that over lengths of the order of several millimeters the pulse propagation shows all features of ballistic transport in a dispersive medium.

Observation of Time-Dependent Specific Heat in Amorphous Si O 2

Abstract: The time-dependent specific heat of amorphous Si${\mathrm{O}}_{2}$ has been measured between 0.1 and 1 K over the range \ensuremath{\sim} 1 sec to 10 \ensuremath{\mu}sec. The decoupling of the roughly $T$-dependent contribution to specific heat is observed in submillisecond time scales, in qualitative agreement with the predictions of the tunneling model. However, the excess ${T}^{3}$-dependent specific heat is not decoupled on the time scales involved.

Phonon magnification and the Gaussian curvature of the slowness surface in anisotropic media: detector shape effects with application to GaAs

Abstract: A new short derivation is given for the magnification of photons, phonons, and other wave phenomena in anisotropic media. The results, when specialized to the phonon case, agree with those of Maris and of Philip and Viswanathan. At nondegenerate points the magnification is shown, regardless of the nature of the wave phenomenon, to be expressible in terms of the Gaussian curvature K at the wave vector q→ of the surface ω (q→)=ω . A representation of K, free of surface coordinates, is given as the cofactor of a 3×3 curvature tensor. The curvature tensor contains information beyond the Jacobian, such as the two principal magnifications for a given mode. Along degenerate directions (such as the [100] direction in cubic crystals) the results, even in the infinitesimal limit, are sensitive to the shape of the detector. Explicit expressions for circular, rectangular and square detectors are given and applied to the case of ballistic phonons in GaAs. Numerical calculation of finite aperture effects indicates significant qualitative as well as quantitative differences with infinitesimal aperture results.

Devices incorporating phonon filters

Abstract: An acoustic superlattice of alternating layers of different acoustic impedance is disclosed as a filter for high frequency phonons. Applications discussed include spectrometers, acoustic imaging apparatus, and cavity resonators.

The study of electron dynamics in n-type CdS by spin-flip Raman scattering

Abstract: A review is presented of the application of spin-flip Raman scattering (SFRS) to the study of electron delocalization in the insulator–metal transition in n-type CdS. The delocalization in the metallic regime appears as a diffusional Dq 2 term in the SFRS linewidth, where q is the light scattering vector, and D is well described for small q by the transport parameters of a non-interacting Fermi gas. The spin Faraday rotation, which is part of the same spin-photon interaction that gives rise to the SFRS, is used to measure selectively, the donor spin susceptibility, χ. Strong focus is placed on the properties of χ in the insulating region where the doped semiconductor is a model ‘amorphous antiferromagnet'. No evidence of any spin-glass-like ordering transition is found at temperatures even far below the percolation threshold. Pure spin diffusion without charge transport is also observed and gives information on spin correlations.

Granular aluminum superconducting bolometer as a molecular beam detector

Abstract: We have tested a granular aluminum superconducting thin film bolometer as a detector of molecular beams. The bolometer is mounted on a liquid He cryostat and designed to operate in a bakeable UHV scattering chamber. The element is shown to be a sensitve detector of the total thermal energy of the molecular beam flux and if operated in tandem with a mass spectrometer allows the average internal energy to be determined.

Symmetry of a Donor Related (DX) Center in AℓxGa1-xAs

Abstract: The phenomena of low temperature persistent impurity photoconductivity in compound semiconductors has attracted considerable recent attention.1,2 For the case of AlxGa1-xAs it involves a non-effective mass like donor-complex labeled a “DX” center. The unoccupied DX center is believed to be resonant with the conduction band, yet sufficiently localized to produce a large lattice relaxation1 (see top of Fig. 1 which shows the configuration coordinate diagram proposed by Lang et al). In this figure the defect activation energy, Eo (∿ 0.1 eV), is small compared to the optical activation energy, Eop (∿ 1.5 eV). When kT

Phonon Magnification in GaAs

Abstract: Our experiments1 on phonon generation by hot electrons in epitaxial layers on GaAs have demonstrated a strong dependence of phonon intensity on phonon propagation direction, phonon polarization and detector shape. The analysis of these experiments is strongly influenced by focussing and defocussing effects whose importance in heat pulse experiments is well known.2, 3 In this paper we present a new expression for the phonon magnification for infinitesimal apertures. We also present graphic illustrations of the transformation of the detector shape from real space to wave vector space for the particular case of GaAs and emphasize the significant difference between finite aperture and infinitesimal aperture results (to which previous calculations have largely been confined).

Phonon Optics, Carrier Relaxation and Recombination in Semiconductors: Case of GaAs Epitaxial Layers

Abstract: The phenomena of carrier-capture and energy transfer processes in semiconductor epitaxial layers are of considerable fundamental and technological importance. The luminous output of semiconductor pn junctions for example, depends on the relative strength of radiative (photon) and non-radiative (phonon or Auger) processes. As another example, the mobility at low temperatures of electrons in an n-epitaxial layer of a semiconductor such as GaAs is determined by ionized impurity scattering but the energy relaxation of the carriers under the application of a field takes place via phonon emission. Until the present time, because of the dominance of impurity scattering, one has not been able to determine the strength of the intrinsic electron-phonon scattering processes from transport measurements.