Phonon Hot Spot Research Articles

First Configurational Study of the CryoAC Detector Silicon Chip of the Athena X-Ray Observatory

The 50 mK cryogenic focal plane anticoincidence detector of the Athena X-ray observatory (CryoAC) is a silicon-suspended absorber sensed by a network of 400 Ir/Au transition edge sensors (TES) and connected through silicon bridges to a surrounding silicon frame plated with gold. The device is shaped by deep reactive ion etching from a single silicon wafer of 500 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m. In operating test of prototypes of the demonstration model of the CryoAC, we have highlighted the importance of the phonon hot spot in the signal generation. A fast response to the cosmic particle of a large area of about 4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a thin 0.5 mm silicon absorber covered by a TES array can be assured by matching the array configuration to the size and properties of the hot spot. In this article, we present the preparatory study of a proposal for measuring the phonon hot spot that is predicted by the particle-absorber interaction model.

Thermal conduction in sub-100 nm transistors

Heat conduction in integrated circuits spans length scales across several orders of magnitude: From the lattice spacing at a few Angstroms to the substrate thickness at hundreds of micrometers. The smaller length scale becomes increasingly important in devices with feature size well below 100nm. This paper provides an overview of sub-continuum electro-thermal transport. We use the phonon Boltzmann transport equation to model heat conduction in the device and show that phonons emitted by hot electrons in the drain create a phonon hotspot. The resulting non-equilibrium leads to increased thermal resistance within the device. At the limits of scaling, the resistance is comparable to that due to the substrate and packaging.

Phonon Hot Spot in Vitreous Silica

Peak arrival times of heat pulse profiles in vitreous silica vary linearly with propagation distances in the energy range from about 20 W/mm2 to 160 W/mm2 at 2.3 K, suggesting the features of quasidiffusive propagation. These arrival times are, however, retarded with increasing pulse energy, which is inconsistent with the simple quasidiffusion picture. We explain this discrepancy in terms of a phonon hot spot which act to efficiently down-convert the high-frequency phonons very near the excitation spot.

The measurement of the temperature inside the hot phonon spot induced by laser excitation of silicon

The heat pulses produced by laser excitation were measured in Si using YBCO sensor. The observed pulse shapes confirm the existence of the phonon hot spot in certain experimental conditions. The exploiting of YBCO bolometer and rather thin Si sample allowed the direct measurement of the hot spot temperature and its life time. Measured hot spot parameters are in satisfactory agreement with those obtained by help of Kazakovtsev-Levinson theory. The Monte-Carlo simulation of phonon propagation was carried out with account of coalescence process. It was found that with increase of excitation energy density, the calculated response peak position shifts noticeably to greater time values.

The effect of hotspot formation on quasiparticle yields in superconducting aluminium films

Existing models show that any hotspot created in an Al film following a particle interaction is rapidly diluted. Quasiparticles and phonons decouple at high energies so that on short time-scales quasiparticle diffusion determines the energy transport. Few quasiparticles have energies near the energy gap and there is little gap suppression. We have measured the initial quasiparticle yields in very thin Al films and find somewhat surprisingly that this yield is reduced. It appears that a short time-scale quasiparticle loss mechanism, probably associated with localised over-injection, exists as quasiparticles scatter to low energies within the phonon hotspot.

Acoustic-phonon emission due to localized photoexcitation of Si: Electron-hole droplets versus the phonon hot spot.

We consider theoretically the evolution of nonequilibrium carriers and phonons created by optical excitation of Si at low temperatures (T<2 K) in light of recent heat-pulse experiments. At low excitation levels (≤20 W/mm 2 ), the detected phonons indicate a quasidiffusive propagation mode, involving anharmonic decay and elastic scattering of relatively high-frequency (≥1 THz) phonons. At intermediate excitation densities (≥20 W/mm 2 ) a transition is observed to a localized source of relatively low-frequency (≤1 THz) phonons. The threshold for this effect is much lower than that predicted for the formation of a hot spot involving only the phonon system

High-resolution optical study of a point-contact-induced phonon hot spot in ruby.

A phonon hot spot is generated in ruby at 1.5 K by a micrometer-sized whisker-film W-W point contact, and subsequently studied by means of high-resolution optical techniques. The phonon distribution in the immediate vicinity of the point contact is imaged directly with micrometer spatial resolution. The method used is to observe the ${\mathit{R}}_{1}$ luminescence following optical excitation of ${\mathrm{Cr}}^{3+}$ to E\ifmmode\bar\else\textasciimacron\fi{}${(}^{2}$E), where advantage is taken of the temperature dependence of the frequency and width of the $^{4}$${\mathit{A}}_{2}$-E\ifmmode\bar\else\textasciimacron\fi{}${(}^{2}$E) optical transition. A model description of the propagating phonon distribution is presented in terms of classical heat conduction close to the point contact and of quasiballistic propagation further away. Results of numerical calculations of the space- and frequency-dependent phonon distribution agree well with experiment.

Channeling of acoustic phonons in silicon: the polarization dependence in elastic scattering

Ballistic phonon flux in crystals at low temperatures is highly directional due to phonon focusing. In this paper, a phonon-imaging experiment is reported which shows that scattered phonons, too, can retain a highly directed flux. Basically, the combination of phonon focusing and the elastic-scattering selection rule act to channel phonons along the ballistic focusing directions. Together with Monte Carlo simulations, the experiments show that the scattered phonon flux is indeed due to elastic scattering processes, which depend on the polarizations of the scattering phonons. Isotropic scattering is inconsistent with the data. The experimental technique shows promise for quantifying the microscopic scattering processes and revealing the dynamics of a phonon hot spot.

Phonon hot spot formation in GaAs under optical excitation

Nonequilibrium phonon currents, generated in GaAs crystals by interband light absorption, are measured. A comparison of these time-of-flight spectra for the specimen in vacuum with those immersed into liquid helium shows that the hot spot forming pattern in the latter case is obscured by excitation energy dependent heat emission into liquid helium. Analyzing the shapes of time-of-flight spectra for different excitation energies in vacuum, we estimate the hot spot formation threshold for GaAs. The experimentally observed ballistic phonon pulse nonlinearities are consistent with the performed theoretical estimations.

Measurement of a phonon hot spot in photoexcited Si

High frequency phonons are produced by the thermalization of photoexcited electronhole pairs in a semiconductor. Inelastic and elastic scattering processes determine the frequency down-conversion and diffusion of this thermal energy. Simple estimates of the anharmonic and isotope scattering processes suggest that the acoustic phonons will undergo a quasi-diffusive propagation process. In particular, non-equilibrium phonons high-resolution phonon-imaging experiments presented here show well defined ballistic pulses and sharp phonon-focusing caustics. We explain this discrepancy in terms of a phonon hot spot which acts to efficiently down-convert the high-frequency phonons very near the excitation spot. We present the first measurements of the size of a phonon hot spot, which depends upon excitation power.

Phonon-wind-driven transport of photoexcited carriers in a semiconductor quantum well.

The in-plane transport of photoexcited carriers in ${\mathrm{GaAsAl}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple quantum wells is measured optically with 10 ps and 4 \ensuremath{\mu}m resolution. The local carrier density is measured by the bleaching of the excitonic absorption line. The expansion of the photoexcited carriers following a 6-ps pump pulse is found to be a strong function of excitation power and ambient temperature. At low powers, with initial carrier densities ${n}_{0}$\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, the spatial distribution is well described by a single expanding Gaussian profile corresponding to an effective diffusion constant ${D}_{\mathrm{eff}}$(${n}_{0}$,T). The power and temperature dependence suggests a phonon-wind driving mechanism. At higher densities (${n}_{0}$&gt;2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) a narrow spatial profile appears which we attribute to ``thermal confinement'' of carriers within a phonon hot spot. After some delay, the ``confined''-carrier distribution is observed to undergo a rapid expansion. This unusual effect is attributed to the ``explosion'' of the phonon hot spot, as predicted by Kazakovtsev and Levinson.

Picosecond imaging of photoexcited carriers in quantum wells: Anomalous lateral confinement at high densities.

We have measured the in-plane motion of photoexcited carriers in semiconductor quantum wells and have discovered several surprising results. The effective diffusivity of the carriers at densities below $n=2\ifmmode\times\else\texttimes\fi{}{10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ is found to depend upon the excitation level, possibly indicating defect-limited diffusion or phonon-wind effects. Above this density the spatial profiles exhibit two distinct components with widely differing diffusivities. We postulate that the slowly diffusing component represents carriers which are "thermally confined" to a phonon hot spot.