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https://hdl.handle.net/2142/20710
Description
Title
Photo-produced phonons in semiconductors
Author(s)
Msall, Madeleine Elizabeth
Issue Date
1995
Doctoral Committee Chair(s)
Wolfe, J.P.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
The techniques of phonon imaging are used in conjunction with photoluminescence measurements to determine the characteristics of optically generated phonon sources in Si, Ge and GaAs at 1.7K. Focused-laser excitation of a semiconductor generates phonons as a byproduct of electronic excitation. A single carrier sheds this excess by the emission of optical phonons which cascade down in frequency, $\nu,$ by the normal anharmonic processes as they propagate in the crystal. This quasidiffusion of phonons is characterized by broad heat pulses at low excitation densities. At higher excitation densities carrier interactions may rapidly thermalize the excess carrier energy without optical phonon emission. The heated carrier distribution can directly emit low-$\nu$ acoustic phonons. The cross-over in energy relaxation mechanisms depends upon the continued production of high-$\nu$ phonons in the presence of a dense, cool plasma. The formation of such a plasma can be ascertained by observation of the electron-hole recombination luminescence.
Experiments with controlled boundary conditions verify that quasidiffusive theory is relevant in all 3 systems under very weak photoexcitation. An analytic solution to the Boltzmann equation for these phonons is presented, along with numerical simulations of the detected phonon flux, which quantitatively agree with the experimental data. The experiments utilize a probe which isolates the excitation surface from the He bath. Experiments with the excitation surface in contact with liquid He show that the heat pulse characteristics are dominated by the large boundary conductance at the He-semiconductor interface.
Beyond the quasidiffusive domain, an extra component of low-$\nu$ phonons is observed. Measurements of the photoluminescence in Si and Ge verify the presence of a dense e-h liquid phase (EHL). It is postulated that acoustic phonons are emitted directly by the EHL (or a dense plasma in GaAs), bypassing the slower production via anharmonic decay. A calculation of the plasma cooling rate, which applies to carriers excited within the plasma in the course of Auger recombination and some of the carriers excited by the laser, is presented. Starting from moderate excitation powers the dense plasma controls the energy balance in the system and is a rich source of low-$\nu$ acoustic phonons. For large populations of low-$\nu$ phonons coalescence processes may be initiated. This may explain changes in the heat pulses in Ge, where the extra component of low-$\nu$ phonons decreases at the highest excitation levels.
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