The scattering of high-frequency acoustic phonons in silicon and GaAs
Shields, Jeffrey Allan
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https://hdl.handle.net/2142/18903
Description
Title
The scattering of high-frequency acoustic phonons in silicon and GaAs
Author(s)
Shields, Jeffrey Allan
Issue Date
1992
Doctoral Committee Chair(s)
Wolfe, J.P.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
phonon scattering
high-frequency acoustic phonons
silicon
GaAs
phonon imaging
Language
en
Abstract
The production and propagation of high-frequency acoustic phonons in silicon and
GaAs are studied at low temperatures (~ 2K) using the phonon imaging technique. In this
technique, a heat source is provided by a focused laser beam which is raster scanned across
the surface of the sample which is immersed in liquid helium. After they propagate across
the sample, the acoustic phonons produced by the heat .source are detected by a small
superconducting detector. The spatial distribution of the ballistic phonon flux is highly
anisotropic due to the phonon focusing effect.
In this thesis, the characteristics of the phonon source produced by the focused laser .
beam have been studied extensively in silicon. The phonon source consists of a small
region in the crystal in which the phonon occupation number is elevated with respect to
the rest of the crystal. The lifetime and frequency distribution of the phonon source are
found to vary dramatically with different boundary conditions, such as whether the surface
is of the sample is metalized or whether the excitation surface is in contact with liquid
helium. The laser power level also has profound effects on the characteristics of the
phonon source, and a threshold of some sort has been observed at low power.
The phonon frequency determines the likelihood that the phonon will traverse the
sample ballistically since the elastic scattering rate depends on the phonon frequency to the
fourth power. A special sample geometry allows the first direct measurement of the elastic
scattering rate in silicon by comparing the experimental data with Monte Carlo calculations
of the elastic scattering.
The phonon focusing p~ttern changes at very high phonon frequencies due to lattice
dispersion. Observations of dispersive effects in the phonon focusing pattern of silicon
have been made and the effect of various lattice dynamics models on the Monte Carlo
calculations of the elastic scattering in silicon are presented.
Phonon scattering from defects in GaAs has been studied and dramatic effects in the
phonon focusing pattern have been observed with near infrared irradiation of the crystal.
Previous studies have indicated that the EL2 defect is changed from its normal state into
a metastable state under irradiation with infrared light. We observe changes in the phonon
scattering after irradiation which indicate a decrease in inelastic phonon scattering with the
transition to the metastable state.
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