Optical properties of size selected nanocrystallites in porous silicon
Yamani, Zain Hassan
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https://hdl.handle.net/2142/31246
Description
Title
Optical properties of size selected nanocrystallites in porous silicon
Author(s)
Yamani, Zain Hassan
Issue Date
1999
Doctoral Committee Chair(s)
Nayfeh, Munir H.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
porous silicon nanocrystallites
photoluminescence
dimer model
Language
en
Abstract
"The work in this thesis concentrated on processing and diagnosing porous silicon
nanocrystallites that have novel chemical, structural and photoluminescent properties.
FTIR and Auger electron spectroscopy showed that while conventional porous films have
incomplete hydrogen passivation and tend to be plagued with defects that compete with
radiative channels, our ""ideal"" porous silicon samples are highly passivated and defect
free. SEM micrographs showed that crystallite sizes in the ideal porous films are smaller
than that prepared by conventional methods.
Ideal porous silicon films were found to have a crystallite size gradient along the
length of the sample, and appeared (under UV illumination) reddish, orangish, yellowish
and greenish as the meniscus was approached. We also presented the results of size
selected microsecond lifetime measurements for different emission colors. Our study
showed that the decay rate depends on wavelength of emission but is quite insensitive to
crystallite size. Reflectivity measurements on ideal porous silicon samples showed nonbulk-
like behavior. A reflectivity study involving copper plating, however, provided
evidence that the loss of crystalline absorption in the porous films is probably not due to a
permanent loss in crystalline structure.
Ill
We have studied the theoretical photoluminescence predictions of the dimer
model. There is a dramatic enhancement in the fluorescence efficiency for sizes below a
critical size of 1.4 nm, the size for which the radiative states become stable against
tunneling and thermal activation. The fluorescence exhibits a large Stokes shift due to the
expansion of the dimers. The emission bandwidth (even for a single crystallite) is found
to be quite wide especially for the ultra small crystallites where it encompasses nearly all
of the visible spectrum and the near infrared.
Our emission, excitation and reflectivity results support the predictions of the
dimer model. However, sufficient calculated lifetime predictions are currently
unavailable to confirm or reject the measured lifetime results."
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