On the strain in silicon nanocrystals
Author(s)
Advisor
Date
2009Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
In this Thesis we present our achievements towards an understanding of atomistic
strain mechanisms and interface chemistry in silicon nanocrystals. The structural
control of silicon nanocrystals embedded in amorphous oxide is currently an
important technological problem. First, our initial attempt is described to
simulate the structural behavior of silicon nanocrystals embedded in amorphous
oxide matrix based on simple valence force fields as described by Keatingtype
potentials. Next, the interface chemistry of silicon nanocrystals (NCs)
embedded in amorphous oxide matrix is studied through molecular dynamics
simulations with the chemical environment being governed by the reactive force
field model. Our results indicate that the Si NC-oxide interface is more involved
than the previously proposed schemes which were based on solely simple bridge or
double bonds. We identify different types of three-coordinated oxygen complexes,
previously not noted. The abundance and the charge distribution of each oxygen
complex is determined as a function of the NC size as well as the transitions
among them.
Strain has a crucial effect on the optical and electronic properties of
nanostructures. We calculate the atomistic strain distribution in silicon NCsup to a diameter of 3.2 nm embedded in an amorphous silicon dioxide matrix. A
seemingly conflicting picture arises when the strain field is expressed in terms
of bond lengths versus volumetric strain. The strain profile in either case
shows uniform behavior in the core, however it becomes nonuniform within 2-
3 ˚A distance to the NC surface: tensile for bond lengths whereas compressive for
volumetric strain. We reconcile their coexistence by an atomistic strain analysis.
Vibrational density of states (VDOS) affects the optical properties of Si-NCs.
VDOS obtained by calculating velocity autocorrelation function (VACF) using
velocities of the atoms is extracted from the molecular dynamics simulations. The
information on bonding topology enables classification of atoms in the system
with respect to their neighbor atoms. With help of this information we separate
contributions of different type of atoms to the VDOS. Calculating VACF of
different type of atoms such as surface atoms and core atoms of nanocrystal,
to the system facilitates understanding of the effects of strain fields and interface
chemistry to the VDOS.
Keywords
siliconvibrational spectra
simulation
monte carlo
molecular dynamics
strain
interface
nanocrystal