Carrier dynamics in silicon and Germanium nanocrystals
Author
Sevik, Cem
Advisor
Bulutay, Ceyhun
Date
2008Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
This is a computational work on the Si and Ge nanocrystals (NCs) embedded
in wide band gap host matrices. As the initial task, extensive ab initio work
on the structural and electronic properties of various NC host matrices, namely,
SiO2, GeO2, Si3N4, and Al2O3 are preformed. The structural parameters, elastic
constants, static and optical dielectric constants are obtained in close agreement
with the available results. Furthermore, recently reported high density cubic
phase of SiO2 together with GeO2 and SnO2 are studied and their stable highdielectric
constant alloys are identified.
Based on the ab initio study of host matrices, two related high field phenomena,
vital especially for the electroluminescence in Si and Ge NCs, are examined.
These are the hot carrier transport through the SiO2 matrix and the subsequent
quantum-confined impact ionization (QCII) process which is responsible for the
creation of electron-hole pairs within the NCs. First, the utility and the validity
of the ab initio density of states results are demonstrated by studying the high
field carrier transport in bulk SiO2 up to fields of 12 MV/cm using the ensemble
Monte Carlo technique. Next, a theoretical modeling of the impact ionization
of NCs due to hot carriers of the bulk SiO2 matrix is undertaken. An original
expression governing the QCII probability as a function of the energy of the hot
carriers is derived.
Next, using an atomistic pseudopotential approach the electronic structures
for embedded Si and Ge NCs in wide band-gap matrices containing several thousand
atoms are employed. Effective band-gap values as a function of NC diameter
reproduce very well the available experimental and theoretical data. To further
check the validity of the electronic structure on radiative processes, direct photon emission rates are computed. The results for Si and Ge NCs as a function of
diameter are in excellent agreement with the available ab initio calculations for
small NCs.
In the final part, non-radiative channels, the Auger recombination (AR) and
carrier multiplication (CM) in Si and Ge NCs are investigated again based on the
atomistic pseudopotential Hamiltonian. The excited electron and excited hole
type AR and CM and biexciton type AR lifetimes are calculated for different
sized and shaped NCs embedded in SiO2 and Al2O3. Asphericity is also observed
to increase the AR and CM rates. An almost monotonous size-scaling and satisfactory
agreement with experiment for AR lifetime is obtained considering a
realistic interface region between the NC core and the host matrix. It is further
shown that the size-scaling of AR can simply be described by slightly decreasing
the established bulk Auger constant for Si to 1.0×10−30cm6
s
−1
. The same
value for germanium is extracted as 1.5×10−30cm6
s
−1 which is very close to the
established bulk value. It is further shown that both Si and Ge NCs are ideal for
photovoltaic efficiency improvement via CM due to the fact that under an optical
excitation exceeding twice the band gap energy, the electrons gain lion’s share
from the total excess energy and can cause a CM. Finally, the electron-initiated
CM is predicted to be enhanced by couple orders of magnitude with a 1 eV of
excess energy beyond the CM threshold leading to subpicosecond CM lifetimes.
Keywords
Si and Ge NanocrystalsHigh-k Oxides
Electronic Structure
Quantum Confined Impact Ionization
Carrier Multiplication
Auger Recombination
Radiative Recombination
High-Field Transport