Templated self-assembly of SiGe quantum dots [Elektronische Ressource] / vorgelegt von Christian Dais

Dais

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2009
• Nombre de lectures: 3
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

Templated Self-Assembly of
SiGe Quantum Dots

Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der
RWTH Aachen University zur Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation


vorgelegt von
Diplom Physiker

Christian Dais

aus
Ludwigsburg, Deutschland



Berichter: Prof. Dr. D. Grützmacher
Prof. Dr. G. Güntherodt

Tag der mündlichen Prüfung: 19. August 2009





Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.
Abstract
This PhD thesis reports on the fabrication and characterization of exact aligned SiGe
quantum dot structures. In general, SiGe quantum dots which nucleate via the
Stranski-Krastanov growth mode exhibit broad size dispersion and nucleate randomly
on the surface. However, to tap the full potential of SiGe quantum dots it is necessary
to control the positioning and size of the dots on a nanometer length, e.g. for
electronically addressing of individual dots. This can be realized by so-called
templated self-assembly, which combines top-down lithography with bottom-up self-
assembly. In this process the lithographically defined pits serve as pre-defined
nucleation points for the epitaxially grown quantum dots. In this thesis, extreme ultra-
violet interference lithography at a wavelength of λ = 13.4 nm is employed for pre-
patterning of the Si substrates. This technique allows the precise and fast fabrication
of high-resolution templates with a high degree of reproducibility. The subsequent
epitaxial deposition is either performed by molecular beam epitaxy or low-pressure
chemical vapour deposition. It will be shown that the dot nucleation on pre-patterned
substrates depends strongly on the lithography parameters, e.g. size and periodicity of
the pits, as well as on the epitaxy parameters, e.g. growth temperature or material
coverage. The interrelations are carefully analyzed by means of scanning force
microscopy, transmission electron microscopy and x-ray diffraction measurements.
Provided that correct template and overgrowth parameters are chosen, perfectly
aligned and uniform SiGe quantum dot arrays of different period, size as well as
symmetry are created. In particular, the quantum dot arrays with the so far smallest
period (35 nm) and smallest size dispersion are fabricated in this thesis. Furthermore,
the strain fields of the underlying quantum dots allow the fabrication of vertically
aligned quantum dot stacks. Combining lateral and vertical dot alignment results in
three-dimensional quantum dot crystals.
The analyzed SiGe quantum dots have a type II band alignment, with holes confined
in the dots and electrons confined in the strained Si in the surrounding of the dots. The
recombination energy of these indirect excitons depends on size, Ge content and strain
distribution of the quantum dots. It will be shown that the structural uniformity of the
created quantum dot structures is reflected in their optical properties, resulting in a
narrow and stable photoluminescence emission with well separated no-phonon and
transversal optical phonon lines. The narrow dot luminescence can be shifted by
varying Ge coverage, dot size or dot period. Furthermore excitation-power dependent
and temperature dependent photoluminescence measurements are discussed. Band
structure calculations indicate that the electronic states of the quantum dot crystals are
electronically coupled at least in vertical direction. For the quantum dot crystal with a
lateral period of 35 nm even a coupling in all three dimensions is calculated. Thus, the
three-dimensional dot arrangement represents not only from the structural but also
from the electronic point of view an artificial crystal.






Figure: SiGe quantum dot crystal consisted of 11 Ge dot layers separated by 10 Si
spacer layers. Extreme ultraviolet interference lithography was utilized to prepare the
pre-patterned substrates with a lateral periodicity of x = 90 nm, y = 100 nm. Molecular
beam epitaxy was used for Si/Ge overgrowth (image is composed of two STEM and one
SFM micrograph).

Table of contents

Abstract I
List of Acronyms III

1 Introduction 1

2 Basics 5
2.1 Growth mechanism of SiGe quantum dots 5
2.2 Structural properties of self-assembled SiGe QDs 7
2.2.1 SiGe island formation and evolution 7
2.2.2 Intermixing of SiGe QDs 11
2.3 Templated self-assembly 12
2.3.1 Lateral ordering 12
2.3.2 Vertical ordering 16
2.4 Optical properties of self-assembled QDs 17

3 Extreme ultra violet interference lithography 21
3.1 Principles of EUV-IL 23
3.2 EUV-IL pre-patterning procedure 26

4 SiGe epitaxy 31
4.1 MBE deposition 31
4.2 LPCVD 33
4.3 Sample treatment prior to epitaxy 34

5 Preparation and structural characterization of SiGe QD arrays 35
5.1 Si buffer layer 36
5.2 SiGe QD arrays: unpatterned vs. patterned 38
5.3 Growth temperature 43
5.4 Variation of the template period from 280 nm down to 35 nm 46
5.5 Template variations 51


I 5.6 SiGe QD crystals 54
5.6.1 SFM and TEM characterization 54
5.6.2 XRD analysis 62
5.7 SiGe QD arrays prepared by LPCVD 65
5.7.1 Deposition of SiGe QDs on Si(100) 65
5.7.2 Selective deposition of SiGe QDs on oxidized Si substrates 68
5.8 Summary 69

6 Optical properties of ordered SiGe QDs 71
6.1 Preliminary considerations 72
6.1.1 Photoluminescence setup 72
6.1.2 Band structure simulations 73
6.2 Photoluminescence of QD arrays 74
6.3 Excitation power dependent PL measurements 84
6.4 Temperature dependence of the QD PL 88
6.5 PL characterization of LPCVD grown QD arrays 90

7 Optical properties of SiGe QD crystals 93

8 Summary and Outlook 103

9 Bibliography 105

Published Work 117
Conference contributions 118
Curriculum vitae 120

II List of Acronyms

Acronym Explanation
CMOS complementary metal-oxide-semiconductor
CVD chemical vapour deposition
EUV-IL extreme ultra-violet interference lithography
EUV-L extreme ultra-violet lithography
FET field effect transistor
FIB focused ion beam
FWHM full width half maximum
Ge Germanium
HAADF high angle annular dark field
HH heavy holes
HSQ hydrogen-silsesquioxane
IL interference lithography
LH light holes
LPCVD low pressure chemical vapour deposition
MBE molecular beam epitaxy
ML monolayer
NP no phonon
PL photoluminescence
PMMA polymethyl-methacrylate
PSI Paul Scherrer Institute
QD quantum dot
RT room temperature
SFM scanning force microscopy
SEM scanning electron microscopy
Si Silicon
SK Stranski-Krastanov
SLS Swiss Light Source
SRH Shockley-Read-Hall
TEM transmission electron microscopy
TO transversal optical
STEM scanning transmission electron microscopy
WL wetting layer
XRD x-ray diffraction
III Introduction

1 Introduction
The general trend in semiconductor technology is to approach towards smaller and
more effective devices. Over decades, CMOS technology has followed the predictions
of Moore’s Law which suggests that the number of transistors on an integrated circuit
is increasing exponentially, doubling approximately every two years. Naturally,
further down scaling of integrated circuits will reach its limit sooner or later and new
concepts are required to continue the successful advancements of semiconductor
technology.
Promising candidates for the introduction into main-stream technology are
semiconductor quantum dots (QDs). QDs represent a solid state-system with charge
carrier confinement in all three dimensions. In analogy to an atom a QD is also often
referred as an artificial atom. Especially semiconductor QDs prepared from silicon
and germanium are of interest, not only because the SiGe material system is a
relatively simple model system to understand the fundamental nucleation mechanism,
but also because of their compatibility to Si microelectronics. The most common way
to produce SiGe QDs is accomplished by strained layer epitaxy via the Stranski-
Krastanov (SK) growth mode. The driving force of this growth mode is the lattice
mismatch of 4.2% between Si and Ge, which causes the formation of self-assembled
SiGe QDs on top of an initial two-dimensional wetting layer. Such dislocation-free
SiGe QDs are in general randomly distributed on the surface and exhibit rather broad
size dispersion. Within the last years enormous efforts have been undertaken to study
and categorize these structures. Especially their optical properties have attracted
strong interest because the localization of carriers in these quasi zero-dimensional
structures improves the light-emitting properties of the indirect band gap materials Si
and Ge. The localization enhances the uncertainty of the carrier momentum and hence
increases the probability of optical transitions without the assistance of phonons.
Nevertheless, SiGe QD-