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Growth and characterisation of group III nitride based nanowires for devices [Elektronische Ressource] / vorgelegt von Ralph Joseph Meijers

185 pages
Growth and Characterisationof Group-III Nitride-basedNanowires for DevicesVon der Fakultät für Mathematik, Informatik und Naturwissenschaften der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Gradeseines Doktors der Naturwissenschaften genehmigte Dissertationvorgelegt vonDiplom-PhysikerRalph Joseph Meijersaus Kerkrade, die NiederlandeBerichter: Universitätsprofessor Dr. Dr. h.c. H. LüthUniversitätsprofessor Dr. G. GüntherodtTag der mündlichen Prüfung: 30. August 2007Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.Mit jedem einfachen Denkakttritt etwas Bleibendes,Substantielles in unsere Seele ein.Bernhard RiemannTable of ContentsTable of Contents v1 Introduction 12 Material Properties 52.1 General Group-III Nitride properties . . . . . . . . . . . . . . . . . . . . . . . 52.2 InN properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Review Nanowires 133.1 Introduction and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.1 Strain engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.2 Fabrication techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2 Origin of anisotropic growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.2.1 Diffusion-dislocation model . . . . . . . . . . . . . . . . . . . . . . . . 163.2.2 Vapour-liquid-solid growth . . . . . . . . . . . . . .
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Growth and Characterisation
of Group-III Nitride-based
Nanowires for Devices
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der Rheinisch-
Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades
eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker
Ralph Joseph Meijers
aus Kerkrade, die Niederlande
Berichter: Universitätsprofessor Dr. Dr. h.c. H. Lüth
Universitätsprofessor Dr. G. Güntherodt
Tag der mündlichen Prüfung: 30. August 2007
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.Mit jedem einfachen Denkakt
tritt etwas Bleibendes,
Substantielles in unsere Seele ein.
Bernhard RiemannTable of Contents
Table of Contents v
1 Introduction 1
2 Material Properties 5
2.1 General Group-III Nitride properties . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 InN properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Review Nanowires 13
3.1 Introduction and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.1 Strain engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.2 Fabrication techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Origin of anisotropic growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.1 Diffusion-dislocation model . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.2 Vapour-liquid-solid growth . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3 Catalyst-free nitride nanowire growth . . . . . . . . . . . . . . . . . . . . . . . 22
3.4 Promising nanowire features for applications . . . . . . . . . . . . . . . . . . . 23
3.5 From Nanoscience to Nanotechnology . . . . . . . . . . . . . . . . . . . . . . . 24
3.5.1 Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.5.2 Realised devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4 Experimental Details 27
4.1 Molecular Beam Epitaxy: basics and theoretical background . . . . . . . . . . 27
4.2 Molecular Beam Epitaxy system . . . . . . . . . . . . . . . . . . . . . . . . . 30
5 Nanowire Growth Mechanism 33
5.1 Morphology as a function of III-V ratio . . . . . . . . . . . . . . . . . . . . . 33
5.2 Nucleation and Growth Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3 Growth interruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6 Characterisation of GaN nanowires 49
6.1 GaN nanowire morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.1.1 Tapering effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.2 Crystal structure and epitaxial orientation . . . . . . . . . . . . . . . . . . . . 59
vvi CONTENTS
6.3 Optical characterisation of GaN nanowires . . . . . . . . . . . . . . . . . . . . 63
6.3.1 GaN cathodoluminescence results . . . . . . . . . . . . . . . . . . . . . 64
6.3.2 GaN photoluminescence results . . . . . . . . . . . . . . . . . . . . . . 74
6.4 Transmission electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 78
7 Characterisation of doped GaN nanowires 87
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7.2 Morphology of doped GaN nanowires . . . . . . . . . . . . . . . . . . . . . . . 88
7.3 Cathodoluminescence of doped GaN nanowires . . . . . . . . . . . . . . . . . 92
7.4 Remark concerning electrical measurements of Si-doped nanowires . . . . . . 94
8 Characterisation of InN nanowires 95
8.1 InN nanowire morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.2 Crystal structure and epitaxial orientation . . . . . . . . . . . . . . . . . . . . 103
8.3 InN photoluminescence results . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
8.4 Transmission electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 114
9 Technological processing and electrical characterisation of nanowires 119
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
9.2 Stability of as-grown nanowires during technological processing . . . . . . . . 121
9.3 Contacting single nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
9.4 Electrical characterisation of GaN nanowires . . . . . . . . . . . . . . . . . . . 127
9.4.1 IV-measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
9.5 Electrical characterisation of InN nanowires . . . . . . . . . . . . . . . . . . . 134
9.5.1 IV-measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
10 Summary and Outlook 137
10.1 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
11 Zusammenfassung 141
List of Figures i
List of Tables xiii
References xv
List of publications xxxiii
Acknowledgments xxxv
Curriculum Vitae xxxviiChapter 1
Introduction
The continuous strive to decrease the size of devices is accompanied by an enormous increase
of the costs especially for lithography steps. In addition conventional device concepts might
not work when quantum mechanical effects govern the behaviour of the device. Therefore
it is necessary to fabricate nanosystems in a low-cost way with sizes smaller than the actual
lithographic resolution. An interesting example is offered by the growth of nanowires. For
some applications this approach could replace conventional device processing, but its main
advantage is the possibility to investigate small device sizes which conventional device pro-
cessing will probably only be able to produce in a few years. Thus, in advance of industrial
1production, one can already address fundamental issues and possible ‘Red Brick Walls’ in
semiconductor roadmaps for very small device sizes and try to find solutions.
In recent years nanowires have attracted a lot of interest in the research community, but
alsocommercial companies likeIntel, IBM,HPand Philips have shown theirinterest forthese
structures with very smalldimensions. They can beproduced in various ways, sometimes also
very cost-effective. Chemically synthesised semiconductor nanowires are interesting building
blocks for nanoscale devices. Although sophisticated device structures have already been
realised [1–6], many fundamental questions dealing with the internal electronic structure and
size dependent transport phenomena remain, but are starting to be addressed. Another
subject of special interest is the extremely large surface in respect to the bulk. In this regard
nanowires areexpectedtobesuitableforthepurposeofstudyingsurfacerelated topics, which
in turn have a strong influence on the physical behaviour of the nanosystem itself. Due to
the limited size, the de Broglie wavelength of electrons is of the same order as the nanowire
diameter and quantum mechanical effects can be expected. This makes them very suitable
for the study of fundamental quantum transport phenomena.
In addition, nanowires offer the unique capability to realise highly lattice mismatched
heterostructures with much more flexibility than in planar films and with a broad variety
of material combinations. Hence, one can expect to cover an extended frequency range of
electromagnetic fields, inparticular theintermediate (THztofar-infrared)wavelength regime,
1‘Red Brick Walls’ in semiconductor roadmaps mark technical areas where further development is expected
to be restricted by fundamental physical or technical limitations and for which no manufacturable solution is
known at present.
12 1. Chapter Introduction
which is a key point for optical as well as communication applications. In addition nanowires
can be fabricated on a wide variety of substrates, including silicon, which make them suitable
forfutureCMOSintegration. Theemployment ofagrownnanowirestructureoffersasuperior
one-dimensional electronic system (as compared to top-down concepts), being suitable for
classical device layouts as well as quantum structures.
Nitrides have been used in many device applications lately, because they are very well
suited for optoelectronics. They feature a direct band gap and cover the visible range of
the electromagnetic spectrum as well as reaching into the ultra-violet allowing for very high
data densities in storage applications [7]. Despite high defect densities nitrides are highly
luminescent and nanowires, which can be grown with more crystalline perfection than planar
films are expected to further improve the performance of optical devices.
GaN nanowires have been grown catalyst-free by molecular beam epitaxy (MBE) in the
group of Calleja [8]. However, the understanding of the growth mechanism, which is essential
to have good control over the growth, has not been thoroughly investigated. A vapour-liquid-
solid (VLS) model has been suggested, but no evidence for a droplet formation has been
presented. Also the physical properties of the nanowires have not been fully explored. Since
nitrides and especially nanowires are predicted to be very good candidates for optoelectronic
applications a good understanding of the material properties as for instance the influence of
doping is required.
Position control of nanowires is an important task for densely-packed device applica-
tions and is usually obtained by patterning the catalyst particle employed in VLS growth
of nanowires. For catalyst-free growth as in the case of nitride nanowires, however, alter-
native approaches to position control have to be investigated and also therefore a thorough
understanding of the growth mechanism is required.
Another important issue in nanowire growth, especially for nitrides where no homosub-
strates are available, is the influence of the substrate and the epitaxial relation to it. The
substrateofchoiceinthesemiconductorindustryisSi(100),buttocomplywiththehexagonal
structure of nitrides Si(111) is often used. The interface between nanowires and substrate is
important for devices where the substrate is used as a bottom contact. In this respect de-
termination of strain which can be relieved by dislocation formation is of crucial importance.
Dislocations usually reduce the mobility of charge carriers and thus the performance of a
device.
Among III-nitrides InN is of great interest. The physical properties reported for InN
scatter a lot and recently there is a lot of scientific interest for the material as a result of
significant improvements in its crystalline quality. InN nanowires could further increase the
obtained material quality and give more reliable values for fundamental physical properties
like the band gap, which is very important for all kinds of optical applications (light emitting
diodes, lasers, solar cells).
Theaimofthisthesisistoaddresssomeoftheissuesmentionedabove. Theunderstanding
of the growth mechanism for nitride nanowires and the influence of the growth parameters
and the substrate is one of the central points. In addition the quality of the nanowires is
studied and optimised. Furthermore, the electrical properties and influence of technological
processing on nitride nanowires are determined.
In Chapter2 the properties of nitrides are reviewed and there is special attention for the3
properties of InN, because of the aforementioned active research in this field. In Chapter3
an overview of the current status of nanowire research is presented as well as models to
explain the one-dimensional growth. Promising features of nanowires are discussed as well
as requirements to integrate them in devices. Chapter4 describes the basics of growth by
molecular beam epitaxy and the MBE system, which was employed to grow the samples for
this thesis.
Experimental results on nitride nanowires are presented in Chapter5, Chapter6, Chap-
ter7, Chapter8 and Chapter9. Firstly, the nucleation, kinetics and growth mechanism is
studied in Chapter5. In Chapter6 the morphology, crystal structure and epitaxial relation to
the substrate as well as the optical properties are determined for GaN nanowires. The same
studiesareperformedforInNnanowiresinChapter8. Chapter7dealswiththedopingofGaN
nanowires and the influence on the morphology as well as the optical properties. Finally, in
Chapter9 the influence of processing steps on nanowires is studied and results of electrical
measurements and deduced physical properties are discussed.
Chapter10 presents a summary with important conclusions of this thesis and an outlook
on future interesting studies.4 1. Chapter Introduction