The influence of energetic bombardment on the structure formation of sputtered zinc oxide films [Elektronische Ressource] : development of an atomistic growth model and its application to tailor thin film properties / Dominik Köhl

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• Physik

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2011
• Nombre de lectures: 21
• Langue: English
• Poids de l'ouvrage: 13 Mo

Extrait

The influence of energetic
bombardment on the structure
formation of sputtered zinc oxide films

Development of an atomistic growth
model and its application to tailor thin
film properties
Von der Fakult¨at fur¨ Mathematik, Informatik und Naturwissenschaften der
RWTH Aachen University zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker
Dominik K¨ohl
aus Gießen
Berichter: Universit¨atsprofessor Matthias Wuttig
Universit¨atsprofessor Dieter Mergel
Tag der mu¨ndlichen Prufung:¨ 17.02.2011
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online
verfug¨ bar.2Contents
1 Introduction 5
2 Zinc oxide - properties, applications and growth 9
2.1 Material properties . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.1 Miller index notation in hexagonal systems . . . . . . . . . 11
2.2 Properties of thin films . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Principles of textured thin film growth . . . . . . . . . . . . . . . 20
2.4.1 Minimization of surface free energy as a driving force for
preferred nucleation? . . . . . . . . . . . . . . . . . . . . . 23
2.4.2 Minimization of strain energy as a driving force for pre-
ferred nucleation? . . . . . . . . . . . . . . . . . . . . . . . 26
2.5 Thin film growth by chemical processes . . . . . . . . . . . . . . . 30
2.5.1 Growth from c solution . . . . . . . . . . . . . . . 30
2.5.2 Growth from chemical vapour . . . . . . . . . . . . . . . . 32
2.6 Thin film growth by physical vapour deposition (PVD) processes . 38
2.6.1 Growth by pulsed laser deposition . . . . . . . . . . . . . . 38
2.6.2 Growth by magnetron sputtering . . . . . . . . . . . . . . 41
2.7 Summary: Texture formation in sputtered zinc oxide thin films . . 48
3 The magnetron sputtering processes 51
3.1 Coater 1: The IBAS setup . . . . . . . . . . . . . . . . . . . . . . 51
3.2 Coater 2: A conventional sputtering setup . . . . . . . . . . . . . 56
4 Thin film analytics 59
4.1 X-ray techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.1.1 X-ray diffraction (XRD) . . . . . . . . . . . . . . . . . . . 59
4.1.2 Interpretation of grazing incidence XRD patterns . . . . . 62
4.1.3 X-ray reflectometry (XRR) . . . . . . . . . . . . . . . . . . 64
4.2 Atomic force microscopy (AFM) . . . . . . . . . . . . . . . . . . . 67
4.3 Transmission electron microscopy (TEM) . . . . . . . . . . . . . . 67
4.4 In-situ stress measurements - wafer curvature method . . . . . . . 67
4.5 Optical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.5.1 Reflectance and transmittance . . . . . . . . . . . . . . . . 71
4.5.2 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.5.3 Modeling of the dielectric function . . . . . . . . . . . . . 72
4.6 Electrical measurements . . . . . . . . . . . . . . . . . . . . . . . 73
3CONTENTS
5 I: Growth of zinc oxide films by IBAS 75
5.1 Growth conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Optimization of film structure . . . . . . . . . . . . . . . . . . . . 80
5.2.1 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . 80
5.2.2 Mechanical growth stresses . . . . . . . . . . . . . . . . . . 81
5.2.3 Surface topography . . . . . . . . . . . . . . . . . . . . . . 84
5.2.4 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . 92
5.3 Discussion and evaluation . . . . . . . . . . . . . . . . . . . . . . 96
5.4 Development of a first growth model . . . . . . . . . . . . . . . . 101
6 II: Applications of modified zinc oxide structures 105
6.1 Homo-epitaxial growth of ZnO and ZnO:Al thin films . . . . . . . 105
6.1.1 Tailoring buffer layers . . . . . . . . . . . . . . . . . . . . 106
6.1.2 Influence of modified buffer layers . . . . . . . . . . . . . . 119
6.2 Generalization of the growth model . . . . . . . . . . . . . . . . . 126
6.3 Hetero-epitaxial growth of Ag thin films . . . . . . . . . . . . . . 132
6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.3.2 Silver structure improvement by modified ZnO seed layers 135
6.3.3 Microscopical growth study of Ag films on ZnO seed layers 147
7 III: Finally unravelling the origin of preferred orientation in zinc
oxide thin films 157
7.1 Blocking (002) preferred orientation by IBAS: a-axis textured ZnO 158
7.2 Discussion: Preferred nucleation vs evolutionary selection and the
influence of energetic oxygen ions . . . . . . . . . . . . . . . . . . 166
7.3 The final growth model . . . . . . . . . . . . . . . . . . . . . . . . 175
8 Summary and outlook 181
References 185
List of figures 195
41
Introduction
The focus of this work is the investigation of the growth of zinc oxide (ZnO)
thin films. This material has attained increasing scientific interest during the
last decade. Particularly, doped zinc oxide films have become the material of
choice in the fabrication of transparent electrodes for silicon thin film solar cells,
which offer a low-cost alternative to wafer-based highly efficient but expensive
modules. Most commonly, zinc oxide is doped with aluminium to obtain a trans-
parentconductingoxide(TCO).Suchfilmsexhibithighopticaltransparencyand
good (but not perfect) electrical conductivity. Nevertheless, this disadvantage is
overcompensated by the low production costs due to large deposits of both zinc
and aluminium in the terrestrial crust.
Another market segment where zinc oxide films are utilized is the fabrication
of low-emissivity architectural glazing. The functionality of these energy saving
windowsarisesfromacombinationofhighopticalreflectivityintheinfraredspec-
tral region with optimum transparency in the visible regime. This prerequisite
could be easily achieved with an arbitrarily complex multi-layer stack. Compe-
tition however requires minimization of production costs. Therefore, typically
one or two very thin silver films in combination with a minimum number of anti-
reflectiveoxidelayersareutilized. Maximizationofproductionefficiencyrequires
tweaking each single layer for optimum performance, which is particularly true
for the silver films. Since the typical thickness of these silver films is almost at
the percolation limit, the electrical and optical properties critically depend on
the morphology of the films and hence on nucleation and grain growth. For this
reason,zincoxidefilmsareutilizedasseedlayersforsilverfilmgrowthsincetheir
close epitaxial relationship significantly promotes the formation of a well-ordered
silver crystal structure with a preferred orientation. Thus, optimizing silver layer
performance requires mastering zinc oxide film growth.
A common feature of these applications of zinc oxide thin films is that if ma-
terial properties can be significantly improved, either the device efficiency can
be maximized and/or the production costs can be minimized. It is therefore
highly desirable to establish a thorough understanding of zinc oxide film growth.
Particularly, this understanding must include the influence of energetic ion bom-
bardment, a feature which is inherent in the coating process.
5CHAPTER 1. INTRODUCTION
For the applications mentioned above zinc oxide is most commonly deposited in
largescaleontoamorphousfloatglasssubstratesbyadepositionprocessfaraway
from thermodynamic equilibrium: sputter deposition. Consequently, films often
exhibit kinetically controlled structures. However, a self-texturing mechanism
of zinc oxide that might originate from thermodynamics typically leads to high
structural order with increasing film thickness. In spite of that mechanism there
is unused potential since films are often weakly textured in the initial growth
stage, a fact which also limits the achievable maximum in the structural order of
thick films. Particularly in the fabrication of low-emissivity coatings, where very
thin zinc oxide films are utilized, device performance critically depends on the
structural quality obtained in the early growth stage of zinc oxide. It is therefore
vital to understand how different process parameters affect structure formation.
It is widely known that additional heating of the substrate (200-300°C) during
film growth improves the structural quality. On the other hand, it is also known
that the bombardment of the growing film with highly energetic oxygen ions,
which is an inevitable feature of the sputtering process, markedly deteriorates
structural order. While substrate heating can compensate for this damage, such
energetic ion bombardment has a detrimental effect on the zinc oxide film struc-
tureatroomtemperature. Thenecessitytoheattheglasssubstrateaddsprocess
costs and reduces the energy balance of the final product.
Intheliteratureonzincoxidethinfilmsitiscomprehensivelyportrayedthatfilms
often exhibit structural inhomogeneities along the growth direction if grown on
amorphous (non-epitaxial) substrates (see e.g. [1]). Also, the detrimental influ-
ence of highly energetic oxygen ion bombardment on film growth in general is
extensively discussed. However, literature on possible positive influences of tai-
loredionbombardmentisrare; justasliteratureonpossiblecorrelationsbetween
different growth stages of the zinc oxide films and t