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@PHDTHESIS{Khl:64301,
author = {Köhl, Dominik},
othercontributors = {Wuttig, Matthias},
title = {{T}he 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},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-CONV-125632},
pages = {197 S. : Ill., graph. Darst.},
year = {2011},
note = {Aachen, Techn. Hochsch., Diss., 2011},
abstract = {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. Another market
segment where zinc oxide films are utilized is the
fabrication of low-emissivity architectural glazing. The
functionality of these energy saving windows arises from a
combination of high optical reflectivity in the infrared
spectral region with optimum transparency in the visible
regime. This prerequisite could be easily achieved with an
arbitrarily complex multi-layer stack. Competition however
requires minimization of production costs. Therefore,
typically one or two very thin silver films in combination
with a minimum number of anti-reflective oxide layers are
utilized. Maximization of production efficiency requires
tweaking each single layer for optimum performance, which is
particularly true for the silver films. For this reason,
zinc oxide films are utilized as seed layers for silver film
growth since their 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 material 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
bombardment, a feature which is inherent in the coating
process. For the applications mentioned above zinc oxide is
most commonly deposited in large scale onto amorphous float
glass substrates by a deposition process far away 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. In
the literature on zinc oxide thin films it is
comprehensively portrayed that films often exhibit
structural inhomogeneities along the growth direction if
grown on amorphous (non-epitaxial) substrates. Also, the
detrimental influence of highly energetic oxygen ion
bombardment on film growth in general is extensively
discussed. However, literature on possible positive
influences of tailored ion bombardment is rare; just as
literature on possible correlations between different growth
stages of the zinc oxide films and the extent of structural
modification/damage caused by ion bombardment. Closing this
gap is the scope of this work. It will be demonstrated that
with a modified, ion beam assisted sputtering (IBAS)
process, zinc oxide films can be deposited which exhibit a
markedly improved crystalline order. Furthermore, it will be
demonstrated that intense energetic oxygen ion bombardment
can be utilized to change film texture from the typical
(002)-self-texture to an a-axis texture where the
(002)-planes are perpendicular to the substrate surface. An
understanding of the underlying mechanisms will be developed
which also facilitates a more detailed understanding of the
action of ion bombardment during zinc oxide film growth. It
will be shown that zinc oxide films are susceptible to the
influence of ion bombardment particularly in the nucleation
regime of growth and that this finding is generally true for
all observed structural changes induced by ion bombardment
with various species, energies and flux densities. These
findings will therefore allow answering fundamental
questions regarding the mechanisms governing structural
evolution of zinc oxide thin films, which is also the scope
of this work. It will be demonstrated not only that the
initial growth stage plays an important role in the
formation of a preferred growth orientation but also that
the action of texture forming mechanisms in subsequent
growth stages is comparatively weak.},
keywords = {Strukturbildung (SWD) / Zinkoxid (SWD) / Reaktives Sputtern
(SWD) / Sputtern (SWD) / Beschichten (SWD)},
cin = {130000 / 131110},
ddc = {530},
cid = {$I:(DE-82)130000_20140620$ / $I:(DE-82)131110_20140620$},
shelfmark = {68.55.jm * 68.55.ag * 81.15.Aa * 81.15.Jj * 81.15.Cd},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-opus-35985},
url = {https://publications.rwth-aachen.de/record/64301},
}
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