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TY - THES AU - Köhl, Dominik TI - 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 CY - Aachen PB - Publikationsserver der RWTH Aachen University M1 - RWTH-CONV-125632 SP - 197 S. : Ill., graph. Darst. PY - 2011 N1 - Aachen, Techn. Hochsch., Diss., 2011 AB - 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. KW - Strukturbildung (SWD) KW - Zinkoxid (SWD) KW - Reaktives Sputtern (SWD) KW - Sputtern (SWD) KW - Beschichten (SWD) LB - PUB:(DE-HGF)11 UR - https://publications.rwth-aachen.de/record/64301 ER -