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The Influence of Energetic Bombardment on the Structure

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The Influence of Energetic Bombardment on the Structure 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¨atf¨urMathematik, 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 m¨undlichen Pr¨ufung:17.02.2011 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verf¨ugbar. 2 Contents 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 chemical 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 3 CONTENTS 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 4 1 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- parent conducting oxide (TCO). Such films exhibit high optical transparency and 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 windows arises from a combination of high optical reflectivity in the infrared spec- 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- 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. 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, 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 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. 5 CHAPTER 1. INTRODUCTION 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. 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- ture at room temperature. The necessity to heat the glass substrate adds process costs and reduces the energy balance of the final product. 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 (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- lored ion bombardment is rare; just as literature on possible correlations between different growth stages of the zinc oxide films and the extent of structural mod- ification/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 nu- cleation 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 6 that the initial growth stage plays an important role in the formation of a pre- ferred growth orientation but also that the action of texture forming mechanisms in subsequent growth stages is comparatively weak. Detailed models of zinc oxide film growth with special respect to the influ- ence of ion bombardment during different growth stages will be developed and discussed. First, the influence of low energy bombardment will be described. Subsequently it will be shown how this knowledge can be used to specifically improve the structural order of ZnO and ZnO:Al films with minimum additional technical efforts by utilizing seed layers. The investigation of the role of ZnO seed layers will allow for a substantive enhancement of the first growth model. Distinct structural improvements in thin silver films grown on modified ZnO seed layers will be demonstrated.
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