Understanding and optimizing the electrical transport in silver thin films for the use as heat insulation glass coatings

• Verständnis und Optimierung des elektrischen Transports in Silber-Dünnfilmen für die Verwendung als Wärmeschutzbeschichtungen

Sittner, Ernst-Roland; Wuttig, Matthias (Thesis advisor); Mergel, Dieter (Thesis advisor)

Aachen (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Large glass facades are one of the central elements in modern architecture. Well known buildings like the Elbphilharmonie in Hamburg get their unique look by the use of large and specially shaped glass panes. In times when climate change is one of the major global challenges, the saving of energy and the avoidance of CO2 emissions are central topics. Bare glass as a building material is highly absorbing to infrared (IR) radiation, leading to an unwanted heat transfer through windows, causing unwanted energy losses or creating additional needs for cooling. Modern multiple glazings, suppress this heat transfer through the window by using thin metallic coatings, which act as IR reflectors and by this make window panes energy-efficient. Ag thin films are typically used as such coatings. The capability to reflect IR radiation is directly connected to the electrical sheet resistance of the metal film. Low electrical resistances give rise to high IR reflectivities. However, for thin metal films, the sheet resistance as well as the resistivity increase with decreasing film thickness. In this work, the resistivity of sputtered Ag thin films is studied. Special focus is set on structural properties and the use of growth templates. A major part of the work deals with the optimization of the Ag thin film resistivity by the use of suitable seed layers. On bulk single crystal substrates, thin Ag films with a biaxial texture and low resistivities could be fabricated. Biaxial texture means, that the thin films have a non-random in-plane and out-of-plane crystallite orientation and, depending on the quality of the texture, can have similar (structural) properties to single crystals. The hetroepitaxial growth of Ag on Si, MgO and ZnO single crystal substrates gives rise to a resistivity reduction of about 20 % compared to standard ZnO/Ag layer stacks. Especially the MgO (200) single crystal growth templates offered good electrical properties. Based on these findings, an ion beam assisted sputter deposition process for MgO seed layers has been established, offering biaxially textured Ag thin films without the aid of a bulk single crystal substrate. Beside optimization, the electrical transport properties in Ag thin films have been studied to identify the most crucial factors increasing the resistivity. By studying strained ZnO seed layers, the role of misfit dislocation formation could be examined, leading to a resistivity reduction, when Ag thin films are grown on strained ZnO seed layers. The comparison of the Ag growth on different single crystal growth templates proved, that structural order and low grain boundary angles are crucial for low Ag resistivities. However, despite good structural properties, electrical properties may still be poor due to an insufficient wetting behavior. Thus, a good wetting behavior is a further crucial parameter for low Ag resistivities. By the use of the electrical models from Mayadas and Shatzkes, the impact of grain boundary scattering and interface scattering on the Ag thin film resistivity could be decoupled for different seed layer/Ag systems. It could be shown, that for all systems studied, both scattering channels play a role. Especially for the MgO (200)/Ag system, low grain boundary and interface resistances could be obtained. In general, it could be shown that the dependence of Ag resistivity on wetting, crystal structure and interface effects is a multi-parameter problem.