Electrical Transport and Scattering Mechanisms in Thin Silver Films for Thermally Insulating Glazing

Thin silver films are widely used in low-emissivity coatings for building glazing due to their high reflectance in the infrared and high transmittance in the visible spectrum. The determining parameter for the infrared reflectance is the electrical conductance of the layer stack - the better the conductance the higher the reflectance. Electrically conductive films of thicknesses smaller than the electron mean free path exhibit a strong increase in the residual resistivity proportional to the inverse of the film thickness. Despite intensive discussions, which have extended over tens of years, it is not understood yet if this conductive behavior originates from electron scattering at interfaces (Fuchs-Sondheimer model) or grain boundaries (Mayadas-Shatzkes model). To achieve a fundamental understanding of the prevailing electron scattering mechanisms, aluminum-doped zinc oxide (ZnO:Al) / Ag / ZnO:Al layer stacks produced by magnetron sputtering were investigated concerning their electronic structure and electrical transport properties. The electronic structure of the layer stacks was probed and analyzed by electron energy-loss spectroscopy. By this technique, plasmonic excitations are observed, which can be categorized into excitations of the electrons in the bulk silver and excitations at the ZnO:Al / Ag interface. The plasmons were analyzed with respect to their dispersion and the peak width, and brought into relation with electrical conductivity measurements by calculating the plasmon lifetime and the electron scattering rate. The difficulty in determining the relative contributions of the interface and grain boundary scattering in experimental conditions is due to the fact that the way in which these scattering mechanisms depend on the film thickness, is very similar. Understanding the electron transport in thin films is of paramount importance, because the differentiation between the scattering mechanisms is a key issue for the improvement of the coatings. In the present work, the solution came from the expected difference in the temperature-dependent behavior of the resistivity between electron scattering at interfaces and electron scattering at grain boundaries. Hence, the resistivity was measured as a function of the temperature on layer stacks with different silver film thickness varying in the range of 4 to 200 nm. The data were analyzed using the extended Mayadas-Shatzkes model involving both electron scattering at interfaces (Fuchs-Sondheimer model), and electron scattering at grain boundaries. The results demonstrate that electron scattering at grain boundaries dominates for all film thicknesses. The basic layer stack was compared to more sophisticated systems, obtained either by adding a thin titanium layer in between silver and ZnO:Al, or by exposing the growing silver film to an oxygen partial pressure (oxidizing the film). Furthermore, the effect of annealing at 250°C was studied for all these systems.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Technische Universität Dresden

Year

2011

Author dc:creator

• Philipp, Martin

Contributors dc:contributor

• Büchner, Bernd
• Jupille, Jacques
• Le Bourhis, Eric
• Klingeler, Rüdiger

• silver
• zinc oxide
• ZnO:Al
• resistivity
• EELS
• plasmon
• surface
• grain boundary
• scattering
• low-emissivity
• Silber
• Zinkoxid
• Widerstand
• Oberfläche
• Korngrenzen
• Streuung