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Ultra-thin Germania and Germania-Silica films: Growth, structure and reactivity


Fuhrich,  Alexander
Chemical Physics, Fritz Haber Institute, Max Planck Society;

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Fuhrich, A. (2019). Ultra-thin Germania and Germania-Silica films: Growth, structure and reactivity. PhD Thesis, Freie Universität, Berlin.

Cite as: https://hdl.handle.net/21.11116/0000-0005-5A47-7
This thesis presents a specto-microscopical study on the preparation of ultra-thin germania and germania-silica films on a Ru(0001) single crystal support and the discussion of their chemical reaction with molecular hydrogen. The two-dimensional GeO2 and GeO2-SiO2 films were investigated by synchrotron based spectro-microscopy applying microscopy, diffraction and spectroscopy. For a calibration of the deposited germanium amount, the growth of germanium on bare Ru(0001) was studied in-situ by LEEM, LEED, XPEEM and XPS. The germanium films were grown at 540 K in UHV. This demonstrated that, at first, a (2x2) germanium layer is formed and further germanium growth leads to the formation of a coexisting (3x3) germanium structure. Afterwards a second layer is formed, however before the (3x3) phase is completely closed. Further germanium growth leads to the formation of three-dimensional germanium islands. Germanium grows at 540 K in UHV on bare Ru(0001) in a Stranski-Krastanov growth mode. The germanium film oxidation was studied in real-time and in-situ. The formation of a partially closed GeO2 layer on top of a closed GeO2 monolayer was found. Furthermore the oxidation of a 0.6 nm thick germanium film is presented. As a result, it was found that a closed GeO2 monolayer on Ru(0001) is formed and that excess GeOx desorbs at temperature above 770 K. Another part of this work is the preparation of ultra-thin mixed GeO2-SiO2 films on Ru(0001). Different preparation recipes are presented showing that the crystallinity of the initially deposited film has a significant influence on the final film. The addition of germania or silica to a crystalline film leads to the coexistence of two separated oxides. The non-connected germania desorbs at temperatures above 970 K. Different silica to germania ratios were prepared. An amount of nominal 0.5 ML germanium was dispersed in 1.5 ML silica matrix. Only crystalline germania-silica films could be prepared. Germania containing silica leads to lower crystallization temperatures compared to pure silica. Finally the effect of hydrogen treatments on the GeO2 and GeO2-SiO2 films were studied in real-time LEEM. Whereas hydrogen completely reduces the GeO2 film. The reduced germanium segregates on the bare Ru(0001) surface. The hydrogen treatment on the mixed GeO2-SiO2 films shows two steps. In the first step the hydrogen removes oxygen with a binding energy of 529 eV. This step is not visible in LEEM. In a second step the hydrogen treatment leads to a partial reduction of the germanium in the silica matrix, the silica is not reduced. However, the Si2p core level shifts by 0.2 eV. The GeO2-SiO2 films can be reoxidized. The reoxidized films are similar to the initial films. Furthermore, the low energy electron beam influences the chemical reaction and enhances locally the reduction of GeO2 by hydrogen.