In situ investigation of growth of gold on crystalline TiO 2 and amorphous Al 2 O 3 substrates
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In situ investigation of growth of gold on crystalline TiO2 and amorphous Al2O3 substrates L. Lauter and R. Abermann Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
ABSTRACT: The growth of thin gold films on highly crystalline TiO2 and amorphous Al2O3 substrates and its dependence on substrate temperature was investigated under UHV-conditions by in situ internal stress measurements. Deposition of gold on amorphous Al2O3 at substrate temperatures between 27°C and 300°C shows a stress vs. thickness curve which indicates island growth at first and the formation of a polycrystalline film at higher thickness. A comparable stress vs. thickness curve is found for the growth of gold on the highly crystalline TiO2 substrate at substrate temperatures below 200°C, again indicating island growth. At higher temperatures, however, a new tensile stress feature at low gold coverage is interpreted to indicate the formation of a strained interface layer. This strain in the gold film is eliminated after deposition of a few monolayers most likely through incorporation of dislocations and defects. The growth stress at higher film thickness is indicative of island film growth. INTRODUCTION: Numerous experiments published earlier have shown that the growth stress of mobile film materials on amorphous is consistent with the growth of polycrystalline films with island formation during the initial growth stage. The film stress built up during the initial nucleation of individual islands is always tensile. This tensile stress reaches a maximum at the end of film coalescence. In this growth stage a continuous film has formed and the growth stress turns compressive with further thickness increase. After deposition a tensile stress change with time is observed which is attributed to restructuring and recrystallization of the polycrystalline film [7]. Its magnitude increases with the thickness of the deposited film and depends on substrate temperature. In a model for the origin of film stress it is assumed that the compressive stress is generated by a compressive strain locked in at the interface between the substrate and deposited film which originates from surface tension effects during film coalescence. As the thickness of the film increases, this compressive strain is transmitted layer by layer to the film surface and the film continues to grow by “strained layer epitaxy”. In view of all the experimental results obtained, it is assumed that this compressive stress dominates all other stress contributions as long as the compressive strain is transmitted from the substrate interface to the surface of the film. On the other hand substrate interface strain can be reduced by formation of dislocations and/or vacancies [3]. During deposition of titanium on highly crystalline TiO2 substrates, however, a novel stress vs. thickness curve was found [1]. In this case a compressive stress is built up at first which turns tensile after deposition of a few monolayers. The initial compressive stress is
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