Subsurface structure & morphology : Epitaxial metal film growth studied by in situ X-ray scattering
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The existence of missing material (vacancies) below the exposed surface was deduced from deposition-angle-dependent experimental evidence. Due to a lack of experimental evidence, theoretical film growth models have largely neglected the incorporation of vacancies. In fact, downward funneling is a key construct introduced to remove vacancies, therefore, the growth mechanisms for vacancies are not well understood. Vacancies can even explain the anomalous change in physical film properties, such as conductivity and film strain. There are two schools of thought concerning how film strain is incorporated during film growth: 1) structural surface morphology and 2) vacancy clusters. Left out of the discussion is the possible interpretation that there is an interplay between surface morphology and vacancies. In the case of islands growth, surface morphology dominates, and in the smooth film growth case, vacancies could dominate. Is it surface morphology or is it vacancies that are responsible for the measured film strain? This is a complicated question that does not have a direct answer. In order for this question to be addressed, the buried interface has to be characterized. It will be shown, through the deposition-angle-dependent characterization of the Ag/Si(111)7x7 system, using X-ray reflectivity, which is sensitive to the buried interface, that missing material at the interface exists. The characterization of the Ag/Ag(001) deposition-angle-dependent "Pearl Necklace" data, using X-ray reflectivity, shows a constant strain below the exposed surface irrespective of the growth conditions: the deposition angle, the coverage, or the surface roughness. This constant strain below the surface, in spite of the growth conditions, is attributed to the buried missing material trapped during film growth.