Carbon-induced germanium dots: Kinetically-limited islanding process prevents coherent vertical alignment

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Pregrowth of a small amount of C on a Si(001) substrate induces small Ge quantum dots. We present a structure where an initial layer of these C-induced Ge dots is followed by five layers of four monolayers Ge, each layer separated by a 2 nm thick Si spacer. Although the critical thickness for planar growth of a single Ge layer is not exceeded, a vertically aligned stack of Ge islands is formed. If we substitute the pure Ge layers by a fivefold stack of C-induced Ge dots, no vertical island correlation is observed. The phenomenon is explained by the strongly kinetically limited process of C-induced Ge island formation, itself. Photoluminescence experiments on stacks of 0.16 monolayers C/2.2 monolayers Ge dots, where we systematically varied the Si spacer thickness and the number of dot layers, suggest that the average dot size increases if the Si spacer is kept thinner than 10 nm.