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

Pregrowth of a small amount of C on a Si(001) substrate induces small Ge qu antum dots. We present a structure where an initial layer of these C-induce d Ge dots is followed by five layers of four monolayers Ge, each layer sepa rated 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 st ack of C-induced Ge dots, no vertical island correlation is observed. The p henomenon is explained by the strongly kinetically limited process of C-ind uced Ge island formation, itself. Photoluminescence experiments on stacks o f 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 aver age dot size increases if the Si spacer is kept thinner than 10 nm. (C) 199 8 American Institute of Physics. [S0003-6951(98)03331-2].