The researchers began by defining the most important parameters for molecular beam epitaxy growth of strictly ordered germanium dots on pit-patterned silicon substrates. They then showed that these growth parameters are closely linked and need to be adjusted with respect to each other for optimal growth. Indeed, the initial pit shape and size, as well the growth conditions of the Si buffer layer, have to be adjusted to provide suitable preconditions – or a solid foundation - for the growth of Ge quantum dots with the desired size, composition and nucleation position.

The team also showed that the two-dimensional Ge wetting layer between pits can act as a stabilizer that prevents the dots from changing shape and inhibits the formation of dislocations in ordered dots. These findings allow perfectly ordered and homogeneous Ge dots to be fabricated on one and the same sample, even if the pit-period is varied from a few hundred nanometres to several microns.

Finally, by showing that the growth of InAs dots on GaAs substrates can also be controlled in this way, the researchers say that many aspects described in their work might be of great use when growing ordered epitaxial quantum dots made from other materials, such as different group III-V semiconductors.

More details of the work can be found in the journal Nanotechnology.