Credit: © 2008 AIP

Quantum dots are potential building blocks for future nanoscale electronic devices, and the controlled formation of arrays is a critical step towards such applications. Nanoscale arrays of germanium quantum dots have been successfully grown on patterned silicon surfaces, which act as a template and are produced using focused ion-beam implantation. However, the quantum-dot coverage is usually low over large areas of the substrate, and the mechanism responsible for focused-ion-beam-initiated growth is poorly understood. Now, Maria Gherasimova and co-workers1 at Virginia University, Rensselaer Polytechnic Institute and the T.J. Watson Research Center have uncovered the factors limiting the synthesis of such large scale arrays.

Using a transmission electron microscope, the team examined the nucleation of germanium islands on silicon surface templates as the distance between the target positions in the array was reduced. The positioning of the germanium quantum dots in their intended sites had a success rate of almost 100% for sites separated by 100 nm or more. For sites positioned closer together, however, partial occupancy was observed.

The researchers found that the competition between neighbouring target sites for the supply of germanium adatoms is the key issue that limits the formation of precise arrays over a large area.