Growth and optical properties of Ge/Si quantum dots formed on patterned SiO2/Si(0 0 1) substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO2 windows patterned by electron-beam lithography on oxidized Si (001) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [113]-faceted pyramids, which reduces the area of the topmost (001) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.