Strain-mediated uniform islands in stacked Ge/Si(001) layers

The role of a Si spacer layer in improving the uniformity of multilayer coherent Ge quantum dots on Si(001) is investigated using scanning tunneling microscopy. Deposition of the Si spacer layer at room temperature and subsequent annealing lead to the formation of (001)-oriented Si mesas on top of Ge islands (pyramids and huts). One mesa forms when the island beneath is pyramid-like, whereas a few mesas. form when the island beneath is an elongated hut. The mesas are found to be preferential nucleation sites for the Ge islands in consecutive stacked layers. The vertical-pyramid-replicating and hut-breaking processes play key roles in the improvement of island size, shape and spacing uniformity.