Self-assembled SiGe quantum rings grown on Si(001) by molecular beam epitaxy

SiGe quantum rings (QRs) were grown by partially capping on Ge quantum dots (QDs) on Si(001). Atomic force microscopy images show the shape transformation from QDs to QRs. Initial capping, with a Si layer thickness less than 2 nm, will result in the decrease of height of QDs and increase of base diameter of QDs. Capped with a Si layer, QDs will change into QRs. The mechanism of transformation from QDs to QRs is discussed. The strain will redistribute after capping, thus the strain energy relief, together with high Ge surface diffusion and Ge surface segregation at a relative high temperature of 680 °C, play the dominant role.