Size tunable strain and interfacial engineering of germanium quantum dots

Spherical Ge quantum dots (QDs) of desire sizes and locations within the Si-containing layers has been demonstrated using thermal oxidation of SiGe nanopillars over buffer Si₃N₄ on the Si substrate. Local environment material of SiO₂ imposes large stress onto the Ge QD, and the compressive strain progressively increases from 0.5 to 4% with a decrease in the QD size from 110 to 60nm. Such a high pressure of 1-4.5 GPa makes a striking transition in the crystalline structure from indirect bandgap diamond to direct bandgap tetragonal for Ge as evidenced by a significant blue shift in the LO phonon Raman lines and diffraction patterns (DPs), when decreasing the QD size. Remarkably, an opposite, red shift of the Raman lines is observed from the Ge QDs when removing the encapsulated layer of SiO₂ from the QDs, indicating the QD experiencing a size-dependent tensile strain. This indicates that strain engineering on the Ge QDs is tunable by the QD size and their interactions with local environments.