Structural and optical properties of three-dimensional Si _1-xGe_x/Si nanostructures

Steady-state and time-resolved photoluminescence (PL) combined with x-ray and Raman measurements have been performed on a series of well-characterized Si_1-xGe_x/Si superlattice samples with an island-like morphology and with precise control over the alloy chemical composition in the range 0.091 ≤ x ≤ 0.61. In the samples with x increasing from 0.091 to 0.53, an increase in the intensity of the Raman signal related to Ge-Ge vibrations correlates with a red shift in the PL peak position and an increase in the activation energy of the PL thermal quenching. Time-resolved PL measurements reveal two PL components with relaxation times of a microsecond and up to 10 ms, respectively. The highest PL quantum efficiency observed (better than 1% at low temperature) is found in the samples with x ≈ 0.5 where carrier recombination presumably occurs at sharp Si/Si_1-xGe _x interfaces which exhibit type-II band alignment, with a small (of the order of several milli-electron volts) barrier for electrons and deep potential wells for holes localized within Ge-rich Si_1-xGe _x islands. In the samples with Ge concentration close to 0.61, we observe a strong, step-like increase in strain and significant evidence of strain-induced Si/Ge interdiffusion resulting in a decrease of the PL quantum efficiency.