Optical Properties of Composition-Controlled Three-Dimensional Si/Si $_{1 - x}$Ge$_{x}$ Nanostructures

We report the Raman, continuous-wave (CW), and time-resolved photoluminescence (PL) measurements in a series of multilayer Si/Si
_1-x
Ge
_x
samples with an island-like morphology and precise control over the chemical composition in the range of 0.096 les x les 0.61. In the samples with x continuously increasing from 0.096 to 0.55, an increase in the intensity of the Raman signal related to the 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 1-10-ms PL components. The highest observed PL quantum efficiency (better than 1% at low temperature) is found in the samples with x~0.5, where the carrier recombination presumably occurs at sharp Si/SiGe interfaces that exhibit type-II band alignment, with a small (to the order of several milli-electron volts) barrier for electrons and deep potential wells for the holes localized within the Ge-rich Si
_1-x
Ge
_x
islands. In the samples with Ge concentration close to 0.61, we observe a strong, step-like increase in the strain, and a significant evidence of strain-induced SiGe interdiffusion that results in the decrease in the PL quantum efficiency