Effect of Si-spacer thickness on optical properties of multistacked Ge quantum dots grown by rapid thermal chemical vapor deposition

Multistacked Ge quantum dots (QDs) with Si spacers of different thicknesses have been grown on (100) Si wafer by rapid thermal chemical vapor deposition and characterized by photoluminescence (PL) and Raman scattering. High-resolution transmission electron microscopy images demonstrate that the Ge QDs are vertically ordered and fully contrasted with the Si spacer. Two major PL emission bands observed from the QDs are attributed to their no-phonon (NP) and transverse-optical phonon replica and are redshifted with increasing the Si-spacer thickness (d). As the temperature increases, the NP PL peak is redshifted when d is 59 nm, but it is blueshifted when d is 76 nm. As d decreases, two Raman peaks of Ge-Ge and Si-Ge modes are shifted to lower frequency, while the peaks of Si-Si modes at 416 and 520 cm-1 remain almost unchanged. It is suggested that these optical properties are strongly related with the correlation between the QDs and the Ge-Si intermixing.