Improvement of photoluminescence efficiency in stacked Ge/Si/Ge quantum dots with a thin Si spacer

In this work, two successive layers of Ge quantum dots separated by a thin Si spacer grown by ultra-high-vacuum chemical vapor deposition were demonstrated. With an optimal thickness of the thin Si spacer, the sandwiched Ge (13.1 ML)/Si (28 ML)/Ge (13.1 ML) quantum dots suppress the coarsening of Ge quantum dots and efficiently increase the uniformity of the quantum dots. Cross-sectional transmission electron microscopy shows that the Si/Ge underlayers provide preferred nucleation sites for the overlayer Ge deposition. Such a modification avoids the formation of Ge superdomes and prevents the occurrence of threading dislocations even at such a thin Si spacer thickness. A stronger photoluminescence intensity of these sandwich Ge/Si/Ge quantum dots was observed compared with that of 26.2 eq-ML Ge quantum dots without using any intermediate Si layer. Furthermore, the narrower width of the photoluminescence spectra indicates that the Ge/Si/Ge dots are more uniform compared with the 13. 1 and 26.1 eq-ML Ge quantum dots. Five-fold bilayers of Ge/Si/Ge/Si(150 ML) have been achieved to enhance the photoemission efficiency.