Strain-compensated Ge/Si1−xCx quantum dots with Si mediating layers grown by molecular beam epitaxy

We fabricated 20-layer-stacked Ge/Si1−xCx quantum dots (QDs) by solid source molecular beam epitaxy, for the realization of Si-based, strain-compensated Ge QDs. We inserted 2-nm-thick Si mediating layers between the Ge QDs and the Si1−xCx strain-compensating spacer layers (SCLs) and evaluated their influence on the structural and optical properties of the obtained system. High density QDs with a size fluctuation of approximately 14% were successfully maintained independent of the number of stacked layers in the case of QDs with Si mediating layers, while the QD size monotonically increased with a number of stacked layers in the case of QDs without Si mediating layers. The sheet density of QDs with Si mediating layers reached 1.2×1011 cm−2. A strong photoluminescence (PL) emission with a line-width of 96.7 meV was obtained for the QDs with Si mediating layers. The improvement of the aforementioned properties was caused by both the absence of C bonds at the surface and the improved surface roughness obtained by introducing Si mediating layers. The QD size increased for the 20-layer-stacked Ge/Si QDs samples grown with 10-nm-thick spacer layers, due to the accumulation of internal strains. By contrast, the size distribution of QDs was almost constant for the range of spacer layer thicknesses used in the present work (between 10 nm and 40 nm), implying that the tensile-strained Si1−xCx SCLs compensated a certain fraction of the strain field created by the compressive-strained Ge QDs.