Influences of Si spacer layers on the structures of Ge/Si quantum dot bilayers

Abstract

The influences of the Si caplayer or spacer layer on the microstructures of the Ge/Si(0 0 1) quantum dots (underneath or on the top of the Si thin layer) are studied by synchrotron radiation X-ray techniques (reflection, diffraction and grazing incident diffraction (GID)) and atomic force microscope (AFM). It is found that the Ge composition varies with the dot size in the freestanding Ge dots sample with a bi-modal size distribution. When a Si caplayer is deposited on the Ge dots, the Ge quantum dots are laterally compressed by the surrounding Si lattice. The strain of the Ge dots spreads into the caplayer and reaches to the surface even if the caplayer is as thick as 70 nm. It is confirmed that this residual strain induced by the buried Ge dots improves the size uniformity of Ge dots grown on the Si caplayer (or spacer layer) surface, so that a uniform size distribution of the Ge dots is observed by AFM and the corresponding strain is measured by GID. However, for a Si caplayer with a thickness larger than 90 nm, no residual strain from the buried quantum dots was observed near the sample’s surface. In this case, the dots grown on the surface show the same features as that grown on a Si substrate wafer (i.e. bi-modal size distribution), so that no strain-mediated influence of the buried dots takes place.