Self-assembling formation of silicon quantum dots with a germanium core by low-pressure chemical vapour deposition

The formation of nanometre-scale silicon dots with a germanium core on an ultrathin SiO2 layer has been studied by controlling the early stages of low-pressure chemical vapour deposition (LPCVD) alternately using pure monosilane and 5% germane diluted with helium. From atomic force microscope observations and x-ray photoelectron spectroscopy measurements, the selective growth of Ge on pregrown Si dots and subsequent complete coverage with a Si cap have been confirmed. Cross-sectional transmission electron microscope images have shown the formation of isolated spherical nanocrystallites with Ge cores in contrast with hemispherical pregrown Si dots, implying a high structural strain at the interface between cladding Si and the Ge core. For multiply stacked structures of the dots with a Ge core, Raman-scattering spectra indicate that compositional mixing occurs partly at the Si/Ge-core interface during LPCVD.