We have studied the compositional mixing and the crystallinity of Si dots with Ge core (similar to20 nm in average dot diameter), which were prepared on thermally grown SiO2 by highly selective low-pressure chemical vapor deposition (LPCVD), as a function of annealing temperature in the range of 540-1000 degreesC. Raman scattering spectra of multiply stacked structures consisting of dots with Ge core and 2 nm-thick SiO2 interlayers indicate that compositional mixing occurs partly at Si/Ge interface during the sample preparation, where the sample was heated up to 600 degreesC. Also, 800 degreesC annealing promotes Si-Ge alloying between Si clad and Ge core, and degrades the crystallinity. By 950 degreesC annealing, SiO2 reduction with diffused Ge atoms to form volatile GeOx units becomes significant, resulting in the generation of Si-Si bonds. Additionally, XPS analysis of annealed single layer structures confirms the incorporation of Ge atoms into Si clad from Ge core and the formation of GeOx at the Si clad surface. (C) 2003 Elsevier B.V. All rights reserved.