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Si0.6Ge0.4 nanocrystals, of diameter <5 nm, embedded in SiO2 in the form of single layers (2.1 × 1012 nanoparticles cm&-2) and five-period multilayers (above 1013 nanoparticles cm&-2) have been fabricated using a low-thermal-budget process consisting of deposition by low-pressure chemical vapor deposition and crystallization by rapid thermal annealing at several temperatures and for different times. The crystallization process was monitored by Raman spectroscopy and transmission electron microscopy. The loss of integrity and compositional changes of the nanoparticles during the annealing process were characterized by Rutherford backscattering spectrometry. During the annealing process, crystallization and Ge out-diffusion have been observed to compete with each other. Annealing of samples with nanoparticles of 4.6 nm diameter at low temperature (750°C) yields poor crystallization of the nanoparticles and causes the Ge to leave them by a pure diffusive mechanism, thus destroying their integrity. At higher temperatures (≥800°C), crystallization takes place in a short period of time (<30 s) and diffusion from the crystallized material is initially hindered. For samples with nanoparticles of 3.3 nm diameter, partial crystallization is detected at 800°C and 900°C and the crystalline quality is improved in both cases as the annealing time increases. Also, the detection capabilities of the Raman spectroscopy system for the detection of a certain density of SiGe nanocrystals of given diameter and composition have been explored and the lower limit estimated.