Unexpected features of the formation of Si and Ge nanocrystals during annealing of implanted SiO2 layers: Low frequency Raman spectroscopic characterization

The present paper reports a study of the influence of heat treatments on the ion-beam synthesis of Si and Ge nanocrystals in SiO2 layers by low-frequency Raman scattering. Low-frequency Raman scattering is used just because the appearance in the glass matrix of crystal nuclei leads to an additional contribution to the density of only low-frequency acoustic vibrational states due to surface vibration modes of the nuclei. Electron microscopy, contrary to expectations, revealed a decrease rather than increase in the size of the crystal nucleus during annealing. Additionally, low-frequency Raman spectra show that the samples do not have a smooth distribution of nanoparticle sizes, as expected, but two different sizes of Si and Ge nanocrystals. This similarity is surprising because Si and Ge have different diffusion coefficients, temperatures of crystallization, meltings, and binding energies. Despite this, in both cases the same mechanism operates during the growth of Si and Ge nanocrystals.