Evolution of vibrational modes of SiO2 during the formation of Ge and Si nanocrystals by ion implantation and magnetron sputtering.

Structural variations of SiOx matrix have been studied with Fourier Transform Infrared Spectroscopy (FTIR) during the formation of Si and Ge nanocrystal. Two frequently used methods, magnetron sputtering and ion implantation have been employed to form SiOx matrix containing excess Si and Ge. The Si-O-Si stretching mode has been deconvoluted to monitor the evolution of SiOx films during the annealing process. The integrated area and the shift in the SiOx peak positions are found to be well correlated with the change of the film stoichiometry and nanocrystal formation. It is shown that the nonstoichiometric SiOx matrix turns into stoichiometric SiO2 as the excess Si and Ge atoms precipitate to form nanocrystals. This process takes place at much lower temperatures for Ge than Si for both ion implantation and magnetron sputtering. FTIR technique is shown to be useful to study the matrix hosting nanocrystals to monitor nanocrystal formation.

Formation of Ge nanocrystals in Al2O3 matrix.

Ge nanocrystals were formed in Al2O3 matrix by implantation of Ge ions into sapphire (alpha-Al2O3) substrates and subsequent annealing. Diagnostic techniques, Raman spectroscopy, XRD, TEM, EDS, and SAED were employed to monitor and study formation of Ge nanocrystals and their evolution during heat treatments. TEM and EDS analysis revealed the diffusion of Ge ions into the substrate during annealing process. While Ge nanocrystals with mean sizes of 15 nm were observed in the heavily implanted region small nanocrystals with mean sizes of 4 nm were identified underneath this region. Some grains of transition aluminas were formed in the implanted region which was amorphized during the implantation. Extensive stress between the transition aluminas and sapphire matrices and its effects on the matrix were detected. The effect of stress on the Raman and XRD spectra of Ge nanocrystals was discussed.