Stress Evolution during Ge Nanoparticles Growth in a SiO₂ Matrix

Superstructures are explored that were obtained by multilayer magnetron deposition at room temperature of 20 SiO₂ and SiO₂:Ge bilayers, each 2 × 4 nm thick, and subsequently annealed in inert N₂ atmosphere at different temperatures in the range of 500-750 °C. The structural and optical changes induced by annealing and the formation and growth of Ge nanoparticles (nps) from early clusters to their full growth and final dissolution were studied by the simultaneous grazing-incidence small- and wide-angle X-ray scattering, transmission electron microscopy, and (time-resolved) photoluminescence (PL). It is shown that in as-deposited multilayers aggregation of small clusters already occurred, and the clusters were reasonably well intercorrelated in the lateral plane. During annealing at T_a = 550 °C or higher temperatures, Ge nps start to form and remain partly amorphous at lower T_a but crystallize completely at about 600 °C. At even higher temperatures, the Ge nps dissolve and Ge diffuses out almost completely, leaving voids in the SiO₂ matrix. Visible PL from the samples was detected and attributed to defects in the nps/matrix interface layers rather than to the nps itself because PL persisted even after Ge nps dissolution.