Fabrication of light-emitting Si quantum dots by pulsed annealing of Si-rich SiO2 layers

Thin Si-rich SiO2 layers have been prepared by implantation of 100 to 190 keV Si ions in thermally grown oxide films. The ion doses provided the excess Si concentration of about 10-15%. KrF excimer laser pulses, flash lamp annealing and rapid thermal annealing were used for the post-implantation heat treatments to form light-emitting Si quantum dots. The pulse durations were 20 ns, 20 ms and 1 s, respectively. Studies were carried out using a spectroscopy of PL excited at room temperature by a nitrogen laser (λ = 337 nm). The excess Si atoms in SiO2 are not free to migrate, but are rather incorporated into the oxide network. The fabrication of Si-ncs necessitates segregation of the Si atoms, availability of sinks to which these atoms may diffuse, availability of Si-phase nucleation centers, growth of the incipient nanoprecipitates, their crystallization, and, finally, the Ostwald ripening Due to the high temperatures resulting from intense light pulses, the different stages in the formation of light-emitting Si quantum dots may occur quite rapidly. Treatment for times as short as 20 ns is already sufficient for the segregation of the excess Si in the SiO2 atomic network and the formation of quantum dots that luminesce in the visible portion of the spectrum (400-600 nm). For laser pulse duration of 20 ns no formation of nanocrystals occurs, which is evidently associated with the insufficient growth time. However, if one creates in advance amorphous Si precipitates of the right size, it is possible to form quantum-size Si nanocrystals by pulsed laser processing. This process occurs most likely via melting rather than in a solid phase, favored by the release of latent heat and the reduction in temperature of melting of the nanoparticles. Photoluminescence band near 800 nm, typical to quantum-size Si nanocrystals, was found after laser annealing. Formation of the luminesing Si nanocrystals in Si-rich SiO2 layers is feasible under the influence of intense light pulses of 20 ms and 1 s duration. In this case the SiO2 layer has not been heated above the Si melting point. Comparison of the nanocrystals formation rate and the estimates of the diffusion-limited growth yields diffusivity of the excess Si in SiO2 that are substantially larger than the values obtained in experiments using stationary annealing. The discrepancies may be explained by the existence of a transient mechanism of rapid growth at the very beginning of the pulsed annealing.