Light-emitting Si nanostructures formed in Si implanted SiO2 layers by pulsed anneals

Thin thermally grown SiO2 layers were implanted with 100 to 190 keV Si ions. 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 analyze their possibilities to synthesize the light-emitting Si nanocrystals (Si-ncs) and therewith the mechanism of Si-ncs formation . The pulse durations were 20 ns, 20 ms and 1 s, respectively. Studies were carried out using the photoluminescence spectroscopy, excited at 20 oC by a nitrogen laser (λ = 337 nm). The formation of Si-ncs necessitates segregation of the Si atoms from the oxide network, availability of sinks to which these atoms may diffuse, availability of Si-phase nucleation centers, growth of the incipient nanoprecipitates, and, finally, their crystallization. Treatment for times as short as 20 ns is already sufficient for the segregation of the excess Si and for formation of nanostructures that luminesce in the visible range (400-600 nm). For laser pulse duration of 20 ns no formation of Si-ncs occurs. However, if one creates in advance amorphous Si nanoprecipitates, it is possible to form the luminescing Si-ncs by the 20-ns pulses. This process occurs most likely via melting, favored by the release of the latent heat. Formation of the luminescing Si-ncs is feasible under the 20-ms and 1-s light pulses. In these cases the SiO2 layer has not been heated above the Si melting point. Comparison of the Si-ncs formation rate with their conjectural diffusion-limited growth suggests the existence of a transient non-diffusional mechanism at the beginning of the pulsed annealing.