Light-emitting Si quantum dots formed by pulsed annealing

Pulsed anneals are of high practical importance as they enable one to perform heat treatment locally while affecting insignificantly the adjacent regions. This is particularly advantageous for the processing of low dimensional devices. The ongoing drive for smaller devices coupled with the discovery of strong luminescence from quantum-sized Si nanocrystals (Si-ncs) has spurred extensive research into the processes of their fabrication. Nowadays a decomposition of supersaturated solid solution of Si in SiO2 layers is mostly employed for this purpose. 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 clusters, and subsequent crystallization. Our work is an attempt to address the dependence of the light-emitting Si nanostructure formation on the length of the annealing light pulses. Implantation of Si ions in thin thermally grown SiO2 layers followed by intense light pulse annealing was used to form Si quantum dots. The ion doses provided the excess Si concentrations of about 10-15%. KrF excimer laser pulses, flash lamp annealing and rapid thermal annealing were used for the post-implantation heat treatments. The pulse durations were 20 ns, 20 ms and 1 s, respectively. Studies were carried out using photoluminescence excited at 20 °C by a N2 laser (λ = 337 nm). Due to the high temperatures resulting from intense light pulses, the different stages in the formation of light-emitting Si nanostructures may occur quite rapidly. Treatment for times as short as 20 ns is already sufficient for the segregation of the excess Si and the formation of low dimensional clusters that emit light in the visible range (400-600 nm). For 20 ns laser pulses of no formation of Si-ncs occurs, which is evidently associated with the insufficient growth time. However, if one creates in advance amorphous Si nanoprecipitates, it is possible to form Si-ncs by pulsed laser processing. The crystallization 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 low dimensional particles. Formation of the luminescing Si-ncs is feasible under the 20 ms and 1 s intense light pulses. In this case the temperatures never exceeded Si melting point. Comparison of the Si-ncs formation rate and the estimates of the expected diffusion-limited grain growth yields diffusivity of the excess Si in SiO2 that are several orders larger, than the values obtained in experiments using stationary annealing. Possible mechanism of Si-ncs formation is discussed.