Properties of Silicon Nanoparticles Produced by Picosecond Pulsed Laser Ablation
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Properties of Silicon Nanoparticles Produced by Picosecond Pulsed Laser Ablation
M. H. Wu, R. Mu, A. Ueda and D. O. Henderson Chemical Physics Laboratory Fisk University Nashville, TN 37208 Abstract Silicon nanocrystals have been fabricated by picosecond pulsed laser ablation. Size control over the nanocrystals can be achieved by careful selection of experimental geometry, laser fluence, laser wavelength, backing gas pressure and distance from plume center. Measurements of optical absorption and photoluminescence confirm that particle size variation does significantly affect bandgap and emission efficiency. Differences between results published previously for nanosecond pulsed laser ablation will also be discussed. Introduction Interest in silicon nanostructures has remained high since the discovery of strong light emission in porous silicon. Research in this area is motivated by both basic questions concerning the nature of carrier dynamics in quantum confined structures and the enticing vision of silicon based photonic devices. Silicon nanocrystals have been produced by a variety of methods, including colloidal chemical techniques, implantation of Si ions into dielectric hosts, strain induced island growth using molecular beam epitaxy or chemical vapor deposition and pulsed laser deposition. The latter technique has several key advantages: low probability of contaminants (the only materials needed in the deposition chamber are the target and the substrate), ability to deposit nanocrystals onto virtually any substrate, ability to control the size of the nanocrystals, and no damage. Reports of formation of silicon nanocrystals by pulsed laser deposition has mainly been limited to nanosecond or longer pulsed lasers. Earlier studies with nanosecond pulses have shown that nanocrystals are reliably produced using energy densities of 1 – 10 J/cm2 and backing gas pressures of 1 – 10 torr. Nanocrystals produced with these pulse parameters have diameters ranging from 1 to 10 nm and relatively small size dispersions. Recent work by Marine et al [1], shows that nanocrystal size can be controlled by varying the laser pulse energy and the distance away from the center of the plume. Other works have shown some effects on average nanocrystals size with gas pressure and target to substrate distance. Despite this progress, relatively little is understood concerning the mechanism of nanocrystal formation during pulsed laser ablation. Numerical modeling based on hydrodynamic theory has been able to reproduce experimental measurements of plume density in silicon ablated by nanosecond pulses, but predictions based on nucleation theory concerning particle formation do not agree well with the experimental data [2]. Additional experiments, including those that illustrate effects of varying the laser pulse duration, are needed to advance the current understanding of the mechanism behind nanocrystal formation.
F13.2.1
Ablation with picosecond pulses provides considerable physical differences from ablation with nanosecond pulses. Shorter
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