engleski

Nanostructuring of a Silicon Surface by Laser Redeposition of Si Vapor

Report on the surface nanostructuring of silicon wafer by self-organization of redeposited Si nanoparticles, at various energy levels, in the vaporization regime of laser-matter interaction. By using the semiconfined configuration, a quasitwo-dimensional turbulent Si vapor field with gradients of pressure and temperature is formed. The turbulent field evolves into point vortices which condense into Si nanodroplets. At a low laser energy of ~ 1.2 J (0.23 GW/cm2), the inertial instability of nanodroplets under gradients of pressure and temperature, cause their intermittent accumulation in the low pressure regions of turbulent field. The solidification of Si nanodroplets into particles and their redeposition, cause a simple two-dimensional low density nanostructuring of Si wafer in the near periphery region, and a high density nanostructuring in the periphery region of the spot. The pattern of redeposited Si nanoparticles in these regions is equivalent to the pattern of point vortices in a two-dimensional turbulent field. Such a pattern of point vortices is obtained by numerical simulation from the two-dimensional Navier-Stokes equation for forced turbulence. The self organization of the coherent point vortex pattern is generated by numerical simulation of the solitary turbulence model based on the nonlinear Schrödinger equation. At the high laser energy of ~ 1.5 and ~ 2.0 J (~ 0.42 and ~ 0.52 GW/cm2, respectively), the transition from simple intermittent two-dimensional nanoparticle organization into a continuous and more complex one, takes place. The nanostructured pattern shows a continuous distribution of Si particles, whose size increases from the periphery towards the center without spatial intermittency, showing a gradient of particle size. In addition, the open- and closed-loops chain clusters appear, with morphology and fractal dimension similar to the chain clusters which grow according to the Meakin-Jullien model of cluster-cluster aggregation. At the higher power density of ~ 0.52 GW/cm2, the chain clusters become connected and tend to compactification. They form a network similar to the one obtained by numerical simulation of two-dimensional turbulence at small Stokes numbers. The silicon surface nanostructured by recondensation in this case comprises only the nanometer sized particles.

laser vaporization ; silicon nanostructuring ; nonlinear dynamics

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