Transitions Between Ordered and Disordered Solid-Melt Patterns Formed on Silicon by Continuous Laser Beams: Competition Between Electrodynamics and Thermodynamics

Abstract

The melting of a semiconductor is usually accompanied by a discontinuous change in linear optical properties. If then a light beam is used to melt a semiconductor the accompanying change in optical properties is a highly nonlinear function of intensity. For example, for silicon at 10µm, the reflectivity of the (dielectric) solid is approximately 0.3 up to the melting temperature (T_m = 1685K). However, uniformly molten (metallic) silicon has a reflectivity which is approximately 0.9. As silicon is heated from just below the melting point to just above it with a small increase in laser intensity, the dramatic change in reflectivity will prevent a uniform molten or solid state from existing since neither state is consistent with the amount of absorbed power. The nonlinear changes in reflectivity accompanying the melting transition therefore leads to a lateral or transverse instability, in which a uniform incident beam will cause a transversely inhomogeneous state to form on the surface.^1–3