Local molten regions formation on silicon surfaces by laser annealing at the melting threshold

Nanosecond (NLA) and microsecond laser annealing (μLA) are promising techniques for contact formation, crystallization, or dopant activation in semiconductors, especially when integrating with thermal budget limitations. When exposed to NLA or μLA, at the boundary between solid-phase and liquid-phase processing conditions, Si or Ge surfaces can be roughened by the formation of local molten regions (LMRs). In this work, Si surfaces were irradiated after various preparations using NLA and μLA, to better understand the formation of LMRs. During the exposure of a Si surface to NLA, LMRs nucleate at the Si/surface oxide interface through the formation of a liquid Si droplet and then extend in depth and laterally. Due to the anisotropy of the melting rate, the LMRs’ edges are close to a square or an equilateral triangle on (100) and (111) surfaces, respectively, with edges along <110> crystalline directions. In the case of (100) Si surfaces exposed to ultraviolet NLA, the molten volume forms a square-based inverted pyramid, truncated at 20-30 nm depth. Upon solidification, some topography is introduced to the initial flat surface. On (100) Si surfaces, the smallest LMRs are monocrystalline pyramids of 12 to 25 nm height, while the largest ones exhibit a castle-like shape with 4 peaks. The presence of a thin surface oxide is mandatory to obtain LMRs with a geometrically well-defined base, and the thickness of this oxide impacts the shape and size of the LMRs. The evolution of the LMRs size as a function of the annealing duration suggests that thermal diffusion limits their extent. • Nanometric local molten regions (LMRs) generated on Si surfaces by nanosecond laser annealing (NLA) at the melting threshold. • LMRs shape evolves from a square-based pyramid to a castle-like nanostructure with four peaks. • The size and shape of LMRs are strongly influenced by the surface oxide thickness, the crystalline orientation and the annealing duration. • The maximum molten depth is approximately 20 nm when using a 160 ns UV laser pulse. • Extra laser pulses duplicate LMRs at the outermost corners of existing LMRs.