Formation and distribution of Au nanoparticles within Si lamellae of the Au-Si eutectic microstructure

Eutectic solidification is a self-driven phase separation process that enables the straightforward synthesis of complex micro- and nanostructures in a single step. However, impurities incorporated into the phases during coupled growth can affect the material properties, posing challenges for its practical application. This study investigates the formation and incorporation of Au nanoparticles (NPs) within Si lamellae formed by the melting and solidification of Au-Si thin films. Our findings show that faster cooling rates of the eutectic melt increase the Au concentration in the Si lamellae beyond its solubility limit. We found a homogeneous distribution of Au NPs throughout the Si bulk, and additional bands of NPs, 2-3 times larger in size than in the bulk, aligned along Σ3 {111} twin boundaries (TBs). We present a theoretical model for Au NPs formation where Au clusters nucleate during solidification in the near-surface liquid layer on the solidification front (SF), serving as sinks for Au atoms and forming the NPs. The advancing SF traps these NPs within the solid Si, creating the homogeneous particle distribution inside the Si lamellae. Au diffusion in defect-free solid Si is slow, even at the eutectic temperature, making Au NPs coarsening in the bulk negligible. However, the activation energy for Au diffusion along TBs is lower than in the bulk, accelerating growth and coarsening along the TBs, producing bands of larger NPs. This study provides a new understanding of the mechanisms governing the formation and incorporation of low-soluble impurity NPs in eutectic microstructures and their evolution along TBs.