Microscopic analysis of thermally-driven formation of Cu-Si alloy nanoparticles in a Cu/Si template

Selective thermal diffusion of Cu into a 100-nm-thick SiO₂-patterned Si(001) substrate was investigated to elucidate the spontaneous formation of Cu-Si alloy nanoparticles. Transmission electron microscopy and energy dispersive X-ray spectroscopy provided the indirect evidence for the formation on the substrate's surface of nanoparticles that served as a catalyst to grow SiO₂ nanowires selectively in window regions. The microstructural analysis revealed that thermal annealing caused selective diffusion of Cu into the Si matrix in window regions only and that the Cu-Si alloy nanoparticles were formed at 900 °C although the diffusion of Cu into Si was already significant at 700 °C. The nanoparticles that were sparsely distributed below the surface of the Si matrix did not serve as a catalyst for growing SiO₂ nanowires, and the chemical composition analysis showed that the nanoparticles at the tip of SiO₂ nanowires were Cu₃Si.