The coexistence of silicon borides with boron-saturated silicon: Metastability of SiB₃

The silicon-rich end of the Si-B phase diagram, defining the silicon boride(s) that coexist in equilibrium with boron-saturated silicon, is poorly known. Understanding this equilibrium has implications for the processing of p⁺ silicon wafers, whose boron concentrations are near the solubility limit. Additionally, silicon boride precipitates produced by boron-ion-implantation and annealing of crystalline silicon have recently been shown to be efficient internal getters of transition metal ions. The experiments described in this paper probe the stability of these silicon borides. A phase with a boron-carbide-like structure, SiB₃, grows from boron-saturated silicon in both the solid and the liquid state. However, SiB₃ is not found to be stable in either circumstance. Rather, SiB₃ is a metastable phase whose formation is driven by the relative ease of its nucleation and growth. The silicon boride that exists in stable equilibrium with boron-saturated silicon is SiB₆. A qualitative understanding of the metastability of SiB₃ comes from recognizing the conflict between the bonding requirements of icosahedral borides such as SiB₃ and the size mismatch between silicon and boron atoms.