Nucleation of boron clusters in implanted silicon

Laser-assisted wide angle tomographic atom probe was employed to investigate clustering of boron in highly doped implanted silicon (B-11, 10 keV, 5x10(15) atoms/cm(2)) at room temperature and at different annealing temperatures (600 degrees C/h, 800 degrees C/h, and 900 degrees C/5 h). The implantation profile shows a maximum of 10(21) atoms/cm(3) at a distance close to 30 nm. The evolution of microstructural features (cluster and matrix composition, cluster size, molar fraction of clusters, and density) was studied as a function of depth. As expected, the number density of clusters shows a maximum (2x10(18) and 1.7x10(18) cm(-3) for 600 and 800 degrees C, respectively) at the implantation peak where the driving force for nucleation is the highest. As expected, the overall number density of clusters decreases when increasing temperature (lower supersaturation). The boron concentration in clusters as well as that in the parent boron-depleted phase was found to follow the same trend as the implantation profile. The boron level in clusters was found higher close to the implantation peak (8.9 at. % at 600 degrees C, 12 at. % at 800 degrees C). The increasing concentration of boron in clusters as a function of temperature suggests the clusters are metastable nuclei of a transient phase, the composition approaches that of equilibrium borides as given by phase diagram (SiB3). This equilibrium boride SiB3 is detected after a heat treatment at 900 degrees C for 5 h. Experiments were confronted to predictions as given by classical nucleation theory. Reasonable agreement was observed.