Nucleation and Growth of Li Precipitates in Si

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Abstract
Precipitation of excess Li in solution in Si monocrystals, preannealed at temperatures of 750°–1340°C, has been measured resistiometrically. Infrared transmission microscopy shows precipitate particles are spherical or nearly so. The precipitates are thermodynamically similar to the equilibrium phase, Li2Si, but the data suggest LiSi. Excess Li content decreases exponentially with time and is Li-diffusion limited; Ham's theory is valid in this system. The number of particles is found to span ∼2×1010 to ∼3×1013/cc over the preanneal temperature range in low O content, ∼5×1015/cc, Si. Nucleation occurs at oxygen-vacancy pairs, OV, which form reversibly at elevated temperatures with binding energy of −0.56 eV; kinetic sluggishness prevents equilibrium at temperatures below about 800°C. A model whereby prenucleation Li+ ion segregation occurs about OV− ions due to electrostatic attraction is examined. Under experimental conditions, a diffuse cloud of roughly 75 Li+ ions could exist at 600°C, and could condense within 1 sec at 100°C. An estimate of precipitate-matrix interfacial energy is about 0.14 eV/site, which in conjunction with nucleation thermodynamics, predicts critical nucleus sizes of 3–39 atoms depending on choice of crystal habit plane.