It was noted that the solid phase epitaxy of Er-implanted amorphous material resulted in the segregation and trapping of Er, and led to the incorporation of up to 2 x 1020/cm3 into monocrystalline material. Segregation occurred in spite of an extremely low Er diffusivity (less than 10-17cm2/s) in bulk amorphous material. A narrow segregation spike (with a measured width of about 3nm) suggested that kinetic trapping was responsible for non-equilibrium concentrations of Er. The dependence of trapping upon temperature, concentration, and impurities instead indicated that thermodynamics controlled the segregation. It was proposed that Er, by analogy with transition metals, diffused interstitially in amorphous Si, but was strongly bound at trapping centers. The binding enthalpy of these trapping sites caused the amorphous phase to be energetically favorable for Er so that, at low concentrations, the Er was almost completely segregated. However, when the concentration of Er in the segregation spike exceeded the amorphous trap center concentration, more Er was trapped in the crystal. A similar segregation and trapping behavior was exhibited by Pr.

J.S.Custer, A.Polman, H.M.Van Pinxteren: Journal of Applied Physics, 1994, 75[6], 2809-17