Segregation and trapping of Er during solid-phase crystallization of amorphous Si on crystalline Si is studied in a concentration range of 1016–5×1020Er/cm3. Amorphous surface layers are prepared on Si(100) by 250 keV Er ion implantation, recrystallized at 600 °C, and then analyzed using high-resolution Rutherford backscattering spectrometry using 2 MeV He+or 100 keV H+. The segregation coefficientkdepends strongly on Er concentration. At Er interface areal densities below 6×1013Er/cm2nearly full segregation to the surface is observed, withk=0.01. At higher Er densities, segregation and trapping in the crystal are observed, withk=0.20. The results are consistent with a model in which it is assumed that defects in thea-Si near the interface act as traps for the Er. ©1997 American Institute of Physics.