Insights into solid phase epitaxy of ultrahighly doped silicon

In this study we investigate the mechanisms of growth and boron (B) incorporation into crystalline silicon (c-Si) during crystallization of amorphous doped silicon (a-Si:B) films. The process developed consists of two steps, first the chemical vapor codeposition at low temperature of Si and B atoms to form a-Si:B layer and second the crystallization of amorphous phase during in situ annealing to incorporate boron atoms on the substitutional sites of c-Si. We find that the crystallization rate linearly increases with the nominal boron concentration ( C B ) up to a critical C B ∗ which corresponds to the maximum concentration of electrically active boron atoms in the crystalline phase. In these conditions, an increase in the crystallization rate by a factor 22 as compared to the intrinsic crystallization rate is obtained. We suggest that this remarkable behavior is attributed to D + charged defects associated to the activated doping atoms in agreement with the generalized Fermi level shifting model. For larger C B , further boron atoms are incorporated in the amorphous phase in the form of ultrasmall clusters that do not contribute to shift the Fermi level of a-Si. As a consequence, for C B > C B ∗ the crystallization rate does not increase any more. We also show that crystallization provides a more complete incorporation of boron atoms already present in a-Si than the codeposition of Si and B atoms in the same experimental conditions (same growth rate and temperature). This result is attributed to the lower kinetic segregation at the amorphous-crystalline (a/c) interface than at the vacuum-crystalline interface. The lower kinetic segregation results from both a higher diffusion barrier of boron atoms at the a/c interface and a lower segregation energy (due to a low a/c interface energy).