Insights into solid phase epitaxy of ultrahighly doped silicon

Abstract

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 (CB) up to a critical CB∗ 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 CB, 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 CB>CB∗ 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).