Local Arsenic Structure in Shallow Implants in Si following SPER: An EXAFS and MEIS study

Solid phase epitaxial regrowth (SPER) has been investigated in the last few years as a possible method to form ultra shallow dopant distributions in silicon with a high level of electrical. Despite the interest for this process, few investigations were related to arsenic. Apart from the fact that it is easier to form shallow distribution with arsenic than with boron, it is also well known that at the moderate temperatures implied by SPER (500-700°C) arsenic easily deactivates, probably by forming inactive clusters around point defects in silicon. In order to have a better understanding of the SPER process for arsenic implanted silicon in shallow regime, an EXAFS (extended x-ray absorption fine structure) and MEIS (medium energy ion scattering) study is reported in this paper. Silicon samples were implanted at 3 keV with arsenic ions (dose was 2E15 at/cm² producing a 11 nm amorphous layer) and then annealed in nitrogen at temperatures ranging from 500 to 700°C to have different levels of recrystallisation. From the comparison of the recrystallised fraction as measured by MEIS with the electrical activation measured by Hall effect it results evident that a full regrowth of the lattice is not reflected by a high electrical activation. The activated arsenic corresponds to less than one third of the apparently substitutional dopant for all the samples analyzed. This lack of activation was further investigated by EXAFS: the samples that according to MEIS are fully recrystallised do not reveal a clear local order around As atoms suggesting that either the As atoms are not yet completely relocated within the lattice sites or a deactivation occurred resulting in a more disordered local structure.