Surface morphologies of excimer-laser annealed BF₂⁺ implanted Si diodes

Laser-induced surface roughness and damage formation in ultra-shallow n⁺-p and p⁺-n junctions, formed by low energy (5 keV) As⁺ and BF₂⁺ implantations in Si, respectively, with a dose of 1 × 10¹⁵ cm^-2 have been investigated by atomic force microscopy (AFM) and Positron Annihilation Doppler Broadening (PADB) technique. The Si surface roughness is found to increase with laser energy density, and reaches a value of 3.5 rm after excimer-laser annealing (ELA) at 1100 mJ/cm². However, anomalous behavior is witnessed for BF₂⁺-implanted Si sample at 800 mJ/cm ², at which energy very high surface protrusions up to 9nm high are observed. By PADB this behavior is correlated to extensive deep microcavity formation in the Si whereby the volatile F₂ fraction can accumulate and evaporate/out-diffuse, leading to Si surface roughening. The consequences for the diode characteristics and contact resistivity are examined.