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 nm after excimer-laser annealing (ELA) at 1100 mJ/cm2. However, anomalous behavior is witnessed for BF2+-implanted Si sample at 800 mJ/cm2, at which energy very high surface protrusions up to 9 nm 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.