Ultrashallow p+-n junctions in Si(111):: electron-beam diagnostics of the surface region

Ultrashallow p(+)-n junctions fabricated in Si(111) are investigated by low- and intermediate-energy electron-beam probing of the surface region in order to determine how the crystallographic orientation of the silicon films affects the mechanisms for nonequilibrium diffusion of boron. A comparative study is made of p(+)-n junctions made on both (111) and (100) silicon with regard to how the irradiation-induced conductivity depends on the energy of the primary electron beam, and also its distribution with area. Using this method, it is possible to determine how the probability of an electron-hole pair being separated by the electric field of the Si(111) and Si(100) p(+)-n junctions varies with depth into the crystal, which experiments show is different, depending on whether diffusive motion of impurities is dominated by the kick-out or dissociative-vacancy mechanisms. It was found that for boron in silicon the kick-out type of diffusion mechanism is strongly enhanced in the [111] crystallographic direction,whereas diffusion in the [100] direction is primarily driven by vacancy mechanisms. It is shown that collection of nonequilibrium carriers in the p(+)-n junction field is strongly enhanced when the diffusion profile consists of certain combinations of longitudinal and transverse quantum wells. (C) 1999 American Institute of Physics. [S1063-7826(99)01301-0].