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

Ultrashallow p(+)-n junctions fabricated in Si(111) are investigated by low - and intermediate-energy electron-beam probing of the surface region in or der 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 wit h regard to how the irradiation-induced conductivity depends on the energy of the primary electron beam, and also its distribution with area. Using th is method, it is possible to determine how the probability of an electron-h ole 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 domina ted by the kick-out or dissociative-vacancy mechanisms. It was found that f or boron in silicon the kick-out type of diffusion mechanism is strongly en hanced in the [111] crystallographic direction,whereas diffusion in the [10 0] direction is primarily driven by vacancy mechanisms. It is shown that co llection of nonequilibrium carriers in the p(+)-n junction field is strongl y enhanced when the diffusion profile consists of certain combinations of l ongitudinal and transverse quantum wells. (C) 1999 American Institute of Ph ysics. [S1063-7826(99)01301-0].