ON LIGHT SUPPRESSED DIVACANCY FORMATION AT PHOTON-ASSISTED ION-IMPLANTATION OF SILICON

The phenomenon of divacancy formation at photon assisted (PA) ion impl antation of silicon is modeled. Photon assistance implies a fraction o f free electron-hole pairs To be generated additionally to that of the ion beam produced. In an n-type silicon crystal, vacancies are assume d to be in an electrically neutral or singly-negative charge states. A vacancy changes the charge state by the capture of an electron or a h ole. Mathematically, the conventional equations which describe the for mation of secondary vacancy complexes from neutral monovacancies are m odified to account for two charge states of the vacancies and the spat ial redistribution of the vacancies in nonmonotonic intrinsic force fi elds inherent to ion implantation. The equations for the vacancy conce ntrations are accompanied by kinetic equations for the concentration o f nonequilibrium carriers. Qualitative analysis of the dependence of t he average vacancy charge and the vacancy flux in an implanted layer u pon the carrier concentration allows us to explain the available exper imental data on the reduced divacancy concentration in silicon implant ed by ions with simultaneous photoexcitation of the crystal electron s ubsystem. It is shown that photon irradiation provides the accumulatio n oi negatively charged vacancies in certain regions of the silicon. A s a consequence, the Coulomb barrier decreases the formation of divaca ncy complexes from these vacancies. (C) 1997 Elsevier Science B.V.