GENERATION OF DIVACANCIES IN SILICON BY MEV ELECTRONS - DOSE-RATE DEPENDENCE AND INFLUENCE OF SN AND P

Silicon samples doped with tin and/or phosphorus have been irradiated at room temperature by 2.0 MeV electrons in order to study the generation of divacancy centers (V2). The samples were analyzed using low-temperature infrared absorption measurements and deep level transient spectroscopy. The production of V2 is found to be very small in the tin-doped samples, which suggests that the generation process hinges strongly on the existence of mobile monovacancies. Substitutional Sn atoms act as efficient traps for migrating vacancies and form stable vacancy-tin centers with a corresponding reduction in the formation of "ordinary" vacancy-related defects. Indeed, at temperatures above approximately 150-degrees-C where the vacancy-tin centers dissociate, a pronounced increase in the V2 concentration occurs. On the other hand, measurements of the V2 concentration in the non tin-doped samples as a function of electron dose using different dose rates reveal that the steady-state concentration of monovacancies during irradiation has no influence on the generation of V2. Thus, pairing of two single vacancies created by different bombarding electrons can be excluded as a major formation mechanism for V2. Evidence is presented for a new model where the formation of V2 is predominantly due to migration and subsequent agglomeration of two single vacancies created directly (electron-atom collision) and indirectly (recoiling atom-atom collision), respectively, by the same impinging electron. Finally, the presence of substitutional phosphorus atoms in high-purity float zone Si samples is shown to enhance the production of V2. The experimental data exhibit excellent relative agreement with computer simulations assuming a defect reaction model where annihilation of vacancy-phosphorus centers by Si self-interstitials is taken into account, i.e., the role of phosphorus is to provide an alternative recombination center for the Si self-interstitials with a corresponding growth of surviving V2 centers.