Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Measurements of Doppler-broadening of annihilation radiation from variable-energy positrons have been applied to examine the nature and distribution of defects in ion-irradiated silicon. Positron measurements were supplemented by ion backscattering/channeling to determine displaced atom distributions, and infrared absorption measurements to determine divacancy concentrations. Silicon (100) wafers were irradiated at 300K with helium ions at energies from 0.25 to 4.0 MeV and fluences from 10{dollar}\sp{lcub}13{rcub}{dollar} to 10{dollar}\sp{lcub}16{rcub}{dollar} cm{dollar}\sp{lcub}-2{rcub},{dollar} and with silicon ions at energies from 0.5 to 5.0 MeV and fluences from 10{dollar}\sp{lcub}11{rcub}{dollar} to 10{dollar}\sp{lcub}15{rcub}{dollar} cm{dollar}\sp{lcub}-2{rcub}.{dollar} Defect annealing was observed through the divacancy annealing stage {dollar}(\sim{dollar}470 to 570K). He-irradiated silicon was restored toward single crystal quality as measured by both infrared and positron methods. For the same anneal, Si-irradiated silicon shows partial restoration of crystallinity (RBS), and removal of the optically-active divacancies, but no change in positron trapping characteristics. Annealing to between 870 and 970K restores the crystal to near pre-implant characteristics. Results are discussed in terms of existing models of damage production during ion irradiation.;Variable-energy positron methods have also been applied to study silicon layers grown by molecular beam epitaxy at low temperatures. The epilayers contain voids of {dollar}\sim{dollar}6 mm diameter, which constitute efficient positron traps. The densities and sizes of voids in the epilayers were determined by transmission electron microscopy. Extremely narrow positron annihilation lineshapes were measured, and attributed to the formation of positronium within the voids. Measurements of the fraction of positrons trapped by voids are compared with the predictions of diffusion-limited trapping theory.;The scope and limitations of defect profiling with variable-energy positrons and suggestions for further development of the technique are discussed.



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