Slow and fast positron studies of defects created in silicon by swift Kr ions

Abstract

Radiation damage of silicon single crystals of (100) and (111) crystal orientation have been measured following irradiation with 246 MeV Kr 7+ ions. Due to the implantation depth of 28 μm, both slow positron beam and conventional positron methods were feasible and were used to study the defects created by the swift heavy ions. The fluence dependence was investigated in the 5×10 11 –1×10 14 ions cm −2 range. We compared the positron beam results with lifetime, Doppler broadening and coincidence Doppler measurements using conventional 22 Na sources. Simultaneous analysis of all measured data show that in all implanted samples silicon divacancy represents the dominant positron trap. Increasing positron trapping is indicated with decreasing temperature, which feature we attribute to negatively charged vacancies. We studied orientation dependence of Doppler parameters in unimplanted silicon substrate and demonstrate the art and technique of using the angle dependence to get defect specific information. The determination of the `zero anisotropy point' offers also means to pinpoint which kind of defect is responsible for the positron trapping. We show that the anisotropy of the electron momentum in the divacancies created by the implantation is small compared to the anisotropy in the silicon lattice.