Generation of vacancy-type point defects in single collision cascades during swift-ion bombardment of silicon

Abstract

Silicon samples of n-type have been implanted with low doses (${10}^{7}$-${10}^{10}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$) of $^{11}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{28}\mathrm{Si}$, $^{74,76}\mathrm{Ge}$, and $^{120}\mathrm{Sn}$ ions using energies between 0.4 and 8.0 MeV. Because of the low doses employed, single-collision cascades prevail, and an analysis of the implanted samples by deep-level transient spectroscopy (DLTS) reveals two dominant point defects of vacancy type, the divacancy (${\mathrm{V}}_{2}$) and the vacancy-oxygen (VO) center. The generation of ${\mathrm{V}}_{2}$ and VO is studied in detail as a function of ion mass, dose, dose rate, sample depth, and sample temperature. Surface-enhanced annihilation of migrating defects is found to play a significant role in depleting the concentration of ${\mathrm{V}}_{2}$ and VO centers in proximity to the surface. The concentration of ${\mathrm{V}}_{2}$ and VO increases linearly with ion dose, but the generation efficiency per incoming ion decreases at high dose rates after displaying a constant value below \ensuremath{\sim}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. This dose rate effect exhibits a dependence on ion mass, and is qualitatively predicted by computer simulations of the defect reaction kinetics, applying a model where the interaction between individual collision cascades is primarily due to fast-diffusing Si self-interstitials. At dose rates \ensuremath{\leqslant}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, the generation of ${\mathrm{V}}_{2}$ centers per ion-induced vacancy in the damage peak region is identical, within \ifmmode\pm\else\textpm\fi{}10%, for all the different types of ions studied. However, the ${\mathrm{V}}_{2}$ centers formed by heavy ions are strongly perturbed, as shown by large deviations from a one-to-one relation between the two DLTS peaks associated with the singly and doubly negative charge state of ${\mathrm{V}}_{2}$, and also by a broadening of the two peaks. In contrast to that for ${\mathrm{V}}_{2}$, the generation of VO centers per ion-induced vacancy decreases with increasing ion mass, consistent with the picture that light ions are more effective in generating point defects than heavy ions. At elevated implantation temperatures, gradual relaxation of lattice strain occurs together with recrystallization of disordered zones, promoting the formation of unperturbed ${\mathrm{V}}_{2}$ centers and increasing the generation efficiency of VO centers.