Radiation defect dynamics in Si at room temperature studied by pulsed ion beams

Abstract

The evolution of radiation defects after the thermalization of collision cascades often plays the dominant role in the formation of stable radiation disorder in crystalline solids of interest to electronics and nuclear materials applications. Here, we explore a pulsed-ion-beam method to study defect interaction dynamics in Si crystals bombarded at room temperature with 500 keV Ne, Ar, Kr, and Xe ions. The effective time constant of defect interaction is measured directly by studying the dependence of lattice disorder, monitored by ion channeling, on the passive part of the beam duty cycle. The effective defect diffusion length is revealed by the dependence of damage on the active part of the beam duty cycle. Results show that the defect relaxation behavior obeys a second order kinetic process for all the cases studied, with a time constant in the range of ∼4–13 ms and a diffusion length of ∼15–50 nm. Both radiation dynamics parameters (the time constant and diffusion length) are essentially independent of the maximum instantaneous dose rate, total ion dose, and dopant concentration within the ranges studied. However, both the time constant and diffusion length increase with increasing ion mass. This demonstrates that the density of collision cascades influences not only defect production and annealing efficiencies but also the defect interaction dynamics.

The role of Frenkel defect diffusion in dynamic annealing in ion-irradiated Si

“…Also plotted in Fig. 3 is the T dependence of the DA efficiency ξ 1,2 , which we define as before refs [7, 8]: ξ  = ( n (0) −  n ∞ )/ n (0). Figure 3 shows that, within experimental errors, ξ increases with T , reflecting a corresponding decrease in n ∞ , which is also clearly seen in n(t off ) dependencies of Fig.…”

“…The large difference between the E a values measured in the present work and those in previous dose rate studies234 could be attributed to the fact that, in the dose rate approach, the E a is effectively extracted from the ξ(T ) dependence. As discussed in detail recently8, for our choice of F on and t on , ξ is the magnitude of the dose rate effect; i.e., the difference between n for continuous beam irradiation with dose rates of F  =  F on and F  → 0. Hence, ξ reflects the fraction of ballistically-generated Frenkel defects that participate in DA processes for any given F on rather than the rate of defect interaction.…”

“…The depth profile of ballistically-generated vacancies was calculated with the TRIM code (version SRIM-2013.00)16 with an atomic concentration of Si of 4.98 × 10 22  atoms cm −3 and a threshold energy for atomic displacements of 13 eV. The pulsing parameters F on and t on were chosen based on previous pulsed beam measurements of Si at room T 78 in order to maximize the DA efficiency and to limit the inter-cascade defect interaction within each pulse. The adjacent pulses were separated by time t off , which was varied between 0.2 and 50 ms.…”

“…1(a) shows a schematic of the time dependence of the instantaneous dose rate and defines pulsing parameters t on , t off , and F on . A more detailed description of the experimental arrangement can be found elsewhere78.…”

“…Here, we use a novel pulsed ion beam technique78910 to measure the T dependence of the DA time constant ( τ ) in Si bombarded with 500 keV Ar ions in a regime of relatively high ion doses when damage accumulation is dominated by inter-cascade DA processes (i.e., by the interaction of mobile defects generated in different collision cascades). Our results reveal two well-defined regions in the Arrhenius plot of the DA rate, with very different E a s of 73 and 420 meV, below and above ~60 °C, respectively.…”