Radiation-enhanced diffusion under conditions of non-steady state and non-homogeneity ofexcess defects

Abstract

Radiation-enhanced diffusion, or RED, has been conventionally studied under theconditions of steady state and homogeneous background of excess defects. Hence MeV ionirradiation and diffusion annealing were conducted simultaneously and the temporal andspatial dependences of the diffusing parameters were ignored. This review covers anew type of RED, i.e. non-steady-state radiation-enhanced diffusion or NSRED.The sequence of steps in NSRED are (i) keV ion irradiation of the substrate tocreate defects, (ii) evaporation of the diffusing materials onto the surface, followedby (iii) diffusion annealing. Using such a sequence, the diffusion region directlyoverlaps with the central region of the ion implantation profile. Ti diffusion inion pre-irradiated MgO(100) was selected as a model diffusion system, ions ofAr+,Ne+,Kr+,Cl+ and Cr+ were used for irradiation and diffusion was conducted in an inert atmosphere. Secondaryion mass spectroscopy (SIMS) was used to depth-profile the diffusing materials. Aphenomenological model based on the concept of depth-dependent diffusion coefficients wasdeveloped to quantify the NSRED results. Monte Carlo (TRIM) simulations were usedto model the implantation. Compared to conventional RED, vacancy clusters,rather than excess mono-vacancies, are the dominant contributors to NSRED,resulting in two unique observations. The first is a post-irradiation annealingeffect, i.e. annealing a pre-irradiated substrate enhances the subsequent diffusion.This is due to the key roles of vacancy clusters in the diffusion enhancement.The second is a chemical effect, i.e. the enhanced diffusion does not only dependon the ballistic behaviours of the irradiating ions, as in conventional RED, buton the chemical properties of the ions as well. This effect is consistent with amodified vacancy-clustering model. The results indicate that NSRED is a promisingtechnique for modification of the optical and mechanical properties of oxides throughmanipulation of doping ion diffusion behaviours in a well-controlled manner.