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

Radiation-enhanced diffusion, or RED, has been conventionally studied under the conditions of steady state and homogeneous background of excess defects. Hence MeV ion irradiation and diffusion annealing were conducted simultaneously and the temporal and spatial dependences of the diffusing parameters were ignored. This review covers a new 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 to create defects, (ii) evaporation of the diffusing materials onto the surface, followed by (iii) diffusion annealing. Using such a sequence, the diffusion region directly overlaps with the central region of the ion implantation profile. Ti diffusion in ion pre-irradiated MgO(100) was selected as a model diffusion system, ions of Ar+, Ne+, Kr+, Cl+ and Cr+ were used for irradiation and diffusion was conducted in an inert atmosphere. Secondary ion mass spectroscopy (SIMS) was used to depth-profile the diffusing materials. A phenomenological model based on the concept of depth-dependent diffusion coefficients was developed to quantify the NSRED results. Monte Carlo (TRIM) simulations were used to 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 annealing effect, 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 depend on the ballistic behaviours of the irradiating ions, as in conventional RED, but on the chemical properties of the ions as well. This effect is consistent with a modified vacancy-clustering model. The results indicate that NSRED is a promising technique for modification of the optical and mechanical properties of oxides through manipulation of doping ion diffusion behaviours in a well-controlled manner. (topical review)