It was noted that radiation-enhanced diffusion was usually studied under conditions of a steady state and an 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 radiation-enhanced diffusion, i.e. non steady-state radiation-enhanced diffusion. The sequence of steps in non steady-state radiation-enhanced diffusion were: keV-ion irradiation of the substrate to create defects, evaporation of the diffusing materials onto the surface, followed by diffusion annealing. Using such a sequence, the diffusion region directly overlapped with the central region of the ion implantation profile. Here, Ti diffusion into ion pre-irradiated MgO(100) was chosen 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 was used to depth-profile the diffusing materials. A phenomenological model based upon the concept of depth-dependent diffusion coefficients was developed in order to quantify the non steady-state radiation-enhanced diffusion results. Monte Carlo simulations were used to model the implantation. Compared to conventional radiation-enhanced diffusion, vacancy clusters, rather than excess mono-vacancies, were the predominant contributors to non steady-state radiation-enhanced diffusion; resulting in 2 unique observations. The first was a post-irradiation annealing effect, i.e. annealing a pre-irradiated substrate enhanced the subsequent diffusion. This was due to the key roles of vacancy clusters in the diffusion enhancement. The second was that the enhanced diffusion did not depend only upon the ballistic behaviours of the irradiating ions, as in conventional radiation-enhanced diffusion, but on the chemical properties of the ions as well. This effect was consistent with a modified vacancy-clustering model. The results indicated that non steady-state radiation-enhanced diffusion was 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.

Radiation-Enhanced Diffusion under Conditions of Non Steady-State and Non-Homogeneity of Excess Defects. M.Lu, C.Lupu, J.W.Rabalais: Journal of Physics – Condensed Matter, 2004, 16[18], R581-602