Ion-Beam-Induced Self-Organisation of Nanostructures at Interfaces

Ion irradiation through an interface between the phases A and B causes atomic displacements which results at low temperatures in a diffusion-like concentration profile. Even if phases A and B are immiscible, a metastable layer of an A/B mixture forms at high ion fluence. A subsequent thermal treatment will activate phase separation in this A/B mixture via nucleation and coarsening. This phase separation process has the potential of self-organisation of nanostructures, where the resulting nanostructure can be tailored by understanding and controlling the reaction pathway.
(i) At first, in this presentation it will be shown how the ion beam mixing of a flat infinite interface can be simulated with the SRIM and TRIDYN programs.
(ii) Then, by means of 3D kinetic lattice Monte-Carlo simulations it will be demonstrated how a thermally activated phase separation of the A/B mixture starts either by formation of nuclei of the minority phase or by spinodal decomposition.
(iii) Simulations for long times show that the subsequent nanostructure evolution is driven by interface minimization, i.e. Ostwald ripening of nanocluster ensembles or coarsening of spinodal structures.
At this stage, a self-organisation process governed by Brailsford's diffusional screening length can evolve, which can be eventually controlled. The A/B interface which re-forms during phase separation plays a central role for self-organisation and self-alignment of nanostructures.
These general mechanisms are effective in ion beam mixing of a thin SiO2 layer buried in Si with the following observations: (i) Zones denuded of Si form during annealing at the upper and lower interface. (ii) Additionally, three, two or one layer of Si nanoclusters form and align with the interface. (iii) If only a tiny volume ~(10nm)^3 of metastable SiOx (such as in an ion beam mixed nanopillar of a Si/SiO2/Si stack) becomes phase separated, the reaction pathway leads always to the existence of a single Si dot for a rather long time period.
This single Si nanodot fabrication becomes even more stable if all boundaries of the tiny SiOx volume are sinks for Si diffusing in SiO2, which can be realized by sideways in-diffusion of oxygen into the nanopillar.
Finally it will be shown, how such a single Si nanodot fabrication process can be used for manufactoring of single electron transisrors working at room temperature.