EXAFS and CEMS investigation of the local structure of Sn implanted SiO2

The ion beam synthesis (IBS) of nanoparticles in amorphous matrix has received great attention in the last years as a promising technique for nanocrystal formation in insulating layers. Metallic and semiconducting nanoclusters embedded in SiO2 have been recognized as potential materials for the production of memory devices and optoelectronic components. Sn nanoclusters has been proposed, together with Si and Ge nanoclusters, as potential candidates for charge storage. One of the critical point for the technological application of nanocrystals is that they must be isolated and similar in size. In addition, their position and spacing, their structure and stability after formation should be controllable.
In this work we address the formation of Sn nanoclusters by ion implantation and thermal treatments in thin SiO2 films (< 100 nm), where the cluster formation mechanism is not well understood yet. The local structure of Sn in SiO2 has been addressed by 119 Sn CEMS and EXAFS. EXAFS and CEMS analyses provided unique information on the local atomic and electronic environment of Sn in SiO2.
Sn 2+ and Sn 4+ oxidized phases or the Sn 0 metallic phase have been found under different annealing conditions. By comparing these data with TEM and RBS analyses we concluded that metallic beta-Sn phase is related to big Sn crystalline clusters formed after annealing, while the oxidized Sn is due to atoms dissolved in the matrix or in small precipitates of SnO2 or SnOx. In addition, the formation mechanism of Sn nanoclusters may be influenced by H2O absorption in the highly damaged SiO2 film after ion implantation, and by in-diffusion of moisture during the annealing. Low energy ion implantation (10–15 keV) showed the possibility for a better control of the cluster size distribution and positioning inside the SiO2 films. This may be relevant for the application of this kind of nanoclusters for charge storage applications in memory devices. Preliminary electrical characterization confirmed the memory effects in MOS capacitors with Sn nanocrystals embedded in the gate oxide.