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Quantitative TEM and STEM study of Pt-nanoparticles coarsening and Ge(Mn)-based ferromagnetic semiconductors

Published on 12 July 2013
Thesis presented July 12, 2013

In this work, different system have been studied using transmission electron microscopy (TEM) methods: Pt nanoparticles (NPs) on amorphous carbon, Ge quantum dots (QDs), Mn incorporation in Ge QDs and GeMn nanocolumns (NCs) embedded in Ge or GeSn matrix. The coarsening of Pt NPs on amorphous carbon film was studied by high resolution TEM (HRTEM) after annealing at temperatures between 200°C and 300°C for periods of up to 160 hours. A significant increase of the average particle size is observed with increasing annealing time for all investigated temperatures. An in-situ annealing experiment reveals two coarsening stages. The first coarsening stage is dominated by Smoluchowski ripening whereas the second coarsening stage is dominated by surface Ostwald ripening. The Arrhenius-type dependence of the derived surface mass-transport coefficients yields an activation energy Ed = 0.84 ± 0.08 eV/atom for the surface diffusion of Pt atoms on an amorphous carbon substrate. Advanced TEM methods have be used to obtain direct determination of composition profiles with atomic resolution and large field of view by correlation of high angle annular dark field (HAADF) and electron energy loss spectroscopy (EELS) signals. This method was used to obtain a direct and precise quantification of Ge concentration at the atomic level for the SiGe system. The Ge concentration profile along the growth direction was explained by Ge surface segregation during the growth with a two-state exchange model. The incorporation of Mn in Ge QDs have been performed by molecular beam epitaxy (MBE) growth of GeMn. At growth temperature of 380°C, SiMn precipitates are formed. Lowering the growth temperature at 220°C allows limiting the lateral segregation of Mn in Ge and incorporating Mn in Ge QDs. Absolute chemical composition by STEM-EELS evidenced that the total atomic density in Ge(Mn) NCs is almost two times higher than in the Ge matrix. Structural analysis by HRTEM shows that the crystalline NCs exhibit a high degree of disorder. Experimental observation can be model with a modified α-phase structure if variants are introduced to cancel reflexions and Ge atoms are substituted by Mn atoms. The structural and magnetic properties of GeSnMn films grown on Ge(001) by low temperature MBE (LTMBE) have been studied. Like in Ge(Mn) films, Mn atoms diffuse during the growth and aggregate into vertically aligned Mn-rich NCs of a few nanometers in diameter. TEM observations in plane view clearly indicate that the Sn incorporation is not uniform with concentration in Mn rich vertical NCs lower than the EELS detection limit. The matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn NCs. The magnetization in GeSnMn layers is higher than in GeMn films. This magnetic moment enhancement in GeSnMn is probably related to the modification of the electronic structure of Mn atoms in the NCs by the Sn-rich shell, which is formed around the NCs.

Ge(Mn) nanocolumns, Pt nanoparticles, Coarsening, Hrtem, Stem, Ferromagnetic semiconductors

On-line thesis.