Metal-rich Au-silicide nanoparticles for use in nanotechnology
(Result of the month 04/2010)

We present a route to functionalize chemically and magnetically silicon surfaces by a local passivation, taking advantage of Stranski–Krastanov growth mode of the Au–Si(111) system. Metal-rich Au-silicide nanoparticles, supported on a Si-rich two-dimensional Au-silicide layer, are obtained. Subsequently deposited Co is used to form magnetic nanostructures. The two Au silicides display a different chemical reactivity with Co enabling the fabrication of localized magnetic Co nanodots. These magnetic nanostructures can be aligned along step bunches of a vicinal Si(111) surface. By varying the growth parameters, the particle density can be tuned from 109 to the low 1012 dots/in.2

STM image of a Au–Si particle, of 3.5 nm height, in the derivative mode, I=0.1 nA, V=+1 V
The topographic STM image in Fig. 1 shows a typical faceted island after Au deposition (1.5 ML, 400 °C). It is truncated by a hexagonal facet on top and consists of six facets that are inclined by about 35° from the [111] direction toward the [11-2] direction of the substrate. A mesh of atoms with a centered unitcell of 0.79 x 0.83 nm2 (white arrows) was observed on the tilted facets.

a) STM image of a vicinal Si(111) surface miscut by 1.5° along [11-2] direction obtained at room temperature after UHV heat treatment, parallel step bunches are separated by equally sized terraces. b) STM image I=0.5 nA, V=+2 V, taken after deposition of 3.5 ML Au at 370 °C. Au-rich islands are aligned along the step-bunches with a density of 2.5*1011 in.−2. Images sizes are 500 nm2.

The periodic arrangement of the islands is obtained through Au deposition onto self-organized Si templates. Vicinal Si(111) reorganizes under heat treatment and transforms into a 1D nanopattern consisting of equally sized terraces separated by step-bunches. Tuning the growth parameters enables the self-alignement of Au silicidesd along the step-bunch edges.

Superstructures observed for Au deposition on Si(111). Covergae corresponds to 0.3 ML in (a) and 0.7 ML in (b). A zoom into the structures is shown in (c) and (d).