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Buffer layers Au(111) / H-Si(111)

by Rosso Michel - published on , updated on

Participants : P. Allongue, R. Cortès and F. Maroun

PhD student : P. Prod’homme (2005)

Gold electrochemical growth on a vicinal H-Si(111) (Fig. 1a) leads to strong epitaxy Au(111)/Si(111) if deposition is conducted at – 2VMSE. The nucleation of the deposit depends however strongly on the gold plating composition. In KAuCN pH 14 one obtains gold nm-islands which are decorating the silicon steps (Fig. 1b). In HAuCl4 pH 4 the nucleation is homogeneous and a continuous atomically smooth Au(111) film is obtained (Fig. 1c). No grain boundary is visible. The flat buffer layers are used to investigate ferromagnetic layers and the seld-organized islands can be used as template to grow magnetic dots.

Figure 1 : AFM images of a vicinal H-Si(111) surface obtained by anisotropic etching in 40% NH4F (a), of a gold deposit realized with a KAuCN pH 14 solution (b) and a HAuCl4 pH 4 solution (c). In (b) the islands are decorating the silicon steps. Note the flatness of the deposit in (c). Steps are monatomic gold steps.

The above difference between the nucleation modes may be rationalized by recalling that a strong hydrogen evolution reaction (HER) occurs in parallel with gold deposition. On the molecular scale, proton reduction (Fig. 2a) induces the desorption of H atoms, which creates preferential adsorption sites for Au adatoms. At this Ph nucleation is homogenous (Fig. 2b, top) since the HER is not site selective. A continuous film is rapidly obtained. The co-reduction of protons seems to favor interlayer mobility of Au on Au. At Ph 14, the HER is, by contrast, wrongly site selective because it requires decomposing water molecules. As a result Au/Si nucleation is highly site dependent at the Si step edges (Fig. 2b, bottom). In absence of a strong HER, a granular deposit is obtained at pH 4.

Figure 2 : (a) Schematic reaction for the HER on H-Si(111); (b) Schemes showing thelocalization of Au adatom adsorption when HER is occurring at the Si surface at pH 4 et 14.

 P. Prodhomme, S. Warren, R. Cortès, H. Jurca, F. Maroun , P. Allongue, Mechanism of Gold Epitaxial growth on H-Si(111): The determining role of hydrogen evolution, ChemPhysChem, 11 (2010) 2992-3001
 P. Prod’homme, F. Maroun, R. Cortès, P. Allongue, Electrochemical growth of ultra flat Au(111) epitaxial buffer layers on H-Si(111), Appl. Phys. Letters 93 (2008) 171901
 P. Allongue et F. Maroun, “Self-ordered electrochemical growth at single electrode surfaces,” J. Phys. Cond. Mater.18 (2006) S97
 M. L. Munford, R. Cortès et P. Allongue, “Electrochemical growth of gold on well defined vicinal H-Si(111) surfaces studied by AFM and XRD”, Surf. Sci., 537 (2003) 95 – 112.