The cathodic deposition of iron-cobalt alloys inside the pores of anodically formed nanoporous silicon (PS) from p-type Si substrate is investigated with respect to the electrolyte composition. The samples were characterized by scanning electron microscopy, energy dispersive spectrometry, Auger electron spectroscopy, and Fourier transform infrared spectroscopy. Results showed that the nucleation of pure cobalt started at the bottom of the pores and the nucleation of pure iron occurred all over the pore walls, leading to a preferential deposition on top surface of the porous layer. Nevertheless, a low concentration of Co 2+ ions (5 at.%) in the electrolyte drastically improved the penetration of iron into the pores. As a result, a good filling of the pores with Co metal as well as with Fe-Co alloys was achieved. It was also shown that the deposition process oxidizes the structure mainly at the pore walls. The results of our investigation indicate that the mechanisms occurring during the electrodeposition of metals on porous p-type silicon substrates are completely different depending on the kind of electrolyte used: pure iron-based electrolyte or cobalt-based solutions. A complete understanding of the deposition process requires further analyses of the carrier transport in PS and of the charge exchange at the Si/electrolyte and PS/electrolyte interfaces. These new results involving the deposition of iron-group materials into porous p-type silicon are useful for future silicon technologies. a) Present address: Département des Matériaux et Composants,

Porous silicon (PS) is characterized by a very large surface that is very reactive with the external environment. In this work, the chemical composition of the internal surface of both freshly anodically formed and electroplated PS with Fe and Co metals was studied. The samples were analyzed by scanning electron microscopy (SEM), Fourier transform infrared absorption spectroscopy (FTIR) and Auger electron spectroscopy (AES) combined to sputter depth profiling. Mesoporous structures with different morphologies were obtained in p-type Si(100) anodized in ethanoic hydrofluoric acid solutions. We showed that the surface of as-prepared PS is practically oxide-free and H-terminated, whereas the metal deposition process oxidizes the pore walls. Nevertheless, silicon atoms from PS surface are likely bonded to iron and cobalt ones.

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This work reports on the use of the cyclic voltametry technique to study the electrochemical behavior of porous p+‐type silicon (PS) contacted by aqueous solutions containing Fe2+ and Co2+ electro‐actives species. Current‐potential (I‐V) measurements were performed on both flat and porous silicon substrates and different Fe2+ concentrations were considered. With flat Si, similar I‐V characteristic behaviors were obtained in 0.1 M Fe2+ and 0. 1 M Co2+ solutions, except that iron can be deposited from a potential more cathodic than cobalt. With porous Si, different I‐V characteristic behaviors were observed and a diffusion‐like current peak was detected in the presence of Fe2+ species. The observed current peak is located near the standard redox potential of iron. However, its intensity is lower than that of Fe2+ ions diffusion current peak and it does not increase as Fe2+ concentration increases. A three‐step carrier transport mechanism was proposed to show that the current peak could be attributed to iron nucleation on the pore walls via electroless deposition. Accordingly the difference observed in PS I‐V characteristics was attributed to the fact that electroless deposition on pore walls occurs at a potential for which the reduction of Co2+ ions is faster than that of Fe2+ ones and it can also occur at the pore bottom via electrodeposition. These results are used to explain the poor pore filling rate with iron, reported elsewhere. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)