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Ideal H-terminated Si(111) surface by anisotropic etching

par Rosso Michel - publié le , mis à jour le

Participants : P. Allongue, R. Cortès, C. Henry de Villeneuve

Silicon dissolution in NaOH may be described on the molecular scale by the reaction shown in Fig. 1. In situ STM observations have confirmed the formation of Si-OH sites and electrochemical characterizations have established that the first step may actually follow two routes : a purely chemical route, which is highly anisotropic for steric reasons and an electrochemical route, which is relatively isotropic. Therefore, conditions favoring the chemical reaction and minimizing the electrochemical reaction lead to surfaces presenting (111) planes.

Our investigations of anisotropic etching of Si(111) in buffer NH4F has shown the following : (i) dissolved oxygen is responsible for triangular pit initiation on (111) terraces on a large scale (Fig. 1a) ; (ii) Exposing only the (111) face of the wafer leads to pits on a smaller scale (Fig. 1b). To obtain etch pit free (111) terraces, dissolved oxygen must be removed and a rough zone must be exposed to solution to act as sacrificial anode. The azimuth of the miscut (determined by XRD) is the last important parameter because it determines the morphology and organization of steps. When the projection of the direction [111] is precisely aligned along <11-2> a vicinal surface with regularly spaced rectilinear steps may be obtained (Fig. 2c). Kinetic Monte Carlo simulations (de J. Kasparian, LASIM) indicate that, in a such a case, the dissolution rate is 107 times slower on terraces sites than on step sites. Such vicinal H-terminated Si(111), as the one shown in Fig. 2c, are routinely used in our current projects about electrodeposition and surface functionalization.

Figure 1 : Reaction model for the dissolution of silicon in NaOH.

Figure 2 : AFM images comparing the topography of H-Si(111) surface prepared by etching in 40% NH4F : (a) with dissolved oxygen in solution ; (b) with the polished faced exposed to solution ; (c) With an oxygen free solution and the unpolished rear face exposed to solution.

Publications :

[1] M. L. Munford, R. Cortes, P. Allongue, "The preparation of ideally flat and ordered H-Si(111) surfaces", Sensors and Materials, 13 (2001) 259-269

[2] P. Allongue, C. Henry de Villeneuve, S. Morin, R. Boukherroub, D. Wayner, "The preparation of H-Si(111) surface in 40% NH4F revisited", Electrochim. Acta, 45 (2000) 4591

[3] P. Allongue, "In-situ STM molecular imaging and LDOS determination at the Si(111) / NaOH interface", Phys. Rev. Lett. 77 (1996) 1986-1989

[4] P. Allongue, V. Costa Kieling, and H. Gerischer, "Etching of Silicon in NaOH Solutions .1. In situ Scanning Tunneling Microscopic Investigation of n-Si(111)," J. Electrochem. Soc. 140 (4), 1009-1018 (1993).

[5] P. Allongue, V. Costa Kieling, and H. Gerischer, "Etching of Silicon in NaOH Solutions .2. Electrochemical Studies of n-Si(111) and n-Si(100) and Mechanism of the Dissolution," J. Electrochem. Soc. 140 (4), 1018-1026 (1993).