Preparation of ideally flat Si surfaces

Ideally flat (111) surfaces do not exist in practice. There are always steps and pits according to the angle of crystal miscut. One possible way to obtain surfaces with an ordered staircase structure consists in using vicinal (111) surfaces with a miscut angle of a few degrees in the proper direction so as to make the lateral flow of steps faster than the nucleation and growth processes of etch pits on terraces (Fig. 5(a)). The rate of dissolution is indeed ~1 s"1 on step dihydrides and 10"2 s"1 on step monohydrides, and ranges between 10"3 and 10"4 s"1 on vertical terrace monohydrides [17]. Using additives is another possibility which is less dependent on crystal orientation. Amount traces of Triton [formula : CiCHs^CHziCHsk-CeBLt -(OCH2CH2)nOH)] added to NaOH tends for instance to flatten the silicon surface on a large scale [18]. Figure 6 illustrates this effect. Unlike in pure NaOH, it is remarkable that the initial spacing between steps is preserved at the end of the series. With Si(100) flatter surfaces may be also obtained, with no formation of pyramids as usually observed after anisotropic etching in nh4f.

Observations may be interpreted in terms of the formation of a monomolecular membrane floating on top of the surface as Fig. 5(b) sketches. The benzene rings favor indeed stacking of molecules on the atomically flat portions of the surface in a sort of self assembly, with the hydrophobic alkyl heads of the molecule in contact with the H-terminated surface (though Van der Waals interactions) while the hydrophilic tails -(OCH2CH2)nOH point towards the solution. The sequence shows that any atomic size

Fig. 5. Schematic technique to prepare ideal Si(l 11) surfaces, (a) Use of the crystal miscut; (b) Use of Triton as additive.
Fig. 6. Time sequence (45 s/image) taken on n-Si(l 11) tilted by 0.7° in Triton/NaOH. Chemical etching is promoted between images 2-6. In other images the surface is cathodically protected. Frames are 1400x1400 A2 (after [18]).

defects beaks this order and results in faster local etching in a self-repairing action which smoothens the surface on a larger scale. That the molecules do not bind chemically on the surface is of great interest for applications.

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