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2.1 Etching mechanism and surface structure

2.2 Preparation of ideally flat surfaces

3 Sulfide passivation of GaAs surfaces

4 Formation of molecular monolayers

5 Conclusions

6 References control of surfaces down to the atomic scale. Multiple examples could be given. In all these fields a better understanding and control of interfacial processes has recently been gained with in-situ local probe techniques which may yield access to the composition and structure of surfaces as well as the dynamics of processes. Preparing ordered semiconductor surfaces by a wet chemical treatment is a challenging problem because one cannot use surface self diffusion of atoms to rearrange the surface as is usually done upon annealing in ultrahigh vacuum (UHV). In the case of silicon, an oxide is first grown at elevated temperature (ca 1050 °C) in dry oxygen such that the interface Si/SiC>2 is as flat as possible on the atomic scale (Fig. 1(a)). The second step is selective stripping of the oxide in HF to obtain a surface in a state close to the fingerprint of the Si/SiC>2 interface (Fig. 1(b)).

Silicon

Silicon

Thermal oxidation Selective oxide stripping Etching

Fig. 1. Schematic preparation to obtain an ideal Si. (a) Oxide formation, (b) selective oxide stripping in HF. The surface is H-terminated with a structure which is close to the fingerprint of the Si/Si02 interface, (c ) after subsequent chemical etching.

The last step is surface anisotropic etching to remove the remaining adatoms and islands and leave terraces atomically flat (Fig. 1(c)). This technique of preparation is specific to Si and is made possible by the high quality of the Si/Si02 interface and the extreme selectivity of Si02 to Si etching in HF (Si is almost not etched in acidic HF). With other materials like compound semiconductors procedures are generally different.

During the last couple of years we have been using in-situ STM/AFM to investigate semiconductor surface etching as well as molecular grafting on these materials. Metal deposition is a younger topic and is not considered further in this contribution.

Imaging semiconductor surfaces in liquids raises several kinds of difficulties which have been recently reviewed [1, 2]. In the case of STM, n-type samples must for instance be cathodically polarized since the contact of a semiconductor with a liquid is a diode and proper tunneling requires the diode be forward-biased so as to reach a sufficient density of electrons at the surface. In the case of AFM, the laser detection may cause uncontrolled photoeffects under reverse bias, such as photocorrosion with n-type substrates. The friction of the tip may also enhance surface reactions locally. Despite these limitations, in-situ STM/AFM imaging semiconductors under potential control, has the great advantage that this minimizes interactions between the tip and the substrate by comparison with imaging at ambient conditions. Unless the atmosphere is sufficiently controlled, ambient conditions STM generally leads to some uncontrolled electrochemical reactions in the layer of contamination, a phenomenon which can however be utilized in nanolithography applications [3,4].

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