An S/XPS study of hydrogen terminated, ordered silicon (100) and (111) surfaces prepared by chemical etching

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Abstract
There is considerable current interest in the passivation of single crystal silicon surfaces, particularly with reference to the fabrication of epitaxially grown solid state device structures. In this paper, we discuss ordered Si(100) and (111) surfaces prepared by chemical etching in solutions of hydrofluoric acid and studied by use of Soft XPS, XPS and LEED. Despite transfer through the atmosphere before being loaded into the UHV of our analysis instruments, the hydrogen terminated surfaces observed are shown to be among the most perfect yet produced by such chemical means. Under optimum conditions, the etch can yield levels of carbon and oxygen impurities of less than 2% of a monolayer and lead to surfaces where the Fermi level is not pinned. The etched surfaces are modified by the hydrogen termination such that they are electrically passivated and ordered into a (1 × 1) periodicity. Upon annealing at (520 ± 10)°C in UHV, the hydrogen is removed, the surface changes to a (2 × 1) reconstruction and also exhibits band bending due to Fermi level pinning by surface strates. The relationship between residual contamination and etching parameters such as time and etchant concentration has been examined, together with the stability of the passivated surface under exposure to wet and dry environments. This has shown that the surface condition is sensitive to etching procedure and that the optimum surface may be obtained by using a 5% non-aqueous solution, or by pre-cleaning a surface using a UV generated ozone exposure prior to an aqueous etch. It has also been shown that a controlled reoxidation of an etched surface using UV-ozone can produce a non-abrupt Si-SiO2 interface. This may form a useful part of a reduced temperature surface preparation, compatible with any subsequent epitaxial overlayer growth.