Confinement effects in π-bonded chains at group IV semiconductor (111) surfaces

The degree of 1D character of surface chains at group IV (111)-2 × 1 reconstructed surfaces is established by surface sensitive optical spectroscopy. Optical experiments on a diamond C(111)-2 × 1 surface show that the absorption peak related to dangling-bond transitions exhibits a marked blueshift upon oxygen exposure of the clean surface. Such behaviour is analogous to that observed on a clean Si(111)-2 × 1 surface. For both surfaces the experimental finding is interpreted in terms of quantum confinement of surface electrons in quasi-one-dimensional π-bonded chains, whose length decreases with oxygen uptake. A different behaviour is observed in Ge(111)-2 × 1, where only a very slight blueshift of the surface-state optical transition is detected upon oxidation. The almost negligible blueshift in Ge(111)-2 × 1 is consistent with a significant coupling between the π-bonded chains resulting in a much less pronounced one-dimensional character of Ge(111)-2 × 1 surface electrons compared to diamond and silicon reconstructed surfaces.