In situ monitoring of the electronic properties and the pH stability of grafted Si 111

Photoluminescence, surface photovoltage, interface capacitance and vibrational infrared spectroscopy measurements have been used to monitor the electronic properties of silicon surfaces protected by covalently grafted organic monolayers. The grafted surfaces present electronic properties equivalent to those of freshly prepared H terminated surfaces. Moreover, the organic layers are found to improve the lifetime of the surface passivation in ambient atmosphere by orders of magnitude, as compared to a hydrogenated surface. In liquid water, the interfaces are stable up to pH 9. Above that value, and though silicon oxidation is barely detectable, one observes a marked decrease in the photoluminescence intensity, a sharp change in the value of the surface photovoltage, and a strong increase in the capacitance peak associated with surface states. These changes are not fully reversible when pH is brought back to acidic values. Complementary experiments on the wetting properties of paraffin films as a function of pH suggest that the abrupt degradation of electronic properties above pH 9 is triggered by interactions between molecular water and the aliphatic chains, which leads to permeation of water at structural defects of the organic monolayer and to localized oxidation of silicon at the interface. This sets an upper limit to the pH range where sensors based on silicon grafted with an organic layer can be use