Charge trapping at chemically modified Si(111) surfaces studied by optical second harmonic generation

Covalent chemical modification of Si(111) surfaces by attachment of organic monolayers with terminal tertbutylhydrazide groups is described. Second harmonic generation studies of the hydrazide-modified surfaces in air show the presence of an electric field induced second harmonic (EFISH) response, indicative of charge trapping at the surface. Charge trapping is associated with the hydrazide group and is not observed for other organic monolayers or oxidized silicon surfaces. Charging is reversible and can be modulated by variation of the gaseous ambient of the sample, including particularly oxygen and water vapor. An interpretation is given in terms of ionosorption of O-2 as an electron trap, with strong stabilization of the electron affinity level of O-2 in the monolayer by polar hydrazide groups and adsorbed water molecules. There is also an indication that the hydrazide group may become ionized, as an electron donor. From the dependence of EFISH on laser irradiance and the observation of a transient EFISH response, it is suggested that ionosorption of O-2 may occur spontaneously in the dark and, in addition, is transiently enhanced by laser photoexcitation of silicon. By comparison with EFISH and high-resolution electron energy loss measurements on oxidized silicon surfaces, the density of charge trapped at hydrazide-modified surfaces has been estimated.