Effects of water adsorption and surface oxidation on the electrical conductivity of silicon nanocrystal films

Electrical conductivities of thin films of silicon nanocrystals (4-6 nm) exhibit high sensitivity to water vapor. Specifically, water adsorption on the surface of silicon nanocrystals films increases their electrical conductivity by a factor of 4 at room temperature and an order of magnitude at 175 K. The increase in conductivity is reversible and can manifest as peaks or hysteresis loops in temperature-dependent conductivity measurements even when the measurements are conducted under vacuum at 10^-5 Torr and in the presence of only residual amounts of water vapor. Hydrogen-terminated silicon nanocrystals are easily oxidized to form submonolayer to a monolayer of chemically bound oxygen on their surfaces when annealed at 300 °C in a glovebox with 0.1 ppm of water vapor. Annealing under vacuum at 300 °C retains H-passivation without oxidation. The electrical conductivity of films made from hydrogen-terminated silicon nanocrystals is 200 times higher than the electrical conductivity of films made from silicon nanocrystals with a monolayer of chemically bound oxygen. However, the conductivities of both types of films increase upon adsorption of water on the nanocrystal surfaces. These findings underscore the importance of controlling silicon nanocrystal surfaces in determining the electrical properties of their thin films.