Surface state density decrease in nanostructured polycrystalline SnO2: modelling and experimental evidence

We developed a model for n-type metal-oxide polycrystalline semiconductors which allows us to calculate the density of surface-chemisorbed oxygen and the band bending, once the Schottky barrier height is measured. From numerical simulation, we estimated for the polycrystalline SnO2 a depletion layer, Λ, of approximately 20 nm; consequently, we deposited films with a mean grain radius, R, larger (30 nm) than Λ, and smaller (10 nm). When R is smaller than Λ, the model predicts a flattening of the band bending, and a decrease in the density of chemisorbed oxygen that leads to the unpinning of the Fermi level. To give evidence of the predicted decrease in the density of surface states, we present and compare electrical measurements in dry air, as well as scanning tunnelling microscopy/spectroscopy results.