Local study of thickness-dependent electronic properties of ultrathin silicon oxide near SiO2/Si interface

Abstract

We have used scanning tunnelling microscopy and spectroscopy (STM/STS) to investigate the electronic properties of ultrathin silicon oxide films on silicon, e.g. surface band gap and band offset versus silicon oxide thickness near the interface. The length of the interfacial electronically transitional region is measured to be about 0.9 nm, in which the silicon oxide surface band gap increases gradually with thickness and its evolution is meticulously observed. Both the conduction and valence band offsets between silicon oxide and silicon increase by about 1.0 eV with only a ∼0.3 nm steric difference in this region. Furthermore, in an oxide thinner than ∼0.6 nm, oxide gap states can be discerned from STS which is attributed to the electronic states extending from the silicon substrate over the Si oxide. On the basis of local layer-resolved electronic properties, we corroborate that a usable oxide thickness should be no less than ∼0.9 nm on the Si(1 1 1) substrate, which corresponds to only three initial oxide layers.