ENERGY-DISTRIBUTION OF SURFACE-STATES IN THE SI BAND-GAP FOR MOS DIODES OBTAINED FROM XPS MEASUREMENTS UNDER BIASES

The energy distribution of surface states in the Si band-gap for MOS diodes has been obtained from X-ray photoelectron spectroscopic (XPS) measurements under biases. The XPS measurements are performed for the (3 nm thick Pt/2.5 nm thick native silicon oxide/n-Si(100)) MOS diodes. The binding energy of the substrate Si 2p peak, E(si(2p)), with respect to that of the Pt 4f peak of the metal overlayer, E(pt(4f)), is decreased by applying a forward bias, while it is increased by applying a reverse bias. These shifts are completely reversible, and attributed to the band-edge shifts caused by accumulation of electric charges in the surface states. By analyzing the amount of the bias-induced shift in E(si(2p)) - E(pt(4f)), measured as a function of the bias voltage, the energy distribution of the surface states present at the native oxide/Si interface is obtained for the first time. The density of the surface states is high near the Fermi level (0.49 eV above the valence band maximum) and decreases toward the valence and conduction bands.