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The energy distribution of interface states at ultrathin oxide/Si(100) interfaces is obtained using a new method, i.e., x-ray photoelectron spectroscopy measurements under biases between the metal overlayer and the Si substrate of the metal-oxide-semiconductor (MOS) devices. Ultrathin thermal oxide layers formed at 450 °C in oxygen have an interface state peak near the midgap and it is attributed to isolated Si dangling bonds with which no atoms in the oxide layer interact. On the other hand, thermal oxide layers formed at 650 °C have a two-peaked structure, one peak above and the other below the midgap, and they are attributed to Si dangling bonds with which an oxygen or Si atom in the oxide layer interacts weakly. The density of the interface states, especially that near the midgap, decreases drastically by cyanide treatment, i.e., the immersion of Si in a KCN solution for a few seconds followed by a rinse in boiling water, performed before the oxide formation. It is suggested that cyanide ions penetrate into the Si, forming Si–CN bonds at structurally imperfect places. The cyanide treatment improves the electrical characteristics of the MOS tunneling diodes. © 1998 American Institute of Physics.