Chemical oxidation of Si by use of azeotrope of nitric acid and water can form 1.4-nm-thick silicon dioxide layers with a leakage current density as low as those of thermally grown SiO2 layers. The capacitance–voltage (C–V) curves for these ultrathin chemical SiO2 layers have been measured due to the low leakage current density. The leakage current density is further decreased to ∼1/5 (cf. 0.4 A/cm2 at the forward gate bias of 1 V) by post-metallization annealing at 200 °C in hydrogen. Photoelectron spectroscopy and C–V measurements show that this decrease results from (i) increase in the energy discontinuity at the Si/SiO2 interface, and (ii) elimination of Si/SiO2 interface states and SiO2 gap states.

A simple method to passivate interface states at ultrathin SiO2/Si interfaces is developed. In this method, ultrathin SiO2-covered Si is immersed in a KCN solution containing crown-ether, followed by a rinse in water at 25 °C. The conductance–voltage measurements show that the interface state density is decreased to ∼1/10 by this crown-ether cyanide treatment. The capacitance–voltage measurements show that contamination by K+ ions is effectively avoided by the inclusion of crown-ether. These results demonstrate that crown-ether molecules effectively capture K+ ions and consequently CN− ions effectively may react with defect states, probably forming Si–CN bonds. The passivation of interface states by the cyanide treatment improves the electrical characteristics of metal–oxide–semiconductor tunneling diodes.

The energy distribution of interface states at ultrathin oxide/Si͑100͒ 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 ͑MOS͒ 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.

Low temperature nitridation of silicon oxide layers by nitrogen plasma generated by electron impact is investigated using x-ray photoelectron spectroscopy (XPS) and synchrotron radiation ultraviolet photoelectron spectroscopy and it is found that a large amount of nitrogen can be incorporated in the layers. The valence band structure of the oxide surface nitrided at 25 °C is similar to that of Si3N4, while that nitrided at 700 °C resembles the mixture of silicon oxide and silicon oxynitride. Measurements of XPS depth profiles show that the nitrogen concentration is high near the surface and the oxide/Si interface.

Crown-ether cyanide treatment, which includes the immersion of Si in KCN solutions containing 18-crown-6 molecules, is found to greatly decrease the leakage current density of Si-based metal–oxide–semiconductor (MOS) diodes. The decrease by one order of magnitude for the single crystalline Si-based MOS diodes is attributable to the elimination of Si/SiO2 interface states by reaction with cyanide ions and formation of Si–CN bonds. The reduction in the leakage current density by two orders of magnitude is caused for polycrystalline Si-based MOS diodes, and this decrease is attributed to the passivation of trap states in poly-Si as well as the interface states.