The properties of charge states in metal-oxide-semiconductor (MOS) system are important for the quality of the device containing ultrathin oxide layers. In present study the MOS structures with ultrathin SiO2 layer were prepared on Si(1 0 0) substrates by using low temperature nitric acid oxidation of silicon (NAOS) method. Properties of the NAOS SiO2 layer and the charge states in the MOS structure were studied by the scanning tunneling microscopy method and by the capacitance-voltage method. Carrier confinement produces the energy and space quantization effects which remarkably influence of charge states in the MOS structure with ultrathin oxide layer. The properties of charge states in an Al/NAOS-SiO2/Si MOS structure were analyzed by solving the Schrodinger-Poisson equations and results were used in a construction of the theoretical model of the MOS capacitance with the interface states. From the comparison of theoretical capacitance model and experimental C-V curve the density of interface states were determined. The influence of fixed charge in NAOS oxide layer was not observed. (c) 2012 Elsevier B.V. All rights reserved.