Sodium Ions at Defect Sites at SiO2/Si Interfaces as Determined by X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is described in its application as a probe for studying defects such as sodium in SiO2 films. A general description is given of key experimental methods in XPS. New techniques are described for applying and monitoring a fixed bias at the surface of the oxide during the XPS measurement. These methods are shown capable of detecting extremely small Na and Cu concentrations in undoped samples (< 1011 cm¿2). In deliberately Na-doped samples, five spectral peaks are distinctly observed and related to different defect states at the vacuum/SiO2 and SiO2/Si interfaces. By applying a bias-temperature stress during the XPS measurements, these peaks change in relative intensity and can be related to the motion of the Na+ ions between different states occurring at the two interfaces. An attempt is made to correlate the observations with previously reported models.