Electron trapping levels in silicon dioxide thermally grown on silicon

Abstract

Photocurrents associated with optical release of photoinjected electrons trapped in thin films of amorphous silicon dioxide have been studied. Temporal and spectral variation of the photocurrents were examined in detail: the effects on spectrally resolved response caused by variations in applied electrical field, wavelength sweep rate, and optical belaching are reported. All measurements were made on metal-oxide-semiconductor capacitors. The experiments were interpreted in terms of a straightforward model of optical excitation and transport of electrons out of localized energy levels in the silicon dioxide band gap. Semi-quantitative analysis indicated that a distribution of states peaked approximately 2·1 eV below the conduction band edge was associated with an electron trapping center distributed rather uniformly throughout the oxide film. In the wet thermal oxide specimens examined, the average density of trapping centers was greater than 10 14 cm −3. A time-stable spread in energy of approximately 0·5 eV was measured, and was attributed to local disorder in the amorphous insulator. The existence of an optically inactive charge distribution in the oxide films, with bulk average density greater than 10 15 cm −3, was indicated by collected charge vs. applied field data.