Description of the SiO2Si interface properties by means of very low frequency MOS capacitance measurements

Two measurement methods of the MOS capacitance under conditions of total reversibility are described. The knowledge of the exact very low frequency MOS capacitance permits us to determine for a given structure the apparent interface density in the whole band gap of silicon. We show theoretically that this apparent inteiface density can contain a contribution of defects unspecific of the interface, for instance: spatial fluctuation of the interface potential or silicon defects introducing a deep level in the band gap. Experimental results show that: (1) the apparent interface density increases sharply towards the band edges ; (2) in the structure having an important flat band translation the apparent interface state distribution can be attributed to the spatial fluctuation of the interface potential; (3) MOS structures having 〈111〉 orientation have an interface state distribution different from 〈100〉 homologous structures; for a 〈111〉 structure we find a peak of apparent interface density probably due to a donor defect in the silicon.