Investigation of defects in deposited oxides with a frequency resolved capacitance technique

In this paper we propose a new tool to investigate defective oxides. The technique measures the differential capacitance of MOS devices under substrate accumulation as a function of the small-signal frequency. In off-stochiometric oxides deposited by plasma-enhanced CVD we measure a consistent increase of capacitance while decreasing frequency. An analytical model of capacitance is developed, starting from the hypothesis that trapped charge hops between defect sites around the Fermi level via a phonon-assisted mechanism. The hopping characteristic time depends on the energy difference and distance between defects and is compared with the inverse frequency. This gives rise to the observed dispersive behavior of the capacitance. Experimental results are successfully reproduced by the proposed model. Defect densities up to 10²⁰ cm^-3 were extracted, with an energy span as low as 0.1 eV and hopping distance around 25 Å