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 te chnique measures the differential capacitance of MOS devices under substrat e accumulation as a function of the small-signal frequency. In off-stochiom etric oxides deposited by plasma-enhanced CVD we measure a consistent incre ase of capacitance while decreasing frequency. An analytical model of capac itance 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 dist ance 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 10(20) cm(-3) were extracted, with an energy span as low as 0.1 eV a nd hopping distance around 25 Angstrom.