Transient capacitance spectroscopy of Na+-induced surface states at the Si/SiO2 interface

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The interface states induced by the drift of Na+ ions to the Si/SiO2 interface are investigated in metal-oxide-semiconductor capacitors made on n-type Si using deep level transient spectroscopy. The results clearly reveal the existence of two types of defects. The dominant ones yield an increase in the density of states mainly near the Si conduction band, resulting in a band tail. These states can be annealed out by pumping of the Na+ ions back to the metal gate. The introduction kinetics of these traps as a function of sodium concentration at the Si/SiO2 interface ([Na+]) exhibits a strong increase near [Na+]∼3×1012 ions/cm2. The other type of defect, created in less significant amounts, accounts for a flat continuum of levels throughout the Si band gap. These traps are little affected by the Na+ ions leaving the Si surface, which is indicative of a more or less irreversible degradation of the Si/SiO2 interface. These results are compared with other authors’ experimental data and are discussed in terms of several possible physical models.