Improvements in the quasi-static capacitance-voltage characterization of semiconductor-insulator interface states (Si/SiO2)

The quasi-static capacitance-voltage determination of the electron density of states as a function of energy at the semiconductor-insulator interface is addressed. The respective effects are analyzed of random and systematic errors in a measured capacitance-voltage characteristic on the interface-state distribution derived. The random errors show up as fluctuations that grow indefinitely in magnitude as the energy approaches either band edge. Systematic errors are manifested in under- and overshoots near the band edges. The most important are the systematic errors associated with the estimation of the insulator capacitance and the identification of the relationship between the semiconductor surface potential and the applied voltage. A method for minimizing the errors is proposed. It is noted that the method should enable one to substantially expand the accessible energy range and to significantly improve the accuracy to which the density of states is evaluated. These advantages are confirmed by an experiment on the Si/SiO2 interface. It is found that the energy range can be made as wide as about 0.9 eV and the accuracy of the semiconductor surface potential can be improved to about 0.1 meV, so that the integrated density of states can be determined to within about 5 × 107 cm-2. It is inferred from the experimental data that the interface states are concentrated near the conduction band edge and are due to positive oxide fixed ions rather than Pb centers. The ions should act as electron traps involved in tunnel electron exchange with the conduction band of the silicon.