Thermally stimulated ionic conductivity of sodium in thermal SiO ₂

Abstract

The effect of Na+ concentration, oxide thickness, applied field, and metal electrode on thermally stimulated ionic conductivity (TSIC) measurements of positive-ion motion in SiO2 grown on single-crystal silicon has been studied. A surface trapping model, assuming blocking electrodes, has been used to analyze TSIC curves. Using a hyperbolic hearing rate (1/T ∝ time), for which analytic expressions for normalized first-order TSIC curves are obtained, both energies E and preexponential factors s appropriate to the detrapping processes are obtained. Two distinct positive-charge peaks are observed in TSIC curves. The magnitude of the low-temperature peak, the α peak, is proportional to the amount of Na + introduced into the sample. At high fields, the magnitude of the high-temperature peak, the β peak, is independent of the evaporated Na + concentration; its origin is uncertain but it may be due to mobile hydrogen. The temperature for the maxima in the TSIC curves, Tm, decreases with field for both the α and β peaks. At Na+ concentrations below 2×1012 Na+/cm2, the β peak is dominant in TSIC curves. Na+ motion from the Au-SiO2 interface during the first heating of Au-SiO2-Si samples follows the simple model for release of Na+ from interface traps. On the other hand, Na+ motion from the Al-SiO2 interface on the first heating of Al-SiO2-Si samples occurs at lower temperatures than for Au-SiO2-Si samples and appears to depend on reaction between Al and SiO2 rather than on a simple trap-release process. Na+ release from traps at the Si-SiO2 interface is similar with both Al and Au electrodes, and occurs at lower temperatures than for Na+ release from the metal-SiO2 interface.