High-temperature ac conductivity of amorphous SiO2:: Fused silica and thin thermal films

High-temperature ionic conductivity of both fused silica and thin films thermally grown on a silicon substrate has been investigated in the temperature ranges 700-1400 and 570-830 K, respectively, by means of impedance spectroscopy. The results for both types of materials may be interpreted as being due to the existence of an extrinsic conduction mechanism related to the presence of dissociated sodium ions. The numerical analysis of the experimental results led to values of 1.3 and 0.6 eV for the dissociation and migration energies, respectively. The dissociation energy turns out to be in good agreement with the value obtained in crystalline quartz and related to the aluminum-alkali ([AlO4-M](0)) center; moreover, good agreement is also found between the mobility value obtained here and literature data evaluated on thin films by different electrical techniques. No influence of hydrogen content on alkali transport was found by considering wet and dry samples, at variance with results previously obtained on crystalline quartz. [S0163-1829(99)14515-6].