Highly mobile oxygen hole-type charge carriers in fused silica
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Minoru M. Freundb) Laboratorium fur Festkorperphysik, ETH-Honggerberg, 8093 Zurich, Switzerland (Received 31 January 1990; accepted 4 April 1991)
Some peculiar positive charge carriers are thermally generated in fused silica above 500 °C. These charge carriers appear to be positive holes, chemically O~~ states, probably arising from dissociation of peroxy defects. The charge carriers give rise to a pronounced positive surface charge which disappears upon cooling but can be quenched by rapid quenching from «800 °C. Reheating to «200 °C remobilizes these charge carriers and causes them to anneal below 400 °C. The generation of positive holes charge carriers may be important to understand failure mechanisms of SiO2 insulators.
Fused silica has a wide band gap, 7.6-7.8 eV.1 As an excellent electrical insulator it has countless technical applications. When exposed to ionizing radiation many defects are produced, which have been studied in great detail, using primarily electron spin resonance (ESR) spectroscopy as an investigative tool (for a recent review see Ref. 2). Most of the defects exist only at low temperatures and readily anneal upon heating to temperatures around 300 K or slightly higher. Among them are O~ states or positive holes, also called oxygen-associated hole centers (OHC). They represent defect electrons in the O2~ matrix and occur in various forms as peroxy radicals, =Si—OO*, as nonbridging-oxygen hole centers (NBOHC), =Si—O*, as self-trapped holes (STH), probably = S i — O * — S i = and O\~, as interstitial O^ molecular ions, and other related defects.3"5 The mobility of free holes at room temperature generated by an x-ray pulse is of the order of 2 x 10~ 5 cm 2 /V • s with an activation energy of 0.16 eV.6 Annealing is accompanied by a transient increase of the electrical conductivity — a process that is of considerable interest for understanding electrical breakdown phenomena in silica.7"9 Rapid quenching from high or very high temperatures represents another way of creating paramagnetic hole centers in fused silica, as some more recent work has shown.10'11 As in the case of radiation damage, the mode of formation appears to be linked to the extreme departure from thermodynamic equilibrium. In this communication we report on a different kind of defects, also presumably O~ states, the formation of which does not require extreme conditions or high energy processes.
^Mailing address: NASA Ames Research Center, Space Science Division, MS 239-4, Moffett Field, California 94035. b) Current address: Department of Physics, University of California, Berkeley, California 94720. J. Mater. Res., Vol. 6, No. 8, Aug 1991
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In order to study these defects, we embarked on a program to measure thermally generated charge carriers using a newly developed technique, Charge Distribution Analysis or CD A.12 CDA is, in principle, a contactless conductivity technique which allows one to detect small concentrations of mobile charge carriers in wide band gap insulator
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