Thermal and radiation induced mobilities of charge-compensating ions in crystalline SiO₂

Abstract

All quartz contains Al³⁺ ions substituting for Si⁴⁺ ions with an adjacent interstitial alkali or a proton providing the necessary charge compensation. When thermally released from their trapping sites the interstitials can migrate along the relatively large c-axis channels. Li⁺ is the predominant charge compensator in commercially grown quartz. If the sample is heated with an electric field applied along the c-axis the alkalis will be swept out and released either by protons or by another alkali. This electro-diffusion or sweeping process can be used to selectively dope the Al³⁺ site. Applied fields of approximately 15 V/cm and 3000 V/cm result in equivalent current densities for alkali ions and hydrogen respectively. Alkalis show activation energies between 0.95 and 1.4 eV; hydrogen shows an activation energy near 1.9 eV. If ionizing radiation is present, hydrogen is mobile at all temperatures; however, the alkalis only become mobile at temperatures above 200 K. A comparison of the radiation induced mobility of sodium and lithium electro-diffused samples shows that sodium becomes mobile under irradiation approximately 10 to 15 K higher than lithium.