A role for oxygen vacancies in quartz luminescence

Abstract

The role that oxygen vacancies can play in the emission of quartz ultraviolet luminescence is investigated. Within a defect pair model approach we highlight the significance of aluminium, titanium and germanium point defects. By comparing known trace element estimates with estimates of the number of luminescent photons that are emitted globally from our bright quartz samples, we confirm that the available impurity defects outnumber the emitted photons by several orders of magnitude. Our conclusion is that another defect species must play a significant role in the observed luminescence sensitivity of quartz. By reviewing the literature on oxygen vacancies, we develop a hypothesis that the well-known oxygen vacancy E′1 centres act as a barrier to migration of holes along the quartz c-channel wall structure, preventing the emission of luminescence. We proceed to examine the processes via which E′1 centres migrate historically under ambient temperature conditions from their source location. By applying known potential barrier information, we contend that a thermally-assisted solar UV exposure mechanism can explain the migration process. In particular, we argue that during such migration the E′1 centres can be trapped at other centres which neutralise them, allowing free passage for subsequent holes en route to the luminescence centres. Such a mechanism predicts that bright luminescence should be observed from quartz extracted from hot sunny locations, compared with lower levels of luminescence in the case of quartz from cool overcast locations. This is consistent with known observations that quartz sensitivity increases with cumulative sunlight exposure.