Tunneling recombinations in scintillators, phosphors, and dosimeters

This review paper provides a description of direct recombination processes between carriers from traps to luminescent centers, involving the occurrence of a tunneling mechanism between their space-correlated localized electronic levels. The experimental evidences that allow to recognize tunneling recombination in phosphorescence time decay and thermoluminescence experiments are first outlined, and compared with those characteristic of recombination processes involving the transfer of carriers in the delocalized bands prior to their recombination. Simple explanations are also proposed for the phenomenological observations in both athermal and thermally assisted tunneling processes. The importance of such recombinations in three material classes - scintillators, persistent phosphors, and dosimeters, is then outlined and discussed in relation to the material requirements in their distinct application fields. For each application, numerous literature examples are reported. Finally, the paper is complemented by a brief illustration of literature investigations describing experimental evidences of direct trap-center recombination different from tunneling, mostly based on the inspection of emission spectra and thermoluminescence glow curve shapes.