Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

Abstract

Insufficient radiation resistance of construction materials is the Achilles heel for thermonuclear energetics. In wide-gap dielectrics, Frenkel defects are created not only because of the knock-out (impact) mechanism but also because of the decay of the electronic excitations formed during the irradiation (i.e. due to nonimpact mechanisms). The processes of the defect creation at the irradiation of highly pure LiF single crystals at 6–8K by 1–30-keV electrons, X-rays, or synchrotron radiation of 12–70eV have been investigated. The annealing processes of these defects in a temperature range up to 200K have been studied as well. In LiF, creation has been revealed for the following: (1) F–H pairs caused by the decay of anion excitons or by the recombination of electrons and holes, (2) F′–H–VK and F–I–VK defect groups at the decay of cation excitons (62eV), or (3) 20-keV electron irradiation. The mechanism of defect creation at the recombination of hot holes and hot electrons has been discussed for α-SiO2 crystals with an energy gap between the subbands of a valence band. One of the possible ways to suppress this mechanism (“luminescent defence”) is doping a material by luminescent impurities able to capture a part of the energy of hot carriers before their relaxation and recombination (e.g. in MgO:Cr).