Effects of simultaneous displacive and ionizing radiation in ionic and covalent crystals

A review is given on the formation process of radiation-induced defects and their stability under simultaneous displacive and ionizing radiation for ionic and covalent crystals, which include MgO•nAl₂O₃ (n = 1.0, 2.4), α-Al₂O₃, MgO, Si, Ge and GaP. In the ionic crystals of MgO-Al₂O₃ system, retardation of dislocation loop formation, preferential formation of bubbles and retardation of amorphization are observed subjected to simultaneous ionizing and displacive radiation. On the other hand, simultaneous irradiation with ions and electrons in the covalent crystals of semiconductors retards the accumulation of amorphous zones and the amorphization of entire specimen. In both ionic and covalent crystals, the stability of the small defect clusters, which are dislocation loops and amorphous zones for the ionic and covalent crystals respectively, plays an important role for the microstructure evolution, such as the retardation of dislocation loops and amorphization. The instability of defect clusters is attributed to electronic exciation for ionic and covalent crystals at lower energy electron irradiataion, whereas displacement damage and subthreshold displacement damage are important for the regrowth of amorphous zones in covalent crystals under irradiation with higher energy electrons.