Displacement Thresholds in ZnO

Two displacement thresholds have been observed in ZnO single crystals. Above the lower energy threshold of 310 keV, the carrier removal rate increases quadratically with increasing electron beam energy. Above the second threshold at 900 keV, a sharp reduction in the free carrier microwave absorption is observed. Accompanying this, the EPR resonances for several iron group impurities and for the F+ center (ionized oxygen vacancy) become activated and are observable. The color centers which have been associated with F centers are also activated. After a thermal anneal to remove these color centers, irradiation at lower energies yields the same energy dependence for the production of F+ centers as is observed with the electrical measurements as well as the same threshold at 310 keV. The lower threshold at 310 keV is associated with oxygen displacements while the upper threshold at 900 keV is associated with zinc displacements. On the basis of these assignments, a value for the maximum atomic recoil energy of 57 eV is obtained for both zinc and oxygen displacements.