Paramagnetic and diamagnetic defects in e − and UV-irradiated TeO 2 single crystal

Abstract

A study is reported of the influence of illumination on generation and decay of point defects in TeO 2 crystals following electron irradiation at ∼400 K. Electron irradiation is believed to cause a large concentration of diamagnetic oxygen vacancies denoted by V O x and a smaller concentration of vacancies with one trapped electron denoted by V O . When the sample is UV illuminated at 330 nm and 77 K or lower, electron spin resonance (ESR) measurements show that the number of V O centers increases and a comparable gain of V O ′ centers (three electrons in the vacancy) occurs. A brief illumination at 660 nm causes the V O ′ signal to disappear and the V O signal to decrease and return to its original value. Changes in the crystal's optical absorption obtained from spectra measured with polarized light are given. When V O ′ centers are removed by bleaching, or by thermal annealing, broad bands at 600 and 700 nm disappear and there are increases in optical absorption at 380, 440 and 480 nm. The source of these bands is discussed. These processes are reversed by a new UV illumination at 330 nm and 77 K. The growth and decay kinetics of V O ′ centers and V O centers measured by ESR indicate the same rates of percentage change in their concentrations. Explanation of these reversible processes supports selected models for the three different vacancy centers.