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Experimental measurements and theoretical calculations are reported which may resolve some of the remaining difficulties associated with photoemission from anion vacancies in thermochemically reduced MgO crystals. The intensities of the 2.3and 3.2-eV luminescence bands are found to be strongly influenced by both the concentrations of H ioris and anion vacancies present, and also by the intensity of the -5.0-eV exciting light. Theoretical calculations predict that for a 'A~g relaxation of the surrounding ions the 'T&"and T&"electronic states of the F center are almost degenerate and -0.05 eV above the 'A~g state. The 'Tj"-+'A&~and the T&"~'A&~transition energies are predicted to be 2.2 and 2.9 eV, respectively.

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The electrical properties, optical-absorption characteristics, and positron-annihilation lifetimes have been determined for nominally pure ZnO single crystals that were thermochemically reduced in Zn vapor in the temperature range between 1100and 1500 K. Electrical-conductivity and Hall-effect measurements indicate that donors are produced as a result of the thermochemical reduction process. Additionally, optical measurements show that the reduction results in an increase in the optical absorption near the two fundamental absorption edges. Positron-annihilation studies reveal that a well-defined positron state having a lifetime of 169+2 ps exists in the reduced crystals, in contrast to the lifetime of 180+3 ps characteristic of colorless, high-resistivity as-grown crystals. The lifetime of 169 ps is attributed to positron annihilation in the bulk material. It is concluded that defects produced by thermochemical reduction of ZnO are not elcient positron traps -indicating that the defects either exist as interstitials or that they are positively charged.

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