EFFECT OF THERMOCHEMICAL REDUCTION ON THE ELECTRICAL, OPTICAL-ABSORPTION, AND POSITRON-ANNIHILATION CHARACTERISTICS OF ZNO CRYSTALS

The electrical properties, optical-absorption characteristics, and pos ition-annihilation lifetimes have been determined for nominally pure Z nO single crystals that were thermochemically reduced in Zn vapor in t he temperature range between 1100 and 1500 K. Electrical-conductivity and Hall-effect measurements indicate that donors are produced as a re sult of the thermochemical reduction process. Additionally, optical me asurements show that the reduction results in an increase in the optic al absorption near the two fundamental absorption edges. Positron-anni hilation studies reveal that a well-defined positron state having a li fetime of 169 +/- 2 ps exists in the reduced crystals, in contrast to the lifetime of 180 +/- 3 ps characteristic of colorless, high-resisti vity as-grown crystals. The lifetime of 169 ps is attributed to positr on annihilation in the bulk material. It is concluded that defects pro duced by thermochemical reduction of ZnO are not efficient positron tr aps-indicating that the defects either exist as interstitials or that they are positively charged.