Effect of thermochemical reduction on the electrical, optical-absorption, and positron-annihilation characteristics of ZnO crystals

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 1100 and 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 1692 ps exists in the reduced crystals, in contrast to the lifetime of 1803 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 efficient positron traps---indicating that the defects either exist as interstitials or that they are positively charged.