Electrical and optical properties of doped and undoped zinc oxide powders

The effects on the diffuse reflectance spectra of ZnO powders by mechanical grinding and by heat treatment were investigated. Diffuse optical reflectance of the powders was measured over near-ultraviolet and visible wavelengths from 350 to 700 nm. An optical absorption band with its center at 400 nm was observed as the result of mechanical grinding. The magnitude of the 400 nm band in the ground ZnO powder was partially reduced by annealing in oxygen. This absorption band is attributed to F{dollar}sp+{dollar} center point defects located at 0.24 eV below the conduction band edge. The decrease in the optical absorption is attributed to the reduction in the concentration of oxygen vacancies.; Optical absorption bands at 390 nm and at wavelengths longer than 400 nm were detected when the ground ZnO powder was heated in either vacuum or zinc vapor. The simultaneous formation of the 390 nm band and the suppression of the 400 nm band at low oxygen vapor pressure are interpreted to occur as the result of the pairing of oxygen vacancies with zinc interstitials. Zinc interstitial and oxygen vacancy pairs are proposed to be associated with the 390 nm band. The defect states from which the 390 nm band originate are located at 3.17 eV below the conduction band edge. The broad absorption band at wavelengths above 400 nm is suggested to be related to more complex defect clusters involving more than one zinc interstitial.; The free electron densities of ZnO powders doped with aluminum as a solid solution were characterized with diffuse infrared reflectance technique. The powders having free electron densities above 10{dollar}sp{lcub}17{rcub}{dollar} cm{dollar}sp{lcub}-1{rcub}{dollar} showed significant resistance to the formation of the 400 nm absorption band. An increase in the free electron density will thereby result in an increase in the Fermi energy of the ZnO powders. This was found to effectively suppress the electron transition which is necessary for the 400 nm optical absorption. The 390 nm absorption was found not to be affected by the presence of the Al doping.