Effect of Ultra-violet Irradiation on the Electrical Conductivity of Zinc Oxide Single Crystals

EXTENSIVE examination of ZnO crystals and powder in the past few years has been reported by Heiland et al.1 for its optical and electronic properties. Present measurements have been initiated because of the effect of the solar ultra-violet energy on thermal control coatings of spacecraft which use ZnO as a pigment with an adequate binder. Previous measurement of electrical conductivity of ZnO, with and without doping, indicated that the increase in conductivity with doping is due to the donors as charge carriers. Optical measurements1,2 indicated that when the specimens have been heated in ZnO vapour the unresolved peak near the absorption edge is more pronounced, and this was attributed by Scharowsky2 to be due to the absorption of zinc ion, while Heiland et al.1 considered this to be due to unspecified lattice defects resulting from the diffusion of atomic zinc. Craeynest et al.3 explains a similar effect by reflexion spectra in powders and proposes a complex centre with an unpaired electron, but he could not detect any band at 380 nm by heating ZnO powders in nitrogen or argon atmosphere.