Surface activity of zinc oxide. Examination of the effect of γ-irradiation by electron spin resonance, electrical conductivity, reflectance spectroscopy and gas adsorption

Abstract

The activity of the n-type semiconductor zinc oxide is considerably affected by γ-irradiation. Studies have been conducted both on powders which are in an “excess zinc” condition as a result of evacuation at high temperatures and on powders which are close to stoichiometry.

In non-stoichiometric zinc oxide the surface activity is dominated by the presence of donor sites and their associated electrons, which are distributed throughout the bulk of the crystallites, and changes in the concentration of these have been monitored by e.s.r., reflectance spectroscopy and electrical conductivity measurements. In more stoichiometric zinc oxide the surface activity is lower but can be stimulated to a greater relative extent by γ-irradiation. The radiation-induced electrical conductivity and the lifetime of the “free” electrons in the conduction band are critically dependent on the surface conditions; adsorbed water acts as an efficient electron-hole recombination centre, oxygen as an electron trap and hydrogen as a hole trap.

Whereas chemically produced non-stoichiometry is associated with donor sites having electrons in levels high in the forbidden gap, the F⁺ centre point defect which is produced by lattice displacements as a result of elastic collisions appears to possess electrons in low energy levels within the forbidden gap.