Electronic Properties of Zinc Oxide

The existence of the latter has been recognized for many years, since Wagner (20) applied his thermodynamic theory of defect oxides to the system zinc oxide-oxygen (21). According to this scheme, at sufficiently high temperature an equihbrium sets in between zinc oxide and oxygen in the gas phase, whereby excess zinc (Zn ) can be accommodated in interstitial positions of the lattice  

If now the interstitial zinc atom loses one electron by thermal ionization of the lattice  

This relation was verified by von Baumbach and Wagner (21) and checked more recently by Bevan and Anderson (22) in an extended range of temperatures and pressures. 

The conductivity measurements show that equilibrium (1) sets in rapidly at temperatures as low as 500°C. Since the melting point of zinc oxide is about 2100°C. and accordingly its Tamman temperature about 900°C., the process under consideration cannot possibly involve the bulk of the crystal because defects could not diffuse rapidly enough through the lattice at such low temperatures. Except at very high temperatures, the defect equilibrium is realized only at the surface of the crystal, that is, in a layer of a few unit cells thickness. 

Finally, another consequence of this situation is the danger of attributing to the surface layer properties which have been established for the bulk material. This applies in particular to semiconducting characteristics of a defect oxide. Thus, with zinc oxide, the surface layer may be nearly stoichiometric and poorly conducting as a result of oxygen adsorption or conversely may present quasimetallic properties after activation in vacuo, irrespective of the composition of the bulk material. 

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In this section this field of study is reviewed with reference to work on zinc oxide and titanium oxide. Two comprehensive articles on the electronic properties of zinc oxide IO4) and of rutile (106) serve very well to provide the physical background. 

The chemical industry has been opening new markets for zinc oxide. Examples are lubricating oil additives, water treatment and catalysts. For photocopying, photoconductivity is a unique electronic property of zinc oxide. 

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