Oxygen gas sensing characteristics at ambient pressure of undoped and lithium-doped ZnO-sputtered thin films

Abstract

Characteristics of sensing oxygen, present in some vol.% concentration in nitrogen at atmospheric pressure, for ZnO thin films, are reported here; these characteristics were obtained by means of simple electrical conductance measurements. The thin films were grown by reactive sputtering, in a 28% O 2/Ar atmosphere, starting from a ZnO target; either pure or 1 mol.% Li doped. The lithium content of the films was measured using nuclear reaction analysis. The crystallographic orientation and the surface structure of the films were studied using X-ray diffraction and reflection high energy electron diffraction respectively. The undoped films showed a higher (002) preferential orientation with respect to the doped films. The conductance of lithium-doped films decreased more than undoped film conductance in the presence of oxygen; the conductance of both thin films followed a power law at operating temperatures higher than 673 K, while at a temperature lower than 673 K conductance vs. oxygen partial pressure followed a linear law. The different response laws of conductance towards oxygen are explained as resulting from the migration of oxygen vacancies from the material bulk towards the surface at temperatures higher than 673 K and as oxygen ionosorption on to the surface sites at temperatures lower than 673 K.