The structural phase transition and mechanism of abnormal temperature dependence of conductivity in ZnTe:Cu polycrystalline thin films

The structure and electrical transport of ZnTe polycrystalline thin films have been studied as a function of doping concentration and post-grown annealing temperature. For undoped ZnTe thin films, the electrical transport characterizes as commonly expected thermal activation of carriers with activation energy 0.22 eV, and the structure is the cubic phase with a (111)-preferred orientation as shown by X-ray diffraction (XRD) measurement. For copper doped ZnTe thin films an abnormal temperature dependence of conductivity has been observed. The conductivity measurement with temperature cycles at top values of 144, 170, 217, and 260 degreesC, has been performed and the conductivity activation energy has been calculated. For copper doped ZnTe thin films, the structural phase transition, i.e. cubic --> hexagonal --> cubic, is observed by XRD measurement with the increase of annealing temperatures. With increasing temperature, a part of the copper atoms in grain boundaries can diffuse into grains and some of copper atoms in grains are thermally ionized and then inhabit the Zn sites of ZnTe lattice in grains, which cause the structural phase transition of ZnTe films and, the variation in grain boundary barrier. The effects of the variation in both grain boundary barrier and average carrier concentration on the changes in conductivity with temperature are discussed taking into consideration of the grain boundary trapping theory. Then the abnormal electrical transport behavior in copper doped films is explained. (C) 2002 Elsevier Science B.V. All rights reserved.