Metal-semiconductor transition in epitaxial ZnO thin films

We report on the formation and transport properties of ZnO thin films which are grown by pulsed-electron beam deposition under a low residual oxygen pressure (10−5 mbar). ZnO films presenting metallic conductivity at room temperature, and a metal-semiconductor transition at low temperature, were epitaxially grown on Al2O3 single crystal substrates for growth temperatures in the 300–450 °C range. These results have been interpreted through the quantum corrections to conductivity in a disordered oxide conductor, implying first a high density of carriers leading to degenerate semiconductor ZnO films, and then a sufficient disorder in these films. These characteristics could be related to the nature of the ZnO films formed by pulsed-electron deposition: a high density of carriers related to an oxygen deficiency in the films and a high density of defects related to the high deposition rate of the pulsed-electron beam deposition method.