Structural insight into nanoscale inhomogeneity of electrical properties in highly conductive polycrystalline ZnO thin films doped using methane

Abstract

Abstract Recently, methane has been demonstrated as an effective n-type dopant for ZnO thin films deposited using the RF-magnetron sputtering method. It was shown that the major electrical doping effect of methane is caused by hydrogen released during methane decomposition. This work investigates the origin of the observed increase in conductivity of methane-doped ZnO films with the increase in thickness. The study is aimed at describing the nature of this thickness-dependent effect through a detailed analysis of the thickness-dependent morphology and crystalline structure. A combination of structural, electrical, and optical characterization revealed a transition from fine-grained films with a random orientation at early stages to partially (002)-textured films with columnar grains at later stages of growth. It is demonstrated that grain/sub-grain boundaries increase the electrical conductivity and that the contribution of such buried inner boundaries increases with increasing thickness. It is proposed that hydrogen diffuses along the grain and sub-grain boundaries during growth, leading to continuous doping of the buried interfaces. This hydrogen diffusion mechanism results in an apparent ‘additional doping’ of thicker films. The results provide new insights into the thickness-dependent conductivity of doped polycrystalline ZnO films mediated by hydrogen diffusion along internal interfaces.