Free charge carriers in hydrogen doped ZnO thin films deposited by pulsed DC magnetron sputtering

ZnO thin films are used as both highly conductive TCO electrode in different optoelectronic devices and as highly transparent, highly resistive layers in applications such as diffusion barriers, buffer layers, and insulating coatings. We report on intrinsic ZnO (i-ZnO) thin films and hydrogen doped ZnO (ZnO:H) thin films deposited by pulsed DC magnetron sputtering using a rotatable, ceramic, intrinsic ZnO target at room temperature. The resistivity of the deposited films could be reduced even below 1.2 mOhm cm by increasing the hydrogen gas flux used as dopant gas. It will be shown that not only the resistivity changes with doping level of the films, but also  important coating  characteristics like surface roughness, deposition rate, electron transport mechanism, optical properties, etc. An optical model based on the effective medium approximation (EMA) and Lorenz-Drude dispersion theory has been applied to investigate the impact of hydrogen gas on free charge carriers in the experimental samples. It will be shown, for instance, how strong the volume fraction of the depletion layer surrounding ZnO crystallites can be influenced by  deposition parameters such as  doping gas flow and deposition DC power.