Electrical transport in Al doped ZnO grown by reactive pulsed magnetron sputtering

The synthesis of transparent conductive oxides (TCO) with high transmittance in the near infrared (IR) spectral range is a key requirement for increasing the power conversion efficiency in thin film solar cells. As the absorption at energies close to the Si band gap (1.1 eV) is caused by the free electron plasma it is necessary to maximize their mobility at moderate densities (~5x10²⁰ cm^-3) in order to simultaneously reach low resistivities of ~2x10^-4 Ohm*cm and improve the IR transmittance.
Therefore a reactive magnetron sputtering method using metallic Zn/Al alloy targets was developed to achieve high carrier mobilities (~45 cm²/Vs) in ZnO:Al thin films. The influence of growth temperature, oxygen partial pressure and target Al concentration on the electrical film properties has been investigated systematically by Hall effect measurements.
Additionally XRD, X-TEM, AFM, ERDA, RBS and spectroscopic ellipsometry were employed to reveal film structure, composition and optical properties. The experimentally observed limit of mobility in polycrystalline ZnO:Al is discussed in terms of ionized impurity scattering and clustering as well as grain boundary limited transport.