AbstractsPhysics

Transparent conductive oxides (TCOs) are key functional materials in existing and future electro-optical devices in the fields of energy efficiency, energy generation and information technology. The main application of TCOs is as thin films transparent electrodes where a combination of maximum electrical conductivity and transmittance in the visible to nearinfrared spectral range is required. However, due to the interdependence of the optical properties and the free electron density and mobility, respectively, these requirements cannot be achieved simultaneously in degenerately doped wide band-gap oxide semiconductors. Therefore, a detailed understanding of the mechanisms governing the generation of free charge carriers by extrinsic doping and the charge transport in these materials is essential for further development of high performance TCOs and corresponding deposition methods. The present work is aimed at a comprehensive investigation of the electrical, optical and structural properties as well as the elemental composition of (Al,Ga) doped ZnO and Nb doped TiO2 thin films prepared by pulsed DC reactive magnetron sputtering. The evolution of the film properties is studied in dependence of various deposition parameters through a combination of characterization techniques including Hall-effect, spectroscopic ellipsometry, spectral photometry, X-ray diffraction, X-ray near edge absorption, Rutherford backscattering spectrometry and particle induced X-ray emission. This approach resulted in the development of an alternative process control method based on the material specific current-voltage pressure characteristics of the reactive magnetron discharge which allows to precisely control the oxygen deficiency of the sputter deposited films. Based on the experimental data, models have been established that describe the room temperature charge transport properties and the dielectric function of the obtained ZnO and TiO2 based transparent conductors. On the one hand, these findings allow the prediction of material specific electron mobility limits by identifying the dominating charge carrier scattering mechanisms. On the other hand, new insight is gained into the origin of the observed transition from highly conductive to electrically insulating ZnO layers upon the incorporation of increasing concentrations of Al at elevated growth temperatures. Moreover, the Al and Ga dopant activation in ZnO have been quantified systematically for a wide range of Al concentrations and deposition conditions. A direct comparison of the Ga and Al doping efficiency demonstrates that Ga is a more efficient electron donor in ZnO. Further, it has been shown that high free electron mobilities in polycrystalline and epitaxial Nb:TiO2 layers can be achieved by reactive magnetron sputtering of TiNb alloy targets. The suppression of rutile phase formation and the control of the Nb dopant activation by fine tuning the oxygen deficiency have been identified as crucial for the growth of high quality TiO2 based TCO layers.