Dopant activation and charge transport limits in transparent conductive (Al,Ga):ZnO and Nb:TiO2

Transparent conductive oxides (TCOs) are a class of wide bandgap metal oxides in which high densities of free charge carriers can be induced in such a way, that a unique combination of high optical transmittance in the visible spectral range and metal-like electrical conductivity is achieved. The influence of the oxygen partial pressure, substrate temperature, dopant concentration and substrate type on the properties (Al,Ga) doped ZnO and Nb doped TiO2 thin films has been studied systematically. The correlations between electrical, structural and optical properties as well as the elemental composition of the films have been characterized by a combination of Hall-effect, XRD, spectroscopic ellipsometry, spectral photometry, RBS and PIXE techniques including complementary XANES and TEM studies. The results are discussed with a focus on understanding the dopant incorporation into the ZnO and TiO2 host materials as well as finding physical limits for the resistivity based on charge transport models for degenerate polar semiconductors.