Origins of limited electrical performance of polycrystalline Cu2O thin-film transistors

In this thesis, cuprous oxide Cu2O was investigated concerning its ability to function as ptype channel in thin-film transistors. The material was chosen for its promising electronic characteristics as bulk single crystal. In order to be competitive with other technological solutions for flexible thin-film electronics, the temperature during fabrication has to remain below 200 C. Following this approach, a tremendous gap between potential and actual electrical performance of Cu2O thin-film transistors is encountered. The aim of this thesis is to show the reasons for this discrepancy. Relevant stages during the fabrication process of a thin-film transistor were analyzed with respect to their impact on the cation oxidation state. These stages included thin film deposition, the study of interface formation to the dielectric layers as well as postdeposition annealing. Semiconducting and dielectric layers were deposited by reactive magnetron sputtering (Cu2O, Cu4O3, CuO, Bi2O3, Al2O3) and atomic layer deposition (Al2O3). An innovative approach for a thickness-dependent characterization of thin films was conducted by a combination of in situ X-ray photoelectron spectroscopy with in situ conductance measurement. Electrical properties of Cu2O films and thin-film transistors were analyzed in dependence of film thickness, temperature, oxygen partial pressure and time. It is shown, that the primary cause for the limited electrical performance is the polycrystalline morphology in conjunction with the material-inherent tendency to oxidation and reduction of the metal cation. On the one hand, metallic Cu(0) depletes the material from hole carriers and causes Fermi level pinning. On the other hand, a high conductivity in the grain boundary is caused by the presence of Cu(II). A model is presented to describe the conductivity at different film thicknesses as a function of grain size