Selective Passivation of Electrical Defects in Metal Oxides

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In 1962 the first solid state metal oxide gas-sensor was reported. Since then metal oxides gained high importance in the fields of sensors due to their excellent chemo-resistive properties. Since 2004 another fast developing field of application for metal oxides was promoted by Hosono who fabricated the first metal oxide based thin-film transistor (TFT). High mobility even in amorphous and nano-crystalline films, transparency in the visible range and solution processability by cheap deposition technology make the class of material highly interesting for innovative electronic applications, e.g. for transparent and smart displays. The dynamic surface structure of metal oxides serves as big advantage for sensor and catalysis applications but represents the major drawback of the material as active layer in transistors. Logic circuits require high stability and reliability even under long term operation. These criteria turned out to be very challenging in metal oxide transistor application. Especially, a high sensitivity of the electrical devices towards humidity was correlated to the significant instability of device characteristics. The effect becomes even more enhanced by the ultra-thin films and their nanoparticular and amorphous character.Especially in cheap and low temperature processes, which are a prerequisite for wide spread applications, an increased density of impurities is incorporated in the material creating a high number of defect states which act as electrically active sites hampering proper transistor operations. Low temperature (< 200 °C) spray deposited ZnO is possible as a large-area coating process, but exhibits a dramatic performance loss with decreasing temperature. Hence passivation of these active sites is not only necessary to improve device stability significantly but also to reduce the process temperature dramatically while keeping the performance constant. In this project various approaches are followed to selective passivate those defects during and/or after deposition. In this respect, Fluor containing molecules are known for their unique properties, e.g. Teflon towards water. AG Wagner and AG Röschenthaler have already demonstrated exceptional passivation properties offered by specific Fluor containing molecules with respect to metal oxide transistor operations. Focus in this project is to systematically modify functional chemical groups in Fluor containing molecules, which allow to understand their passivation mechanism and to identify superior passivation procedures and strategies for metal oxide transistors.