Radiotracer Diffusion Measurements of Noble Metal Atoms in Semiconducting Organic Films
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Radiotracer diffusion measurements of noble metal atoms in semiconducting organic films Michael Scharnberg, Jörn Kanzow, Klaus Rätzke, Rainer Adelung, and Franz Faupel Chair for Multicomponent Materials, CAU Kiel, Kaiserstrasse 2, 24143 Kiel, Germany Stephan Meyer, Jens Pflaum 3. Physikalisches Institut, University Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany ABSTRACT The application of organic field effect transistors (OFETs) for large scale low-cost electronic devices has lead to intense research. Diindenoperylene (DIP) thin films on SiO2 are a prominent system due to their high structural out-of-plane order. While bottom contact OFET structures can be realized easily, preparation of top contacts might cause diffusion of metal atoms (typically Ag or Au) deep into the organic film changing the injection properties at the interface. These properties are of great importance for device fabrication. Therefore, only by understanding the diffusion behaviour of metals into the organic layer, formation of well defined interfaces and control of their properties will become possible. For a better understanding of the diffusion of noble metal atoms into crystalline organic films, a radiotracer technique has been used to obtain diffusion profiles for Ag and Au diffusion in crystalline DIP films. For Ag diffusion in DIP, the decrease in Ag concentration of four orders of magnitude within the first few nanometers indicates that most of the metal atoms remain near the surface while small amounts can penetrate deep into the thin film and can even accumulate at the interface between organic film and the silicon substrate. A comparison with diffusion profiles obtained for polymers indicates that the interplay between diffusion and immobilization by aggregation also determine the diffusion behaviour of metals in organic crystalline materials. Latest experiments support this interpretation of the diffusion profiles. Single atoms are highly mobile in the organic crystalline material due to the weak interaction between the metal and the organic material. Therefore, most of the single atoms that penetrate into the material do so during the initial phase of the deposition. When more and more atoms arrive at the surface, cluster formation sets in. Due to the high cohesive energy of the metal the atoms can not leave the cluster and become immobilized. After deposition of a closed surface layer no further metal diffusion should be observed. With the knowledge about the diffusion processes gained by the radiotracer measurements control of process parameters and development of barrier layers in sub-monolayer range should be possible.
INTRODUCTION The radiotracer method used for these experiments has been successfully applied to study diffusion in various systems including metals, metallic glasses and also polymers with high resolution [1,2,3,4], but has not yet been applied to study metal diffusion in crystalline organic materials. These materials are of technological importance in the field of organic electronics. Devices
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