Organic Field-Effect-Transistors for the Study of Correlation Phenomena
Final Report Abstract
The intention of this work was to prepare and investigate new kinds of 2- dimensional systems. Therefore, we studied both single crystals of the organic semiconductors tetracene and perylene as well as graphene mono- and doublelayers. These systems differ fundamentally, for example, organic semiconductors have big baud gaps, whereas in graphene, valence and conduction bands touch. Furthermore, electrons in organic semiconductors follow a parabolic dispersion relation, whereas in graphene, their energy depends linearly on momentum. But there are also similarities between these systems: Both are based on benzene rings which form a poly-aromatic system. Additionally, in both cases the semiconductors are ambipolar, meaning that by varying the gate voltage, not only the charge carrier density can be manipulated but in principle it is possible to switch from electron to hole transport in one sample. We were successful in preparing 2-dimensional systems out of perylene and tetracene crystals in a field effect transistor geometry. In both cases however, measurements at low temperature were not possible, which prevented to observe hallmarks of 2-dimensional transport. In tetracene crystals a phase transition occurs during cooling. The phase transition is accompanied by a change of volume, which generates cracks and destroys transistor action. We demonstrated that tetracene serves well as a model system for sample preparation and contact optimization on organic single crystals, but it is unsuitable to study charge carrier transport at low temperatures. Perylene, on the other hand, promises high carrier mobilities at low temperatures due to very low trap densities, however a suitable contact material that allows for efficient charge injection at low temperature remains to be dientified. To prevent problems due to thermal stress, contacting the electrodes with thin gold foils for example might be a possibility. Furthermore, intermediate layers between perylene and contact metals could enhance hole injection, so that the high intrinsic carrier mobilities can be deployed. But above all, achieving electron injection into perylene would be a big improvement, since SCLC clearly shows In graphene, we demonstrated SdH oscillations and the quantum Hall effect. To be able to observe further effects of 2-dimensionaI transport, primarily lower concentrations of imperfections are necessary. Some progress was made to reduce the intrinsic doping level through room temperature evacuation and sample annealing up to 140° C. We found evidence that the substrate has a negative influence on transport. Therefore, free standing graphene flakes by etching away the SiO2 or at least adequate intermediate layers between graphene and gate isolation could further boost the quality of charge carrier transport. Finally, compressibility studies using a single electron transistor were performed on graphene. These studies confirmed a theoretical prediction that at the charge neutrality point, where the average carrier density is zero, electron and hole puddles exist. These electron and hole puddles are likely an important ingredient to understand the so-called minimal conductivity in graphene. Even though the system is on average charge neutral, it can support a non-zero current.
Publications
- Comparative study of the growth of sputtered aluminium oxide films on organic and inorganic substrates, Thin Solid Films 516, 6377 (2008).
- Investigation of the shift of Raman modes of graphene flakes, Phys. Stat. Sol. (b) 244, 4143 (2007).
- Mechanism for the enhancement of the thermal stability of organic thin films by aluminium oxide capping layers, J. Mater. Res. 21, 455 (2006).
- Observation of electron-hole puddles in graphene using a scanning singleelectron- transistor, Nature Physics 4, 144 (2008).
