| AVS 54th International Symposium | |
| Surface Science | Monday Sessions |
| Session SS2-MoA |
| Session: | Electronic and Vibrational Structure |
| Presenter: | S. Gaan, Carnegie Mellon University |
| Authors: | S. Gaan, Carnegie Mellon University R. Duca, Carnegie Mellon University R. Feenstra, Carnegie Mellon University |
| Correspondent: | Click to Email |
Pentacene (Pn) is a promising material for organic field effect transistors because of its relatively high mobility and its tendency to form ordered structures. In this work we have used scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the electronic properties of Pn thin films. The substrates were hydrogen-etched SiC, oxidized by exposure to 4000 L of molecular oxygen at 700°C. The Pn was deposited from a crucible by vacuum sublimation, with the sample at room temperature. On a large scale, the Pn films display a layered morphology with dendritic edges, consistent with prior results.1 Within each layer, the molecules form ordered arrangements; it is on these regions that STS was performed. We obtain a HOMO-LUMO gap of about 2 eV.2 Effects of degradation of Pn molecules were seen in the spectra, producing smearing of the band edges. Using the STM, with various tip-sample separations, widely different currents were injected into the sample in order to probe transport of the charge carriers. The tunnel current was found to saturate at positive bias for all tip sample separations, thus revealing a spreading resistance type of effect in Pn films. In other words, a quasi Fermi level exists in the sample and varies as a function of distance from the apex of the probe tip. In order to determine current saturation at negative bias, we examined the dependence of the current on tip-sample separation, at constant voltage. Significant deviation from ideal vacuum tunneling was observed, which also indicates transport limited tunnel current. To explain the observed transport limitation in the tunnel current we propose a simple two resistor model, one resistor for the vacuum and one for the sample, with the assumption that the sample resistance is constant. With different injection levels and voltages we obtain reasonable fits to the data. Using this model we are able to deduce resistivities of the sample for both filled and empty states, 3.3x102 and 2x104 Ω cm, respectively. Work is underway to interpret these results in terms of possible transport mechanisms in our Pn thin films.
1Heringdorf et. al., Nature, 412, 517 (2001).
2Repp et. al., Phys. Rev. Lett., 94, 026803 (2005).