"Carrier Transport in Pentacene Probed by Scanning Tunneling Spectroscopy"*S. Gaan, S. Nie, and R. M. Feenstra1+, Carnegie Mellon University By performing tunneling spectroscopy measurements (using a single probe-tip) over a wide range of tunnel currents, the transport of carriers within semiconducting samples can be investigated. We have performed such studies for pentacene thin films on SiC, with comparative studies performed on various other SiC(0001) surfaces including the Si-rich 3x3 and √3x√3-R30° structures (Mott-Hubbard insulators) as well as graphene on SiC. Pentacene on oxidized SiC surface was deposited by sublimation in ultra-high-vacuum, with a resulting dendritic growth arrangement shown in Fig. 1. Current-voltage curves acquired with various tip-sample separations, Δs, reveal a distinct saturation in the current at positive voltage, as shown in Fig. 2. (The behavior is similar to that recently reported for isolated Si dangling bonds on a B-doped Si(111) surface [1]). This current saturation reveals a transport limitation in the LUMO band of the molecules, whereas no such limitation is found for the HOMO band at negative sample voltages. We interpret this limitation in terms of a space-charge limited type of current injected into the LUMO band, producing a high density of carriers that lead to additional band bending in the sample. The transport limitation is quantified by measurements of the decay constant, k, for the tunnel current, with k->0 when the current saturates. For comparison, we examine other SiC(0001) surfaces prepared by thermal treatment of SiC in the presence of a Si flux, including the 3x3, √3x√3-R30°, and graphene structures. We find good correlation of the measured k values with the expected conductivities of the surfaces. * Supported by the National Science Foundation + Corresponding author, feenstra@cmu.edu [1] M. Berthe et al., Science 319, 436 (2008). |