In recent years, substantial progress has occurred in the field of molecular electronics . In this paper, charge transport through molecule-semiconductor junctions is probed with ultra-high vacuum (UHV) scanning tunneling microscopy (STM). The presence of the semiconductor band gap enables new manifestations of resonant tunneling through individual molecules, such as unipolar negative differential resistance (NDR). Furthermore, by doping the substrate, the majority charge carrier can be tailored, thus allowing asymmetry to be designed into the I-V curve. By demonstrating these effects on silicon, molecular electronic devices have the potential of being interfaced with conventional integrated circuit technology.
Three organic molecules are considered on the Si(100) surface: styrene, cyclopentene, and TEMPO. In all cases, room temperature I-V curves on individual molecules mounted on degenerately n-type Si(100) show NDR at negative sample bias. On the other hand, at positive sample bias, the I-V curves do not show NDR, although a discontinuity in the differential conductance is observed. With degenerately p-type Si(100) substrates, NDR is observed at positive sample bias while the discontinuity in the differential conductance occurs at negative sample bias. These empirical observations can be qualitatively explained with the energy band diagram for a semiconductor-molecule-metal junction . More sophisticated theoretical treatments also confirm the experimental data .
In addition, cryogenic variable temperature UHV STM has been used to probe isolated cyclopentene molecules adsorbed to degenerately p-type Si(100) . I-V curves taken at 80 K show NDR at positive sample bias in agreement with room temperature data. Due to the enhanced stability of the STM at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. In this manner, I-V curves are demonstrated to be reproducible and possess negligible hysteresis for a given tip-molecule distance. On the other hand, measurements with variable tip position show that the NDR voltage increases with increasing tip-molecule distance. Using a one-dimensional capacitive equivalent circuit, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-metal junction. This model also provides a quantitative estimate for the alignment of the molecular orbitals with respect to the substrate Fermi level. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices.
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Purdue University, West Lafayette, IN