Probing Molecules in Integrated Silicon−Molecule−Metal Junctions by Inelastic Tunneling Spectroscopy

Abstract

Molecular electronics has drawn significant attention for nanoelectronic and sensing applications. A hybrid technology where molecular devices are integrated with traditional semiconductor microelectronics is a particularly promising approach for these applications. Key challenges in this area include developing devices in which the molecular integrity is preserved, developing in situ characterization techniques to probe the molecules within the completed devices, and determining the physical processes that influence carrier transport. In this study, we present the first experimental report of inelastic electron tunneling spectroscopy of integrated metal-molecule-silicon devices with molecules assembled directly to silicon contacts. The results provide direct experimental confirmation that the chemical integrity of the monolayer is preserved and that the molecules play a direct role in electronic conduction through the devices. Spectra obtained under varying measurement conditions show differences related to the silicon electrode, which can provide valuable information about the physics influencing carrier transport in these molecule/Si hybrid devices.

Molecules on Si: Electronics with Chemistry

“…[68] In other work powerful techniques for characterizing Si MOMS were used and, in part, adapted. [69][70][71][72] Significant contributions have been made to transport modeling for several Si-molecule systems. [60,73,74] Interfacial monolayers affect the junction electrostatics by the interfacial dipole they create/change (see Sections 3.2, 3.3, and 4.2).…”

“…Excellent reviews on the chemistry of wet [77][78][79] and gas-phase [10,31,80,81] adsorption have been published. The SiÀH bond needs to be activated for the reaction, which can be done by heating (150À200 8C), [44,71] UV illumination, [31,69] Lewis acid-mediated initiators, [42] or electrochemistry. [42,43] Alternatively, H is replaced by Cl, a better leaving group that can react with short Grignard reagents.…”

“…There are specialized electrical characterization methods that provide finer interface details, such as inelastic electron tunneling spectroscopy (IETS) to reveal molecular vibrations involved in the transport, [71,72,98] noise analysis to learn about interface states [73] and ballistic emission electron microscopy (BEEM) to study spatial inhomogeneities for transport. [99,100] Here we focus on macroscopic transport characteristics such as current and capacitance measurements.…”

“…[71] Still, one of the most basic requirements of work with molecules directly bound to Si is that oxidation must be avoided as much as possible. The reason is that oxidation drastically changes the interface charge and energetics, via surface states that can dominate transport already at concentrations below 0.01 at%, [42,97] i.e., well below the XPS detection limit.…”

“…This improves the accuracy of the transport measurement, because of smaller net capacitance and a decrease in the ratio of setup (series) to junction resistance. [62,69,71,89] The question here is how clean and flat the Si surface is between the patches of SiOx or photoresist, i.e., after patterning. Common spectroscopies (IR, XPS) cannot be used, as they generally require significantly larger areas.…”