NANO LETTERS
Evaluations and Considerations for Self-Assembled Monolayer Field-Effect Transistors
2003 Vol. 3, No. 2 119-124
C. R. Kagan,* A. Afzali, R. Martel, L. M. Gignac, P. M. Solomon, A. G. Schrott, and B. Ek IBM T. J. Watson Research Center, Yorktown Heights, New York 10598 Received November 22, 2002
ABSTRACT We elucidate the key chemical and physical requirements necessary for the future successful design and fabrication of molecular field-effect devices. We show that the molecular assembly, device fabrication, and electrical measurements of reported self-assembled monolayer fieldeffect transistors (SAMFETs) cannot be reproduced. Carrier tunneling and device electrostatics place minimum molecular lengths of L > 2.5−3 nm and minimum gate dielectric thickness tdielectric j L/1.5 for such devices. In conflict with reported SAMFET device characteristics, for the values of L and tdielectric in these structures, it is fundamentally impossible to either turn the devices off or to obtain a significant field-effect. Synthesis, assembly, and characterization of functionalized molecular systems and fabrication and characterization of appropriately scaled device structures may enable the successful preparation of a molecular field-effect transistor.
As scaling in Si devices approaches fundamental physical limits, alternative computational devices are being widely explored. Molecular electronics offers the exciting promise of scaling devices to molecular dimensions and of engineering devices with a broad range of characteristics by building function at the molecular level. Figure 1 shows a general schematic of a molecular device. Synthetic chemistry provides the flexibility to build (1) functionality at the end(s) of the molecule to direct the assembly and interconnection of molecules to electrode surfaces, (2) optional tunnel barriers to tune the electronic coupling between the electroactive part of the molecule and the electrodes, (3) molecular bridges that mediate charge transfer between the electrodes and the active center, and (4) the active center of the molecule that provides the desired device function. Aviram and Ratner first recognized in 1974 the concept of chemically tailoring molecules to create analogues of traditional semiconductor devices.1,2 While the vision of molecular electronics is to build the functionality of circuits into the molecule3 and a present goal is to develop electronics based on small numbers of molecules, the immediate challenges are to understand how to design and assemble molecules into devices with desired electronic characteristics and how to fabricate device test structures with well-defined and reproducible characteristics that allow confident assessment of the underlying device chemistry and physics. Recently two-4-8 and three-terminal9-14 molecular devices have been reported, offering intriguing potential alternatives for memory and electronic applications. In this letter, we 10.1021/nl0259075 CCC: $25.00 Published on Web 12/19/2002
© 2003 American Chemical Society
Figure 1. General schematic of a molecular device. Synthetic chemistry provides flexibility to build functionality at the molecular level. Note the inseparability of the molecule and the electrodes.
report our investigation of recently published self-assembled monolayer field-effect transistors [SAMFETs]9-12 (Figure 2a), containing monolayers of molecules such as 1,1′biphenyl-4,4′-dithiol in “trench” (Figure 2b) and planar (Figure 2c) Si structures. This work was carried out in an effort to understand the remarkable, yet puzzling published results on the organization of dithiol monolayers, the structure of fabricated devices, and the electrical characteristics and device physics of these structures. While we now understand that the published results are fraudulent,15 here, we describe the fabrication, characterization, and electrical measurements of these molecules, assemblies, and structures to clear up any misconception about the feasibility of the reported claims
Figure 2. Geometry for field-effect transistors scaled so tdielectric ∼ L to achieve gate modulation of the transistor channel. (a) General structure whether (b) rotated 90° in the “trench” geometry or (c) turned upside down in a “planar” geometry.
and to use this example to elucidate the limitations and pitfalls of these approaches and the key requirements necessary to successfully fabricate molecular devices. Trench and planar device structures were fabricated using highly doped Si wafers that serve as the gate electrode of the transistor. Trench structures were fabricated in (100) p+ Si wafers by reactive-ion etching16,17 and in (110) n+ Si wafers by anisotropic etching.18 The gas chemistry used to fabricate trenches in (100) p+ Si wafers was adjusted to etch vertical sidewalls into the Si wafer (Figure 3a). Anisotropic wet etchants were used to etch (110) n+ Si wafers in the 〈110〉 direction to obtain vertical sidewalls defined by the slow-etching {111} planes19 (Figure 3b). The lithography must be accurately aligned to 1 mA. Thus, it is physically unfeasible to shut off devices with these short channel lengths, in contrast with reported results.9-11 Molecules with lengths J2-3 nm must be designed and synthesized to define the separation between electrodes in vertical and lateral device structures. Incorporating insulating spacers, as depicted in Figure 1, may be used to tune the electronic coupling between the molecule and the electrodes, thereby increasing the resistance of molecular devices in the off state.36 FETs, whether single-crystal transistors, such as Si metaloxide semiconductor field-effect transistors (MOSFETs) typically working in inversion or thin-film transistors (TFTs) typically working in accumulation, are based on a gate field modulating the conductance of the semiconducting channel to turn the device “off” and “on.” This requires the field generated by the gate electrode, spaced from the semiconducting channel by a dielectric layer of thickness tdielectric, to penetrate the channel of length L (Figure 2). Gate modulation of the drain-source current and the characteristic long-channel transistor behavior, arising from “pinching off” of the channel, is achieved in Si device structures scaled to dimensions so that typically L J 1.5tdielectric.37 Scaling the fabrication of an FET to molecular dimensions is no small feat. The gate electrode must be fabricated in proximity so tdielectricjL (assuming similar dielectric constants for the gate insulator and the molecular channel, see ref 37) to achieve gate field penetration and modulation of the molecular channel. SAMFETs reported in refs 9-12 have dimensions such that tdielectric ∼ 23L. In such structures, the gate field is dominated by the drain field, as described by Poisson’s equation, and no significant gate effect is to be expected. While this is especially true for isotropic molecules that respond equally to gate and drain fields, even for anisotropic molecules, since there is always a component of the drain field in the direction of the gate field, the drain field may still dominate. Our research on both reported SAMFET structures and similar device structures with a-Si channels confirm this basic understanding of electrostatics. While tdielectric j L, tdielectric must also be sufficiently large to prevent significant carrier tunneling giving rise to gate leakage currents, a well-known problem in Si technology that is presently driving large research efforts in “high-k” dielecNano Lett., Vol. 3, No. 2, 2003
trics.38 In conclusion, the physics of device structures reported in refs 9-12 is such that significant gate field effects are unattainable. This example elucidates some of the design rules necessary to fabricate molecular devices and in particular a molecular transistor based on field modulation of a molecular channel by a third electrode. (1) Molecules must be synthesized, purified, and characterized to reliably attribute device characteristics to the molecule and not to low-level impurities that may nonetheless dominate electrical characteristics, particularly in device structures with low-yields. While synthetic routes are known for many molecules of interest, developing routes that are easily separated from byproducts and reaction materials is necessary to reduce impurities. (2) The surface preparation,39 the structure of electrodes and insulators of the device, and the molecular assembly on these surfaces must be characterized at the molecular and device area level to understand molecular order, orientation, and domain formation and therefore the origin of charge transport in such devices. (3) The molecule-device structure must be designed and characterized both structurally and electrically to ensure that (a) the gate field penetrates the molecular channel, requiring the dielectric thickness to be comparable to the molecular length; (b) the molecule is in contact with the gate dielectric, or else the space between the molecule and the surface acts to increase the dielectric thickness; (c) the molecule is sufficiently long and chemically functionalized and the gate dielectric is sufficiently thick to limit tunneling between source and drain electrodes and ensure an “OFF” state of the device and between source-gate and drain-gate to limit gate leakage currents. As discussed above, tunneling and scaling arguments result in a minimum molecular length L > 2.5-3 nm and a minimum gate dielectric thickness tdielectric j L/1.5. Figure 2a shows a schematic structure of an FET that obeys these design rules-whether the structure is rotated 90° (Figure 2b) or turned upside-down (Figure 2c). The above represents a short, but challenging list of requirements that must be addressed to make significant progress in molecular electronics. Many mechanisms, such as conformational changes in and reduction-oxidation of molecular systems may be envisioned and synthetically engineered (as in Figure 1) into molecules to attain the desired switching characteristics in molecular devices. Fabricating suitable structures requires the development of processing schemes and characterization tools to attain welldefined molecule-device structures from which one can understand the device physics. Molecular electronics continues to be of great interest in the exploration of future analog logic and memory devices and circuits, despite recent misconduct. There is no scientific reason to lose faith in the tremendous promise of this field. As shown in this paper, SAMFET devices as previously reported and widely heralded as a major breakthrough in molecular electronics, defy limitations presented by carrier tunneling and device electrostatics. Future experiments and devices must be designed with such requirements and considerations in mind. 123
Acknowledgment. The authors thank IBM’s ASTL and CSS facilities for silicon fabrication, T. Domenicucci for TEM EDS maps, C. Lin and F. Meyer zu Heringdorf for help taking AFM images, J. Vichiconti and P. Andry for a-Si deposition, V. Derycke for sharing the STM load, Y. Zhang for developing the RIE recipe, M. Prikas for building vacuum parts, D. Medeiros for GC-MassSpec measurements, H. Hovel for ellipsometry measurements, and J. Tsang and P. Chaudhari for SERS measurements. Special thanks to R. Tromp, for the encouragement to write and patience to read and edit many versions of this manuscript, and to D. Frank, N. Lang, Ph. Avouris, and H.-S. P. Wong for many helpful discussions.
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NL0259075 Nano Lett., Vol. 3, No. 2, 2003
