A Brief Overview of Nanoelectronic Devices

James C. Ellenbogen, Ph.D.

To be presented at the 1998 Government Microelectronics Applications Conference (GOMAC98)

Arlington, VA, 13-16 March 1998

January 1998

A Brief Overview of Nanoelectronic Devices

James C. Ellenbogen, Ph.D.

MP 98W0000024

January 1998

Sponsor MITRE MSR ProgramProject No. 51MSR89GDept. W062

Approved for public release;distribution unlimited.

Copyright © 1998 by The MITRE Corporation.All rights reserved.

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A BRIEF OVERVIEW OF NANOELECTRONIC DEVICES

James C. Ellenbogen, Ph.D.Nanosystems Group

The MITRE CorporationMcLean, VA 22102

e-mail: nanotech@mitre.orgWWW: http://www.mitre.org/research/nanotech

ABSTRACT

This paper surveys and explains nanometer-scale,quantum-effect alternatives to micron-scale, bulk-effecttransistors in digital circuits. The status of R&D andrecent important advances are reviewed briefly.

I . INTRODUCTION

For the past forty years, electronic computers have grownmore powerful as their basic sub-unit, the transistor, hasshrunk. However, the laws of quantum mechanics, plusthe limitations of materials and fabrication techniquessoon are likely to inhibit further reduction in the minimumsize of today's bulk-effect semiconductor transistors.Researchers have projected that as the overall size of thebulk-effect, semiconductor transistor is aggressivelyminiaturized to approximately 0.1 micron (i.e., 100nanometers) and beyond, the devices may no longerfunction as well.

Thus, in order to continue the miniaturization of integratedcircuits well into the next century, it is likely that present-day, micron-scale or microelectronic device designs will bereplaced with new designs for devices that take advantageof the quantum mechanical effects that dominate on themuch smaller, nanometer scale. (Note that 1 nanometer,abbreviated nm, is one billionth of a meter or 10 atomicdiameters.)

This paper briefly surveys such quantum-effect,nanometer-scale alternatives to micron-scale, bulk-effecttransistors for use in electronic digital circuits. Longer, in-depth reviews of this topic appear elsewhere [1-3].

The topic of nanoelectronic devices subdivides into twobroad areas, as follows:

• Solid-state quantum-effect nanoelectronic devices

• Molecular electronic devices

The comments above are summarized and amplified inFigure 1, as well as in the sections of this paper that follow.

I I . SOLID-STATE QUANTUM-EFFECTNANOELECTRONIC DEVICES

Solid-state quantum-effect nanoelectronic devices are thenearer-term alternative for continuing to increase thedensity and speed of information processing. Thistechnology, which changes the operating principles, butnot the solid fabrication medium for integrated circuits,

might extend Moore's Law of electronics miniaturization toproduce devices down to 25 nm in length, beyond thedomain of the bulk-effect transistor. Present-day field-effect transistors (FETs) on commercial integrated circuitsare approximately 1 micron or 1000 nm across, and eventhe most optimistic proponents of aggressivelyminiaturized bulk-effect FETs predict that they will ceaseto function effectively when their gate lengths dip below 25nm, which corresponds to an overall device length ofapproximately 100 nm.

By contrast, quantum-effect switching devices tend tofunction better when they are made smaller. Solid statequantum-effect nanoelectronic devices might be made assmall as approximately 12 to 25 nanometers across andstill function effectively if they could be fabricated reliablyand uniformly on that scale. Presently, mass fabricationon this small scale does present a set of formidablechallenges, no matter what the device design.

However, a number of solid-state nanoelectronic switchingdevices have been fabricated and demonstrated, and pro-totype solid-state nanoelectronic processors already areoperational, as well [4,5]. Moreover, the drastic decreasein size that could result from the wide-scale introduction ofsuch nanoelectronic devices might produce ultra-fast, lowpower integrated circuits with as many as 100 billion oreven 1 trillion switching devices on a single CPU chip, aswell as Terabyte nonevanescent memories on a chip.

Such great advances would require, however, the reliable,uniform mass fabrication of features only 5 to 10nanometers wide in solids. This is at least twenty-fivetimes as small as is achievable using UV lithography, thepresent industrial method of choice. More modest, near-term goals do not require such extreme precision, but takeadvantage of other very desirable properties of solid-statequantum-effect devices, such as low power consumptionand high speed.

A . Types of Solid-State NanoelectronicDevices

As is indicated in Figure 1 and explained in greater detailelsewhere in the literature [1], there are 3 basic categoriesof solid-state nanoelectronic devices: (1) Quantum Dots(or "artificial atoms"), (2) Resonant Tunneling Devices, and(3) Single-Electron Transistors (SETs). A small "island"composed of semiconductor or metal in which electronsmay be confined is the essential structural feature that allthese quantum-effect, solid-state nanoelectronic deviceshave in common. The island in a nanoelectronic deviceassumes a role analogous to that of the channel that forms

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Aggressively MiniaturizedSemiconductor Transistors

Quantum-EffectNanoelectronic Devices

Alternatives for Nanometer-ScaleElectronic Switches

Figure 1. Taxonomy of nanoelectronic devices

Solid-StateNanoelectronic Devices Molecular Electronics

“Hybrid” Micro-Nano-Electronic Devices

Quantum Dots(QDs)

Resonant Tunneling Devices

Single-ElectronTransistors (SETs)

Electrons on island havezero

classical d.o.f.

Electrons on island haveone or two

classical d.o.f.

Electrons on island havethree

classical d.o.f.

Resonant Tunneling Diodes (RTDs)

Resonant TunnelingTransistors (RTTs)

beneath the gate in a familiar microelectronic field-effecttransistor when it is switched on. The composition, shape,and size of the island gives the different types of solid-state nanoelectronic devices their distinct properties.Controlling these factors permits the designer of thedevice to employ quantum effects in different ways tocontrol the passage of electrons on to and off of theisland.

B . Resonant-tunneling devices

Here, we focus primarily on explaining the operation ofresonant tunneling devices, because they employquantum effects in their simplest form [1]. Presently,these devices usually are fabricated from layers of twodifferent III/V semiconductor alloys, such as the pairGaAs and AlAs. The simplest type of resonant tunnelingdevice is the resonant tunneling diode (RTD).

As depicted in Figure 2(a), a resonant-tunneling diode ismade by placing two insulating barriers in asemiconductor, creating between them an island orpotential well where electrons can reside. Resonant-tunneling diodes are made with center islandsapproximately 10 nanometers in width. Wheneverelectrons are confined between two such closely spacedbarriers, quantum mechanics restricts their energies toone of a finite number of discrete "quantized" levels. Thisenergy quantization is the basis for the operation of theresonant-tunneling diode.

The only way for electrons to pass through the device is to

"tunnel," quantum mechanically, through the two barriers.The probability that the electrons can tunnel is dependenton the energy of the incoming electrons compared to theenergy levels on the island of the device. As illustrated inFigure 2(b), if the energy of the incoming electrons differsfrom the energy levels allowed inside the potential well onthe island, then current does not flow. The device isswitched "off."

However, when the energy of the incoming electrons alignswith that of one of the internal energy levels, as shown inFigure 2(c), the energy of the electrons outside the well issaid to be "in resonance" with the allowed energy inside thewell. Then, current flows through the device--i.e., thedevice is switched "on."

By adding a small gate electrode over the island of an RTDone may construct a somewhat more complex resonanttunneling device called a resonant tunneling transistor(RTT). In this three-terminal configuration, a small gatevoltage can control a large current across the device.Because a very small voltage to the gate can result in arelatively large current and voltage across the device,amplification or "gain" is achieved. Thus, an RTT canperform as both switch and amplifier, just like theconventional bulk-effect transistor.

Unlike conventional bulk effect transistors which usuallyhave only two switching states, "on" and "off," resonant-tunneling devices like RTDs and RTTs can have severalswitching states. This occurs because the quantum wellon the island may exhibit several possible energy levels.

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~10-100nm

(a)

MetalContact

MetalContact

DrainIslandSource

Tunnel Barriers

(b)

Energy

Distance

“Potential Energy Well”Occupied

ConductionBand

•

Lowest-Energy(N+1)-ElectronStates in Well

(c)

TransmittedElectrons

OccupiedConduction

BandEnergy

Distance

•

Figure 2. Operation of Solid-StateResonant Tunneling Diode

Thus, as the voltage bias on an RTD is increased fromzero, where it is "off," the device initially switches "on"when the first energy level comes in resonance with theincident electrons. Then, it switches off again as the biasis increased further, past resonance. However, if there isa second energy level in the quantum well, then the devicecan switch on again as the bias voltage of the RTD (or thegate voltage of an RTT) is increased still further. Thismultistate switching behavior permits each device to"count higher" and represent more logic states.

C . "Hybrid" Microelectronic-NanoelectronicResonant Tunneling Transistors

Purely nanometer-scale RTTs tend to be difficult tofabricate with sufficient uniformity in large quantitiesbecause of their relatively complex structure, small size,and the sensitivity of the effects they employ. However,there are also nanoelectronic devices which are "hybrids"of solid-state quantum-effect devices combined withmicron-scale transistors. One such hybrid transistor-likedevice, the RTD-FET, is constructed by building tiny,nanometer-scale quantum-effect RTDs into the drain (orsource) of a bulk-effect micron-scale FET. A schematic ofsuch a device is shown in Figure 3.

Such a hybrid RTT can exhibit multistate switching be-havior of the type described in the preceding section for

Figure 3. “Hybrid”Microelectronic-NanoelectronicResonant Tunneling Transistor

0.5 µm

RTDs

SiO2

p-Si

Gate

Gate OxideDrainSource

purely nanometer-scale RTTs. For this reason, the hybriddevice can represent more logic states than a pure bulk-effect, microelectronic FET with the same "footprint" on theintegrated circuit. Thus, the density of the logic can beincreased using the multistate switching characteristics ofhybrid micro-nanoelectronic RTTs, without appreciablyincreasing the density of the devices on an integratedcircuit. Also, these hybrid RTTs share the advantages oflow power and high speed exhibited by the purelynanoelectronic RTTs.

Most important, fabricating circuits with this relativelylarge, hybrid type of three-terminal RTT is easier thanfabricating circuits with the much tinier, complexstructures for purely nanoelectronic RTTs. For this reasona number of groups are experimenting with such devices.In recent months, robust prototype circuits based uponsuch hybrid devices have been demonstrated [4,5].Hybrid logic can be viewed as an important, practicalengineering step on an evolutionary path towardnanoelectronics. It should accelerate the availability ofquantum-effect devices in integrated circuits with verydense functionality.

I I I . MOLECULAR ELECTRONIC DEVICES

Ultimately, however, it will be desirable to build ultra-dense, low-power nanoelectronic circuits made from purelynanometer-scale switching devices and wires. Molecularelectronics--using individual covalently bonded moleculesto act as wires and switching devices--is the longer-termalternative for achieving this increase in density and forcontinuing Moore's Law down to the nanometer scale.Individual molecular switching devices could be as smallas 1.5 nanometers across, with densities of approximately1012 devices per sq. cm. This decrease in size couldresult in Terabyte memories on a chip and in excess of onetrillion switching-devices on a single CPU chip. A primaryadvantage of molecular electronics is that molecules arenatural nanometer-scale structures that can be madeabsolutely identical in vast quantities (approximately 1023at one time). Also, synthetic organic chemistry--the

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Energy

Distance

Tunnel Barrier

UnoccupiedOccupied {

{

Unoccupied

Occupied

HS • • • SHCH2 CH2

TRANSMITTEDELECTRONS

Figure 4. Molecular Electronic Resonant Tunneling Diode

chemistry of carbon-based compounds--offers moreoptions for designing and fabricating these intrinsicallynanometer-scale devices than the technology presentlyavailable for producing solid-state chips with nanometer-scale features. It has been shown during the past fewyears that small organic molecules can conduct andcontrol electricity [6-11], that large organic biomoleculesare conductive [12], and that individual molecules of thenewly discovered molecular forms of pure carbon calledbuckminsterfullerenes ("buckyballs" and "buckytubes")have interesting and useful electrical properties [13-16].

Great progress is being made in understanding andharnessing the electrical properties of individual organicmolecules. There has been particular experimentalprogress in demonstrating that individual molecules canconduct electrical current to function as wires and thatthey also can act as switches to turn this current on andoff [6-11,13-16]. No logic circuits using these molecularswitching devices have been fabricated yet. However,designs for such circuits have been proposed recently[17,18].

To some extent, the research and development ofmolecular electronic switches is recapitulating that for thecorresponding solid-state devices, except on a muchsmaller scale. Thus, it is not surprising, that molecularelectronic switching proposals and prototypes may becategorized into molecular analogs of solid-state diodesand analogs for solid-state three-terminal FETs.

A molecular analog of the solid state RTD that is discussedabove in Section II.B is shown in Figure 4. This molecularRTD first was fabricated by Tour [9] and thendemonstrated by a group led by Reed [10].

In the molecular structure depicted at the top of Figure 4,the hexagonally shaped components are known asbenzene "rings." Chains of these rings, such as appear atthe right and left ends of the molecule in the figure havebeen shown to function as conductive "molecular wires"[6,7]. The "CH2" groups (termed "methylene" groups), onthe other hand, act as insulators or "barriers" to electronflow. They can trap electrons on the single benzene ring

between them, which forms an island. This molecularsandwich-like structure, with the island in the middle,produces a potential energy well like that sketched at thebottom of Figure 4.

Except that it is 10 to 100 times smaller, the molecularpotential well in Figure 4 plainly resembles the wellillustrated in Figure 2(b) for the solid state RTD. Thissimilarity is the reason why the molecule sketched inFigure 4 behaves like an RTD.

I V . CONCLUSION: CHALLENGES

Despite enormous recent progress in the fabrication anddemonstration of nanoelectronic devices, many challen-ges remain. For solid state nanoelectronics, one of themost important challenges is to be able to produce reliablyand uniformly in silicon the characteristic nanometer-scalefeatures required for nanoelectronics: nanometer-scaleislands, barriers, and "heterojunctions" between islandsand barriers. Up to now, solid state nanoelectronicdevices largely have been fabricated in III/V semicon-ductor compounds such as gallium arsenide and aluminumarsenide. It is believed, however, that silicon nanoelect-ronics would go a long way toward permitting the inexpen-sive mass manufacture of nanoelectronic devices.

For molecular electronics, one of the great challenges is todevelop two-terminal and three-terminal devices than canbe incorporated in circuits. Then, it must be demonstratedthat such molecular switches can be used to make reliablelogic, such as has been proposed recently [17,18].

Using either the solid-state or the molecular electronicapproach, in order to fulfill the promise of nanoelectronicsit will be necessary to refine greatly both the fabricationtechniques and the architectural concepts to permit theuseful assembly of small, low-power computing structuresthat contain trillions of nanoelectronic switching devices.

ACKNOWLEDGEMENTS

The author wishes to thank the members of the MITRENanosystems Group for their collaboration in the

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nanoelectronics research investigations upon which thispaper is based. He also wishes to thank many of his othercolleagues at MITRE for their longstanding support of thisresearch, notably David Lehman, Steven Huffman, EdwardPalo, William Hutzler, and Karen Pullen. Thanks also aredue to numerous colleagues outside MITRE for theirencouragement, copies of their publications, andexplanations of their research. This work was funded by agrant from The MITRE Corporation.

REFERENCES

[1] D. Goldhaber-Gordon, M.S. Montemerlo, J.C. Love,G.J. Opiteck, and J. C. Ellenbogen, "Overview ofNanoelectronic Devices," Proceedings of the IEEE,vol. 85, no. 4, April 1997, pp. 521-540. See alsoreferences cited in the extensive bibliography in thiswork.

[2] M.S.Montemerlo, J.C. Love, G.J. Opiteck, D. J.Goldhaber-Gordon, and J. C. Ellenbogen,"Technologies and Designs for ElectronicNanocomputers," MITRE Technical Report No.96W0000044, The MITRE Corporation, McLean, VA,July 1996. See also references cited in theextensive bibliography in this work.

[3] "The Nanoelectronics and Nanocomputing HomePage," The MITRE Corporation 1996-1998, McLean,VA. Available on the World-Wide Web at the URL:http://www.mitre.org/research/nanotech. This largeWWW site provides references 1 and 2 as down-loadable documents.

[4] B. Brar et al., "High Speed A/D Converters Based onResonant Tunneling Technology," Proceedings of the1998 Government Mocroelectronics Conference(GOMAC98), Arlington, VA, 13-16 March 1998.Following paper in this volume.

[5] S.Kulkarni and P. Mazumder, "Full Adder CircuitUsing RTDs and MOSFETs," Proceedings of the 1998Government Mocroelectronics Conference(GOMAC98), Arlington, VA, 13-16 March 1998.Following paper in this volume.

[6] L.A. Bumm et al., "Are Single Molecular WiresConducting?" Science, Vol. 271, 22 March 1996, pp.1705-1707.

[7] C. Zhou et. al., "Mesoscopic Phenomena Studiedwith Mechanically Controllable Break Junctions atRoom Temperature," in J. Jortner and M. Ratner(eds.), Molecular Electronics, Blackwell ScienceLtd., London, UK, 1997, pp. 191-213.

[8] C. Zhou et. al., "Nanoscale metal/self-assembledmonolayer/metal heterostructures," Applied Phys.Lett., v. 71, no. 5, 4 Aug. 1997, pp. 611-613.

[9] J. Tour, "Chemical Synthesis of Molecular ElectronicDevices," Presentation at the 1997 DARPA ULTRAReview Conference, Santa Fe, NM, October 1997.This presentation discussed the synthesis of amolecular RTD.

[10] M. Reed, "Self-Assembly-Based Approaches toMicroelectronic Fabrication and Devices,"Presentation at the 1997 DARPA ULTRA ReviewConference, Santa Fe, NM, October 1997. Thispresentation discussed the experimentalmeasurement of a molecular RTD.

[11] R. M. Metzger et al., "Unimolecular ElectricalRectification in HexadecylquinoliniumTricyanoquinodimethanide", J. Am. Chem. Soc., 15October 1997.

[12] E.K. Wilson, "DNA: Insulator or Wire?," Chemical andEngineering News, 17 February 1997, pp. 33-39.

[13] C. Joachim and J. Gimzewski, "AnElectromechanical Amplifier Using A SingleMolecule," Chem. Phys. Lett., Vol. 265, pp. 353-357,1997.

[14] S. J. Tans et al., Individual single-wall carbonnanotubes as quantum wires, Nature, Vol. 386, 3April 1997, p. 474.

[15] J. W. G. Wildöer et al., "Electronic structure ofatomically resolved carbon nanotubes," Nature , Vol.391, 1 January 1998, p. 59.

[16] S. J. Tans, A. R. M. Verschueren, and C. Dekker,"Single nanotube-molecule transistor at roomtemperature," submitted to Nature.

[17] J. C. Ellenbogen and J. C. Love, "Architectures formolecular electronic computers: 1. Logic structuresusing molecular electronic diodes," Manuscript inpreparation.

[18] J. C. Ellenbogen and J. C. Love, "Architectures formolecular electronic computers: 2. Logic structuresusing molecular electronic FETs," Manuscript inpreparation.