Nanoelectronics
Nanoscale electronic devices can be made with either top-down technology or bottom-up
technology. The most advanced top-down devices are made using silicon technology.
These circuits are terribly complex involving millions of transistors and many layers of
metal interconnects that connect the transistors together. The transistors are fabricated on
the surface of a silicon wafer and the metal wiring layers are deposited on top of the
transistors.
Left: A cross section of an integrated circuit showing the wiring
layers. The lower metal wiring layers are used for local connections
and the upper layers are used for global connections. Right: An
electron microscope image of a silicon transistor.
A Pentium 4 chip. Reference:
www.intel.com/products/desktop/processors/pentium4/index.htm.
34
The CMOS process steps. Reference: Prospects for Single Molecule
Information Processing Devices, Yasuo Wada, Proceedings of the
IEEE, Vol. 89 (2001).
The dimensions of integrated circuits have been decreasing exponentially ever since they
were first introduced in the 1960's. Every four years, the smallest features decrease by one
half. The exponential decrease was first pointed out by Gordon Moore of Intel and is
known as Moore's law. The smallest lateral features of integrated circuits fabricated today
are 90 nm. It is expected that this shrinking will continue for another decade but
exponential decreases in feature size cannot continue indefinitely. Transistors cannot be
made smaller than atoms. The decreasing feature sizes in integrated circuits and the
problems that need to be overcome to continue shrinking circuits are described in detail in
the International Technology Roadmap for Semiconductors (ITRS roadmap).
35
Fig. 5.2. A table of chip characteristics from the ITRS roadmap.
Recently there have been a number of articles in popular science journals claiming that
silicon technology is facing insurmountable problems and it will soon be replaced by some
other technology based on carbon nanotubes or molecular electronics. This is nonsense.
Silicon technology is the dominant technology for information processing and there is no
other technology that even comes close to delivering the same performance. Especially in
high performance applications like microprocessors, silicon technology has no realistic
challengers.
This does not mean that there is no future for bottom-up technologies in electronics. In
bottom-up electronics, electronic components like diodes or transistors are synthesized
chemically and then arranged into circuits. Because the devices are synthesized
chemically, the position of every atom in the device is known and the devices can be
optimized on the atomic scale. Chemical synthesis is also a process that can be scaled up
easily. Once it is possible to make transistors by chemical synthesis, more transistors will
be made in one day than it will ever be possible to make by photolithography. After diodes
and transistors are produced chemically, there will be great incentive to try to couple them
into circuits by chemical means. Since so many devices can be produced simultaneously
by chemical synthesis, the more complexity that can be included in a circuit chemically,
the lower the costs of the final circuit will be. This could well be the successor to Moore's
law. Instead of improving electronics by making the components smaller, we will start
with atomically precise components and build ever more complex circuits out of them.
36
This somewhat futuristic vision of bottom-up electronics has lead to a flurry of research on
chemically synthesized electronic components and self-assembly methods. Devices made
of molecules, nanotubes, nanowires, nanocrystals, macromolecular assemblies, or
collections of these structures have been studied. Bottom-up electronics is still in a
primitive state and electronic devices fabricated by bottom-up techniques are not yet being
widely used in applications. This is an exciting and rapidly developing field. Most of the
results that are being published describe the fundamental electron transport mechanisms
that occur in nanostructures. Unfortunately, sometimes the terminology can be confusing
for someone new to the field.
The words 'transistor' and 'electronics' don't always have the same meaning. Most books,
articles, and conferences on "nanoelectronics" or "molecular electronics" are not really
concerned with electronics in the conventional sense. Molecular electronics is a term that
is usually used to describe the fundamental study of electrical current flow through
molecules. The molecules may or may not have any practical circuit application. In this
research community, a transistor is any three terminal device that modulates a current.
Such a 'transistor' is often not a transistor at all from the electrical engineering point of
view. The breakthroughs in nanoelectronics that we read about in popular science journals
are typically breakthroughs in our understanding of electronic transport but not in
electronics. At this point, how current flows through molecules is not very well understood
and possibly the breakthroughs in electronic transport will lead someday to new
developments in electronics. However, most popular journals over hype progress in our
understanding of electronic transport and present it as advancements for electronics.
Presumably this is done to increase magazine sales.
Here is what Edwin A. Chandross said about the molecular electronics hype:
Molecular electronics hype
The proponents of this "technology" have buried us in hype for
several years. They promised workstations that will operate for
decades powered by only a small battery (1), 100 high-end
workstations on a grain of sand (2), and, more recently, 1000
Pentiums on the same base (3). Although it has been admitted that
logic will require devices with gain (4), no such molecular device
has yet been identified. Such unreasonable advertising should have
aroused the skepticism even of novices.
Edwin A.
Chandross
Letter to
Science ,
Science vol.
303, pp. 11361137 (2004).
Serious questions about this field have been raised for several years
after talks by proponents at American Chemical Society and
Materials Research Society meetings. Results that are claimed to
represent technology must be subjected to examination of
manufacturability and device reliability, issues that have been nearly
completely ignored. Any device that cannot be made reliably in the
lab is unlikely to become the basis of a technology in 5 years,
despite what Jim Heath said about molecular-based memory at the
UCLA debate in September (5).
Much of the present situation is a result of publication, primarily in
the press, and of reports that do not include important experimental
details. The lack of full papers is a prime characteristic of the field of
molecular electronics, and this makes it impossible to fully evaluate
the experiments. It is time to demand much more information.
37
Edwin A. Chandross
MaterialsChemistry LLC,
14 Hunterdon Boulevard,
Murray Hill, NJ 07974,
USA.
References
1. J. Heath, S. Williams, Chem. Eng. News 77, 6 (1999).
2. See www.chem.ucla.edu/dept/Faculty/heath/.
3. J. R. Heath, in Reducing the Time from Basic Research to
Innovation in the Chemical Sciences: A Workshop Report to the
Chemical Sciences Roundtable (National Academies Press,
Washington, DC, 2003), pp. 56-63.
4. J. F. Stoddart, J. R. Heath, Chem. Phys. Chem. 3, 519 (2002).
5. Much of the UCLA debate is now available on the California
Nanosciences Institute Web site
(www.cnsi.ucla.edu/mainpage.html). A number of issues in
molecular electronics are addressed in detail in my presentation.
An interesting example of a transistor that is not really a transistor is the report of a
transistor made using a single C60 molecule (Hongkun Park, Jiwoong Park, Andrew K. L.
Lim, Erik H. Anderson, A. Paul Alivisatos, and Paul L. McEuen, "Nanomechanical
oscillations in a single-C60 transistor," Nature 407 p. 57 (2000)). This article describes
some excellent research on electronic transport through a C60 molecule. A single C60
molecule was placed between two metal electrodes and the current through the molecule
was measured. The electron transport excited a bouncing ball mode where the C60 bounced
on the substrate. Consequences of this vibration were observed in the current
measurement.
Fig. 5.3. The electrical characteristics of a C60 transistor. Reference:
Hongkun Park, Jiwoong Park, Andrew K. L. Lim, Erik H.
Anderson, A. Paul Alivisatos, and Paul L. McEuen,
"Nanomechanical oscillations in a single-C60 transistor," Nature 407
p. 57 (2000).
38
The C60 transistor can also be evaluated for its suitability in circuit applications. There are
several properties that a transistor should have. First of all, it should have signal gain. It
must be possible to amplify the signal that represents the information in a circuit. Usually,
a signal is represented by a voltage. In this case, the transistor should exhibit voltage gain.
The voltage gain can be calculated from the published electrical characteristics.
In this transistor, a signal is input at the gate, Vg, and the output appears across the source
and drain electrodes. The output voltage is labeled V in the Fig. 5.3. The voltage gain is the
change in the output voltage divided by a change in the input voltage at constant current.
The maximum voltage gain seems to occur for gate voltages between 7.4 V and 7.7 V. In
this region a change in gate voltage of 0.3 V causes a change in output voltage of about 10
mV. The voltage gain is thus 0.03. A gain smaller than 1 indicates that the transistor
attenuates the signal instead of amplifying it. This transistor is thus not suitable for
building complex circuits. Most molecular transistors do not exhibit voltage gain. In fact,
the smallest transistors that do show voltage gain greater than 1 are silicon transistors.
In circuits, the output of one transistor is often connected to the input of the next transistor.
The output voltage of the C60 transistor is plotted in the range from -70 mV to +70 mV.
These are the output voltages that could be used to drive the input of the next transistor.
Unfortunately, the only gate voltages that are given in the figure are between 5.9 V and 7.7
V. From the circuit point of view, it would be more interesting to plot the response of the
transistor for gate voltages between -70 mV and +70 mV because those are the only
voltages that this device can generate.
Figure 5.4 shows the current-voltage characteristics of a silicon transistor made by Intel.
The distance between the source and drain electrodes in this case is 15 nm. Notice that the
gate voltages plotted fall in the range of the source-drain voltage. This is the information
that is needed to design circuits with the transistor.
Fig. 5.4. The current-voltage characteristics of silicon transistor
made by Intel.
39
Another important characteristic of a transistor is the leakage current. A transistor can be
thought of as a sort of switch. For certain gate voltages the switch is closed and a current
will flow between the source and drain electrodes. For other gate voltages the switch is
open and ideally no current flows between the source and drain electrodes. Nevertheless, a
small leakage current flows when the transistor is nominally open. Small transistors tend
to have higher leakage currents than larger transistors because the source and drain
electrodes are closer together in a small transistor. This leakage current dissipates energy
and causes the circuit to heat up. The heat generated by the leakage current is one of the
factors that limits the density of transistors in a circuit. The transistor density, n, times the
leakage current, Ileak, times the voltage across the transistors is the power dissipated per
unit area. An acceptable total power density is about 10 W/cm². Most of the power that is
dissipated when the transistors are actively switching open and shut but a upper limit on
the density of the transistors can be found by assuming that all of the transistors are open
and the only power dissipation is due to the leakage current. This can be expressed as the
inequality,
n < 10/(IleakVsupply) [transistors/cm²].
A typical supply voltage is about 1 V and silicon circuits have already reached densities of
100M transistors/cm². This puts an upper limit on the leakage currents of a few nanoamps.
The drive current of a transistor is also an important characteristic to consider. This is the
current that the transistor can deliver when the switch is closed. Voltage levels are often
used to represent information in electronics and the time it takes to change the voltage of a
wire from one voltage to another is capacitance of the wire times the voltage difference
divided by the drive current of the transistor that is charging the wire. The higher the drive
current, the faster the electronics. The drive current in silicon electronics is about 1 mA.
This drive current has been maintained even as the transistors have been made smaller.
The capacitance of the components decreases as they become smaller and the supply
voltage can be decreased which has resulted in an increase in the switching speed as
transistors have been made smaller. Most non-silicon transistors that have been
investigated have a significantly lower drive current than 1 mA. Either more of these
transistors would have to be used in parallel to achieve the same circuit speed or the
circuits will be significantly slower than what is possible with silicon transistors.
Plastic electronics
The term molecular electronics is sometimes applied to devices made from organic
semiconductors. This is also called plastic electronics. The idea is to develop organic
conductors, semiconductors, and insulators that can be easily processed. These materials
can be stamped or printed to electronic circuits. The goal is to produce electronic circuits
as quickly and as cheaply as printing a full color page. Organic semiconductors typically
have charge carrier mobilities 100 to 10000 times lower than crystalline silicon devices.
This means that to make a device that is provides the same current drive as a silicon
transistor, the organic device has to be 100 to 10000 times larger. This fact combined with
the limited resolution of cheap printing processes results in slow devices with much lower
densities than those achieved by silicon technology. However, there are some applications
like smart price tags where plastic electronics are useful. Another conceivable application
of plastic electronics is to make inexpensive displays. As of January 2004, the most
complicated plastic electronic circuits contain about 2000 transistors and operate at a
switching speed of 5kHz. The individual transistors have a channel length of 2.5 μm and a
width of about 500 μm. For more details see the publication: Flexible active-matrix
displays and shift registers based on solution-processed organic transistors, G. H.
Gelinck, H. Edzer A. Huitema, E. Van Veenendaal, E. Cantatore, L. Schrijnemakers, J. B.
P. H. Van Der Putten, T. C. T. Geuns, M. Beenakkers, J. B. Giesbers, B.-H. Huisman, E. J.
40
Meijer, E. Mena Benito, F. J. Touwslager, A. W. Marsman, B. J. E. Van Rens and D. M.
de Leeuw, Nature Materials 3, pp.106-110 (2004) doi: 10.1038/nmat1061.
Electronics on a flexible substrate.
Molecular Memories
It has often been remarked that molecular memories will probably be made before logic
circuits and much progress has been made lately. In one approach, molecules are
sandwiched between a parallel set of wires running vertically and a parallel set of wires
running horizontally. Some molecules can assume two conformations and it is possible by
applying voltage pulses to the molecules to switch between these conformations. If the
molecule has a low electrical resistance in one conformation and a high electrical
resistance in another conformation, one bit of information can be stored in the molecule.
This information can be read out by measuring the resistance of the molecule. The
information storage is non-volitile; the information is not lost when the power to the
memory chip is turned off.
Rotaxane molecules can assume two configurations. One has a low
electrical resistivity and the other has a high resistivity. This can be
used to build a molecular memory.
41
In this scheme it is important that the molecules have a nonlinear current-voltage
characteristic like a diode. This is necessary so that the information from an individual
molecule can be read. Suppose that a large current flows through the molecule if it is in
one conformation when 1 V is applied across it, little current flows across it if is in the
other conformation and little current flows through the molecule when 0.5 V is applied
across it in either conformation. Then if one vertical wire is held at -0.5 V and one
horizontal wire is held at +0.5 V while all of the other wires are held at 0 V, only the
molecule at the intersection of the wires to which voltage has been applied will determine
the current that flows. Of course, when 0.5 V is applied across the molecules there will be
a small leakage current. This leakage current will limit the size of the array because the
current through all of the leaking molecules must be less than the current through the
molecule that is intended to be read.
Carbon nanotube transistors
Carbon nanotubes have been used to make transistors. Nanotubes can carry very high
current densities.
Left: An AFM image of a carbon nanotube transistor. Right: An
artist's impression of a carbon nanotube transistor.
Semiconducting nanowires
Semiconducting nanowires are cylindrical single crystals with a diameter of 10 nm - 100
nm and a length of several microns. It is possible to grow complex three-dimensional
semiconducting structures in nanowires with sub nanometer precision. Different materials
can be combined in nanowires in layers and shells to define p-n junctions, quantum dots,
or one-dimensional conducting channels. The large surface to volume ratio of
semiconducting nanowires make them attractive for applications as sensors and for
contacts. Prototype transistors, logic circuits, light-emitting diodes, and lasers have been
made with these nanowires.
One possible route to applications is to develop processes where many nanowires can be
fabricated cheaply and then arrange these components into circuits. For instance, diodes
can be made from semiconducting nanowires and then these diodes can be aligned in
solution using the same kinds of mechanisms that form cell membranes. The many diodes
in parallel could then be transferred onto an electrode to make lighting panels or solar
cells. Nanowires could also be used to make transistors in large area electronics where the
channel of a transistor would consist of many nanowires in parallel. Flow assembly could
be used to align the nanowires. In this case, the nanowires would compete with plastic
42
electronics where applications such as electronic paper, smart packaging, and sensors
require cheap circuits on flexible substrates. The combination of nanowire LEDs and
nanowire transistors could potentially be used to produce cheap and high quality displays.
Another route towards applications would be incorporate semiconducting nanowires in
silicon technology. In nanowires, it is possible to epitaxially grow materials on top of each
other that do not grow epitaxially in large areas. This is because the cross section of the
nanowires is so small that less strain is built up and larger lattice mismatches are possible.
This makes it possible to incorporate more materials in silicon circuits than would
otherwise be possible. This may provide a route to incorporate III-V optical devices in
silicon. The properties such as the band gap of a semiconductor can be tuned by changing
the diameter of the nanowires. This gives further flexibility in the fabrication of optical
devices using nanowires. The large surface-to-volume ratio of nanowires makes them
potentially useful for making low ohmic contacts and it could be useful for integrating
sensitive sensors with silicon circuits.
A nanowire consisting of InAs layers and
InP layers. The transmission electron
microscope picture on the right shows that
the periodicity of the atomic lattice is
maintained across the interfaces between
the two materials. Reference: Onedimensional heterostructures in
semiconductor nanowhiskers, M. T. Björk,
B. J. Ohlsson, T. Sass, A. I. Persson, C.
Thelander, M. H. Magnusson, K. Deppert,
L. R. Wallenberg, and L. Samuelson,
Applied Physics Letters, Vol. 80, pp. 10581060, (2002).
Self-assembled circuits
Modern silicon circuits that are produced by top-down technologies have very complicated
architectures and it is difficult to see how this sort of complexity could be achieved by
self-assembly. A self-assembled circuit would likely have a simple, regular architecture.
43
This can perhaps be best explained by giving some examples. Suppose we wanted to build
a cheap display that would self-assemble itself. The display would consist of many
identical pixel elements, each capable of exchanging information with its neighbors and
displaying color information. These pixel elements would be designed so that, under the
right conditions, they arrange themselves into a two-dimensional planar array. Image
information would then be passed to some random pixel near the lower left corner. The
image information would specify the color that should be displayed and the position (x,y)
where that color should be displayed. Each element would be programmed to subtract 1
from the x coordinate and pass the information to the right and to subtract 1 from the y
coordinate and pass the pass the information up. If an element receives information with
the coordinates (0,0), that element displays the color information. In this architecture, there
is no complex wiring, only nearest neighbor connections. No pixel element knows its
position yet it displays the correct information. The elements could be powered by
sandwiching them between two planar electrodes that act as power and ground. If the
elements were produced purely by chemical synthesis, large cheap displays could be
made.
A memory could be made using similar elements to those described above but instead of
displaying the information, small chunks of information would be stored in each element.
The information would be stored until a request for that information was passed to the
element to pass the information to the edge of the array where it could be read. If a global
signal were given that every element should pass their current information to the right and
receive new information from the left, large amounts of information could be written into
or read out of the memory quickly. A real breakthrough in memory storage would be
achieved if this kind of memory could be made using a three-dimensional array of
elements.
A self-assembled processor could be based on the same principles. A processor element
would store two kinds of information, data and instructions. The processing element would
manipulate the data based on the instructions and pass information to its neighbors. In
architectures such as this consisting of many identical elements, it is easier to implement
error correction and fault tolerance since every element can take on the duties of any other
element. It is also conceivable that nanomachines could be made that implement repairs by
replacing faulty elements with new ones.
Bottom-up nanoelectronics has not yet matured to the stage that it is possible to fabricate
the pixel elements, memory elements or processor elements described here. Present
research on bottom-up electronics is still at the stage of trying to manufacture individual
devices like transistors. Combining these devices into circuits will have to wait until more
is known about the devices.
References
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A very influential paper that is often cited as the first to discuss using molecules as
electronic components is A. Aviram and M. A. Ratner, "Molecular rectifiers," Chem.
Phys. Lett. Vol. 29 pp. 277-283 (1974).
Gordon Moore, Cramming More Components Onto Integrated Circuits, Electronics,
April 19, 1965.
Computing with Molecules - A Scientific American article by Mark Reed and James
Tour.
Hybrid Semiconductor-Molecular Nanoelectronics by Konstantin Likharev.
Molecules Get Wired - The "Breakthrough of the year 2001" in Science magazine.
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Scaling CMOS to the limit - Special issue of the IBM Journal of Research and
Development Vol. 46, Nos. 2/3, 2002.
Electronics below 10 nm - by Konstantin Likharev.
HP announces molecular electronics breakthrough IOP PhysicsWeb September 2002.
Electronics using hybrid-molecular and mono-molecular devices, C. Joachim, J. K.
Gimzewski and A. Aviram, Nature 408 p. 541 (2000).
Prospects for Single Molecule Information Processing Devices, Yasuo Wada,
Proceedings of the IEEE, Vol. 89 (2001).
Carbon Nanotube Electronics, Ph. Avouris, J. Appenzeller, R. Martel, and S. J. Wind,
Proceedings of the IEEE, Vol 91 p. 1772 (2003).
Molecular electronics: from devices and interconnect to circuits and architecture,
M.R. Stan, P.D. Franzon, S.C. Goldstein, J. C. Lach, M. M. Ziegler, Proceedings of
the IEEE, Vol 91 p. 1940 (2003).
Fabrication, Assembly, and Characterization of Molecular Electronic Components, B.
A. Mantooth and P. S. Weiss, Proceedings of the IEEE, Vol 91 p. 1772 (2003).
Limits to binary logic switch scaling-a gedanken model, V. V. Zhirnov, R. K. Cavin,
J. A. Hutchby, G. I. Bourianoff, Proceedings of the IEEE, Vol 91 p. 1934 (2003).
High Performance Silicon Nanowire Field Effect Transistors, Yi Cui, Zhaohui Zhong,
Deli Wang, Wayne U. Wang, and Charles M. Lieber, Nano Letters, 3 (2), 149 -152,
2003.
Britney's Guide to Semiconductor Physics
MOLECULAR ELECTRONICS: Next-Generation Technology Hits an Early Midlife
Crisis, Robert F. Service, Science Volume 302, pp. 556-559 (2003).
Conductance Switching in Single Molecules Through Conformational Changes, Z. J.
Donhauser, B. A. Mantooth, K. F. Kelly, L. A. Bumm, J. D. Monnell, J. J. Stapleton,
D. W. Price, Jr., A. M. Rawlett, D. L. Allara, J. M. Tour, and P. S. Weiss, Science
vol. 292 pp. 2303-2307 (2001).
MOLECULAR ELECTRONICS: Nanodevices Make Fresh Strides Toward Reality,
Robert F. Service, Science vol. 302 p. 1310 (2003).
More on Molecular electronics, Edwin A. Chandross, Science vol. 303, pp. 11361137 (2004).
Problems
1. A carbon nanotube transistor allows 1 μA of current to pass through it when it is
switched on and 10 pA leaks through it when it is switched off. The voltage across the
transistor is 1 volt. The output of the transistor charges a wire with a capacitance to ground
of 1 fF. How many of these transistors could be placed in 1 cm2? How long will it take the
transistor to change the voltage on the wire at the output by 1 volt?
2. DNA has an information density of about 1 Gigabit/meter. By laying DNA strands next
to each other, a bit density of 1014 bits/cm2 would be possible. This kind of bit density
should be achievable with molecular memories. You could read the bits for instance with
an STM. If information were stored in three dimensions, the bit density would be about
1021 bits/cm3. How would you read these bits? If you think it is impossible to store (and be
able to read) 1021 bits/cm3, what three dimensional bit density would be possible?
45