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Joining the dots
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NEWS
Self-assembling organic field,effect transistors
Bell labs' scientists Hendrik Sch6n and Zhenan Bao, part of the team working on molecular-scale organic transistors. (Photo courtesy of Lucent Technologies. Copyright © 2001 Lucent Technologies.)
Researchers from Lucent Technologies' Bell Labs take a further step towards possible molecular electronics with their development of a self-assembled monolayer (SAM) organic fieldeffect transistor (FET). Reporting in the October 18
issue of Nature [2001, 413, 713-716], Hendrik SchOn, Hong Meng and Zhenan Bac demonstrate a transistor where the thickness of a SAM defines the channel length and is a factor of 10 smaller than demonstrated by
lithographic techniques. Using a highly doped silicon substrate as the gate electrode, lithography and anisotropic etching are used to create vertical steps, onto which layers of Si02 (gate insulator), gold (source and drain electrodes), and SAMs of thiolor isocysnide-terminated conjugated molecules are deposited to form the device. One of the major difficulties associated with such molecular devices has been fabricating electrodes and the attaching of electrical contacts. "We solved the contact problem by letting one layer of organic molecules self-assemble on one electrode first, and then placing the second electrode above it," explains Bao. "For the self assembly, we simply make a solution of the organic semiconductor, pour it on the base, and the molecules do
the work of finding the electrodes and attaching themselves." Such a chemical self-assembly technique could be easy and inexpensive to implement on a large scale although reproducibility and scale-up potential remains to be demonstrated. Using the SAMFETs, the researchers built a voltage inverter and demonstrated a gain of six. In all, six different organic SAMs were tried, all showing similar current-voltage characteristics with no degradation effects observed up to 105 cycles. Neither resonant tunnelling nor charging effects are exhibited. The researchers believe that the device structure could be applied to other molecular systems, without, says Penn State's Paul Weiss "many of the difficulties inherent in other nanofabricstion approaches."
"Now we have proven that you can link quantum dote together, but the next thing will
be to make them do things, to control the spins in a doublequantum dot," says Chang.
Joining the dots Quantum computing is being heralded as the next revolution in the on-going drive to process information more quickly and increase memory capacity. In such a computer, quantum dote could be used as the switches, just as transistors are now. Researchers at Purdue University have linked quantum dote so that information can be transferred between them. Reporting in Science [(2001) 293, 2221-2223], Albert M. Chang, Heejun Jeong and Michael R. Melloch have succeeded in linking two dote, controlling the number of electrons in each dot, and detecting the associated spin.
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The extremely fine circuits they used - only 50 nm wide - were fabricated on gallium arsenide using electron beam lithography. "As far as we know, no other groups have been able to do such fine lithography," says Chang. Although other devices potentially suitable for quantum computing have been demonstrated, Chang believes that this method may be easier to scale-up into real applications. "The advantage of semiconductors is that, once you've proven the feasibility, you've got the whole semiconductor industry's expertise behind you," he explains•
November/December 2001
!
t!
Purdue researcherscreate quantum dots with extremely fine circuitry, using ~lectronbeam lithography. The quantum dots (in the center), each only 180 nm in diameter, hold puddles of about 20-40 electrons. (Courtesy of Albert Chang, Purdue University.)
NEWS
Self-assembling organic field,effect transistors
Bell labs' scientists Hendrik Sch6n and Zhenan Bao, part of the team working on molecular-scale organic transistors. (Photo courtesy of Lucent Technologies. Copyright © 2001 Lucent Technologies.)
Researchers from Lucent Technologies' Bell Labs take a further step towards possible molecular electronics with their development of a self-assembled monolayer (SAM) organic fieldeffect transistor (FET). Reporting in the October 18
issue of Nature [2001, 413, 713-716], Hendrik SchOn, Hong Meng and Zhenan Bac demonstrate a transistor where the thickness of a SAM defines the channel length and is a factor of 10 smaller than demonstrated by
lithographic techniques. Using a highly doped silicon substrate as the gate electrode, lithography and anisotropic etching are used to create vertical steps, onto which layers of Si02 (gate insulator), gold (source and drain electrodes), and SAMs of thiolor isocysnide-terminated conjugated molecules are deposited to form the device. One of the major difficulties associated with such molecular devices has been fabricating electrodes and the attaching of electrical contacts. "We solved the contact problem by letting one layer of organic molecules self-assemble on one electrode first, and then placing the second electrode above it," explains Bao. "For the self assembly, we simply make a solution of the organic semiconductor, pour it on the base, and the molecules do
the work of finding the electrodes and attaching themselves." Such a chemical self-assembly technique could be easy and inexpensive to implement on a large scale although reproducibility and scale-up potential remains to be demonstrated. Using the SAMFETs, the researchers built a voltage inverter and demonstrated a gain of six. In all, six different organic SAMs were tried, all showing similar current-voltage characteristics with no degradation effects observed up to 105 cycles. Neither resonant tunnelling nor charging effects are exhibited. The researchers believe that the device structure could be applied to other molecular systems, without, says Penn State's Paul Weiss "many of the difficulties inherent in other nanofabricstion approaches."
"Now we have proven that you can link quantum dote together, but the next thing will
be to make them do things, to control the spins in a doublequantum dot," says Chang.
Joining the dots Quantum computing is being heralded as the next revolution in the on-going drive to process information more quickly and increase memory capacity. In such a computer, quantum dote could be used as the switches, just as transistors are now. Researchers at Purdue University have linked quantum dote so that information can be transferred between them. Reporting in Science [(2001) 293, 2221-2223], Albert M. Chang, Heejun Jeong and Michael R. Melloch have succeeded in linking two dote, controlling the number of electrons in each dot, and detecting the associated spin.
12
~
The extremely fine circuits they used - only 50 nm wide - were fabricated on gallium arsenide using electron beam lithography. "As far as we know, no other groups have been able to do such fine lithography," says Chang. Although other devices potentially suitable for quantum computing have been demonstrated, Chang believes that this method may be easier to scale-up into real applications. "The advantage of semiconductors is that, once you've proven the feasibility, you've got the whole semiconductor industry's expertise behind you," he explains•
November/December 2001
!
t!
Purdue researcherscreate quantum dots with extremely fine circuitry, using ~lectronbeam lithography. The quantum dots (in the center), each only 180 nm in diameter, hold puddles of about 20-40 electrons. (Courtesy of Albert Chang, Purdue University.)