Molecular Quantum-dot Cellular Automata (QCA): Beyond Transistors
Transcript of Molecular Quantum-dot Cellular Automata (QCA): Beyond Transistors
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Craig S. LentUniversity of Notre Dame
Molecular Quantum-dot Cellular Automata (QCA): Beyond
Transistors
Supported by National Science Foundation
ND Collaborators: Greg Snider, Peter Kogge, Mike Niemier, Marya Lieberman, Thomas Fehlner, Alex Kandel,Alexei Orlov, Mo Liu,Yuhui Lu
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Outline of presentation• Introduction and motivation• QCA paradigm• QCA implementations
– Metal-dot– Semiconductor-dot– Magnetic– Molecular
• Circuit and system architecture• Summary
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Goal: Electronics at the single-molecule scale
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The Dream of Molecular Transistors
Why don’t we keep on shrinking transistors until they are each a single molecule?
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Dream molecular transistors
V
off
low conductance state
V
high conductance state
on I
1 nm
fmax=1 THz
Molecular densities: 1nm x 1nm 1014/cm2
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Transistors at molecular densitiesSuppose in each clock cycle a single electron moves from power supply (1V) to ground.
V
Frequency (Hz) 1014 devices/cm2 1013 devices/cm2 1012 devices/cm2 1011 devices/cm2
1012 16,000,000 1,600,000 160,000 16,000
1011 1,600,000 160,000 16,000 1,600
1010 160,000 16,000 1,600 160
109 16,000 1600 160 16108 1600 160 16 1.6107 160 16 1.6 0.16106 16 1.6 0.16 0.016
Power dissipation (Watts/cm2)
ITRS roadmap: 7nm gate length, 109 logic transistors/cm2 @ 3x1010 Hz for 2016
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The Dream of Molecular Transistors
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New paradigm: Quantum-dot Cellular Automata
Revolutionary, not incremental, approach
Beyond transistors – requires rethinking circuits and architectures
Use molecules, not as current switches, but as structured charge containers.
Represent information with molecular charge configuration.
Zuse’s paradigm• Binary• Current switch • charge configuration
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Quantum-dot cellular automataRepresent binary information by charge configuration of cell.
“0”
“null”
“1”QCA cell
• Dots localize charge
• Two mobile charges
• Tunneling between dots
• Clock signal varies relativeenergies of “active” and “null” dots
active
Clock need not separately contact each cell.
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“null”
Quantum-dot cellular automata
Neighboring cells tend to align in the same state.
“1”
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Quantum-dot cellular automata
Neighboring cells tend to align in the same state.
“1” “1”
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Quantum-dot cellular automata
Neighboring cells tend to align in the same state.
“1” “1”
This is the COPY operation.
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QCA cell-cell response function
Neighbor Polarization Neighbor Polarization
Cel
l Pol
ariz
atio
n
Cel
l Pol
ariz
atio
nClock: off Clock: on
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Majority Gate
“1”
“1”
“0”
“null”
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Majority Gate
“1”
“1”
“0”
“1”
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Majority Gate
Three input majority gate can function as programmable 2-input AND/OR gate.
“A”
“C”
“B”
“out”
MABC
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QCA single-bit full adder
Hierarchical layout and design are possible.Simple-12 microprocessor (Kogge & Niemier)
result of SC-HF calculation with site model
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Outline of presentation• Introduction and motivation• QCA paradigm• QCA implementations
– Metal-dot– Semiconductor-dot– Magnetic– Molecular
• Circuit and system architecture• Summary
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QCA devices exist
“dot” = metal island
electrometers
70-300 mK
Al/AlOx on SiO2
Metal-dot QCA implementation
Greg Snider, Alexei Orlov, and Gary Bernstein
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Metal-dot QCA cells and devices
• Majority Gate
MABC
Amlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284, pp. 289-291 (1999).
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QCA Shift Register
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QCA Shift Register
Gtop
Gbotelectrometers
VIN+
VIN–
VCLK1 VCLK2
D1 D4
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Metal-dot QCA devices exist
• Single electron analogue of molecular QCA• Gates and circuits:
– Wires– Shift registers– Fan-out– Power gain demonstrated– AND, OR, Majority gates
• Work underway to raise operating temperatures
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Power Gain in QCA Cells• Power gain is crucial for practical devices
because some energy is always lost between stages.
• Lost energy must be replaced.– Conventional devices – current from power supply– QCA devices – from the clock
• Unity power gain means replacing exactly as much energy as is lost to environment.
Power gain > 3 has been measured in metal-dot QCA.
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GaAs-AlGaAs QCA cell
• Dots defined by top gates depleting 2DEG
• Direct measurement of cell switching
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Silicon P-dot QCA cell
• Dots defined by implanted phosphorus
• Single-donor creation foreseen
• Direct measurement of cell switching
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Magnetic QCA
• Dots defined by magnetic domains
• Room temperature operation
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Molecular QCA
“dot” = metal island70 mK
Mixed valence compounds
“dot” = redox center
room temperature+
Metal tunnel junctions
Key strategy: use nonbonding orbitals (π or d) to act as dots.
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Experiments on molecular double-dot
Thomas Fehlner et al. (Notre Dame chemistry group)Journal of American Chemical Society,125:15250, 2003
Ru Ru
Fe Fe
“0” “1”
Fe group and Ru group act as two unequal quantum dots.
trans-Ru-(dppm)2(C≡CFc)(NCCH2CH2NH2) dication
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Surface attachment and orientation
N
Si Si3.8 Α
2.4 Α106o
PHENYL GROUPS“TOUCHING” SILICON
Molecule is covalent bonded to Si and oriented vertically by “struts.”
Si(111)
molecule Si-N bonds
“struts”
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FeRu Fe Ru Fe
Ru
Si
HgFe
Ru
Si
HgFe
Ru
Si
HgFe
Ru
ac C
apac
itanc
e
voltage
excited stateswitching
Ener
gy
ground state
Applied field equalizes the energy of the two dots
When equalized, capacitance peaks.
appliedpotential
Measurement of molecular bistability
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
C(oxidized) C(reduced) ∆C
VHg (V)C
(nF)
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
∆C (nF)
layer of molecules
Ru
Fe
Ru
Fe
2 counterion charge configurations on surface
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Longer molecular double-dot
Isopotentialsurface
HOMO orbital
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Double-dot click-clack
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
0.9
1.0
1.1
1.2
1.3
COxidized Cas prepared ∆C
VHg (V)
C (n
F)
-0.2
-0.1
0.0
0.1
∆C (nF)
Ph2P
RuPPh2
Ph2P
PPh2
Ph2P
RuPPh2
Ph2P
PPh2
N
N
H2N
NH2
3+
d1=1.8 nmdm=2.8 nm
Hua & Fehlner
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Square 4-dot QCA molecules
0.6 nm
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Imaging molecular double-dot
structure toluene solution Goal: single-molecule imaging on surfaces
Kandel group
Molecules are pulse-injected from solution into vacuum onto a clean, crystalline gold [Au(111)] surface.
Ru-Ru molecule with no surface binding. Not mixed-valence species.
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Ru2 clusteringSome clustering and alignment of molecules occurs automatically during deposition. (50 nm image shown.)
We should be able to compare isolated molecules with those in larger clusters.
Experimental conditions: 0.5 V, 20 pA, 298 K
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Molecular motion
Changing tunneling conditions (from 1.0 V, 20 pA to 1.0 V, 100 pA) increases tip/molecule interaction.
We observe a change in orientation for one Ru2 molecule.
This suggests the possibility of using the STM tip for controlled manipulation of these molecules on the surface.
Experimental conditions: 250 ×180 Å, 1.0 V, 20 (100) pA, 298 K
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Imaging charge localization
Neutral molecules Mixed-valence molecules
Preliminary results for Fe-Fe fabricated by Claude Lapinte (Rennes)
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Single-atom quantum dots
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Outline of presentation• Introduction and motivation• QCA paradigm• QCA implementations
– Metal-dot– Semiconductor-dot– Magnetic– Molecular
• Circuit and system architecture• Summary
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Field-clocking of QCA wire: shift-register
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Computational wave: majority gate
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Computational wave: adder back-end
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Permuter
Deep pipe-lining at very small scale
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Wider QCA wires
Redundancy results in defect tolerance.
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Molecular circuits and clocking wires
Next: zoom out to dataflow level
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Universal floorplan
Peter Kogge
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QCA design tools
Design tools are starting to enable new systems ideas.
QCADesigner
Konrad WalusU. British Columbia
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System + Application Architectures
Grounded in device physics & simulation Incorporate clock driven dataflow
AB
CD
AB
Device architecture maps well to many system architectures…
A A’ B B’ C C’
AB
AC
AND Plane OR Plane
AB + BC + AC
BC
Reconfigurable General PurposeSystolic
Good for FIR, FT, Matrix multiply, graph algorithms, etc.
Mike Niemier
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Summary
• QCA offers possible path to limits of downscaling –molecular computing.– General-purpose computing– New architecture– Low power dissipation which is essential
• Single-electron metal-dot QCA devices exist.• First steps in molecular-scale QCA• Clear path but much research remains to be done.
– Rethinking architecture to match problem– Chemistry, physics, electrical engineering, computer science
Thanks for your attention.