Download Quantum-dot Cellular Automata: Beyond the Transistor Paradigm

Molecular Quantum-dot Cellular
Automata (QCA): Beyond
Transistors
Craig S. Lent
University of Notre Dame
ND Collaborators: Greg Snider, Peter Kogge, Mike Niemier,
Marya Lieberman, Thomas Fehlner, Alex Kandel,
Alexei Orlov, Mo Liu,Yuhui Lu
Supported by National Science Foundation
Center for Nano Science and Technology
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
V
on
I
1 nm
low conductance
state
high conductance
state
fmax=1 THz
Molecular densities: 1nm x 1nm Æ 1014/cm2
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Transistors at molecular densities
V
Suppose in each clock cycle a single electron
moves from power supply (1V) to ground.
Power dissipation (Watts/cm2)
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
16
108
1600
160
16
1.6
107
160
16
1.6
0.16
106
16
1.6
0.16
0.016
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
Represent information with molecular charge
configuration.
Zuse’s paradigm
3• Binary
2• Current switch
3• charge configuration
Revolutionary, not incremental, approach
Beyond transistors – requires rethinking circuits and
architectures
Use molecules, not as current switches, but as
structured charge containers.
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Quantum-dot cellular automata
Represent binary
information by charge
configuration of cell.
•
QCA cell
Dots localize charge
•
Two mobile charges
•
Tunneling between dots
•
Clock signal varies relative
energies of “active” and “null” dots
active
“1”
“0”
“null”
Clock need not separately contact each cell.
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Quantum-dot cellular automata
Neighboring cells tend to
align in the same state.
“1”
“null”
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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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Clock: off
Neighbor Polarization
Cell Polarization
Cell Polarization
QCA cell-cell response function
Clock: on
Neighbor Polarization
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Majority Gate
“0”
“null”
“1”
“1”
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Majority Gate
“0”
“1”
“1”
“1”
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Majority Gate
A
B
C
“B”
M
“out”
“A”
“C”
Three input majority gate can function as programmable 2-input
AND/OR gate.
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QCA single-bit full adder
result of SC-HF calculation
with site model
Hierarchical layout and design are possible.
Simple-12 microprocessor (Kogge & Niemier)
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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
Metal-dot QCA implementation
Al/AlOx on
SiO2
electrometers
70-300 mK
“dot” = metal island
Greg Snider, Alexei Orlov, and Gary Bernstein
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Metal-dot QCA cells and devices
• Majority Gate
A
B
C
M
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
D1
D4
Gtop
V+
IN
VCLK1
VIN–
VCLK2
Gbot
electrometers
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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
Metal tunnel junctions
“dot” = metal island
70 mK
“dot” = redox center
Mixed valence compounds
room temperature+
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
Fe
Fe
Ru
Ru
“0”
“1”
trans-Ru-(dppm)2(C≡CFc)(NCCH2CH2NH2) dication
Fe group and Ru group act as two unequal quantum dots.
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Surface attachment and orientation
molecule
N
Si-N bonds
Si
106o
3.8 Α
2.4 Α
Si
Si(111)
PHENYL GROUPS
“TOUCHING” SILICON
“struts”
Molecule is covalent bonded to Si and oriented vertically by “struts.”
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Measurement of molecular bistability
layer of molecules
Fe
Ru
Ru
Fe
0.15
1.7
Ru
C(oxidized)
C(reduced)
∆C
1.6
Hg
Hg
1.5
Hg
Fe
Fe
Fe
Ru
Ru
Ru
Si
ground state
applied
potential
-0.05
1.3
Si
1.2
switching
excited state
1.1
-0.10
-0.15
-0.20
1.0
voltage
-1.5
-1.0
Fe
When equalized, capacitance peaks.
2 counterion charge configurations on surface
0.05
0.00
1.4
Si
0.10
∆C (nF)
ac Capacitance
Fe
C (nF)
Energy
Applied field equalizes the energy of the two dots
Ru
-0.5
0.0
0.5
VHg (V)
1.0
-0.25
1.5
Fe
Ru
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Longer molecular double-dot
Isopotential
surface
HOMO
orbital
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Double-dot click-clack
COxidized
Cas prepared
∆C
1.3
3+
H2N
0.0
1.1
-0.1
1.0
Hua &
Fehlner
Ph2
P
P
Ph2
dm=2.8 nm
d1=1.8 nm
Ph2
P
Ru
P N
Ph2
Ph2
P
P
Ph2
NH2
0.9
-1.5
Ph2
P N
Ru
P
Ph2
∆C (nF)
C (nF)
1.2
0.1
-0.2
-1.0
-0.5
0.0
0.5
1.0
1.5
VHg (V)
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Square 4-dot QCA molecules
0.6 nm
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Imaging molecular double-dot
Kandel group
structure
toluene solution
Goal: single-molecule imaging on surfaces
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 clustering
Some 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
0.5and
V, Technology
20 pA, 298 K
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Nano Science
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
QCADesigner
Konrad Walus
U. British Columbia
Design tools are starting to enable new systems ideas.
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System + Application Architectures
Mike Niemier
Grounded in device physics & simulation
Incorporate clock driven dataflow
B
A
D
C
B
A
Device architecture maps well to many system architectures…
Reconfigurable
AND Plane
Systolic
General Purpose
OR Plane
BC
AB
AC
A A’ B B’ C C’
AB + BC + AC
Good for FIR, FT,
Matrix multiply, graph
algorithms, etc.
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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.
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