Quantum dot cellular automata

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Quantum- dot cellular automata

• Computing by field polarization
• Author Gary H. Bernstein

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• Introduction
• What Is A Quantum-Dot?
• What is QCA ?
• Clocked QCA
• QCA Implementation
• Metal Tunnel Junction QCA
• Molecular and Magnetic QCA
• QCA Circuits

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Some History

• For more than 30 years, the microelectronics
  industry has enjoyed dramatic improvements in the
  speed and size of electronic devices.
• This trend has long obeyed Moores law, which
  predicts that the number of devices integrated on
  a chip will double every 18 months.

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Some History

• Since the early 1970s the device of choice for
  high levels of integration has been the field
  effect transistor (FET), this are tiny transistor
  amplifier in which the current flow between two
  terminals is controlled by the electric field
  generated inside the silicon by an external
  voltage on the surface.
• The FET of today is a vast improvement over that
  of 1970, but it is still used as a current
  switch.

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The Problem

• At gate lengths below 0.1 nm FETs will begin to
  encounter fundamental effects (mainly issues of
  power dissipation) that make further scaling
  difficult.
• Based on the Semiconductor Industry
  Association's
• 2003 - drawn gate lengths of 65 nm and etched at
  45 nm.
• By 2010, gate lengths of less than 20 nm will be
  in production.

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Solution Idea

• possible way for the microelectronics industry to
  maintain growth in device density is to change
  from the FET-based paradigm to one based on
  nanostructures
• instead of fighting the effects that come with
  feature size reduction, these effects are used to
  advantage.

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One nanostructure paradigm, is the

• Quantum-Dots these are nanostructures created
  from standard semi conductive materials such as
  InAs/GaAs. These structures can be modeled as
  3-dimensional quantum wells.
• they exhibit energy quantization effects even at
  distances several hundred times larger than the
  material system lattice constant.

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Quantum-Dots

• Electrons, once trapped inside the dot, do not
  alone possess the energy required to escape
• We can use quantum physics to our advantage
  because the smaller a quantum dot is physically,
  the higher the potential energy necessary for an
  electron to escape.

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Quantum-Dots

• researchers were especially captured by the
  quantum dot in that it represented the ultimate
  limit to size scales envisioned for semiconductor
  devices.
• Example quantum dot pyramid created with
  InAs/GaAs.

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Quantum - Dot Introduction (cont)

• In 1993, the notion of Quantum-Dot Cellular
  Automata - (QCA) in which ,quantum dots respond
  to the charge state of their neighbours was
  intreduced.
• This notion was offering the first computation
  scheme where quantum dots play the central role
  in Von Neumann type computing.
• This notion was envisioned that semi-conductor
  quantum dots confined by leaky barriers would
  lead to QCA architectures

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Quantum-Dot

• This notion failed ,it was found to be too
  cumbersome and prone to charge fluctuations
  caused by impurities and defects.
• so, a new notion of using metal tunnel junctions
  type for charge confinement was approached. (we
  will be back for this later)

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• What is QCA ?
• " Quantum-Dot Cellular Automata"

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Quantum-Dot Cellular Automata - (QCA)

• Rather than using voltages on transistors to
  encode information, QCA exploits interacting
  electric or magnetic field polarization to effect
  Boolean logic functions .
• For charge-based QCA, no current, other than a
  small displacement current, flows during
  computation. For magnetic QCA, magnetic dipole
  interactions effect computing ( I will explain
  more later )

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Quantum-Dot Cellular Automata - (QCA)Structure

• A basic QCA cell consists of four quantum dots in
  a square array coupled by tunnel barriers.
• Electrons are able to tunnel between the dots,
  but cannot leave the cell.
• If two excess electrons are placed in the cell,
  Coulomb repulsion will force the electrons to
  dots on opposite corners

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Quantum-Dot Cellular Automata - (QCA) Structure
(cont)

• There are two energetically equivalent ground
  state polarizations, which can be labeled logic
  "0" and" 1."

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Quantum-Dot Cellular Automata - (QCA) Structure
(cont)
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Quantum-Dot Cellular Automata - (QCA) Structure
(cont)

• A wire - is a line of cells. Since the cells
  are capacitively coupled to their neighbors, the
  ground state of the wire is for all cells to
  have the same polarization.

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QCA - Structure (cont)

• In this state, the electrons are as widely
  separated as possible, giving the lowest possible
  energy.
• To use the line, an input is applied at the left
  end of the line, forcing it to change to one
  polarization.
• Since the first and second cell are now of
  opposite polarization, with two electrons close
  together, the line is in a higher energy state
  and all subsequent cells in the line must flip
  their polarization to reach the new ground state.

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Quantum-Dot Cellular Automata - (QCA) Logic
Elements

• The mapping of a combinational logic problem onto
  a QCA system can be accomplished by finding
  arrangements of QCA cells that implement the
  basic logic functions AND, OR and NOT (inverters
  ).
• with AND/OR gates and NOT (inverters) it is
  possible to implement all combinational logic
  functions.

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Quantum-Dot Cellular Automata - (QCA) The logic
NOT (inverter)

• the input is first split into two lines of cells
  then brought back together at a cell that is
  displaced by 45 from the two lines. The 45
  placement of the cell produces a polarization
  that is opposite to that in the two lines

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Quantum-Dot Cellular Automata - (QCA) The logic
majority gates

• In this gate the three inputs "vote" on the
  polarization of the central cell, and the
  majority wins.

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Quantum-Dot Cellular Automata - (QCA) The logic
AND and OR gates

• With one input fixed, e.g. A, the majority gate
  functions as either an AND gate (AO) or an OR
  gate (AI)

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Quantum-Dot Cellular Automata - (QCA) fan-out
structure

• When the input of one of these structures is
  flipped, the new ground state of the system is
  achieved when all of the cells in all branches
  flip

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QCA single-bit full adder
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Quantum-Dot Cellular Automata - (QCA)Advantage

• A situation where only few cells flip over is
  impossible in a wire\line.
• An important advantage of QCA devices is the no
  need for long interconnect lines Since the cells
  communicate to their nearest neighbors.
• The inputs applied to the cells at the edge of
  the system and the computation proceeds until the
  output appears at cells at the edge of the QCA
  wire.

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Refinement to the basic QCA Clocked QCA

• To get a clocked system the switching of the
  cells is accomplished at the control of an
  additional clock signal.
• The clock controls the barriers, between the
  quantum dots. As such, they control the rate at
  which electrons are able to tunnel between the
  dots in the cell and therefore switch the
  polarization of the cell.

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Refinement to the basic QCA Clocked QCA

• When the clock signal is high the potential
  barriers between the dots are low and the
  electrons effectively spread out in the cell
• As the clock signal is switched low, the
  potential barriers between the dots are raised
  high and the electrons are localized such that
  they take on the polarization of their
  neighbours.
• clock high -gt cell is unlatched,
• clock low -gt cell is latched.

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Schematic of a clocked six-dot QCA cell.
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QCA - Implementation
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QCA -Implementation

• The concept of a QCA cell is generic in that it
  can be implemented in several different ways and
  there have been proposals for
• Metal Tunnel Junctions
• Molecular Implementation
• Magnetic Implementation
• Each of these implementations has certain
  advantages/disadvantages and none have yet been
  completely developed.

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Metal Tunnel Junctions

• In 1987 Metal Tunnel Junctions structures
  exhibiting single electron properties were
  fabricated.
• This system has been used to demonstrate several
  basic QCA cell and logic functions.

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Metal Tunnel Junctions QCA "dots - structure

• These "dots" relied on their ultra small
  capacitance, which was a consequence of their
  very small size, to reveal measurable voltage
  changes with charge variations of only a single
  electron
• In these system, dots AI metal islands that
  are separated by tunnel junctions, on which
  reside the electrons involved in polarization.
• The AI metal islands are created by complicated
  process.

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Tunnel junctions by shadow evaporation
Oxidation of aluminum
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Metal Tunnel Junctions QCA

• Due to the very small capacitance of the islands
  (on the order of 10-16 F), a change in the
  population of even one electron results in a
  measurable difference in potential.
• The tunnelling of electrons on and off the
  islands can be controlled by an external gate.
  Such a 3-terminal device is referred to as a
  single electron transistor (SET).

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Metal Tunnel Junctions QCA

• At every move, the electron population on the
  islands has changed by precisely one electron.
  after one electron has been added to the islands,
  it is energetically unfavorable for another
  electron to populate it until the gate bias is
  changed to allow another electron on.
• cells are consisted of a series combination of
  two dots capacitively coupled to another pair of
  dots and up to four SETs serving as electrometers
  determine the internal charge states of the dots.

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Metal-dot QCA implementation
70 mK
dot metal islands
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Metal Tunnel Junctions QCA three-dot cell

• A three-dot cell can function alone as a latch
  to hold data
• Multiple tunnel junctions (MTJ) are inserted
  between the dots to prevent electrons from
  leaving the end dots on which they are trapped by
  the energy barrier created by the clocked middle
  dot.

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Metal Tunnel Junctions QCA A three-dot cell can
function alone as a latch to hold data
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metal tunnel junctions structuresAdvantages

• The metal tunnel junctions type for charge
  confinement were found to be
• easier to fabricate.
• more reliable.
• easier to analyze and model.

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metal tunnel junctions structuresDisadvantages

• The metal island implementation does not have the
  structural properties for a scalable design, and,
  as a result, was only meant as a proof-of-concept
  implementation
• The metal-islands are on the order of 1 µm in
  dimension, and therefore, the system had to be
  cooled to extremely low temperatures for the
  electron switching to be observable.

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Molecular QCA

• The ultimate size limit of computing is a single
  molecule .
• The molecular implementation of QCA offers many
  advantages including
• highly symmetric cell structure
• very high operating speeds
• room-temperature operation
• very high device density
• Any current-switched device at reasonable speeds
  would melt the chip at those densities.

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Molecular QCA Structure

• The role of dots in molecular QCA is played by
  redox centers within the molecule.
• A redox center can add an electron or lose an
  electron without breaking chemical bonds.
• many variations of molecules have been designed
  that incorporate redox active corners connected
  by tunnel junctions, as well as chemical moities
  to serve as surface attachment sites.

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Molecular QCA Structure

• Full 4-dot cells

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A three-dot Molecular QCA cell

• three-dot cell Three charge configurations of
  molecule

DOT
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Molecular QCA process

• A practical molecular QCA system would consist of
  lithography at the few nm scale, molecules that
  attach in the channels, and self assembly in
  lines to form circuits with sufficiently few
  errors.

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Molecular QCA Implementation
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Molecular QCA

• these are the challenges still facing ahead
• the selective placement of molecules on a surface
• the realization of a mechanism for performing I/O
  operations with single molecules
• determining which molecules are most suitable
• for QCA operation.
• design of a clocking technology that can provide
  the clocking zone granularity required for
  complex circuit design.

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Magnetic QCA

• The magnetic implementation uses nano-scale
  magnets to act as the cells, and encodes the
  polarization in the magnetic vector of each of
  the nanomagnets.
• Although this technology is referred to as
  magnetic quantum cellular automata, the term
  quantum in this case represents the quantum
  mechanical nature of the exchange interaction and
  not electron tunneling, as in the electronic QCA.

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Magnetic QCA

• nanomagnetic QCA could easily operate at room
  temperature and are within present fabrication
  techniques.
• Unfortunately, magnetic QCA does not appear to
  have the necessary switching speed to compete
  with today's computers but may be an alternative
  for creating memory.

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SUMMARY AND CONCLUSIONS QCA Circuits

• Power dissipation Clocked QCA operates very
  close to the theoretical limits.
• Speed The absolute limit of switching speed is
  determined by tunnelling times
• Operating speeds are expected to be around 100
  MHz .

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SUMMARY AND CONCLUSIONS QCA Circuits

• Density This is a difficult issue to address
• For molecular QCA, 1013 cells cm-2. This far
  exceeds any current projections for CMOS.
• Magnetic QCA of 20 nm dimensions would achieve
  perhaps l011 cells cm-2
• Operating temperature Magnetic QCA, Molecular
  QCA can operate at room temperature while Metal
  Tunnel Junctions not

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SUMMARY AND CONCLUSIONS

• We have reviewed the paradigm of QCA and
  discussed a few experiments in metal tunnel
  junction QCA.
• Some limitations to the adoption of the
  technology were discussed.
• As with any research program in an unexplored
  field, the future utility will likely not be in
  the precise form studied, but rather from further
  refinement of the concepts

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CAD software
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cad software
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Credit

• http//people.atips.ca/walus/tutorials/QCATutoria
  l.html - tutorial qca
• Clocked Molecular Quantum-Dot Cellular Automata\
  Craig S. Lent and Beth Isaksen
• Computer Arithmetic Structures for Quantum
  Cellular Automata K. Walus, G. A. Jullien, V. S.
  Dimitrov University of Calgary/Electrical and
  Computer Engineering, Calgary, Canada
• Clocking of molecular quantum-dot cellular
  automata Kevin Hennessy and Craig S. Lenta)
  Department of Electrical Engineering, University
  of Notre Dame, Notre Dame, Indiana 46556
• Quantum-dot cellular automata Review and recent
  experiments invited G. L. Snider, A. O. Orlov, I.
  Amlani, X. Zuo, G. H. Bernstein, C. S. Lent, J.
  L. Merz, and W. Porod