Quantum dots cellular automata material implementationby apoorva bhattDr. Peter MoranEE5900 Material Science Seminar1AGENDAWHAT ARE QUANTUM DOTS ?

WHAT IS QCA (QUANTUM DOTS CELLULAR AUTOMATA)?

FABRICATION TECHNOLOGY

APPLICATIONS OF QCA

QUESTIONS OR COMMENTS?

Beginning course details and/or books/materials needed for a class/project.2WHAT ARE QUANTUM DOTS?In order to understand Quantum Dots, we must first understand what are called ExcitonEXCITON An exciton is a bound state of an electron (and hole) which are attracted to each other by electrostatic Coulomb ForceWHY BOTHER? In order to implement a system that encodes information in the form of electron position it becomes necessary to construct a vessel in which an electron can be trapped and "counted" as there or not there

Fig 1: Two exciton transitions in a single quantum dot, showing the ground state, the excitons and the biexciton (image credit: X Liet al.2003 Science301809).

A schedule design for optional periods of time/objectives. 3BAND GAP STRUCTURE FORMING EXCITONS

FIG 2 A) Periodic Potential (demarcation between the electron-hole) in the energy bands forming quantum dots B) Exciton Energy Levels

BEHIND THE SCENESWhat are semiconductor quantum dots (QDs)? Traps for matter waves, artificial pseudo-atoms, entities with discrete energy levels one-dimensional, time independent Schrdingers equation

Electron in atom: E = 1-10 eV, L = 0.1 nmExciton in semiconductor quantum dot: E 0.1 eV, L 10 nm,

Simulated Quantum dot generation

SOURCE: - http://www.onlineinvestingai.com/blog/2009/02/04/whats-the-spin-on-quantum-dots/ National Institute of Nanotechnology at the University of Alberta6A quantum dot does logical interpretation (switching) by establishing a region of low potential surrounded by a ring of high potential.

Such rings are able to trap electrons of sufficiently low energies/temperature and are sometimes called potential wells

There are several ways to implement quantum dots but apparently the most common, and the ones used here metal.

Nanometer-scale dots are constructed from Aluminum using electron beam lithography techniques.SIMPLIFIED APPROACH

UNDERSTANDING QUANTUM DOTSA quantum dot is a nanometer sized structure that is capable of trapping electrons in three dimensions

Quantum dots are made by creating an island of conductive material surrounded by insulating material

Electrons that enter the quantum dot will be confined because of the high potential required to escape

A quantum dot can have anything from a single electron to a collection of several thousands

This technique enables us to use the position of the electron as the logic determination factor (i.e. switching would be possible at very high speed)

This would enable us to work at higher frequencies which is not possible using current technology

Quantum dots will become the backbone of future microelectronic and photonic devices:because of their unique properties due to quantum confinement of electrons in 3-dimensionsthis results in interesting electronic and optical propertiesNeuro-quantum structures (Quantum Mind)Single-electron devices, for instance transistors (SET)Tunable lasersPhotodetectorsSensorsQuantum Computing: Quantum Dot Cellular AutomataWhat are their Applications?

WHY ARE QUANTUM DOTS IMPORTANT?

9A quantum dot is a 3 dimensional structure that is formed due to strain in the crystal lattice between two layers of epitaxial grown films. Quantum dots are of particular interest in this project due to their unique ability to hold charge in 3-dimensions. Quantum dots also contain unique optical properties, where they are currently being used for future designs of tunable lasers. Quantum dots are extremely small, only on the order of 10-100nm in width and they form out of a variety of materials, although the particular combination in which this project is interested in is the study of InAs quantum dots grown on top of a GaAs substrate. REALISING QUANTUM DOTS1) Semiconductor with smaller bandgap embedded into matrix with large bandgap 2) Just right size, large enough to accommodate an exciton, small enough in all directions for quantum confinement3) No structural defects such as dislocationsKEY ISSUE: uniformities of size, shape, chemical composition, strain distribution, crystallographic phase, mutual alignment, Optoelectronics (no contacting problem, only problem QD array homogeneity) - Active medium in lasers, (In,Ga,Al)As based 1.3 m!

- Far infrared detectors

- Novel device concepts such as quantum cellular automata

10Producing dots of small positional and size variability usually involves the use of electron beam lithography, which is similar to conventional lithography except that patterns are traced out using an electron beam rather then using a mask and light.

Conventional lithography is not capable of creating devices at that scale since the wavelength of light used is greater then the required feature size. The image below shows three different quantum dot structures:

As we can see the shape of a quantum dot is not necessarily round and varies depending on the process and application.

Mass production of Quantum Dots?

WHAT IS QUANTUM DOT CELLULAR AUTOMATA (QCA) ? - INTRODUCTION

COURTESY : - YOUTUBE VIDEO POSTED BY UNIVERSITY OF ALBERTA, CA http://www.phys.ualberta.ca/~wolkow/news.phpQuantum-Dot Cellular AutomataRepresent binary information by charge configurationA cell with 4 dotsTunneling between dotsPolarization P = +1Bit value 12 extra electronsPolarization P = -1Bit value 0Bistable, nonlinear cell-cell responseRestoration of signal levels

cell1cell2cell1cell2Cell-cell response function

Neighboring cells tend to align.Coulombic coupling

Patterning of Quantum DotsCURRENT: random surface patterningFUTURE: patterned surface arrays?

14Quantum Cellular Automata is a promising future for quantum dot electronic devices, although the development of this field is still facing one large hurdle, which is the patterning of quantum dots. Quantum dots currently form randomly on the surface of a material and vary in size. Quantum dot size variation also means a variation in the electronic properties of the quantum dot. Therefore, it is very crucial to learn how to spatially order quantum dots and control their size and shape for uniformity.QCA The Four Dot DeviceUses electrons in cells to store and transmit dataElectrons move between different positions via electron tunnelingLogic functions performed by Coulombic interactions

Quantum Dots operate as Cellular Automata2 extra electrons are introduced to the quantum cell

Electrons have the ability to tunnel from one quantum dot to the next

Repelling force of electrons moves the charge to opposite corners of the quantum cell, resulting in two possible arrangements, representing binary 0 and 1Quantum Celle-e-binary 0e-e-binary 1

16This slide addresses one of the most relevant applications of quantum dots as relevant to this project, although it has nothing to do with the research performed at the U of A. As I stated before, quantum ots have the unique ability to hold charge in 3-D. Making use of this property, a new form of computing has been envisioned by Dr. Craig Lent at Notre Dame titled Quantum Cellular Automata, which is a new form of logic gates using quantum dots. This works off of the basic architecture of a quantum cell. A quantum cell is made up of four quantum dots at equal distance apart, forming a square. There are two extra electrons held within the quantum cell. Electrons within the quantum dots have the ability to tunnel from one dot to the other, therefore the repulsing electrons will always choose to be as far apart as possible, allowing only two arrangements for the quantum cell, which represents the binary numbers 0 and 1.Quantum Dot Wireless Logic: five dot model of Lent and PorodLent and Porod of Notre Dame proposed a wireless two-state quantum dot device called a cellEach cell consists of 5 quantum dots and two electrons

eeState 1eeState 0

Very similar to four-dot modelThe two electrons repel each other, causing them to move to opposite corners of the device

This yields two states of equal energy in the cellQuantum Dots: Five dot Modeleeee

Quantum Dot WireBy placing two cells adjacent to each other and forcing the first cell into a certain state, the second cell will assume the same state in order to lower its energy

eeeeeeeeThe net effect is that a 1has moved on to the next cellBy stringing cells together inthis way, a pseudo-wire canbe made to transport a signalIn contrast to a real wire,however, no current flows

Other QCA Structures-- Wires90-degree wire

45-degree wireNormal and inverted signal available on the same wire

Observe that in this logic an inverter costs nothing!

Metal-dot QCA cells and devicesdot = metal islandelectrometers70 mKAl/AlOx on SiO2

Metal-dot QCA implementationGreg Snider, Alexei Orlov, and Gary Bernstein

Metal-dot QCA cells and devicesDemonstrated 4-dot cell

A.O. Orlov, I. Amlani, G.H. Bernstein, C.S. Lent, and G.L. Snider, Science, 277, pp. 928-930, (1997).

1234

Metal-dot QCA cells and devicesMajority Gate

MABCAmlani, A. Orlov, G. Toth, G. H. Bernstein, C. S. Lent, G. L. Snider, Science 284, pp. 289-291 (1999).

QCA The CircuitFundamental circuit is shown aboveThis is a 90-degree wire45-degree wires can also be constructedBinary value alternates between polarization +1 and 1 as it travels down the wireRipper cells can be placed to get the actual binary value or complemented value from the wire

OutputInput BInput AProgram Line01111*by simply changing the program line to 1, the device is transformed to an OR gateSpecial cases of Majority25Different combinations and layouts of quantum cells perform the functions of todays logic gates. Represented in the picture above,an AND gate is illustrated. For an AND gate the program line cell is held at binary 0. If input A and B are set at binary 1, this forces the middle cell to also be a binary one because the input A and B cell repulsion forces are greater than the force at the program line cell. This therefore sets the output at one also, therefore performing the AND gate function. This quantum dot AND gate can be quickly changed to an OR gate by switching the program line from 0 to 1.

FABRICATION OF QCAGenerally speaking, there are four different classes of QCA implementations: Metal-Island, Semiconductor, Molecular, and Magnetic

Metal-IslandThe Metal-Island implementation was the first fabrication technology created to demonstrate the concept of QCA. It was not originally intended to compete with current technology in the sense of speed and practicality, as its structural properties are not suitable for scalable designs. The method consists of building quantum dots using aluminum islands. Earlier experiments were implemented with metal islands as big as 1 micrometer in dimension. Because of the relatively large-sized islands, Metal-Island devices had to be kept at extremely low temperatures for quantum effects (electron switching) to be observable.

Semiconductor

Semiconductor(orsolid state) QCA implementations could potentially be used to implement QCA devices with the same highly advancedsemiconductor fabricationprocesses used to implement CMOS devices.

Cell polarization is encoded as charge position, and quantum-dot interactions rely on electrostatic coupling.

However, current semiconductor processes have not yet reached a point where mass production of devices with such small features (~20 nanometers) is possible.Serial lithographicmethods, however, make QCA solid state implementation achievable, but not necessarily practical. Serial lithography is slow, expensive and unsuitable for mass-production of solid-state QCA devices. Today, most QCA prototyping experiments are done using this implementation technology.

REFERENCES1. WIKIPEDIA (www.wikipedia.com)

2. YOUTUBE & UNIVERSITY OF ALBERTA, CANADA FOR THE MULTIMEDIA VIDEOS

3. ADDER AND MULTIPLIER DESIGN IN QUANTUM DOT CELLULAR AUTOMATA IEEE TRANSACTION JUNE 2009 BY Heumpil Cho, Member, IEEE, and Earl E. Swartzlander, Jr., Fellow, IEEE

4. QUANTUM CELLULAR AUTOMATA: THEORY AND APPLICATIONS BY CARLOS A. PEREZ DELGADO

Questions/Discussions

NOW IM SCARED !!!

An opportunity for questions and discussions.29