Quantum Well Wire & Dots

7/31/2019 Quantum Well Wire & Dots

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Presented By-: Mohammad Rameez & Balaji Venkatesan


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Quantum confinement

Trap particles and restrict their motion

Quantum confinement produces new materialbehavior/phenomena

Engineer confinement- control for specific

applicationsStructures

(Scientific American)

Quantum dots (0-D) only confinedstates, and no freely moving ones

Nanowires (1-D) particles travel only along

the wireQuantum wells (2-D) confines particleswithin a thin layer

http://www.me.berkeley.edu/nti/englander1.ppthttp://phys.educ.ksu.edu/vqm/index.html
http://phys.educ.ksu.edu/vqm/index.htmlhttp://phys.educ.ksu.edu/vqm/index.htmlhttp://phys.educ.ksu.edu/vqm/index.html


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Quantum Confinement

Excitons have an average physical separationbetween the electron and hole, referred to asthe Exciton Bohr Radius this physical distanceis different for each material. In bulk, thedimensions of the semiconductor crystal aremuch larger than the Exciton Bohr Radius,allowing the exciton to extend to its natural

limit. However, if the size of a semiconductorcrystal becomes small enough that itapproaches the size of the material's ExcitonBohr Radius, then the electron energy levelscan no longer be treated as continuous - theymust be treated as discrete, meaning thatthere is a small and finite separation between

energy levels. This situation of discrete energylevels is called quantum confinement .


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Quantum Confinement in Nanostructures

Confined in:

1 Direction: Quantum well (thin film)Two-dimensional electrons

2 Directions: Quantum wire

One-dimensional electrons

3 Directions: Quantum dot

Zero-dimensional electrons

Each confinement direction converts a continuous k in a discrete quantum number n.

kx

nz

ny

ny

nz

nx

kx

ky

nz


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Some Basic Physics Density of states (DoS)

in 3D:

Structure Degree of

Confinement

Bulk Material 0D

Quantum Well 1D 1

Quantum Wire 2D

Quantum Dot 3D (E)

dE

dN

E

E1/

dE

dk

dk

dN

dE

dN

DoS

V

k

kN

3

3

)2(34

statepervol

volspacek)(


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Quantum Dots


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What are Quantum Dots??

A quantum dot is a semiconductor whose excitons are confined in allthree spatial dimensions. As a result, they have properties that arebetween those of bulk semiconductors and those of discretemolecules.

A crystal of semiconductor compound (eg. CdSe, PbS) with a diameteron the order of the compound's Exciton Bohr Radius. Quantum dotsare between 2 and 10 nanometers wide (10 and 50 atoms).

An electromagnetic radiation emitter with an easily tunable bandgap.


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Continued

In an unconfined (bulk) semiconductor, an electron-hole

pair is typically bound within a characteristic length

called the Bohr exciton radius. If the electron and hole

are constrained further, then the semiconductor'sproperties change. This effect is a form of quantum

confinement, and it is a key feature in many emerging

electronic structures.The Quantum dot is such an

electronic structure which is based on the principle ofQuantum confinement.


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Artificial Atoms

QuantumDotsaremorecloselyrelatedtoindividualatomsratherthanbulkmaterialsbecauseoftheirdiscretequantizedenergylevelsinsteadofenergybands.Thereforetheyarealsoknownasartificialatoms.


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Why Q Dots?

Their optical and electronic qualities are costly to adjust,because their bandgap cannot be easily changed. Theiremission frequencies cannot be easily manipulated byengineering. Q Dots exist in a quantum world, whereproperties are modulated according to needs.

Technological advancements have made it possible tomake semiconductors with tunable bandgaps, allowing forunique optical and electronic properties and a broad range ofemission frequencies.

Traditional semiconductors haveshortcomings, they lack versatility.


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Quantum Dots - A tunable range ofenergies

Because quantum dots' electron energy levels are discreterather than continuous, the addition or subtraction of just afew atoms to the quantum dot has the effect of altering theboundaries of the bandgap.

Changing the geometry of the surface of the quantum dotalso changes the bandgap energy, owing again to the smallsize of the dot, and the effects of quantum confinement.


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Fabrication Of Quantum Dots

QDotscanbesynthesizedindifferentways,theseare-----

Colloidal Synthesis: Three components precursors, organicsurfactants, and solvents In this form of synthesis precursor

molecules are dissolved in solvent. Solution is then heated at largetemperature to start creating monomers. Once the monomers reach

a high enough super saturation level, the Nano crystal growth starts

with a nucleation process by rearranging and annealing of atoms.

For this process the temperature control is necessary.

And is done via heat or laser.

Due to strong quantum confinement, the nanocrystals

show size-tunable absorption and luminescence.By control of the surface chemistry, we produced

photo chemically stable nanocrystals


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Fabrication Continued

Viral Assembly: In 2002 it was found that using geneticallyengineered M13 bacteriophage virusesQ Dots can be created. It is

known that viruses can recognize specific semiconductor surfaces

Through the method of selection by combinatorial phage display.

Therefore using this property and controlling the solution ionic

strength and by applying outside magnetic field we can createnanocrystals in a controlled environment.


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Fabrication Continued..

Electrochemical Assembly: Highly ordered arrays of quantum dots may alsobe self-assembled by electrochemical techniques. A template is created by

causing an ionic reaction at an electrolyte-metal interface which results in thespontaneous assembly of nanostructures, including quantum dots, onto themetal which is then used as a mask for mesa-etching these nanostructures on achosen substrate.

Cadmium-free quantum dots CFQD: In many regions of the world there isnow a restriction or ban on the use of heavy metals in many household goodswhich means that most cadmium based quantum dots are unusable forconsumer-goods applications. A range of restricted, heavy metal-free quantumdots has been developed showing bright emissions in the visible and near infra-red region of the spectrum and have similar optical properties to those of CdSequantum dots.


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Applications and uses

Information processing and Computing

Quantum dots have also been implemented as qubits for quantum

information processing.By applying small voltages to the leads, the flow

of electrons through the quantum dot can be controlled and thereby precise

measurements of the spin and other properties therein can be made. With

several entangled quantum dots,or qubits, plus a way of performing

operations, quantum calculations and the computers that would perform

them might be possible.


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Biology and Medicinal sciences:

Qdots replacing organic dyes,

Usage of quantum dots for highly sensitive cellular imaging,

Extraordinary photostability of quantum dot probes is the real-time tracking of

molecules and cells over extended periods of time, thus is used cancer technology


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Photovoltaic devices And Nano

crystal solar cellSemiconductor nanoparticles that exhibit size and

compositionally tunable bandgaps. Therefore,different types and sizes of quantum dots,engineered to perfectly match and absorb thelight of the solar spectrum, can be brought

together into the same cell Alternative quantumdot based solar cells approaches including,luminescent concentrator cells, quantum dot dyesensitized solar cells, multiple exciton generation,

and intermediate band solar cells.


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Electronic applications they have been proven to operate

like a single-electron transistor and show the Coulombblockade effect

Security inks

Due to its Colloidal properties Q Dots can be mixed

into inks which incorporate quantum dots, nanoscalesemiconductor particles,which can be tuned to emit

light at specific wavelengths in the visible and

infrared portion of the spectra


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Security Ink with Q Dots emmiting greenlight. Combining multiple quantum dots andother pigments to create unique fluorescentspectral barcodes that identify any object or document upon illuminated


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Quantum Wire


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1D nanostructures represent the smallestdimension structure that can efficiently transportelectrical carriers

1D nanostructures can also exhibit critical device

function, and thus can be exploited as both thewiring and device elements in futurearchitectures for functional nanosystems

In this regard, two material classes:semiconductor nanowires (NWs)

carbon nanotubes (NTs)

have shown particular promise

Introduction


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Background

What is Quantum Wire?

A strip of conducting material

about 10nm or less in width and

thickness that displays quantummechanical effects.

- from Science andTechnology Dictionary

Essential Difference?

Not copying quantum info, buttransported-destroy at source then

recreating at destination.

Fig. 3. Illustration of carbon nanotubefrom www.spacedaily.com/news/nanotech-05zn.html

Fig. 4. A carbon nanotube between two electrodes

from http://www.mb.tn.tudelft.nl
http://www.spacedaily.com/news/nanotech-05zn.htmlhttp://www.mb.tn.tudelft.nl/http://www.mb.tn.tudelft.nl/http://www.spacedaily.com/news/nanotech-05zn.htmlhttp://www.spacedaily.com/news/nanotech-05zn.htmlhttp://www.spacedaily.com/news/nanotech-05zn.html


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Why nanowires?

They represent the smallest dimension forefficient transport of electrons and excitons, andthus will be used as interconnects and criticaldevices in nanoelectronics and nano-

optoelectronics. (CM Lieber, Harvard)

General attributes & desired properties

Diameter 10s of nanometers

Single crystal formation -- common crystallographic orientation along thenanowire axis

Minimal defects within wire

Minimal irregularities within nanowire arrays


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Nanowire fabrication

Challenging!

Template assistance

Electrochemical deposition Ensures fabrication of electrically continuous wires since only takes

place on conductive surfaces

Applicable to a wide range of materials

High pressure injection Limited to elements and heterogeneously-melting compounds with

low melting points

Does not ensure continuous wires

Does not work well for diameters < 30-40 nm

CVD

Laser assisted techniques


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Magnetic nanowires

Important for storage device applications

Cobalt, gold, copper and cobalt-coppernanowire arrays have been fabricated

Electrochemical deposition is prevalent

fabrication technique


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Nanoscale size exhibits the following properties different fromthose found in the bulk:

quantized conductance in point contacts and narrow channelswhose characteristics (transverse) dimensions approach theelectronic wave length

Localization phenomena in low dimensional systems

Mechanical properties characterized by a reduced propensity forcreation and propagation of dislocations in small metallic samples.

Conductance of nanowires depend on

the length,

lateral dimensions,

state and degree of disorder and

elongation mechanism of the wire.

Quantum and localization of nanowire conductance


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NanoElectronic Applications of nanowires

The most important application of nanowires innanoelectronics is using them asjunctions or asmulti-segment nanowires or crossednanodevices.

Potential application of nanowires is in:

very dense logic dense memory optoelectronics sensing devices


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Quantum wells


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Quantum wells

A quantum well is a potential well that confinesparticles, which were originally free to move in three

dimensions, to two dimensions.forcing them to occupy a planar region.The effects of quantum confinement take place when

the quantum well thickness becomes comparable at the

de Broglie wavelength of the carriers (generallyelectrons and holes), leading to energy levels called"energy subbands", i.e., the carriers can only havediscrete energy values.


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2-D Quantum Confinement

Bulk Semiconductors

A B A

Epitaxial Layers

Conduction Bands

Valence Bands

Conduction Band

Valence Band

DiscreteEnergyLevels

A B A50 nm 50 nm5 nm

Quantum Well


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Figure 8.2. Schematic energy band diagram of GaAs/GaAlxAs1-x quantum

well. An electron (represented by its wavefunction y) can be considered

as partially confined in the quantum well of width equal to the GaAs

thickness. The barrier height DE is equal to the difference in the energies

of the bottom of the conduction band Ecfor the two layer materials. E

vis

the energy of the top of the valence band and Egap is the band gap energy.

y

Egap for

AlxGa1xAs

Egap for

GaAs

Ec

Ev

DE

thickness of

GaAs layer


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Multiple Quantum Wells

BulkSemiconductor A

Bulk SemiconductorB

Semiconductor Heterostructure

Quantum Well Bandstructure

Grown atom-by-atomin an MBE machine

(Molecular Beam Epitaxy)


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Transitions

Bound to BoundBound to ContinuumBound to Quasi- Bound


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Bound-to-Continuum

Excited bound state is situated in the contunuum

Photoexcited eletrons escape without tunneling Low bias voltage

Low dark current


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Bound-to-Bound

Photo-excitation to another bound state withinsame energy band

Excited carriers escape out of well by tunneling


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Fabrication

Quantum wells are formed in semiconductors by having a material, like galliumarsenide sandwiched between two layers of a material with a widerbandgap,like aluminium arsenide.

These structures can be grown by molecular beam epitaxy orchemical vapordeposition with control of the layer thickness down to monolayers.
https://en.wikipedia.org/wiki/Gallium_arsenidehttps://en.wikipedia.org/wiki/Gallium_arsenidehttps://en.wikipedia.org/wiki/Bandgaphttps://en.wikipedia.org/wiki/Aluminium_arsenidehttps://en.wikipedia.org/wiki/Molecular_beam_epitaxyhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Monolayerhttps://en.wikipedia.org/wiki/Monolayerhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Chemical_vapor_depositionhttps://en.wikipedia.org/wiki/Molecular_beam_epitaxyhttps://en.wikipedia.org/wiki/Molecular_beam_epitaxyhttps://en.wikipedia.org/wiki/Aluminium_arsenidehttps://en.wikipedia.org/wiki/Bandgaphttps://en.wikipedia.org/wiki/Gallium_arsenidehttps://en.wikipedia.org/wiki/Gallium_arsenide


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In optical devices such as laser diodes. They are also used to make HEMTs (High Electron Mobility Transistors), which areused in low-noise electronics.

Quantum well infrared photo detectors are also based on quantum wells, and areused for infrared imaging.

Applications
https://en.wikipedia.org/wiki/HEMThttps://en.wikipedia.org/wiki/Quantum_well_infrared_photodetectorhttps://en.wikipedia.org/wiki/Infrared_imaginghttps://en.wikipedia.org/wiki/Infrared_imaginghttps://en.wikipedia.org/wiki/Infrared_imaginghttps://en.wikipedia.org/wiki/Quantum_well_infrared_photodetectorhttps://en.wikipedia.org/wiki/Quantum_well_infrared_photodetectorhttps://en.wikipedia.org/wiki/Quantum_well_infrared_photodetectorhttps://en.wikipedia.org/wiki/HEMT


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QUESTIONS


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Thank you

Quantum Well Wire & Dots

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