quntum wells, wires and dots pk.pptx

7/27/2019 Quntum wells, wires and dots pk.pptx

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Quntum wells, wires and

dotsPresented By

Prashant KumarM.Tech, 1st Year

NST


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Quantum confinement

It is defined as trapping of particlesand restricting their motion.

StructuresQuantum dots (0-D) only confined states, and

no freely moving ones

Nanowires (1-D) particles travel only along

the wireQuantum wells (2-D) confines particles within

a thin layer


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Quantum confinement


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Change in density of states


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Why Q Dots?

Traditional semiconductors have their shortcomings , theylack versatility.

Their optical and electronic qualities are costly to adjust,because their bandgap cannot be easily changed.

Their emission frequencies cannot be easily manipulated byengineering.

Q Dots exist in a quantum world, where properties aremodulated according to needs.

Technological advancements have made it possible to makesemiconductors with tunable bandgaps, allowing forunique optical and electronic properties and a broad rangeof emission frequencies.


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Fabrication Of Quantum DotsColloidal Synthesis: Three components

precursors, organic surfactants, and solvents Inthis form of synthesis precursor molecules aredissolved in solvent.Solution is then heated at large temperatureto start creating monomers. Once themonomers reach a high enough supersaturation level, the Nanocrystal growth startswith a nucleation process by rearranging andannealing of atoms.

For this process the temperature control isnecessary. And is done via heat or laser.

Due to strong quantum confinement, thenanocrystals how size-tunable absorption andluminescence.

By control of the surface chemistry, weproduced photo chemically stable nanocrystals


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Viral Assembly: In 2002 itwas found that usinggenetically engineered M13bacteriophage viruses Q Dotscan be created.

It is known that viruses canrecognize specificsemiconductor surfacesthrough the method ofselection by combinatorialphage display.

Therefore using this propertyand controlling the solutionionic strength and byapplying outside magneticfield we can createnanocrystals in a controlledenvironment.


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Fabrication Continued

Electrochemical Assembly: Highly ordered arrays ofquantum dots may also be self-assembled byelectrochemical techniques. A template is created bycausing an ionic reaction at an electrolyte-metal interfacewhich results in the spontaneous assembly ofnanostructures, including quantum dots, onto the metalwhich is then used as a mask for mesa-etching thesenanostructures on a chosen substrate.

Cadmium-free quantum dots CFQD: In many regions ofthe world there is now a restriction or ban on the use ofheavy metals in many household goods which means thatmost cadmium based quantum dots are unusable forconsumer-goods applications. A range of restricted, heavymetal-free quantum dots has been developed showingbright emissions in the visible and near infra-red region ofthe spectrum and have similar optical properties to thoseof CdSe quantum dots.


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Security inks : Due to its Colloidal properties Q Dots can be mixedinto inks which incorporate quantum dots, nanoscalesemiconductor particles , which can be tuned to emit light atspecific wavelengths in the visible and infrared portion of thespectra .

Biology and Medicinal sciences: Q dots replacing organic dyes,for highly sensitive cellular imaging, Extraordinary photostabilityof quantum dot probes is the real-time tracking of molecules andcells over extended periods of time, thus is used cancertechnology


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Quantum Wire

A strip of conducting material about 10nmor less in width and thickness thatdisplays quantum mechanical effects.


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Conductance of nanowires dependon

the length, lateral dimensions,

state and degree of disorder and

elongation mechanism of thewire.


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Nanowire fabrication

Template assistance

Electrochemicaldeposition

High pressureinjection

CVD

Laser assisted

techniques

N El t i


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NanoElectronicApplications of nanowires

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

Potential application of nanowires is in:

very dense logic

dense memory

optoelectronics

sensing devices


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Quantum wells

In 1972, Charles H. Henry, a physicist and newly-appointed Headof the Semiconductor Electronics Research Department at BellLaboratories, had a keen interest in the subject of integratedoptics, the fabrication of optical circuits in which the light travels in

waveguides. A quantum well is a potential well that confines particles, which

were originally free to move in three dimensions, to twodimensions.

forcing them to occupy a planar region.

The effects of quantum confinement take place when the quantumwell thickness becomes comparable at the de Broglie wavelengthof the carriers (generally electrons and holes), leading to energylevels called "energy subbands", i.e., the carriers can only havediscrete energy values.


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2-D Quantum Confinement

A B A

Bulk Semiconductors

A B A

Epitaxial Layers

50 nm 50 nm 50 nm

Conduction Bands

Valence Bands

Valence Band

Conduction Band

DiscreteEnergyLevels

Quantum Well


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Multiple Quantum Wells

Bulk Semiconductor A Bulk Semiconductor B

Semiconductor Heterostructure

Quantum Well Bandstructure

Grown atom-by-atom

in an MBE machine

(Molecular Beam Epitaxy)

A multi-quantum welllayer structure used as adetector is called aQWIP (Quantum WellInfrared Photodetector)


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Transitions Of quantum wells

Bound to Bound

Bound to Continuum

Bound to Quasi- Bound


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Bound-to-Continuum

Excited bound state is situated in thecontunuum

Photoexcited eletrons escape withouttunneling

Low bias voltage

Low dark current


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Fabrication

Quantum wells are formed in semiconductors byhaving a material, like gallium arsenidesandwiched between two layers of a material witha wider bandgap , like aluminium arsenide.

These structures can be grown by molecularbeam epitaxy or chemical vapor deposition withcontrol of the layer thickness down

to monolayers.


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AcknowledgementDr. A. Kasi Viswanath Sir

References Introduction to nanotechnology, Charles P. Poole,Jr

Solid-State Electronics 44 (2000) 2207-2212

JACS Communications

PHYSICAL REVIEW LETTERS


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Thank You

quntum wells, wires and dots pk.pptx

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