ФИЗИКО-ХИМИЧЕСКИЕ ОСНОВЫ НАНОТЕХНОЛОГИИ

ФИЗИКО-ФИЗИКО-ХИМИЧЕСКИЕХИМИЧЕСКИЕ

ОСНОВЫОСНОВЫНАНОТЕХНОЛОГИИНАНОТЕХНОЛОГИИ

Профессор Н.Г. Профессор Н.Г. РамбидиРамбиди

4. Квантовые колодцы, 4. Квантовые колодцы, квантовые нити, квантовые нити, кквантовые ямывантовые ямы

Плотность электронных Плотность электронных состояний в твердом телесостояний в твердом теле

Электрон в одномерной Электрон в одномерной ямеяме

Плотность электронных Плотность электронных состояний в твердом телесостояний в твердом теле

Квантовые колодцыКвантовые колодцы

Quantum WellsQuantum Wells

10 nm thick

10 cm

10 cm

Nanoscale Thin Film

e- moves freely

e- confined

Quantum Well

Quantum Well• Enclosed region of negative energy• Electrons confined• Can exist in one, two, three dimensions• Common example is a square well – sharpest boundary• Discrete energies of electrons• Narrow gap semiconductor between wide gap semiconductors• Surfaces states may be considered like quantum well states

Quantum Well ApplicationsQuantum Well ApplicationsMulti-spectral long wavelength quantum Multi-spectral long wavelength quantum well infrared photodetectors: infrared well infrared photodetectors: infrared radiation photoionize trapped carriers in radiation photoionize trapped carriers in quantum wells; for medical applications, quantum wells; for medical applications, locating hot spots in fires, observing locating hot spots in fires, observing volcanoesvolcanoes

Quantum well lasers: quantum Quantum well lasers: quantum confinement effects increase luminescence confinement effects increase luminescence efficiencyefficiency

Quantum well LEDsQuantum well LEDs

Quantum well Hall (magnetic) sensorsQuantum well Hall (magnetic) sensors

JPL’s QWIP detects minute differences associated with

blood flow changes

http://www.jpl.nasa.gov/technology/shrinking/tiny_tech.html

Quantum well laser mounted on the head of

a pin:

http://wwwrsphysse.anu.edu.au/admin/pgbrochure/quantum.html

SuperlatticesSuperlattices Alternating layers of thin films additional periodicity causes Alternating layers of thin films additional periodicity causes interesting effectsinteresting effects When When ll , , λλ > > b b → phonon wave and particle effects→ phonon wave and particle effects Electron transport well understoodElectron transport well understood Limited understanding of phonon transportLimited understanding of phonon transportAre layers thin enough for electrons/phonons to tunnel?Are layers thin enough for electrons/phonons to tunnel?Is coherence maintained for mini-band formation?Is coherence maintained for mini-band formation?

Si0.76Ge0.24 / Si0.84Ge0.16 superlattice:

In a crystal, atomic periodicity leads to band formation In a superlattice, engineered periodicity leads to minibands

Electron tunneling

b

Superlattice ApplicationsSuperlattice ApplicationsMagnetic superlattices for magneto-optical Magnetic superlattices for magneto-optical recording: large perpendicular anisotropies and recording: large perpendicular anisotropies and enhancement of Kerr rotations provide unique enhancement of Kerr rotations provide unique propertiesproperties

Giant magnetoresistanceGiant magnetoresistance

Superlattice field effect transistorsSuperlattice field effect transistors

Thermoelectric materialsThermoelectric materialsMetallic superlattice for GMR

Commercially available superlattice thermoelectric device from RTI

Superlattice used in thermoelectrics:

http://www.rti.org/pubs/anser.pdfhttp://www.nuee.nagova-u.ac.jp/labs

Квантовые нитиКвантовые нити

Квантовые ямыКвантовые ямы


Квантовые ямы – миниатюрные устройства, Квантовые ямы – миниатюрные устройства, которые содержат немного свободных которые содержат немного свободных электроновэлектронов

Типичные размеры лежат в области от Типичные размеры лежат в области от нанометров до нескольких микрометровнанометров до нескольких микрометров


В квантовой яме могут быть от одного до В квантовой яме могут быть от одного до нескольких тысяч электроновнескольких тысяч электронов

Размеры и форма ямы и число электронов Размеры и форма ямы и число электронов можно точно контролироватьможно точно контролировать


Так же, как и в атоме, энергетические Так же, как и в атоме, энергетические уровни в квантовых ямах дискретныуровни в квантовых ямах дискретны

Структура уровней сходна с уровнями 3Структура уровней сходна с уровнями 3DD потенциальной ямыпотенциальной ямы

В квантовой яме свойства могут В квантовой яме свойства могут существенно измениться если удалить даже существенно измениться если удалить даже один электронодин электрон


В отличие от атомов квантовые ямы легко В отличие от атомов квантовые ямы легко присоединять к электродам и создавать на присоединять к электродам и создавать на их основе различные устройстваих основе различные устройства

Semiconductor Band GapsSemiconductor Band Gaps Energy states in an atom Energy states in an atom

correspond to bands in a correspond to bands in a semiconductorsemiconductor

In between the valence In between the valence and conduction bands, and conduction bands, there are no states where there are no states where an electron can existan electron can exist

Electron-hole pairs (EHPs) Electron-hole pairs (EHPs) can form by thermal or can form by thermal or photo excitationphoto excitation

Electrons in the conduction Electrons in the conduction band are free to conduct band are free to conduct electricityelectricity

Different semiconductors Different semiconductors have different band gapshave different band gaps

EC

EV

EG

The Energy Levels of Quantum The Energy Levels of Quantum DotsDots

The Quantum Dot band gap is smaller than the The Quantum Dot band gap is smaller than the surrounding material, so electrons will tend to “fall” into surrounding material, so electrons will tend to “fall” into the dot to reach a lower-energy configurationthe dot to reach a lower-energy configuration

Because the Quantum Dots are so small (20-30 nm), Because the Quantum Dots are so small (20-30 nm), quantum mechanics govern how an electron will behave quantum mechanics govern how an electron will behave in the dotin the dot

EC

EV

EG

e-

E electron

hole

The Quantum DotThe Quantum Dot

Confinement in all three Confinement in all three dimensions dimensions E Exx, E, Eyy, ,

and Eand Ezz are quantized are quantized

(discrete)(discrete) Higher probability of Higher probability of

recombination means recombination means greater radiative greater radiative emissionemission

“Electronic Structure of InAs Pyramidal Quantum Dots”: http://www.sst.nrel.gov/research/InAs.html

Ee = Ez + Ex + Ey with all E discrete

Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy (MBE)

Substrate wafers Substrate wafers transferred to high transferred to high vacuum growth chamber vacuum growth chamber (red arrow)(red arrow)

Elements kept in K-Cells Elements kept in K-Cells at high tempat high temp

Shutters over cells open Shutters over cells open to release vaporized to release vaporized elements, which deposit elements, which deposit on sampleon sample

Adapted from: Farrow, R.F.C., ed. Molecular Beam Epitaxy: Applications to Key Materials. Noyes Publications, Park Ridge, NJ, 1995.

Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy (MBE)

More About MBEMore About MBE

The temperature of each K-Cell controls the The temperature of each K-Cell controls the rate of deposition of that element (Ga, In, Al, rate of deposition of that element (Ga, In, Al, etc.)etc.)

As and P can also be flowed into chamberAs and P can also be flowed into chamber Precise control over temperatures and shutters Precise control over temperatures and shutters

allows very thin layers to be grown (~1 allows very thin layers to be grown (~1 ML/sec)ML/sec)

RHEED patterns indicate surface morphologyRHEED patterns indicate surface morphology

Fabrication of WellsFabrication of Wells

Lattice matched Lattice matched AlGaAs grown on GaAs AlGaAs grown on GaAs substratesubstrate

Thin layer of GaAs (~10 Thin layer of GaAs (~10 nm)nm)

Another layer of Another layer of AlGaAs to finish the AlGaAs to finish the wellwell

d2

d1

z

d1 = d2

Fabrication of DotsFabrication of Dots

Thick layer of GaAsThick layer of GaAs

Begin growing InAs Begin growing InAs (greater lattice (greater lattice constant)constant)

Crystal strain forces Crystal strain forces dot formationdot formation

Cap dots with layer of Cap dots with layer of GaAsGaAs

d2

d1

z

d1

Epitaxy: Patterned GrowthEpitaxy: Patterned Growth

Growth on patterned Growth on patterned substrates substrates Grow QDs in pyramid-Grow QDs in pyramid-

shaped recessesshaped recesses Recesses formed by Recesses formed by

selective ion etchingselective ion etching Disadvantage: density of Disadvantage: density of

QDs limited by mask QDs limited by mask patternpattern

T. Fukui et al. GaAs tetrahedral quantum dot structures fabricated using selective area metal organic chemical vapor deposition. Appl. Phys. Lett. May, 1991

Epitaxy: Self-Organized GrowthEpitaxy: Self-Organized Growth Self-organized QDs through epitaxial growth strainsSelf-organized QDs through epitaxial growth strains

Stranski-Krastanov growth mode (use MBE, MOCVD)Stranski-Krastanov growth mode (use MBE, MOCVD) Islands formed on wetting layer due to lattice mismatch (size ~10s nm)Islands formed on wetting layer due to lattice mismatch (size ~10s nm)

Disadvantage: size and shape fluctuations, orderingDisadvantage: size and shape fluctuations, ordering Control island initiation Control island initiation

Induce local strain, grow on dislocation, vary growth conditions, combine with Induce local strain, grow on dislocation, vary growth conditions, combine with patterningpatterning

AFM images of islands epitaxiall grown on GaAs substrate.

(a) InAs islands randomly nucleate.

(b) Random distribution of InxGa1 xAs ring-shaped islands.

(c) A 2D lattice of InAs islands on a GaAs substrate.

P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.

QCA: Physics BasicsQCA: Physics Basics

Cell

-

Empty

Containing electron

Tunnel – allows electrons to move between dots

QuantumDot

Using cells w/2 electrons.

? Possible configurations? ?

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-

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-

Only 2, since electrons

repel each other.

“Low energy state”

“0”

“1”

QCA: Physics BasicsQCA: Physics Basics

Adjacent cells’ electrons also repel each other.

Consumes/generates no energy.

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or

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QCA: WiresQCA: Wires

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Adjacent cells in low-energy state

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One cell “fixed” to

some value.

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Electrons move into low-energy state. Value propogates.


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Same whenrotated to vertical.


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Same idea whencells rotated 45º.

Note complementation!

QCA: Wire CrossingsQCA: Wire Crossings

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Such wires cross w/o

interference.

QCA: Wire CrossingsQCA: Wire Crossings

--

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-Same value

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QCA: GatesQCA: Gates

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Fixed inputs

Output

What function computed??

?Majority.

I.e., most common input value.

aa bb cc maj.maj.

00 00 00 00

00 00 11 00

00 11 00 00

00 11 11 11

11 00 00 00

11 00 11 11

11 11 00 11

11 11 11 11

Clocked Molecular Quantum-dot Clocked Molecular Quantum-dot Cellular AutomataCellular Automata

Molecular quantum-dot proposed byMolecular quantum-dot proposed by

Lent – IsakcsenLent – Isakcsen

AllylAllyl

AlkylAlkyl

Clocked Molecular Quantum-dot Clocked Molecular Quantum-dot Cellular AutomataCellular Automata

Allyl groups serve as Allyl groups serve as dots with his red-ox dots with his red-ox centre that can be centre that can be achieved cy hallsachieved cy halls

Alkyl groups serve as Alkyl groups serve as tunnel barrier that tunnel barrier that halls can pass throughhalls can pass through

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