NST 621 Nanomaterials.ppt

7/29/2019 NST 621 Nanomaterials.ppt

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Introduction to Nano-materials

As part of ECE-758 Introduction

to Nanotechnology


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Outline

What is nano-material and why we areinterested in it?

Ways lead to the realization of nano-materials

Optical and electronic properties of nano-materials

Applications


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What is nano-material ?

Narrow definition: low dimension semiconductorstructures including quantum wells, quantumwires, and quantum dots

Unlike bulk semiconductor material, artificialstructure in nanometer scale (from a few nms to

a few tens of nms, 1nm is about 2 mono-layers/lattices) must be introduced in addition tothe naturally given semiconductor crystallinestructure


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Why we are interested in nano-material?

Expecting different behavior of electrons in theirtransport (for electronic devices) and correlation(for optoelectronic devices) from conventional

bulk material


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Stages from free-space to nano-material

Free-space

Schrdinger equation in free-space:

Solution:

Electron behavior: plane wave

,...3,2,1,/2 lLlk

1)/(

Etrkik e

0

22

2||

m

kE

trtrtim

,,

2

0)2(


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Bulk semiconductor

Schrdinger equation in bulk semiconductor:

Solution:

Electron behavior: Bloch wave

trtr t

irV

m ,,0

2

0

)](2

[

)()( 00 RlrVrV

r

erV

2

0 )(

kneEtrki

kn

)/(

effm

kE

2

|| 22


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Nano-materialSchrdinger equation in nano-material:

with artificially generated extra potential contribution:

Solution:

trtrnano

tirVrV

m,,0

2

0

)]()(

2

[

)(rVnano

knrFekn

iEt

kn

)(

,

/


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Electron behavior:

Quantum well 1D confined and in parallel plane 2DBloch wave

Quantum wire in cross-sectional plane 2D confined and1D Bloch wave

Quantum dot all 3D confined


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A summary on electron behavior

Free space plane wave with inherent electron mass continued parabolic dispersion (E~k) relation density of states in terms of E: continues square root

dependence

Bulk semiconductor plane wave like with effective mass, two different type of

electrons identified with opposite sign of their effective mass,i.e., electrons and holes

parabolic band dispersion (E~k) relation density of states in terms of E: continues square rootdependence, with different parameters for electrons/holes indifferent band


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A summary on electron behavior Quantum well

discrete energy levels in 1D for both electrons and holes plane wave like with (different) effective masses in 2D parallel

plane for electrons and holes dispersion (E~k) relation: parabolic bands with discrete states

inside the stop-band

density of states in terms of E: additive staircase functions, withdifferent parameters for electrons/holes in different band

Quantum wire discrete energy levels in 2D cross-sectional plane for both

electrons and holes plane wave like with (different) effective masses in 1D for

electrons and holes dispersion (E~k) relation: parabolic bands with discrete states

inside the stop-band density of states in terms of E: additive staircase decayed

functions, with different parameters for electrons/holes indifferent band


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A summary on electron behavior Quantum dot

discrete energy levels for both electrons and holes dispersion (E~k) relation: atomic-like k-independent discrete

energy states only density of states in terms of E: -functions for electrons/holes


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Electrons in semiconductors: highly mobile, easilytransportable and correlated, yet highlyscattered in terms of energy

Electrons in atomic systems: highly regulated interms of energy, but not mobile


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Electrons in semiconductors: easily controllableand accessible, yet poor inherent performance

Electrons in atomic systems: excellent inherentperformance, yet hardly controllable oraccessible


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Answer: take advantage of both semiconductorsand atomic systems Semiconductor quantumdot material


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Why we are interested in nano-material? Detailed reasons:

Geometrical dimensions in the artificial structure can be tuned tochange the confinement of electrons and holes, hence to tailorthe correlations (e.g., excitations, transitions andrecombinations)

Relaxation and dephasing processes are slowed due to thereduced probability of inelastic and elastic collisions (much

expected for quantum computing, could be a drawback for lightemitting devices) Definite polarization (spin of photons are regulated) (Coulomb) binding between electron and hole is increased due

to the localization Increased binding and confinement also gives increased

electron-hole overlap, which leads to larger dipole matrixelements and larger transition rates Increased confinement reduces the extent of the electron and

hole states and thereby reduces the dipole moment


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Ways lead to the realization of nano-material Required nano-structure size:

Electron in fully confined structure (QD with edge size d), its allowed(quantized) energy (E) scales as 1/d2 (infinite barrier assumed)

Coulomb interaction energy (V) between electron and other chargedparticle scales as 1/d

If the confinement length is so large that V>>E, the Coulomb interactionmixes all the quantized electron energy levels and the materialshows a bulk behavior, i.e., the quantization feature is not preservedfor the same type of electrons (with the same effective mass), butstill preserved among different type of electrons, hence we have(discrete) energy bands

If the confinement length is so small that V


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Ways lead to the realization of nano-material Required nano-structure size:

Similar arguments can be made about the effects oftemperature, i.e., kBT ~ E?

But kBT doesnt change the electron eigen states, instead,it changes the excitation, or the filling of electrons intothe eigen energy structure

If kBT>E, even E is a discrete set, temperature effect still

distribute electrons over multiple energy levels and dilutethe concentration of the density of states provided by theconfinement, since E can never be a single energy level

Therefore, we also need kBT


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Ways lead to the realization of nano-material

Required nano-structure size:The critical size is, therefore, given by V(dc)=E(dc)>kBT (25meV at room

temperature).

For typical III-V semiconductor compounds, dc~10nm-100nm (around20 to 200 mono-layers).

More specifically, if dc100nm, full bulk

(mix-up).

On the other hand, dc must be large enough to ensure that at least oneelectron or one electron plus one hole (depending on applications)state are bounded inside the nano-structure.


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Ways lead to the realization of nano-material

Current technologies Top-down approach: patterning etching

re-growth

Bottom-top approach: patterning etching selective-growth

Uneven substrate growth: edge overgrowth,V-shape growth, interface QD, etc.

Self-organized growth: most successfulapproach so far


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Electronic Properties

Ballistic transport a result of much reducedelectron-phonon scattering, low temperaturemobility in QW (in-plane direction) reaches arather absurd value ~107cm2/s-V, with

corresponding mean free path over 100m

Resulted effect electrons can be steered,

deflected and focused in a manner very similarto optics, as an example, Youngs double slitdiffraction was demonstrated on such platform


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Low dimension tunneling as a collective effectof multiple nano-structures, resonance appearsdue to the phase-matching requirement

Resulted effect stair case like I-Vcharacteristics, on the down-turn side, negativeresistance shows up


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If excitation (charging) itself is also quantized(through, e.g., Coulomb blockade), interactionbetween the excitation quantization and thequantized eigen states (i.e., the discrete energy

levels in nano-structure) brings us into acompletely discrete regime

Resulted effect a possible platform tomanipulate single electron to realize variousfunctionalities, e.g., single electron transistor(SET) for logical gate or memory cell


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Optical Properties

Discretization of energy levels increases thedensity of states

Resulted effect enhances narrow bandcorrelation, such as electron-hole recombination;for QD lasers, the threshold will be greatlyreduced


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Optical Properties

Discretization of energy levels reducesbroadband correlation

Resulted effect reduces relaxation anddephasing, reduces temperature dependence;former keeps the electrons in coherence, whichis very much needed in quantum computing;

latter reduces device performance temperaturedependence (e.g., QD laser threshold andefficiency, QD detector sensitivity, etc.)


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Optical Properties

Quantized energy level dependence on size(geometric dimension)

Resulted effect tuning of opticalgain/absorption spectrum


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Optical Properties

Discretization of energy levels leads to zerodispersion at the gain peak

Resulted effect reduces chirp, a very muchneeded property in dynamic application ofoptoelectronic devices (e.g., optical modulators

or directly modulated lasers)


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Applications

Light source - QD lasers, QC (QuantumCascade) lasers

Light detector QDIP (Quantum Dot Infrared

Photo-detector) Electromagnetic induced transparency (EIT) to

obtain transparent highly dispersive materials

Ballistic electron devices Tunneling electron devices

Single electron devices


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References Solid State PhysicsC. Kittel, Introduction to Solid State Physics,

Springer, ISBN: 978-0-471-41526-8

Basic Quantum MechanicsL. Schiff, Quantum Mechanics, 3rdEdition, McGraw Hill, 1967, ISBN-0070856435

On nano-material electronic properties W. Kirk and M. Reed,Nanostructures and Mesoscopic Systems, Academic Press, 1991,ISBN-0124096603

On nano-material and device fabrication techniques T. Steiner,Semiconductor Nanostructures for Optoelectronic Applications,

Artech House, 2004, ISBN-1580537510

On nano-material optical properties G. Bryant and G. Solomon,Optics of Quantum Dots and Wires, Artech House, 2005, ISBN-1580537618

NST 621 Nanomaterials.ppt

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