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Different Electronic Materials. Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …) Insulators: SiO 2 , Al 2 O 3 , Si 3 N 4 , SiO x N y , ... Conductors: Al, Au, Cu, W, silicide, ...

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Different Electronic Materials


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different electronic materials
Different Electronic Materials
  • Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …)
  • Insulators: SiO2, Al2O3, Si3N4, SiOxNy, ...
  • Conductors: Al, Au, Cu, W, silicide, ...
  • Organic and polymer: liquid crystal, insulator, semiconductor, conductor, superconductor
  • Composite materials: multi-layer structures, nano-materials, photonic crystals, ...
  • More: magnetic, bio, …
insulators conductors semiconductors inorganic materials
Insulators, Conductors, SemiconductorsInorganic Materials

E

E

E

conduction

band

conduction band

empty

-

Band

gap

partially-filled

band

electron

hole

Band

gap

Forbidden

region

Eg < 5eV

Eg > 5eV

+

valence

band

valence band

filled

Insulator Semiconductor Conductor

Si: Eg = 1.1 eV

Ge: Eg = 0.75 eV

GaAs: Eg = 1.42 eV

SiO2: Eg = 9 eV

electronic properties device function of molecules
Electronic properties & device function of molecules
  • Electrons in molecule occupy discrete energy levels---molecular orbitals
  • Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are most important to electronic applications

Bandgap of molecule: Eg = E(LUMO) - E(HOMO)

  • Organic moleculeswith carbon-based covalent bonds, with occupied bond states ( band) as HOMO and empty antibonding states (* band) as LUMO
slide4

Lower energy by delocalization:

 Benzene Biphenyl

Conducting Polymers

Polyacetylene: Eg ~ 1.7 eV

 ~ 104 S cm-1

Polysulphur nitride (SN)n

 ~ 103-106 S cm-1

Poly(phenylene-vinylene) (PPV)

High luminescence efficiency

diodes and nonlinear devices molecule with d a structure c 16 h 33 q 3cnq
Diodes and nonlinear devicesMolecule with D--A structure C16H33Q-3CNQ

D

A

Highly conductive zwitterionic D+--A- state at 1-2V forward bias Reverse conduction state D---A+ requires bias of 9V

I-V curve of Al/4-ML C16H33Q-3CNQ LB film/Al structure

slide6

Negative differential resistance (NDR): electronic structural change under applied bias, showing peak conductance

2’-amino-4-ethynylphenyl-4’ethynylphenyl-5’-nitro-1-benzennthiol

Self-assembled layer between Au electrodes

NDR peak-to-valley ratio ~ 1000

slide7

Molecular FET and logic gates

Molecular single-electron transistor:

Could achieve switching frequency > 1 THz

slide8

Assembly of molecule-based electronic devices

“Alligator clips” of

molecules:

Attaching functional atoms

S for effective contact to Au

High conductance through leads but surface of body is insulating

slide9

Self-assembled Molecular (SAM) Layers

Carene on Si(100)

Simulated STM images

for (c)

for (a)

0.1 ML 1-nitronaphthalene adsorbed on Au(111) at 65 K

Ordered 2-D clusters

slide11

Conventional Organic Electronic Devices

Organic Thin Film Transistors (OTFT)

Organic Light Emitting Diode (OLED)

For large-area flat-panel displays, circuit on plastic sheet

slide12

Printing:

Soft-lithographic process in fabrication of organic electronic circuits

slide13

Unique electronic & opto-electronic properties of nanostructures

  • DOS of reduced dimensionality (spectra lines are normally much narrower)
  • Spatial localization
  • Adjustable emission wavelength
  • Surface/interface states

Effective bandgap blue-shifted, and adjustable by size-control

slide14

Optical properties of quantum dot systems

Excitons in bulk semiconductors

An e-h pair bound by Coulomb potential

H-atom like states of exciton in effective-mass approximation:

M = me*+ mh*, ħK: CM momentum

 = me*mh*/(me*+ mh*) reduced mass

Bohr radius of the exciton:

Bohr radius of electron or hole:

(a0 = 0.529 Å)

aB = ae + ah

slide15

In GaAs (me*= 0.067m0, mhh*= 0.62m0, r = 13.2)

Binding energy (n = 1): 4.7 meV, aB = 115 Å

Generally, binding energy in meV range, Bohr radius 50-400 Å

Excitons in QDs

Bohr radius is comparable or even much larger than QD size R

Weak-confinement regime:

R >> aB, the picture of H atom-like exciton is still largely valid:

slide16

Strong confinement regime (R << ae and ah): model of H atom-like exciton is not valid, confinement potential of QD is more important.

Lowest energy e-h pair state {1s, 1s}:

slide17

Production of uniform size spherical QDs

Controlled nucleation & growth in supersaturated solution

All clusters nucleate at basically same moment, QD size distribution < 15%

QDs of certain average size are obtained by removing them out of solution after a specific growth period

Further size-selective processing to narrow the distribution to  5%

slide18

Similar nucleation and growth processes of QDs also occur in glass (mixture of SiO2 and other oxides) and polymer matrices

Ion implantation into glass + annealing

Mono-dispersed nanocrystals of many semiconductors, such as CdS, CdSe, CdTe, ZnO, CuCl, and Si, are fabricated this way

Optimal performance of QDs for semiconductor laser active layers requires 3D ordered arrays of QDs with uniform size

In wet chemical QDs fabrication: proper control of solvent composition and speed of separation

slide19

In SK growth of QDs: strain-mediated intra- and inter-layer interactions between the QDs

Aligned array of GaN QDs in AlN

slide20

Passive optic devices with nanostructures:Photonic Crystal

An optical medium with periodic dielectric parameter r that generates a bandgap in transmission spectrum

slide21

Luminescence from Si-based nanostructures

Luminescence efficiency of porous Si (PSi) and Si QDs embedded in SiO2 ~ 104 times higher than crystalline Si

Fabrication of PSi: electrochemical etching in HF solution, positive voltage is applied to Si wafer (anodization)

Sizes of porous holes: from nm to m, depending on the doping type and level

slide22

Nano-finger model of PSi: from Si quantum wires to pure SiO2 finger with increasing oxidation

Emission spectrum of PSi: from infrared to the whole visible range

slide23

Remarkable increase in luminescence efficiency also observed in porous GaP, SiC

Precise control of PSi properties not easy

Si-based light emitting materials and devices

Digital Display

slide24

Atomic structures of carbon nanotubes

Stable bulk crystal of carbon Graphite

Layer structure: strong intra-layer atomic bonding, weak inter-layer bonding

3.4 Å

1.42 Å

slide25

Enclosed structures: such as fullerene balls (e.g., C60, C70) or nanotubes are more stable than a small graphite sheet

Trade-off: curving of the bonds raises strain energy, e.g., binding energy per C atom in C60 is ~ 0.7 eV less than in graphite

MWNT, layer spacing ~ 3.4 Å

SWNT

slide26

Index of Single-wall Carbon Nanotubes (SWNT)

Armchair (n, n)

Zigzag (n, 0)

General (m, n)

slide27

Synthesis of CNTs by Laser vaporization: Pulsed laser ablation of compound target (1.2% at. Co-Ni + 98.8% C)

High yield (~70%) of SWNT ropes

slide28

Carbon arc discharge: ~500 Torr He, 20-25 V across 1-mm gap between 2 carbon rods

Plasma T > 3000C, CNT bundles deposited on negative electrode

With catalyst (Co, Ni, Fe, Y, Gd, Fe/Ni, Co/Ni, Co/Pt)

 SWNTs

Without catalyst

 MWNTs

slide29

Vapor-phase synthesis: similar to CVD

Substrate at ~ 700-1500C decorated with catalyst (Co, Ni or Fe) particles, exposed to hydrocarbon (e.g. CH4, C6H6) and H2

Aligned CNTs grow continuously atop of catalyst particles

Regular CNT arrays on catalyst pattern

Useful for flat panel display

slide30

Growth mechanisms of C nanotubes

1) C2 dimer addition model: C2 dimer inserted near pentagons at cap

2) Carbon addition at open ends: attach C2 at armchair sites and C3 at zigzag sites

Functions of catalyst clusters: stabilizing terminators, cracking of hydrocarbons

Fit the controlled CVD process, the open-end is terminated by a catalyst cluster

slide31

Structural identification of nanotubes: with TEM, electron diffraction, STM

HRTEM: number of shells, diameter

STM: diameter, helicity of nanotube out-shell, electronic structure

slide32

Electronic properties of SWNTs

SWNTs: 1D crystal

If m - n = 3qmetallic

Otherwise semiconductor

Zigzag, dt = 1.6nm

=18, dt = 1.7nm

Bandgap of semiconducting SWNTs:

=21, dt = 1.5nm

=11, dt = 1.8nm

Armchair, dt = 1.4nm

= 1.42 Å,  5.4 eV, overlap integral

STM I-V spectroscopy

slide33

Junctions between SWNTs:homojunctions, heterojunctions, Schottky junctions, but how to connect and dope?

SWNT connections: insert pentagons and heptagons

Natural SWNT Junctions

slide34

Doping of semiconductor SWNTs

N, K atoms  n-type; B atoms, oxygen  p-type

SWNT CMOS inverter & its characteristics

slide35

Other nanotubes and nanowires

GaN nanowires

BN nanotubes

p-Si/n-GaN nanowire junction

Si nanowires