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, SemiconductorsInorganic Materials

E

valence band filled

conduction band empty

Forbiddenregion Eg > 5eV

Bandgap

E

conduction band

Eg < 5eVBandgap

+

-electronhole

E

valence band

partially-filledband

Insulator Semiconductor ConductorSi: Eg = 1.1 eVGe: Eg = 0.75 eVGaAs: Eg = 1.42 eV

SiO2: Eg = 9 eV

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 molecules with carbon-based covalent bonds, with occupied bond states ( band) as HOMO and empty antibonding states (* band) as LUMO

Lower energy by delocalization:

 

 

 Benzene Biphenyl

Conducting PolymersPolyacetylene: 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 C16H33Q-3CNQ

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

AD

Self-assembled layer between Au electrodes

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

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

NDR peak-to-valley ratio ~ 1000

Molecular FET and logic gates

 Molecular single-electron transistor:

Could achieve switching frequency > 1 THz

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

Self-assembled Molecular (SAM) Layers

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

Ordered 2-D clusters

Carene on Si(100) Simulated STM images for (c) for (a)

Self-assembled patterns of trans-BCTBPP on Au(111) at 63 K

Interlocking with CN groups

Organic Thin Film Transistors (OTFT)

Organic Light Emitting Diode (OLED)

Conventional Organic Electronic Devices

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

Printing:

Soft-lithographic process in fabrication of organic electronic circuits

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

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:

(eV) 20m2n

.613222

rMgEE

K M = me*+ mh

*, ħK: CM momentum

= me*mh

*/(me*+ mh

*) reduced mass

Bohr radius of the exciton:

00 a

mrBa

(a0 = 0.529 Å)

Bohr radius of electron or hole:

0*,

0, a

hem

mrhea

aB = ae + ah

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:

(eV) 20m2n

.61322

22

rMRgEE

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}:

04

28.1 *1

*1

22

22)(

Rre

hmemRgERE

Production of uniform size spherical QDs

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%

Controlled nucleation & growth in supersaturated solution

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

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

Aligned array of GaN QDs in AlN

Passive optic devices with nanostructures: Photonic Crystal An optical medium with periodic dielectric parameter r that generates a bandgap in transmission spectrum

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

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

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

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

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

Index of Single-wall Carbon Nanotubes

(SWNT)

Armchair (n, n)

 

Zigzag (n, 0)

General (m, n)

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

Carbon arc discharge: ~500 Torr He, 20-25 V across 1-mm gap between 2 carbon rods Plasma T > 3000C, CNT bundles deposited on negative electrode

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

Without catalyst MWNTs

Vapor-phase synthesis: similar to CVDSubstrate at ~ 700-1500C 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

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

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

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

HRTEM: number of shells, diameter

Electronic properties of SWNTs SWNTs: 1D crystal If m - n = 3q metallic Otherwise semiconductor

Zigzag, dt = 1.6nm

=18, dt = 1.7nm

=21, dt = 1.5nm

=11, dt = 1.8nm

Armchair, dt = 1.4nm

STM I-V spectroscopy

Bandgap of semiconducting SWNTs:

tdCCat

gE

= 1.42 Å, 5.4 eV, overlap integral

CCa t

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

SWNT connections: insert pentagons and heptagons

Natural SWNT Junctions

Doping of semiconductor SWNTs

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

SWNT CMOS inverter & its characteristics

Other nanotubes and nanowires BN nanotubes GaN

nanowires

p-Si/n-GaN nanowire junctionSi nanowires