Different Electronic Materials

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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,

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Insulators, Conductors, SemiconductorsInorganic
Materials
E
E
E
conduction band
conduction band empty
-
Band gap
partially-filled band
electron hole
Band gap
Forbidden region
Eg lt 5eV
Eg gt 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
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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

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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
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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
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Negative differential resistance (NDR)
electronic structural change under applied bias,
showing peak conductance
2-amino-4-ethynylphenyl-4ethynylphenyl-5-nitro-
1-benzennthiol
Self-assembled layer between Au electrodes
NDR peak-to-valley ratio 1000
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Molecular FET and logic gates  Molecular
single-electron transistor Could achieve
switching frequency gt 1 THz
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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
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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
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Self-assembled patterns of trans-BCTBPP on
Au(111) at 63 K
Interlocking with CN groups
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Conventional Organic Electronic Devices
Organic Thin Film Transistors (OTFT)
Organic Light Emitting Diode (OLED)
For large-area flat-panel displays, circuit on
plastic sheet
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Printing Soft-lithographic process in
fabrication of organic electronic circuits
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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
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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, hK CM momentum ? memh/(me
mh) reduced mass
Bohr radius of the exciton
Bohr radius of electron or hole
(a0 0.529 Å)
aB ae ah
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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 gtgt aB, the picture of
H atom-like exciton is still largely valid
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Strong confinement regime (R ltlt 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
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Production of uniform size spherical QDs
Controlled nucleation growth in supersaturated
solution
All clusters nucleate at basically same moment,
QD size distribution lt 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
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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
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In SK growth of QDs strain-mediated intra- and
inter-layer interactions between the QDs
Aligned array of GaN QDs in AlN
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Passive optic devices with nanostructures
Photonic Crystal
An optical medium with periodic dielectric
parameter ?r that generates a bandgap in
transmission spectrum
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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
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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
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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
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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
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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
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Index of Single-wall Carbon Nanotubes (SWNT)
Armchair (n, n)   Zigzag (n, 0) General (m, n)
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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
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Carbon arc discharge 500 Torr He, 20-25 V
across 1-mm gap between 2 carbon rods Plasma T
gt 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
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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
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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
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Structural identification of nanotubes with TEM,
electron diffraction, STM
HRTEM number of shells, diameter
STM diameter, helicity of nanotube out-shell,
electronic structure
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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
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Junctions between SWNTs homojunctions,
heterojunctions, Schottky junctions, but how to
connect and dope?
SWNT connections insert pentagons and heptagons
Natural SWNT Junctions
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Doping of semiconductor SWNTs N, K atoms ?
n-type B atoms, oxygen ? p-type
SWNT CMOS inverter its characteristics
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Other nanotubes and nanowires
GaN nanowires
BN nanotubes
p-Si/n-GaN nanowire junction
Si nanowires