Transcript Document

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
E
E
conduction band
empty
Band
gap
Forbidden
region
Eg > 5eV
valence band
filled
Insulator
SiO2: Eg = 9 eV
E
conduction
band
Band
gap Eg <
valence
band
electron
hole
5eV
partially-filled
band
+
Semiconductor
Si: Eg = 1.1 eV
Ge: Eg = 0.75 eV
GaAs: Eg = 1.42 eV
Conductor
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
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
Biphenyl
Diodes and nonlinear devices
Molecule with D--A
structure
C16H33Q-3CNQ
D
Highly conductive zwitterionic D+--Astate 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

A
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
Molecular FET and logic gates
Molecular single-electron transistor:
Could achieve
frequency > 1 THz
switching
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
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
Self-assembled patterns of trans-BCTBPP on Au(111) at 63 K
Interlocking
with CN
groups
Conventional Organic Electronic Devices
Organic Thin Film
Transistors (OTFT)
Organic Light Emitting
Diode (OLED)
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:
2K 2


E  Eg 
13.6
(eV)
2M
n 2m  r2
0
Bohr radius of the exciton:
rm
0a
a 
B
 0
(a0 = 0.529 Å)
M = me*+ mh*, ħK: CM momentum
 = me*mh*/(me*+ mh*) reduced mass
Bohr radius of electron or hole:
a 
e,h
rm
0a
0
*
m
e,h
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:
2 2


E  Eg 
13.6
(eV)
2
2
2
2MR
n m r
0
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}:




2
2
2


1
1
1
.
8
e


E ( R)  E g 

 *
 4  R
2
*
r
2R  me m 
0
h


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%
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 > 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
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
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
HRTEM: number of
shells, diameter
STM: diameter, helicity of nanotube
out-shell, electronic structure
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
Bandgap of semiconducting
SWNTs:
ta
E g  C C
dt
a
C C = 1.42 Å,
STM I-V
spectroscopy
t  5.4 eV,
overlap integral
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
GaN
nanowires
BN nanotubes
Si nanowires
p-Si/n-GaN
nanowire
junction