Transcript Group 2 - Index of

Nanowires and
Light Emitting Diodes (LEDs)
Nanowires
What Is A Nanowire?
Any solid material in
the form of wire with
diameter smaller than
about 100 nm.
Transmission
electron
micrograph of
an InP/InAs
nanowire
Nanowire quantization
Confinement of a particle in two directions
leads to additional energy quantization
and leaves only one degree of freedom.
E
ε13
ε12
Scanning electronic microscope image of free-standing InP quantum wires (from
Thomas Mårtensson, Patrick Carlberg, Magnus Borgstrom, Lars Montelius, Werner
Seifert, and Lars Samuelson, Nano Letters, Vol. 4, No. 4, pp. 699-702 (2004 ) ).
ε11
0
kx
Adapted from lecture summary #06 from Dr. Mitin’s EE240 Lecture
How are they Made?
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The Scanning Tunneling Microscope
(STM) can be used to image surfaces
or to manipulate atoms.
Nanowires can be made by crashing
the tip of an STM into a substrate and
then retracting. A bias voltage can be
applied between the tip and sample
allowing current and resistance
measurements.
The STM can also be used to locally
oxidize a pattern onto thin films of Si
which in return is used to pattern
nanometer sized wires
single rows of Indium
atoms on a Silicon surface.
The picture was taken with
a Scanning Tunneling
Microscope
schematic of the process used to
fabricate the wires
Nanowires can also be made using lithography techniques.
The advantage to this method is that many wires can be
made at once. Wires of 50nm width have been fabricated.
Another method of fabrication is the
capillarity-induced filling of carbon
nanotubes. Many materials have been used
to fill the carbon nanotubes such as Pb, Ni,
Cr, Ge, S, Dy, etc.
Nanoporous templates
The picture above shows an
example. One can see the concentric
carbon nanotubes encapsulating the
PbO center.
Electron deposition
Growing Nanowires
 Put iron nanopowder crystals on a silicon
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surface
Put in a chamber
Add natural gas with carbon (vapor
deposition)
Carbon reacts with iron and forms a
precipitate of carbon that grows up and out
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self-assembly is the
most important
fabrication
technique, because
of the large number
of structures you
can create quickly
http://www.rpi.edu/dept/materials/COURSES/N
ANO/bartolucci/index.html
Diodes
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Current flows in only one
direction
P-N Junction and Depletion
Region
Forward bias, Reverse bias
Reverse Bias
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Forward Bias
Reverse bias prevents current from flowing
Forward bias gives electrons additional energy
to overcome depletion region barrier
Light Emission
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Electron recombines with hole under forward
bias, Photon of light is emitted with characteristic
frequency
E = hw (eV) , frequency emitted is proportional to
the voltage applied multiplied by the elementary
charge
Electrons and holes combine radiatively (with a
photon) or nonradiatively (with a phonon) in the
depletion region
E2
E1   ( E2 -E1 ) /
.
ħω = E2 – E1
Adapted from lecture summary #20_1 from Dr. Mitin’s EE240 Lecture
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Electron recombination in direct bandgap
semiconductors emits photons (light)
Recombination in indirect bandgap emits
phonons to allow for momentum
conservation
Adapted from lecture summary #08 from Dr. Mitin’s EE240 Lecture
Radiative Recombination Rates
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Rate of
recombination is
proportional to
both the electron
and hole
concentrations
Spontaneous
Recombination vs
Hole
Concentration
with multiple
models
Non-Radiative (Phonon) Recombination Rates
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“Deep Levels” (ShockleyRead Equation)
Auger Recombination
Surface Recombination
Optical Properties, Efficiency, and Temperature
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Not all light emitted escapes
semiconductor Reabsorption
and Extraction Efficiency
material
Fraction of light that escapes = (1/2) (1-cosφc)
Fraction can be increased by a factor of 2-3 by
encapsulation
Homojunction vs Heterojunction
Types of LED Materials
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Aluminum Gallium Arsenide (AlGaAs) - red and infrared
Aluminum Gallium Phosphide (AlGaP) – green
Gallium Arsenide Phosphide (GaAsP) - red, orange-red, orange, and
yellow
Gallium Nitride (GaN) - green, pure green (or emerald green), and blue
Indium Gallium Nitride (InGaN) - near ultraviolet, bluish-green and blue
Silicon carbide (SiC) as substrate – blue
Zinc Selenide (ZnSe) – blue
Diamond (C) - ultraviolet
Zinc Oxide
GaN Nanowires
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ZnO Nanowire
Zinc Oxide (ZnO) Research
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P-type ZnO nanowires were once difficult to synthesize
ZnO emits high quality light
Efficient for imaging, data storage and biological/chemical
sensing
 Much lower manufacturing cost then Gallium Nitride (GaN)
LEDs
Traditional Lighting (Incandescent Bulbs)
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Tungsten Filament heats up as
current passes through it
Atoms vibrate, electrons are
temporarily boosted to higher
energy levels
Drop of electrons from higher
levels to lower ones creates
light
Advantages: Cheap to
produce, automatically create
white light
Disadvantages: High
percentage of energy going
towards heat, not as durable or
as long lasting as LED’s
Fluorescent Lamps
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Voltage difference across tube causes
electrons to flow
Mercury atoms are converted from
liquid to gas state
Electrons collide with gaseous mercury
atoms, exciting them to a higher
energy state and releasing light
Majority of the light is ultraviolet;
phosphor coating of tube converts this
ultraviolet light into visible white light
Advantages: More efficient than
traditional lighting, longer lifespan
Disadvantages: Operating
temperature, “flicker” at twice the
operating frequency, safe disposal of
mercury
LEDs
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Positive voltage is applied across
the pins to excite electrons/holes in
the diode
Electrons that jump energy levels
emit light of ONE specific energy
(and frequency)
LED housing designed to reflect as
much of this light forward as
possible
Can be much more efficient than
both incandescent and fluorescent
lighting
Efficiency
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LEDs shipping from manufacturers in 2006 are
approaching the efficiency of compact
fluorescents (CFs)
 Standard
CF’s – 60 lumens/watt
 LEDs – 50-60 lumens/watt
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Compare to Standard 100 watt incandescent
 17
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lumens/watt
LED’s projected to reach 150 lumens/watt within
10 years
Flexible
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Organic LEDs (OLEDs) are lighter and flexible.
Some possible future applications of OLEDs
 Inexpensive, flexible (rollable) displays
 Wall decorations
 Night vision (cheaper)
 Luminous cloth or clothing
 Imagine
a screen on your jacket arm.
(Not So) Future Applications
of OLEDs
Flexible computer and media
screens. Can be easily rolled up
for convenient storage.
OLEDs can be woven, or
possibly sprayed, onto articles of
clothing. Allows people to bring
their media wherever they go.
DieMount Spotlight LEDs
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With traditional LEDs
valuable light loss occurs
LEDs design allows
almost all the light to be
captured
Light is projected at a
solid angle of +/- 3-4°
Low operating current
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Longer lasting, lower
power consumption
Applications
Water Treatment
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Ultraviolet (UV) radiation causes damage to the
genetic structure of bacteria making them
incapable of reproduction.
Reduce bacteria levels in flowing raw sewage by
60% using ultraviolet LEDs
Hydro-Photon’s chamber reduces the level of
e-coli in contaminated water
 Reduces e-coli by 99.99% flowing at 300 ml/minute
 These rates are close to values required for
individual water treatment systems
References
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Tom Harris. "How Light Emitting Diodes Work". January 31,
2002 <http://www.howstuffworks.com/led.htm> (April 26,
2007)
Craig Freudenrich, Ph.D.. "How Light Works". July 10,
2000 <http://www.howstuffworks.com/light.htm> (April 26, 2007)
Schubert, E. Fred. Light-Emitting Diodes. 2nd. Cambridge: Cambridge University Press, 2006.
Gavryushin, V. "The Diode." Functional Combinations in Solid States. 06 June 2006. 26 Apr 2007
<http://www.mtmi.vu.lt/pfk/funkc_dariniai/diod/index.html.>
Frensley, William R. " Heterostructure and Quantum Well Physics." 21 May 1995. University of
Leeds. 26 Apr 2007
<http://www.utdallas.edu/~frensley/technical/hetphys/node1.html>.
"Nanowire Research, LED Breakthrough." TechNews. 03 Jan 2007. 26 Apr 2007
<http://www.technologynewsdaily.com/node/5579>.
Narayan, A L. "ZnO nanowires promise more efficient LEDs." Optics.org. 17 Jan 2007. 26 Apr 2007
<http://optics.org/cws/article/research/26830>.
Hogan, Hank. "A Single Dot Marks the Spot for Nanowire LEDs." Photonics Spectra. April 2007.
Laurin
Publishing.
26 Apr 2007. <http://www.photonics.com/content/spectra/2007/April/LED/87162.aspx>.
"LED basic knowledge." LED Professional. 26 Apr 2007
<http://www.ledprofessional.com/content/view/241/78/#1.2%20Blue%20and%20white%20
LEDs>.
Wyckoff, Susan. "What is a Light-Emitting Diode?." Experiments By Exploration. 1997.
Department of Physics
and
Astronomy, Arizona State University. 26 Apr 2007
<http://acept.la.asu.edu/courses/phs110/expmts/exp13a.html>.
Bai, Yuan Qiang Bai, Ying Dai, Zhong Lin Wang, Yue Zhang. “Bicrystalline zinc oxides nanowires. Chemical Physics Letters. 9 May
2003
Atwee, Tarek, Sandra Borner, Andreas Pohlkotter, Wolfgang Schade. “Zinc oxide nanowires.” TU Clausthal:
LaserAnwendungsCentrum. 26 Apr 2007. <http://lac.tu-clausthal.de/foreschung/zinc-oxide/nanowires/>
“Organic LED Displays (OLEDS). 26 Apr 2007. <http://www.ideasstorming.tw/blog/domotoro7176/idea840>
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Patch, Kimberly. “Crossed nanowires make Lilliputian LEDs.” TRN: The Latest Technology Research News. 17
Apr 2007. <http://www.trnmag.com/Stories/011701/Crossed_nanowires _make_Lilliputian_LEDs_TRN_0011701>
Jan 2001. 26
Savage, Neil. “Efficiency Jump for White OLEDs.” Technology Review. 20 Nov 2006. 26 Apr 2007.
<http://www.technologyreviews.com/Printer_friendly_article.aspx?id=17808>
“Cheaper LEDs from Breakthrough in Zinc Oxide (ZBO) Nanowire Research, Nano Letters Study Says.” NanoTechwire,com. 6 Jan
2007. 26 Apr 2007. <http://www.nanotechwire.com/news.asp?nid=4187&ntid=115&pg=1>
“Spotlight LED.” 29 March 2007. 26 Apr 2007. <http://optics.org/cws/product/P000002170>
Garfinkel, Simson. “LED Lights.” Technology Review. 6 Dec 2002. 26 Apr 2007.
<http://www.technologyreviews.com/printer_friendly_article.aspx?id=13048.