Nanowires and Light Emitting Diodes (LEDs)

What Is A Nanowire?Any solid material inthe form of wire withdiameter smaller thanabout 100 nm.

Transmissionelectronmicrograph ofan InP/InAsnanowire

Nanowires

Nanowire quantization

Confinement of a particle in two directions leads to additional energy quantization and leaves only one degree of freedom.

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 ) ).

0 kx

ε11

ε12

ε13

E

Adapted from lecture summary #06 from Dr. Mitin’s EE240 Lecture

How are they Made? 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

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.

schematic of the process used to fabricate the wires

Electron depositionNanoporous templates

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.

The picture above shows an example. One can see the concentric carbon nanotubes encapsulating the PbO center.

Put iron nanopowder crystals on a silicon 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

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/NANO/bartolucci/index.html

Growing Nanowires

Diodes Current flows in only one

direction P-N Junction and Depletion

Region

Forward bias, Reverse bias

Reverse bias prevents current from flowing Forward bias gives electrons additional energy

to overcome depletion region barrier

Reverse Bias Forward Bias

Light Emission 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

2 1( ) / .E -E

ħω = E2 – E1

E2

E1

Adapted from lecture summary #20_1 from Dr. Mitin’s EE240 Lecture

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 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 “Deep Levels” (Shockley-

Read Equation) Auger Recombination Surface Recombination

Optical Properties, Efficiency, and Temperature

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

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

Zinc Oxide (ZnO) Research 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

GaN Nanowires ZnO Nanowire

Traditional Lighting (Incandescent Bulbs)

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

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

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

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

Compare to Standard 100 watt incandescent 17 lumens/watt

LED’s projected to reach 150 lumens/watt within 10 years

Flexible

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

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 Longer lasting, lower

power consumption Applications

Water Treatment

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

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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> Patch, Kimberly. “Crossed nanowires make Lilliputian LEDs.” TRN: The Latest Technology Research News. 17 Jan 2001. 26

Apr 2007. <http://www.trnmag.com/Stories/011701/Crossed_nanowires _make_Lilliputian_LEDs_TRN_0011701> Savage, Neil. “Efficiency Jump for White OLEDs.” Technology Review. 20 Nov 2006. 26 Apr 2007.

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