Light Emitting Diodes
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Transcript of Light Emitting Diodes
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Light Emitting Diodes
NanoLab 2003
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Outline
• Motivation/Applications: Why LED’s?• Background• Fabrication• Testing• Conclusions
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Motivation/Applications: Why LED’s?
• Wide range of colors • Efficient and Reliable
– Saves money• Requires less money to operate• Generates less heat
– Good for electronics– Reduced AC costs
• Last longer
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Why Do We Care? Efficiency (lumens/watt)
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Light Bulbs vs LED’s•Light Bulbs
–Filament• Sudden
Failure-Breaks/Burns down
–Recent bulbs last up to two years at ~20 lumens/watt
–Fluorescent tubes last about 7500 hrs at ~80 lumens/watt
•LED’s–No filament
• Gradual Failure-Intensity decrease over time
–Last from 50,000 to 100,000 hrs (5-10 yrs)
–Recent LED’s (orange,red) have efficiency of ~100 lumens/watt
– Generate little heat• Reduced A/C costs
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Applications•Communication (fiber optics)•Blue Laser Diodes
–Video Recording–Data Storage–Televisions–Video Games–High Density DVD’s–DVD-ROM drive
•Extra Motivation:–First company to produce efficient, reliable, cost-
effective WHITE LED’s will make lots of money.
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Isolated Atoms > Crystal > Artificial Atom
isolated atom
Diamond lattice
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Background-Band Gaps and Lattice Constants
• Lattice mismatch reduces efficiency
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Background-Band Gaps and Lattice Constants
Bandgap energy vs lattice constant of various III-semiconductors at room temperature.
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What are the III-Vs
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Background-Band Gaps and Lattice Constants
Room-temperature bandgap energy vs lattice constant of common elemental and binary semiconductors.
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Background -Lattice Mismatch
• Lattice mismatch reduces efficiency
Two crystals with mismatched lattice constants resultion in dislocation at or near the interface between the two semiconductors.
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Background: pn Junctions and Recombination• Electron from
donor material recombines with hole in acceptor material.
• Produces photon with energy hv equal to that of the band gap.
• Smaller band gaps give infrared/red light; larger band gaps give blue/UV light
Carrier distribution in pn homojunctionsCarrier distribution in pn homojunctions
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Background: pn Junctions and Recombination
• Electron and holes are trapped in the quantum wells.• Such spatial overlap gratly enhances the
recombination rate - brightness, efficiency.
Heterojunction under forward biasHeterojunction under forward bias
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Background: Ohmic Contacts
• http://nina.ecse.rpi.edu/shur/Ch3/sld043.htm
Contacts
Relatively little resistance
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DopingHole in lower energy band allows for easier travel for electrons
Electrons forced to higher, partially filled band electron moves easier
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Making our Samples• We are working with two
different samples– GaAsP/GaAs– GaAs/GaAs
• We dope the sample with ZnAs (p-type) using the quartz ampoule method– ZnAs and our sample are cleaned
using TCE, Acetone, and Methanol
– Our quartz is cleaned using 2.5% HF
– Seal the ZnAs and our sample in quartz with vacuum
– Bake for 15 minutes for roughly 2 m of diffusion
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Making the Samples• We use a black wax (softening point at
T~140oC) and 1% Bromine in Methanol etch to make contacts
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Test LED’s using curve tracer• Check to see that
device actually works
• Find turn-on voltage
• P=VI, the less power it takes to operate the device, the better
Red LED at 1.5V, 16mA
Current I (mA)
Voltage V (V)
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The Setup
SpectraPro
Sample
Laser
Optic cable
Lens
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The Setup Continued
SpectraPro Setup Curve tracer
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Gratings for SpectraPro
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One of our LED’s
Red LED, 1.5V,15mA
Current (mA)
Voltage (V)
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Testing Our Sample• Use SpectraPro-150 to test wavelength, relative
intensity, and spectral length of our LED
Red LED Testingwithlasers
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Some LED’s
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Intensity v Wavelength
0500010000150002000025000300003500040000
400 450 500 550 600 650 700 750
Wavelength (nm)
Inte
nsity
Intensity
White LED: RGB
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Conclusions
• Several samples were made– Most did not reach a turn-on voltage when
applying a current using the curve tracer– One LED was in the infrared range the other red – The two LEDs that did turn on were not all that
efficient.
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References
• Photos from Jason Rausch• E. Fred Schubert
– www.lightemittingdiodes.org• Craford, M.George and Stringfellow, G.B.
High Brightness Light Emitting Diodes. Academic Press, 1997.
• Professor Colin J Humphreys– www.sterlinggroup.org.uk/lecture2001.htm