Light Emitting Diodes
NanoLab 2003
Outline
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Motivation/Applications: Why LED’s?
Background
Fabrication
Testing
Conclusions
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
Why Do We Care? Efficiency (lumens/watt)
Light Bulbs vs LED’s
• Light Bulbs
–Filament
• Sudden FailureBreaks/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
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, costeffective WHITE LED’s will make lots of money.
Isolated Atoms > Crystal > Artificial Atom
Diamond
lattice
isolated
atom
Background-Band Gaps and Lattice Constants
• Lattice mismatch reduces efficiency
Background-Band Gaps and Lattice Constants
Bandgap energy vs lattice constant of various IIIsemiconductors at room temperature.
What are the III-Vs
Background-Band Gaps and Lattice Constants
Room-temperature bandgap energy vs lattice constant of
common elemental and binary semiconductors.
Background -Lattice Mismatch
Two crystals with mismatched lattice constants resultion in dislocation at
or near the interface between the two semiconductors.
• Lattice mismatch reduces efficiency
Background: pn Junctions and Recombination
Carrier distribution in pn homojunctions
• 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
Background: pn Junctions and Recombination
Heterojunction under forward bias
• Electron and holes are trapped in the quantum wells.
• Such spatial overlap gratly enhances the
recombination rate - brightness, efficiency.
Background: Ohmic Contacts
Contacts
Relatively little resistance
•
http://nina.ecse.rpi.edu/shur/Ch3/sld043.htm
Doping
Hole in
lower
energy
band
allows
for easier
travel for
electrons
Electrons
forced to
higher,
partially
filled
band
electron
moves
easier
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 mm of diffusion
Making the Samples
• We use a black wax (softening point at
T~140oC) and 1% Bromine in Methanol
etch to make contacts
Test LED’s using curve tracer
• Check to see that
device actually works
Current I (mA)
• Find turn-on voltage
Voltage V (V)
Red LED at 1.5V, 16mA
• P=VI, the less power it
takes to operate the
device, the better
The Setup
SpectraPro
Laser
Sample
Optic cable
Lens
The Setup Continued
SpectraPro Setup
Curve tracer
Gratings for SpectraPro
One of our LED’s
Current (mA)
Voltage (V)
Red LED, 1.5V,15mA
Testing
Our
Sample
Intensity v Wavelength
• Use SpectraPro-150 to test wavelength, relative
Green Laser
intensity, and spectral length of our LED
150000
Intensity
Red LED
50000
0
400
Intensity v Wavelength
Red
Laser
450
500
550
600
650
700
Wavelength (nm)
500000
Intensity
Testing
with
Intensity
lasers
100000
400000
300000
Intensity
200000
100000
0
400
450
500
550
600
Wavelength (nm)
650
700
750
Intensity
Intensity
700000
600000
500000
Some
Intensity
LED’s
400000
300000
200000
100000
0
400
Intensity v Wavelength
450
500
550
600
650
700
750
Wavelength (nm)
12000
10000
8000
6000
Intensity
4000
2000
0
400
450
500
550
600
650
Wavelength (nm)
700
750
Intensity
Intensity v Wavelength
40000
35000
30000
25000
20000
15000
10000
5000
0
400
Intensity
450
500
550
600
650
Wavelength (nm)
White LED: RGB
700
750
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.
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