Light Emitting Diodes

NanoLab 2003

Outline

• 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

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

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.

Isolated Atoms > Crystal > Artificial Atom

isolated atom

Diamond lattice

Background-Band Gaps and Lattice Constants

• Lattice mismatch reduces efficiency

Background-Band Gaps and Lattice Constants

Bandgap energy vs lattice constant of various III-semiconductors 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

• Lattice mismatch reduces efficiency

Two crystals with mismatched lattice constants resultion in dislocation at or near the interface between the two semiconductors.

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

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

Background: Ohmic Contacts

• http://nina.ecse.rpi.edu/shur/Ch3/sld043.htm

Contacts

Relatively little resistance

DopingHole 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 m 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

• 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)

The Setup

SpectraPro

Sample

Laser

Optic cable

Lens

The Setup Continued

SpectraPro Setup Curve tracer

Gratings for SpectraPro

One of our LED’s

Red LED, 1.5V,15mA

Current (mA)

Voltage (V)

Testing Our Sample• Use SpectraPro-150 to test wavelength, relative

intensity, and spectral length of our LED

Red LED Testingwithlasers

Some LED’s

Intensity v Wavelength

0500010000150002000025000300003500040000

400 450 500 550 600 650 700 750

Wavelength (nm)

Inte

nsity

Intensity

White LED: RGB

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