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Light Emitting Diodes

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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

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  1. Light Emitting Diodes NanoLab 2003

  2. Outline • Motivation/Applications: Why LED’s? • Background • Fabrication • Testing • Conclusions

  3. 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

  4. Why Do We Care? Efficiency (lumens/watt)

  5. 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 Light Bulbs vs LED’s

  6. 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.

  7. Diamond lattice Isolated Atoms > Crystal > Artificial Atom isolated atom

  8. Background-Band Gaps and Lattice Constants • Lattice mismatch reduces efficiency

  9. Background-Band Gaps and Lattice Constants Bandgap energy vs lattice constant of various III-semiconductors at room temperature.

  10. What are the III-Vs

  11. Background-Band Gaps and Lattice Constants Room-temperature bandgap energy vs lattice constant of common elemental and binary semiconductors.

  12. 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.

  13. Carrier distribution in pn homojunctions 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

  14. 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.

  15. Background: Ohmic Contacts Contacts • http://nina.ecse.rpi.edu/shur/Ch3/sld043.htm Relatively little resistance

  16. Doping Hole in lower energy band allows for easier travel for electrons Electrons forced to higher, partially filled band electron moves easier

  17. 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

  18. Making the Samples • We use a black wax (softening point at T~140oC) and 1% Bromine in Methanol etch to make contacts

  19. Test LED’s using curve tracer Current I (mA) • Check to see that device actually works • Find turn-on voltage • P=VI, the less power it takes to operate the device, the better Voltage V (V) Red LED at 1.5V, 16mA

  20. The Setup SpectraPro Laser Sample Optic cable Lens

  21. The Setup Continued Curve tracer SpectraPro Setup

  22. Gratings for SpectraPro

  23. One of our LED’s Current (mA) Voltage (V) Red LED, 1.5V,15mA

  24. Testing Our Sample RedLED Testing with lasers • Use SpectraPro-150 to test wavelength, relative intensity, and spectral length of our LED

  25. Some LED’s

  26. White LED: RGB

  27. 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.

  28. 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