Light Emitting Diodes Nano Lab 2003 Outline MotivationApplications

Light Emitting Diodes Nano. Lab 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, 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 Relatively little resistance • http: //nina. ecse. rpi. edu/shur/Ch 3/sld 043. 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 – Ga. As. P/Ga. As – Ga. As/Ga. As • We dope the sample with Zn. As (p-type) using the quartz ampoule method – Zn. As and our sample are cleaned using TCE, Acetone, and Methanol – Our quartz is cleaned using 2. 5% HF – Seal the Zn. As 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~140 o. C) and 1% Bromine in Methanol etch to make contacts

Test LED’s using curve tracer • Check to see that device actually works Current I (m. A) • Find turn-on voltage V (V) Red LED at 1. 5 V, 16 m. A • P=VI, the less power it takes to operate the device, the better

The Setup Spectra. Pro Laser Sample Optic cable Lens

The Setup Continued Spectra. Pro Setup Curve tracer

Gratings for Spectra. Pro

One of our LED’s Current (m. A) Voltage (V) Red LED, 1. 5 V, 15 m. A

Testing Our Sample • Use Spectra. Pro-150 to test wavelength, relative intensity, and spectral length of our LED Red LED Testing with lasers

Some LED’s

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/lecture 2001. htm