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 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 bias

Background: Ohmic Contacts Contacts Relatively little resistance

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  m 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 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 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 – Craford, M.George and Stringfellow, G.B. High Brightness Light Emitting Diodes. Academic Press, Professor Colin J Humphreys –