The ABCs of LEDs – Epitaxy, Efficacy, Film, & Flux
The first week of this series discussed Attach Process and Boule, and last week talked about Conductivity and Defects. This week we will touch on a few topics with both letters “E” and “F”.
E - Epitaxy
Epitaxy is a technique that involves growing a thin crystalline film of one material on top of another crystalline material, such that the crystal lattices match—at least approximately. In other words, epitaxy is when additional layers of semiconductor crystals are grown on the surface of the wafer. Defects from substrate and epitaxial processes can impact device performance, reliability, and yield.
Luminaire efficacy, sometimes referred to as wall plug efficacy, is a metric of how well the total fixture converts electrical energy into photons. The efficacy at steady state is calculated by dividing the total luminous flux measured in lumens by the total input power (lm/W). Efficacy is a good figure of merit for system performance since it is influenced by electrical, photometric, optical, and thermal performance. You can have a great light engine that produces a high lm/W output, but the total system may have an overall low efficacy due to high optical loss of an external lens or poor power efficiency of the current driver circuit providing drive current to the LEDs.
F - Film
As mentioned above, epitaxy involves growing thin layers of crystalline film on top of each other. If the epitaxial film has a different lattice constant from that of the underlying material, the mismatch will result in stress in the thin film. Epitaxial film stress can increase electron/hole mobility, which can lead to higher performance in the device. On the other hand, a film under stress tends to have a large number of defects.
Photometric testing includes measurements of total radiant flux, luminous flux, chromaticity, correlated color temperature (CCT), and color rendering index (CRI). Radiant flux, expressed in watts, is a measurement of the total power of electromagnetic radiation (light) emitted from the luminaire or lamp. Luminous flux is a weighted measurement based on human visual perception. Mathematically, the measured radiant flux is convolved with the filter response of the human eye and is measured in units of lumens. Photometric measurements should be made at a recurring one-minute interval over a sufficient period to allow the fixture to reach thermal equilibrium.
One consideration is that everything is reflowed freestanding, so the die floats which keeps the LEDs from bumping into each other. The amount of flux that you use relative to the size of the LED component you are placing is important. Then you put down as many LEDs as required for the application, and go through the ramp and reflow process. This process is best achieved using a high-accuracy Pulsed Heat System (PHS) and platform, so parts are not moved after the die are in place.
LEDs Magazine: “Improve LED manufacturing via in-line monitoring and SPC”, http://ledsmagazine.com/features/10/7/10.
Solid State Technology Journal: “The gleam of well-polished sapphire”, http://electroiq.com/blog/2013/01/the-gleam-of-well-polished-sapphire/.
LEDs Magazine: “LED system evaluation yields quality analysis”, http://ledsmagazine.com/features/10/9/8.
Photo credit: Sandia National Laboratories, "Improving Epitaxial Growth for LEDs", apps1.eere.energy.gov/buildings/publications/pdfs/ssl/koleske_ssl08.pdf.
Palomar Technologies, Inc.