Progress in LED lighting

In a paper released in 2008(1), Purdue University researchers(2) revealed a method for radically reducing the manufacturing cost of white light LED lamps.  The method involves a technique for creating LEDs on low-cost, metal-coated silicon wafers, where the silicon substrate is “metallized” with a built-in reflective layer of zirconium nitride.  This creates a a separate mirror-like collector to reflect light that otherwise would be lost.

Earlier methods of fabricating white light LED lamps were based on a sapphire substrate process, which is relatively expensive.

The problem that the Purdue researchers solved was that zirconium nitride is unstable in the presence of silicon, meaning it undergoes a chemical reaction that changes its properties.  The solution was to place an insulating layer of aluminum nitride between the silicon substrate and the zirconium nitride.

An important part of this solution is the fact that the fabrication process uses techniques already common in the electronics industry including reactive sputter deposition and organometallic vapor phase epitaxy.  The reactive sputter deposition process involves bombarding the metals zirconium and aluminum with positively charged ions of argon gas in a vacuum chamber. The argon ions cause metal atoms to be ejected, and a reaction with nitrogen in the chamber resulting in the deposition of aluminum nitride and zirconium nitride onto the silicon surface. The gallium nitride is deposited by organometallic vapor phase epitaxy in a reactor at temperatures of about 1,000 degrees Celsius, or 1,800 degrees Fahrenheit.

As the zirconium nitride, aluminum nitride and gallium nitride are deposited on the silicon, they arrange themselves in a crystalline structure matching that of silicon.  This is called epitaxial growth, or the ordered arrangement of atoms on top of the substrate.

Since the process is based on  silicon, manufacturing of white LED lamps can be scaled up quickly using processes already in use for manufacturing integrated circuits from silicon wafers.

Incandescent bulbs are about 10 percent efficient; they convert 10 percent of electricity into light and 90 percent into heat.  White LED lamps have been produced with the sapphire process with efficiencies from 47 percent to 64 percent.  LED lamps produced with the new process should have similar efficiencies.

The Purdue researchers expect that white LED lamps should be available soon at a price around $5 per unit.  Given their long life and low power usage;  these lamps should quickly gain a large share of the lighting market.

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(1) Organometallic vapor phase epitaxial growth of GaN on ZrN/AlN/Si substrates

(2) Advance brings low-cost, bright LED lighting closer to reality

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