Towards low-cost LED lighting
You all know that incandescent bulbs are pretty inefficient, converting only 10% of electricity into light -- and 90% into heat. Light-emitting diodes, or LEDs, could soon replace incandescent and compact fluorescent bulbs in our homes. They are more efficient and environmentally friendly. But LED lights are currently too expensive because they are using a sapphire-based technology. Now, Purdue University researchers have found a way to build low-cost and bright LEDs for home lighting. According to the researchers, the LED lights now on the market cost about $100 while LED lights based on their new technology could be commercially available within a couple of years for a cost of about $5. It would also help to cut our electricity bill by about 10%, but read more...
You can see above how "Timothy Sands, at left, director of Purdue's Birck Nanotechnology Center in Discovery Park, and graduate student Mark Oliver, operate a 'reactor' in work aimed at perfecting solid-state lighting, a technology that could cut electricity consumption by 10 percent if widely adopted." (Credit: Purdue News Service; photo by David Umberger) Here is a link to a larger version of this photo.
This research work has been led by Timothy Sands, Professor of Materials Engineering at Purdue University. Sands is also the Director of the Birck Nanotechnology Center at Discovery Park and manages the Heterogeneous Materials Integration Research Group. Several members of this group participated to this project, including Mark Oliver, Graduate Research Assistant.
The research team is not using expensive sapphire-based technology, but cheaper silicon-based one. "In the new silicon-based LED research, the Purdue engineers 'metallized' the silicon substrate with a built-in reflective layer of zirconium nitride. 'When the LED emits light, some of it goes down and some goes up, and we want the light that goes down to bounce back up so we don't lose it,' said Sands. Ordinarily, zirconium nitride is unstable in the presence of silicon, meaning it undergoes a chemical reaction that changes its properties. The Purdue researchers solved this problem by placing an insulating layer of aluminum nitride between the silicon substrate and the zirconium nitride. 'One of the main achievements in this work was placing a barrier on the silicon substrate to keep the zirconium nitride from reacting,' Sands said."
And here are additional details provided by Emil Venere from Purdue University. "The Purdue team used a technique common in the electronics industry called reactive sputter deposition. Using the method, the researchers bombarded the metals zirconium and aluminum with positively charged ions of argon gas in a vacuum chamber. The argon ions caused metal atoms to be ejected, and a reaction with nitrogen in the chamber resulted in the deposition of aluminum nitride and zirconium nitride onto the silicon surface. The gallium nitride was then deposited by another common technique known as organometallic vapor phase epitaxy, performed in a chamber, called a reactor, at temperatures of about 1,000 degrees Celsius, or 1,800 degrees Fahrenheit."
For more information, this research work has been published in Applied Physics Letters under the title "Organometallic vapor phase epitaxial growth of GaN on ZrN/AlN/Si substrates" (Volume 93, Issue 2, Article 023109, July 14, 2008).
And for your 'reading' pleasure, here is the abstract. "An intermediate ZrN/AlN layer stack that enables the epitaxial growth of GaN on (111) silicon substrates using conventional organometallic vapor phase epitaxy at substrate temperatures of ~1000 °C is reported. The epitaxial (111) ZrN layer provides an integral back reflector and Ohmic contact to n-type GaN, whereas the (0001) AlN layer serves as a reaction barrier, as a thermally conductive interface layer, and as an electrical isolation layer. Smooth (0001) GaN films less than 1 μm thick grown on ZrN/AlN/Si yield 0002 x-ray rocking curve full width at half maximum values as low as 1230 arc sec."
If I had only read this paragraph, I doubt I would have published this post...
Sources: Emil Venere, Purdue University News, July 17, 2008; and various websites
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