A new breakthrough in solid-state lighting has the potential to hasten the spread of general-use LED lighting. Scientists at Purdue University have figured out how to manufacture LED solid-state lights on regular metal-coated silicon wafers, which could dramatically lower the costs of LED lighting production.
“If you replaced existing lighting with solid-state lighting, following some reasonable estimates for the penetration of that technology based on economics and other factors, it could reduce the amount of energy we consume for lighting by about one-third. That represents a 10 percent reduction of electricity consumption and a comparable reduction of related carbon emissions,” says Timothy D. Sands, professor of Materials Engineering and Electrical and Computer Engineering at Purdue.
What makes traditional LEDs so expensive is that gallium nitride crystal, which is used in sapphire-based blue and green LEDs, is currently too expensive for widespread domestic-lighting use. It costs at least 20 times more than conventional incandescent and compact fluorescent light bulbs.
One reason for the high cost is that the sapphire-based LEDs require a separate mirror-like collector to reflect light that ordinarily would be lost. The new LEDs can be made using standard silicon wafers and existing, less expensive processes. This would make them competitive with incandescent and CFLs.
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.
The new techniques yield a crystalline structure aligned to the crystalline silicon. Thus the new LEDs are less prone to defects and will perform more efficiently. The silicon-based LEDs also dissipate heat more effectively than sapphires, reducing the potential for damage during operation.
LEDs that are currently available convert electricity to light with an efficiency of 47 percent to 64 percent. It is predicted that LED produced with Purdue's process would have an efficiency in the high-end of that range, compared to about 10 percent for incandescent. Sands expects affordable LEDs for general illumination to be on the market within two years.