Wednesday, May 26, 2010
Compared to silicon, semiconductors like gallium arsenide can be made into solar cells that convert more sunlight into electricity and transistors that are faster than their silicon counterparts. But devices made from these materials are expensive.
Now a new method for making large-area devices from gallium arsenide promises to bring down costs by eliminating manufacturing steps and wasting less materials. Researchers have used the method to make high-performance image sensors, transistors, and solar cells. Semprius, a Durham, NC, company, is using it to make solar modules that should be on the market by the end of the year.
Gallium arsenide solar cells convert twice as much of the energy in sunlight into electricity compared to silicon cells, says John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign, who led the research. Gallium arsenide is also being eyed by microchip manufacturers such as Intel as a potential replacement for silicon.
The problem with gallium arsenide, however, is its price tag. To make a gallium arsenide solar panel today, manufacturers grow a semiconductor crystal on an expensive template in a high-vacuum, high-temperature chamber. The gallium arsenide is then diced into thin pieces, assembled, and bonded. This process destroys the underlying template, which is necessary to create a high-quality crystal. And making only a single layer of gallium arsenide at a time is inefficient--it takes more time to load and unload the vacuum chamber than it does to grow the crystal.
To address the problem, Rogers developed a method for growing multiple layers of devices at one time, and a way to release them from the substrate without destroying it. "Once the substrate is in the chamber at the right temperature, we grow a multilayer stack," explains Rogers. The stack alternates a device layer with a sacrificial layer. After all the layers are put down, the stack is etched in a chemical bath that eats away at the sacrificial layer, made of aluminum arsenide, releasing thin rectangular films of gallium arsenide. As the gallium arsenide films are released, they're picked up and placed on a substrate.