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nanotimes News in Brief

10-05/06 :: May/June 2010

Solar Energy // Semiconductor Manufacturing Technique Holds Promise for Solar Energy

R

esearchers at University of Illinois at Urbana- Champaign, USA, Semprius, Inc., and Hanyang

University, South Korea, describe in the journal Nature materials and fabrication concepts for photo- voltaics and optoelectronics, through the use of films of GaAs or AlGaAs grown in thick, multilayer epita- xial assemblies, then separated from each other and distributed on foreign substrates by printing.

University of Illinois professors John Rogers and Xiuling Li explored lower-cost ways to manufacture thin films of gallium arsenide that also allowed versa- tility in the types of devices they could be incorpora- ted into.

“If you can reduce substantially the cost of gallium arsenide and other compound semiconductors, then you could expand their range of applications,” said Rogers, the Lee J. Flory Founder Chair in Engineering Innovation, and a professor of materials science and engineering and of chemistry.

Typically, gallium arsenide is deposited in a single thin layer on a small wafer. Either the desired device is made directly on the wafer, or the semiconductor- coated wafer is cut up into chips of the desired size. The Illinois group decided to deposit multiple layers of the material on a single wafer, creating a layered, “pancake” stack of gallium arsenide thin films.

“If you grow 10 layers in one growth, you only have to load the wafer one time,” said Li, a professor of electrical and computer engineering. “If you do this in 10 growths, loading and unloading with tempera- ture ramp-up and ramp-down take a lot of time. If you consider what is required for each growth – the machine, the preparation, the time, the people – the overhead saving our approach offers is a significant cost reduction.”

Next the researchers individually peel off the layers and transfer them. To accomplish this, the stacks al- ternate layers of aluminum arsenide with the gallium arsenide. Bathing the stacks in a solution of acid and an oxidizing agent dissolves the layers of aluminum arsenide, freeing the individual thin sheets of gallium arsenide. A soft stamp-like device picks up the layers, one at a time from the top down, for transfer to ano- ther substrate – glass, plastic or silicon, depending on the application. Then the wafer can be reused for another growth.

“By doing this we can generate much more material more rapidly and more cost effectively,” Rogers said. “We’re creating bulk quantities of material, as oppo- sed to just the thin single-layer manner in which it is typically grown.”

The group describes its methods and demonstrates three types of devices using gallium arsenide chips

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