Gallium arsenide (GaAs) and related compounds claim to offer nearly twice the efficiency as silicon in solar cells. But solar cells based on these materials are expensive to make.
‘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 temperature 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.’
In a paper to be published online May 20 in the journal Nature, the group describes its methods and demonstrates three types of devices using gallium arsenide chips manufactured in multilayer stacks: light sensors, high-speed transistors and solar cells. The authors also provide a detailed cost comparison.
Another advantage of the multilayer technique is the release from area constraints, especially important for solar cells. As the layers are removed from the stack, they can be laid out side-by-side on another substrate to produce a much larger surface area, whereas the typical single-layer process limits area to the size of the wafer.
The group deposited multiple layers of the material on a single wafer, creating a layered, “pancake” stack of gallium arsenide thin films.
”Next the researchers individually peel off the layers and transfer them. To accomplish this, the stacks alternate 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,” according to the group.
The paper’s co-authors include two scientists from Semprius Inc., a North Carolina-based startup company that is beginning to use this technique to manufacture solar cells.
Semprius is developing concentrator photovoltaic (CPV) modules for large-scale solar power generation. Semprius’ micro-transfer printing technology enables CPV modules constructed from a large array of very small gallium arsenide-based, multi-junction solar cells. Module cost is minimized by using high concentration ratio.
Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices1, 2 to radio-frequency electronics3, 4 and most forms of optoelectronics5, 6. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.