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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 wafer size.
“For photovoltaics, you want large area coverage to catch as much sunlight as possible. In an extreme case we might grow enough layers to have 10 times the area of the conventional route,” Rogers said.
“You really multiply the area coverage, and by a similar multiplier you reduce the cost, while at the same time eliminating the consumption of the wafer,” he said.
Among the paper’s co-authors are two scientists from Semprius, a North Carolina- based startup company that is beginning to use this technique to manufacture solar cells. A shift from silicon-based panels to more efficient GaAs models could make solar power a more cost-effective form of alternative energy.
The group now plans to explore more potential device applications and other semiconductor materials that could adapt to multilayer growth.
Neutrons pep up PHEMT performance
A small dose of radiation improves the electrical characteristics of PHEMT devices used in the military industry.
French researchers have discovered that neutrons can actually improve some aspects of PHEMT performance, such as knee voltages and leakage currents.
“We were very surprised by this result”, commented team member Bertrand Boudart from the University of Caen Lower Normandy. Previous studies by other researchers revealed that the electrical performance of PHEMT deteriorates at doses in excess of 1 x 1015 neutrons/cm².
Understanding how PHEMT characteristics are influenced by neutron bombardment is important, because these devices are widely used in the military, nuclear and space industries.
MBE growth created the epitaxial structure (see figure), and Ni/AuGe/Ni/Au deposition formed the n- and p- contacts. The cap was then selectively etched away, before electron-beam lithography defined a Al/Ni/Au gate. A SiNx dielectric passivated the devices.
After irradiation of 1.2 x 1010 neutrons/cm², the maximum drain-source current (IDS) increased from 121 to 128mA at a drain source voltage (VDS) of 2.5V and gate-source voltage (VGS) of 0.4V. The “knee voltage” (Vk) which is the value of VDS at the end of the saturation zone, decreased from 0.6 to 0.46V.
The researchers claim that these gains to PHEMT performance resulted from improvements in the ordering of structural impurities at the metal-semiconductor interface.
Further radiation doses of up to 3.3 x 1010 neutrons/cm² had little effect on the peak drain-sourcecurrent, the knee voltage and the leakage current.
Coating the devices with a 1mm-thick sheet of lead ensured that gamma rays did not influence electrical performance of the devices.
Further details on this research are in the original publication which is available online:
June
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