Solar ♦ news digest


When the crystal lattices don’t line up, there is a mismatch between the materials. Researchers usually deposit III-V materials on top of silicon wafers in a thin film that covers the wafer, but the mismatch causes strain and introduces defects, degrading the device performance.


Instead of a thin film, the Illinois team grew a densely packed array of nanowires, tiny strands of III-V semiconductor that grow up vertically from the silicon wafer.


InGaAs: Solar cells (bottom) made with arrays of nanowires. Engineers can tune the performance by using nanowires of differing composition and thickness (top). (Graphic by Xiuling Li)


The nanowire geometry provides the additional benefit of enhancing solar cell performance through greater light absorption and carrier collection efficiency. The nanowire approach also uses less material than thin films, reducing the cost.


Professor Xiuling Li’s group developed a method for growing semiconductor nanowires on silicon wafers that holds promise for advanced device applications, including solar cells. (Graphic by Xiuling Li)


“The nanowire geometry offers a lot more freedom from lattice-matching restrictions by dissipating the mismatch strain energy laterally through the sidewalls,” Li said.


The researchers found conditions for growing nanowires of various compositions of the III-V semiconductor indium gallium arsenide. Their methodology has the advantages of using a common growth technique without the need for any special treatments or patterning on the silicon wafer or the metal catalysts that are often needed for such reactions.


“This work represents the first report on ternary semiconductor nanowire arrays grown on silicon substrates, that are truly epitaxial, controllable in size and doping, high aspect ratio, non-tapered, and broadly tuneable in energy for practical device integration,” said Li, who is affiliated with the Micro and Nanotechnology Laboratory, the Frederick Seitz Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at the U. of I.


Li believes the nanowire approach could be applied broadly to other semiconductors, enabling other applications that have been deterred by mismatch concerns. Next, Li and her group hope soon to demonstrate nanowire-based multi-junction tandem solar cells with high quality and efficiency.


The Department of Energy and the National Science Foundation supported this work.


More details of this work have been published in the paper, “InxGa1-xAs Nanowires on Silicon: One- Dimensional Heterogeneous Epitaxy, Bandgap Engineering, and Photovoltaics,” by Jae Cheol Shin et al, Nano Letters, Articles ASAP (As Soon As Publishable), Publication Date (Web): October 3, 2011 (Letter). DOI: 10.1021/nl202676b


November/December 2011 www.compoundsemiconductor.net 189


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