Growing inexpensive silicon microwires
CAMBRIDGE, Mass. – A new, simpler process has been developed
that turns silicon into precisely sized and spaced microwires with the potential
for practical commercial applications such as solar cells, transistors, integrated
circuits, sensors and batteries.
Researchers from MIT and Pennsylvania State University have developed
a technique for producing the microwires in a highly controlled way that could be
scaled up for industrial processes. Unlike previous methods, this one not only provides
control over size and spacing but also has the potential to be rendered on any curved
The method involves heating and intentionally contaminating the
surface of a silicon wafer with copper, which would diffuse into silicon. While
the silicon slowly cools, the copper diffuses out, forming droplets on the surface.
When the wafer is placed into an atmosphere of silicon tetrachloride gas, silicon
microwires begin to grow outward wherever copper droplets are present on the surface.
Silicon in the gas dissolves into the copper droplets and precipitates out onto
the silicon surface below. The silicon buildup elongates, forming microwires ranging
from 10 to 20 μm in diameter.
The spacing of the wires was controlled with textures created
on the wafer’s surface – tiny dimples that form centers for the copper
droplets – while the temperature used for the diffusion process controlled
the size of the wires. Unlike other production methods, the size and spacing of
the wires can be controlled independently.
The small size of the microwires, which have the potential to
reach conversion efficiencies that are close to those of conventional solar cells,
could also reduce costs by using smaller amounts of expensive silicon.
The findings appeared in Small, Jan. 25, 2011 (doi: 10.1002/smll.201002250),
and the researchers said they still are determining the best combinations of temperature
profiles, copper concentrations and surface patterning for various applications.
The work was supported by the US Department of Energy, the Chesonis
Family Foundation and the National Science Foundation.
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