Growing inexpensive silicon microwires

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 three-dimensional surface.

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.