U.S. Patent No. 8,143,143, issued on March 27, 2012 to Bandgap Engineering Inc. of Waltham, MA, discloses a process for fabricating silicon nanowires.
According to the ’143 patent, silicon nanowires have been previously grown “from the bottom up” using various deposition techniques performed under vacuum conditions, and formed by removing material from bulk silicon “from the top down” using various plasma etching techniques performed under vacuum conditions. However, the costs and limited scalability of these techniques has hindered their use. Existing solution-based processes to etch silicon wafers in a direction normal to the surface have had limited success in achieving diameters that are less than 100 nanometer, which the ’143 patent describes as being of value “to a variety of electronic, optoelectronic, electrochemical and electromechanical applications” since “it is within the sub-100 nm range that silicon begins to demonstrate novel properties distinguishable from the properties of bulk silicon.”
The ’143 patent discloses a solution-based etching process that deposits sub-100 nm nanoparticles and a silver film onto the silicon wafer, which is then exposed to an etchant aqueous solution of HF and an oxidizing agent. The silicon wafer is etched in the regions between the nanoparticles, leaving an array of sub-100 nm silicon nanowires standing up on the silicon wafer. For example, the electron microscope micrographs above show a field of silicon nanowires that have diameters ranging from 12-70 nanometers. The ’146 patent mentions various uses of this material, including as an interfacial layer between bulk silicon and another material, and novel LED and transistor applications. Of particular interest to Bandgap Engineering are photovoltaic applications, which utilize the quantum confinement in the silicon nanowires to form intermediate band photovoltaic (IBPV) materials for solar cells, and as anodes in lithium ion batteries.
According to its website, “Bandgap’s nanowire-enhanced solar cell designs combine low-cost processing with crystalline silicon to yield high-efficiency products” which are made possible by their “highly tunable silicon nanowires.” They tout that their high-efficiency photovoltaics can reduce reflection of incident light and can “dramatically increase the optical absorption of silicon.” The company is also developing their silicon nanowires for “high-capacity Li-ion battery anodes.”
According to the USPTO database, the ’146 patent is Bandgap Engineering’s second U.S. patent, the first being issued in July 2011 (U.S. Pat. No. 7,973,995) for an optoelectronic device having a nanowire array and a host material intermingled with the nanowire array and containing light-scattering or absorption/luminescence centers.
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