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III-V compound semiconductor nanowire arrays are promising candidates for photovoltaics applications due to their high volumetric absorption. Uniform nanowire arrays exhibit high absorption at certain wavelengths due to strong coupling into lossy waveguide modes. Previously, simulations predicted near-unity, broadband absorption in sparse semiconductor nanowire arrays (<5% fill fraction) with multi-radii and tapered nanowire array designs [1].
Herein, we experimentally demonstrate near-unity broadband absorption in InP nanowire arrays via a scalable, epitaxy-free fabrication method, using nanoimprint lithography and ICP-RIE to define nanowire arrays in bulk InP wafers. In addition to mask pattern design (wire radius and spacing) and etch chemistry (wire taper), appropriate selection of a hard mask for the InP etch is critical to precise dimension control and reproducibility. Polymer-embedded wires are removed from the bulk InP substrate by a mechanical method that facilitates extensive reuse of a single bulk InP wafer to synthesize many polymer-embedded nanowire array thin films.
Arrays containing multiple nanowire radii and tapered nanowires were successfully fabricated. For both designs, the polymer-embedded arrays achieved ~90% broadband absorption (λ=400-900 nm) in less than 100 nm planar equivalence of InP. The addition of a silver back reflector increased this broadband absorption to ~95%. The repeatable process of imprinting, etching and peeling to obtain many nanowire arrays from one single wafer represents an economical manufacturing route for high efficiency III-V photovoltaics.
[1] K.T. Fountaine, C.G. Kendall, Harry A. Atwater, “Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation,” Opt. Exp. (2014).
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