Optical reflectance reduction of textured silicon surfaces coated with an antireflective thin film

Abstract

Solar cells and optical detection devices often incorporate surface texturing and antireflective (AR) thin films to reduce reflection and enhance optical absorption. Using micromachining techniques, three different silicon surfaces were fabricated, optically characterized, and analyzed relative to their ability to reduce optical reflectance. The fabricated surfaces consisted of: randomly sized and spaced pyramids (RSSPs), deep vertical-wall grooves (DVWGs), and porous silicon (PS). Three regions of the optical spectrum were investigated: visible (500 ≤ λ ≤ 900 nm), near-infrared (1.25 ≤ λ ≤ 2.5 μm), and mid-infrared (2.5 ≤ λ ≤ 12.5 μm). A highly-polished, single-crystal silicon wafer was used as a reference surface. The RSSP surface reduced the reflectance by more than 69% across the entire measured spectrum. The DVWG surface reduced the reflectance by 85% in the visible region, 34% in the near-infrared range, and 14% over the mid-infrared wavelengths. “Thin” (pore depths less than 1 μm) and “thick” (pore depths greater than 5 μm) PS surfaces were investigated. The “thick” PS surfaces manifested a 91% reflectance reduction in the visible region, a 7% reduction in the near-infrared range, and a 53% reduction over the mid-infrared wavelengths. To further enhance the optical reflectance properties of the textured silicon surfaces in the mid-infrared region, a 1.53 ± 0.03 μm thick yttrium oxide AR thin film was deposited on the textured and reference samples. The AR-coated RSSP sample manifested the most significant improvement compared to the AR-coated silicon reference sample. Specifically, the reference sample manifested R avc = 0.277 with R σ = 0.04, and the RSSP sample yielded R avc = 0.024 with R σ = 0.017.