OPTICAL REFLECTANCE REDUCTION OF TEXTURED SILICON SURFACES COATED WITH AN ANTIREFLECTIVE THIN-FILM

Solar cells and optical detection devices often incorporate surface te xturing and antireflective (AR) thin films to reduce reflection and en hance optical absorption. Using micromachining techniques, three diffe rent silicon surfaces were fabricated, optically characterized, and an alyzed relative to their ability to reduce optical reflectance. The fa bricated surfaces consisted of: randomly sized and spaced pyramids (RS SPs), deep vertical-wall grooves (DVWGs), and porous silicon (PS). Thr ee regions of the optical spectrum were investigated: visible (500 les s than or equal to lambda less than or equal to 900 nm), near-infrared (1.25 less than or equal to lambda less than or equal to 2.5 mu m), a nd mid-infrared (2.5 less than or equal to lambda less than or equal t o 12.5 mu m). A highly-polished, single-crystal silicon wafer was used as a reference surface. The RSSP surface reduced the reflectance by m ore than 69% across the entire measured spectrum. The DVWG surface red uced the reflectance by 85% in the visible region, 34% in the near-inf rared range, and 14% over the mid-infrared wavelengths. ''Thin'' (pore depths less than 1 mu m) and ''thick'' (pore depths greater than 5 mu m) PS surfaces were investigated. The ''thick'' PS surfaces manifeste d a 91% reflectance reduction in the visible region, a 7% reduction in the near-infrared range, and a 53% reduction over the mid-infrared wa velengths. To further enhance the optical reflectance properties of th e textured silicon surfaces in the mid-infrared region, a 1.53 +/- 0.0 3 mu m thick yttrium oxide AR thin film was deposited on the textured and reference samples. The AR-coated RSSP sample manifested the most s ignificant improvement compared to the AR-coated silicon reference sam ple. Specifically, the reference sample manifested R(ave) = 0.277 with R(sigma) = 0.04, and the RSSP sample yielded R(ave) = 0.024 with R(si gma) = 0.017.