Mid-infrared (2.5 ≤ λ ≤ 12.5 μm) optical absorption enhancement of textured silicon surfaces coated with an antireflective thin film

Abstract

High-performance solar cells and optical detection devices frequently incorporate microscopic surface texturing and antireflective (AR) thin films to reduce the reflection of incident radiation and, thus, enhance optical absorption. Using conventional electrochemical and single-crystal silicon micromachining techniques, porous silicon (PS) and textured surfaces composed of randomly spaced and sized pyramids (RSSPs) were fabricated and optically characterized over the mid-infrared (2.5 ≤ λ ≤ 12.5 μm) portion of the optical spectrum. The utility of a 1.53 ± 0.03 μm thick yttrium oxide (Y 2O 3) AR thin film was also investigated in an attempt to enhance optical absorption. The optical measurements were accomplished using a 21 ° incident illumination angle (measured with respect to the sample's normal) and a Bomem® total integrating sphere to quantify the total (specular and diffuse) reflectance ( R). A highly-polished, uncoated, single-crystal silicon wafer was used as a reference surface ( R ave = 0.436 with R σ = 0.033). The performance of the uncoated PS samples revealed R ave = 0.205 with R σ = 0.078, and the RSSP samples manifested R ave = 0.090 with R σ = 0.003. The AR coating significantly improved the performance of the reference and the RSSP textured surfaces: reference sample, ( R ave = 0.251 with R σ = 0.040; RSSP samples, ( R ave = 0.024 with R σ = 0.017). The AR coating did not improve the mid-infrared optical performance of the PS samples; however, the R characteristics for the 0.5 ≤ λ ≤ 2.5 μm portion of the optical spectrum were reduced by more than 50%.