Junjie Hu And Zhongyang Ge
Design and Fabrication of a Three-Layer Coating with Gradient Refractive Index Film for Broadband Antireflection on Silicon
A three-layer gradient refractive index antireflective coating was designed using numerical optimization and fabricated with co-sputtering on a silicon substrate. The topology chosen to optimize was a low index layer of low index (MgF2, n=1.38); a gradient index layer between MgF2 (n=1.38) and TiO2 (n=2.5); and a high index layer of TiO2. The calculated reflectance of different stacks of various layer thicknesses at a variety of angles from normal to oblique incidence was combined with a software optimization algorithm to determine the optimal, low-reflectance, stack to the standard Air Mass (AM) 1.5 solar spectra. The film was deposited using a radio frequency (RF) co-sputtering process with MgF2 and TiO2 targets. The average reflectivity at normal incidence bare silicon from 400 nm to 1000 nm was reduced from ~35% to ~4% after coating. This coating, achieved with a very simple co-sputtering process and readily available materials, is a significant improvement over the antireflective performance of single layer gradient index coatings. Broadband and broad angle coatings based on gradient index coatings could potentially benefit the performance of silicon solar cells, organic light emitting devices, or other devices which need to efficiently absorb or emit light.