Broadband antireflective germanium surfaces based on subwavelength structures for photovoltaic cell applications

We fabricated the germanium (Ge) subwavelength structures (SWSs) using gold (Au) metallic nanopatterns dewetted by rapid thermal annealing and inductively coupled plasma etching in SiCl(4) plasma for Ge-based photovoltaic cells. Using the optimized Au nanopatterns as an etch mask, the Ge SWSs were formed by varying the etching parameters to achieve the better antireflection properties. The reflectance of Ge SWSs depended strongly on their period, height, and shape which are closely related to the refractive index profile between air and the Ge substrate. The tapered cone Ge SWSs reduced considerably the reflectance compared to the samples with a truncated cone shape as well as the Ge substrate due to the linearly graded refractive index distribution from air to the Ge substrate. The Ge SWS with the tapered cone shape and high height exhibited a dramatic decrease in the reflectance (i.e., <10%) over a wide wavelength region of 350-1800 nm, thus leading to a low solar weighted reflectance of ~3.6%. The reflectance was also lower than ~8.8% at a wavelength of 633 nm in the incident angle range of 15-85°. The measured reflectance data of Ge SWSs showed similar trends to the calculated results in a rigorous coupled wave analysis simulation.