Abstract

Nanostructured silicon can reduce reflectance loss in optoelectronic applications, but intrinsic silicon cannot absorb photons with energy below its 1.1 eV bandgap. However, incorporating a high concentration of dopants, i.e., hyperdoping, to nanostructured silicon is expected to bring broadband absorption ranging from UV to short‐wavelength IR (SWIR, <2500 nm). In this work, we prepare nanostructured silicon using cryogenic plasma etching, which is then hyperdoped with selenium (Se) through ion implantation. Besides sub‐bandgap absorption, ion implantation forms crystal damage, which can be recovered through flash lamp annealing. We study crystal damage and broadband (250–2500 nm) absorption from planar and nanostructured surfaces. We first show that nanostructures survive ion implantation hyperdoping and flash lamp annealing under optimized conditions. Secondly, we demonstrate that nanostructured silicon has a 15% higher sub‐bandgap absorption (1100–2500 nm) compared to its non‐hyperdoped nanostructure counterpart while maintaining 97% above‐bandgap absorption (250–1100 nm). Lastly, we simulate the sub‐bandgap absorption of hyperdoped Si nanostructures in a 2D model using the finite element method. Simulation results show that the sub‐bandgap absorption is mainly limited by the thickness of the hyperdoped layer rather than the height of nanostructures.