Black Silicon IR Photodiode Supersaturated With Nitrogen by Femtosecond Laser Irradiation

Micro-ripple and micro-bead structures are formed on a silicon (Si) surface after irradiation with femtosecond laser pulses in nitrogen (N2) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 1020 cm−3, are doped into the textured black Si layer via laser ablation. The N-doped Si exhibits strong below-bandgap infrared absorption from 1.1 to $2.5~\mu \text{m}$ , which remains nearly unchanged after annealing for 30 min at 873 K. The mechanism of this thermally stable infrared absorption is analyzed by first-principles calculations. According to the transmission electron microscopy results, multiple phases (including single crystalline, nanocrystalline, and amorphous phases) are observed in the laser-irradiated layer. Hall Effect measurements prove that N-dopants induce a low background free-carrier concentration ( $\sim 1.67\times 10^{16}\,\,\text {cm}^{-3}$ ). Finally, a Schottky-based bulk structure photodiode is made. This broadband photodiode exhibits good thermal stability and a photo-responsivity of 5.3 mA/W for $1.31~\mu \text{m}$ at a reverse bias of 10 V.