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 (N
₂
) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 10
²⁰
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 μm, 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 (~1.67 × 10
¹⁶
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 μm at a reverse bias of 10 V.