Sub-bandgap photo-response of non-doped black-silicon fabricated by nanosecond laser irradiation.

Non-doped black silicon (b-Si) is fabricated on the surface layer of a near-intrinsic Si substrate by nanosecond (ns) laser direct writing in an argon (Ar) atmosphere. The non-doped samples exhibit a near-unity sub-bandgap (1100∼2500  nm) absorptance of more than 50%. Amazingly, the resistivity of the ns laser irradiated b-Si layer is about five orders of magnitude lower than that of the unprocessed Si substrate. The carrier density of the b-Si layer is about 1×1018   cm-3 , according to the Hall effect measurement. Temperature-dependent Hall effect measurements show that the non-doped b-Si layer exhibits an energy level of 0.026 eV below the conduction band minimum (CBM). At last, Si infrared photodiodes are made based on the difference of carrier concentration between the ns laser-processed b-Si layer and the high-resistivity Si substrate. The responsivity of the b-Si photodiode for 1310 nm is up to 256  mA/W at a 10-V reverse bias, which is much higher than that of the reported pure Si bulk-structure photodiodes.