Sub-bandgap absorption and photo-response of molybdenum heavily doped black silicon fabricated by a femtosecond laser.

Molybdenum (Mo)-doped black silicon (Si) is obtained by using femtosecond laser irradiation. The concentration of Mo atoms at the depth from 10 to 200 nm has exceeded 10. In contrast, the carrier concentration in the Mo-doped layer is lower than 10. The surface morphologies with ripple and conical spike microstructures are formed by changing the pulsed laser fluences. The Mo-doped Si samples exhibit a sub-bandgap (1100∼2500) absorptance of more than 60% at a wavelength of 1310 nm. A Mo-doped Si photodetector is made, and the responsivity of the device for 1310 nm is up to 76 mA/W at a -10 bias.