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 1019cm-3. In contrast, the carrier concentration in the Mo-doped layer is lower than 1015cm-3. 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∼2500nm) 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 -10V bias.