Improving the Processing Efficiency of Femtosecond Laser Sulfur Hyperdoping of Silicon by Diffractive Beam Shaping

We demonstrate and compare two approaches for reducing processing time for ultrashort pulse laser surface functionalization with application to femtosecond laser hyperdoping of silicon with a laser pulse duration of 800 fs and an irradiation wavelength of 1030 nm. In the first, we use a Gaussian intensity distribution and increase the repetition rate from 1 kHz to 1002 kHz while keeping all other parameters and thus the accumulated fluence constant. We find that the sub-bandgap absorptance of the material, which we take as target measure, decreases above a repetition rate of 250 kHz. This suggests an inherent limitation of this approach. The second approach is characterized by the use of a line-shaped intensity distribution which is achieved by diffractive beam shaping using a phase-only spatial light modulator. This process proves to be suitable for laser hyperdoping of silicon with a 22-fold enhanced area processing rate while maintaining a sub-bandgap absorptance of above 80 %abs.