Femtosecond-laser Microstructuring of Silicon: Dopants and Defects

Abstract:

This dissertation deals with the incorporation of elements into silicon using a femtosecond laser in order to understand the source for below-band gap absorptance. Previous experimental results indicate that irradiation of silicon with a femtosecond laser in the presence of sulfur hexafluoride (SF6) leads to unique optical properties. The absorptance for above-band gap radiation is increased to 95%; the more interesting result is that the below-band gap absorptance goes from nearly 0% to 90%. In the first set of experiments performed, we irradiated silicon in the presence of H2S, SiH4, and H2. The absorptance for samples prepared in H2S is identical to that of samples prepared in SF6; the other samples have a trailing edge of absorptance for energies below the band gap. This result indicated that sulfur played a crucial role in the below-band gap absorptance. The next set of experiments involved incorporating selenium and tellurium from a powder source to investigate possible dependence of the optical properties on the size of the dopant (selenium and tellurium have the same valence, but are larger in atomic size than sulfur). Incorporation of these two elements also leads to near-unity absorptance for below-band gap radiation. A comparison of the composition and the optical properties before and after annealing showed that the source for below-band gap absorptance is likely due to both the incorporated chalcogen and defects. The final set of experiments deals with the incorporation of elements from other families. These studies bolster the results of the previous research and provide furtherdetails on the interaction of the dopant with the laser- modified surface. We speculate on some requirements the dopants must satisfy (i.e. atomic size and valence onfiguration) and propose further research that can be done in this area. These experiments provide significant insight into the optical absorption mechanism and show that this material has great potential for devices that operate in the infrared portion of the spectrum, such as infrared photodiodes and solar cells.