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Femtosecond-laser Microstructuring of Silicon: Dopants and Defects
Michael A. Sheehy
Ph.D. Thesis, Harvard University, 2004, 109 pages export citation
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.
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