Person:

Franta, Benjamin

Hyperdoped black silicon fabricated with femtosecond laser irradiation has attracted interest for applications in infrared photodetectors and intermediate band photovoltaics due to its sub-bandgap optical absorptance and light-trapping surface. However, hyperdoped black silicon typically has an amorphous and polyphasic polycrystalline surface that can interfere with carrier transport, electrical rectification, and intermediate band formation. Past studies have used thermal annealing to obtain high crystallinity in hyperdoped black silicon, but thermal annealing causes a deactivation of the sub-bandgap optical absorptance. In this study, nanosecond laser annealing is used to obtain high crystallinity and remove pressure-induced phases in hyperdoped black silicon while maintaining high sub-bandgap optical absorptance and a light-trapping surface morphology. Furthermore, it is shown that nanosecond laser annealing reactivates the sub-bandgap optical absorptance of hyperdoped black silicon after deactivation by thermal annealing. Thermal annealing and nanosecond laser annealing can be combined in sequence to fabricate hyperdoped black silicon that simultaneously shows high crystallinity, high abovebandgap and sub-bandgap absorptance, and a rectifying electrical homojunction. Such nanosecond laser annealing could potentially be applied to non-equilibrium material systems beyond hyperdoped black silicon.

This thesis presents a range of advances in the fabrication of femtosecond laser textured and hyperdoped silicon, a material platform with potential applications in photovoltaics, photodetectors, light-emitting diodes, lasers, and potentially other optoelectronic devices.

After providing background and a review of the state of hyperdoped black silicon research in Chapter 1, we explore a range of fabrication approaches in Chapter 2, including laser texturing near and below the melting threshold of silicon, laser texturing and hyperdoping using scanned pulses, fabrication with thin films, control of the dopant concentration on textured substrates, and removal of surface material using chemical etching.

In Chapter 3, we review the material microstructure of hyperdoped black silicon, including the morphology, the presence and origin of high-pressure material phases, and the incorporation of dopants from thin films.

In Chapter 4, we explore the use of laser annealing to increase the crystallinity of hyperdoped black silicon, addressing a longstanding challenge in the field. We show that nanosecond laser annealing can be used on a wide variety of textures— from at least 10 micrometers in size to sub-micrometer in size—to produce high crystallinity and high optical absorptance simultaneously. Furthermore, we see that nanosecond laser annealing can reactivate the sub-bandgap absorptance after it has been deactivated by thermal annealing. We close Chapter 4 by exploring the use of fs laser pulses to anneal hyperdoped black silicon.

Finally, in Chapter 5, we discuss advances in the thesis, outstanding challenges in the research field, and an outlook for applications.