Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing

Benjamin Franta,Hemi H Gandhi,Silvija Gradečak,Eric Mazur,Michael J Aziz,Paul H Rekemeyer,David Pastor

doi:10.1063/1.4937149

Benjamin Franta, Hemi H Gandhi + Show 5 more

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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 above-bandgap 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.

Highlights

Hyperdoped black silicon is silicon that is both supersaturated with low-solubility dopants (i.e., “hyperdoped”) and that possesses a light-trapping surface
We investigated the microstructure of the hyperdoped black silicon using cross-sectional bright field transmission electron microscopy (BF-TEM) and selected area diffraction (SAD)
Nanosecond laser annealing produces three notable effects: smoothing the surface on the nm scale, converting amorphous and pressure-induced silicon phases to conventional crystalline silicon (Si-I), and placing or maintaining the dopant atoms in optically active, non-equilibrium positions. These effects are analogous to the effects of ns pulsed laser melting on flat silicon hyperdoped by ion implantation, which include crystallizing the surface and placing or maintaining the dopant atoms in optically active, non-equilibrium positions

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Introduction

Hyperdoped black silicon is silicon that is both supersaturated with low-solubility dopants (i.e., “hyperdoped”) and that possesses a light-trapping surface (the “black” in black silicon). Silicon that is hyperdoped but optically flat can be fabricated by pulsed laser melting of ion implanted silicon 11 12 or by laser irradiation of silicon using irradiation parameters that hyperdope the silicon but avoid the formation of selforganized surface structures.[13] Black silicon that is not hyperdoped can be fabricated by chemical etching of silicon,[14 15] by direct laser texturing,[16 17] or by laser irradiation of silicon using irradiation parameters that produce self-organized surface structures without the presence of dopants.[18 19]

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