Optical hyperdoping: black silicon

B. Franta, D. Pastor, H. H. Gandhi, P. Rekemeyer, S. Gradečak, M. J. Aziz, and E. Mazur. 2015. “Simultaneous high crystallinity and optical absorptance in hyperdoped black silicon using nanosecond laser annealing.” J. Appl. Phys., 118, Pp. 225303–. Publisher's VersionAbstract
Hyperdoped black siliconfabricated with femtosecond laserirradiation 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.
M. A. Sheehy, B. R. Tull, C. M. Friend, and E. Mazur. 2006. “Chalcogen doping of silicon via intense femtosecond-laser irradiation.” Mat. Sci. Eng. B, 137, Pp. 289–294. Publisher's VersionAbstract
We have previously shown that doping silicon with sulfur via femtosecond- laser irradiation leads to near-unity absorption of radiation from ultraviolet wavelengths to below band gap short-wave infrared wavelengths. Here, we demonstrate that doping silicon with two other group VI elements (chalcogens), selenium and tellurium, also leads to near-unity broadband absorption. A powder of the chalcogen dopant is spread on the silicon substrate and irradiated with femtosecond-laser pulses. We examine and compare the resulting morphology, optical properties, and chemical composition for each chalcogen-doped substrate before and after thermal annealing. Thermal annealing reduces the absorption of below-band gap radiation by an amount that correlates with the diffusivity of the chalcogen dopant used to make the sample. We propose a mechanism for the absorption of below band gap radiation based on defects in the lattice brought about by the femtosecond laser irradiation and the presence of a supersaturated concentration of chalcogen dopant atoms. The selenium and tellurium doped samples show particular promise for use in infrared photodetectors as they retain most of their infrared absorptance even after thermal annealinga necessary step in many semiconductor device manufacturing processes.
M. T. Winkler. 2009. “Non-Equilbrium Chalcogen Concentrations in Silicon: Physical Structure, Electronic Transport, and Photovoltaic Potential”. Publisher's VersionAbstract
This thesis explores the structure and properties of silicon doped with chalcogens beyond the equilibrium solubility limit, with a focus on the potential presence of an impurity band and its relevance to photovoltaics. The investigations that we report here shed new light on the electronic role of sulfur dopants in particular, and also provide new evidence of a semiconductor-to-metal transition consistent with the formation of an electron-conducting impurity band. The thesis is divided into three primary studies. First, we describe doping silicon with a single fs-laser pulse. We find that irradiation above the melting threshold is sufficient for doping a thin layer of silicon to non-equilibrium sulfur concentrations. Next, we explore the interaction of many fs-laser pulses with a silicon substrate. Temperature-dependent electronic transport measurements indicate metallic conduction, while a form of Fermi level spectroscopy and optical absorption data indicate the presence of an impurity band located 200 − 300 meV below the conduction band edge. Third, we investigate silicon doped to non- equilibrium concentrations using a different technique: ion-implantation followed by pulsed laser melting and crystal regrowth. We determine one of the sulfur states present at low sulfur dose. Additional transport measurements point to the presence of a semiconductor-to- metal transition at sulfur doses corresponding to implanted sulfur concentrations just above 10^20 cm−3 . Finally, in the appendices of this thesis, we describe methods to laser-dope silicon while avoiding the development of significant surface roughness that typically characterizes such samples. Additionally, we present the status of investigations into laser-doping silicon with selenium to non- equilibrium concentrations.
J. E. Carey, C. H. Crouch, M. Shen, and E. Mazur. 2005. “Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes.” Opt. Lett., 30, Pp. 1773–1775. Publisher's VersionAbstract
We investigated the current-voltage characteristics and responsivity of photodiodes fabricated with silicon that was microstructured using femtosecond-laser pulses in a sulfur-containing atmosphere. The photodiodes we fabricated have a broad spectral response ranging from the visible to the near-infrared (4001600 nm). The responsivity depends on substrate doping, microstructuring fluence, and annealing temperature. We obtained room- temperature responsivities as high as 100 A/W at 1064 nm, two orders of magnitude higher than standard silicon photodiodes. For wavelengths below the band gap, we obtained responsivities as high as 50 mA/W at 1330 nm and 35 mA/W at 1550 nm.
T. R. Polte, M. Shen, J. Karavitis, M. Montoya, J. Pendse, S. Xia, E. Mazur, and D. E. Ingber. 2007. “Nanostructured magnetizable materials that switch cells between life and death.” Biomaterials, 28, Pp. 2783–2790. Publisher's VersionAbstract
Development of biochips containing living cells for biodetection, drug screening and tissue engineering applications is limited by a lack of reconfigurable material interfaces and actuators. Here we describe a new class of nanostructured magnetizable materials created with a femtosecond laser surface etching technique that function as multiplexed magnetic field gradient concentrators. When combined with magnetic microbeads coated with cell adhesion ligands, a microarray of virtual adhesive islands that can support cell attachment, resist cell traction forces and maintain cell viability. A cell death (apoptosis) response can then be actuated on command by removing the applied magnetic field, thereby causing cell retraction, rounding and detachment. This simple technology may be used to create reconfigurable interfaces that allow users to selectively discard contaminated or exhausted cellular sensor elements, and to replace them with new living cellular components for continued operation in future biomedical microdevices and biodetectors. Keywords: magnetic particles, magnetic gradient concentrator, culture substrate, apoptosis, mechanical force, cell shape
M. J. Smith, M. T. Winkler, M. Sher, Y. Lin, E. Mazur, and S. Gradečak. 2011. “The Effects of a Thin Film Dopant Precursor on the Structure and Properties of Femtosecond-laser Irradiated Silicon.” Appl. Phys. A, 105, Pp. 795–800. Publisher's VersionAbstract
Femtosecond (fs) laser irradiation of a silicon substrate coated with a thin film is a flexible approach to producing metastable alloys with unique properties, including near-unity sub-band gap absorptance extending into the infrared. However, dopant incorporation from a thin film during fs-laser irradiation is not well understood. We study the thin film femtosecond-laser doping process through optical and structural characterization of silicon fs-laser doped using a selenium thin film, and compare the resulting microstructure and dopant distribution to fs-laser doping with sulfur from a gaseous precursor. We show that a thin film dopant precursor significantly changes the laser-material interactions, modifying both the surface structuring and dopant incorporation processes and in turn affecting p-n diode behavior.
Z. Huang, J. E. Carey, M. Liu, X. Guo, E. Mazur, and J. C. Campbell. 2006. “Microstructured silicon photodetector.” Appl. Phys. Lett., 89, Pp. 033506–033508. Publisher's VersionAbstract
Photodetectors fabricated on microstructured silicon are reported. The photodetectors exhibited high photoresponse; at 3 V bias, the responsivities were 92 A/W at 850 nm and 119 A/W at 960 nm. At wavelengths longer than 1.1 m, the photodetectors still showed strong photoresponse. A generation- recombination gain mechanism has been proposed to explain the photoresponse of these photodiodes. From measurements of the noise current density, the calculated gain was approximately 1200 at 3 V bias.
M. Shen, C. H. Crouch, J. E. Carey, and E. Mazur. 2004. “Femtosecond laser-induced formation of submicrometer spikes on silicon in water.” Appl. Phys. Lett., 85, Pp. 5694–5696. Publisher's VersionAbstract
We fabricate submicrometer silicon spikes by irradiating a silicon surface that is submerged in water with 400-nm, 100-fs laser pulses. These spikes are less than a micrometer tall and about 200 nm wide one to two orders of magnitude smaller than the microspikes formed by laser irradiation of silicon in gases or vacuum. Scanning electron micrographs of the surface show that the formation of the spikes involves a combination of capillary waves on the molten silicon surface and laser-induced etching of silicon. Chemical analysis and scanning electron microscopy of the spikes shows that they are composed of silicon with a 20-nm thick surface oxide layer.
M. T. Winkler, D. Recht, M. Sher, A. J. Said, E. Mazur, and M. J. Aziz. 2011. “Insulator-to-metal transition in sulfur-doped silicon.” Phys. Rev. Lett., 106, Pp. 178701–. Publisher's VersionAbstract
We observe an insulator-to-metal (I–M) transition in crystalline silicon doped with sulfur to non-equilibrium concentrations using ion implantation followed by pulsed laser melting and rapid resolidification. This I–M transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect data measured for temperatures T > 1.7 K both indicate that a transition from insulating to metallic conduction occurs at a peak sulfur concentration between 1.8 and 4.3 × 1020 cm–3. Conduction in insulating samples is consistent with variable range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.
T. Sarnet, T. J. - Derrien, R. Torres, P. Delaporte, F. Torregrosa, M. Sher, Y. Lin, B. Franta, G. Deng, and E. Mazur. 2013. “Black silicon for photovoltaic cells: towards a high-efficiency silicon solar cell.” In . EU PVSEC 2013, 28th European Photovoltaic Solar Energy Conference and Exhibition. Publisher's VersionAbstract
Laser-created Black Silicon has been developed since 1998 at Harvard University. The unique optical and semiconducting properties of black silicon first led to interesting applications for sensors (photodetectors, thermal imaging cameras, etc.) Other applications like photovoltaic solar cells have been rapidly identified, but it took more than ten years of research and development before demonstrating a real improvement of the photovoltaic efficiency on an industrial multi-crystalline solar cell. This paper is a short review on recent research on the use of black silicon for photovoltaic cells.
M. J. Smith, M. Sher, B. Franta, Y. Lin, E. Mazur, and S. Gradečak. 2012. “The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecond laser irradiation.” J. Appl. Phys., 112, Pp. 083518–. Publisher's VersionAbstract
Surface texturing of silicon using femtosecond (fs) laser irradiation can reduce the surface reflectivity to less than 5%, enables control over the resulting surface morphology, and uses little material. The laser-induced damage that occurs in parallel with surface texturing, however, can result in increased recombination currents that inhibit device performance. In this work, we investigate the light- material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. We identify resolidification-induced stresses as the mechanism responsible for driving sub- surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation.
Y. Lin, N. Mangan, S. Marbach, T. M. Schneider, G. Deng, S. Zhou, M. Brenner, and E. Mazur. 2015. “Creating femtosecond-laser-hyperdoped silicon with a homogeneous doping profile.” Appl. Phys. Lett., 106, Pp. 062105–. Publisher's VersionAbstract
Femtosecond-laser hyperdoping of sulfur in silicon typically produces a concentration gradient that results in undesirable inhomogeneous material properties. Using a mathematical model of the doping process, we design a fabrication method consisting of a sequence of laser pulses with varying sulfur concentrations in the atmosphere, which produces hyperdoped silicon with a uniform concentration depth profile. Our measurements of the evolution of the concentration profiles with each laser pulse are consistent with our mathematical model of the doping mechanism, based on classical heat and solute diffusion coupled to the far-from-equilibrium dopant incorporation. The use of optimization methods opens an avenue for creating controllable hyperdoped materials on demand.
M. Sher, M. T. Winkler, and E. Mazur. 2011. “Pulsed-laser hyperdoping and surface texturing for photovoltaics.” MRS Bulletin, 36, Pp. 439–445. Publisher's VersionAbstract
We describe two ways in which pulsed lasers can be used to increase efficiency in photovoltaic devices. First, pulsed-laser hyperdoping can introduce dopants into a semiconductor at non-equilibrium concentrations, which creates an intermediate band within the bandgap of the material and modifies the absorption coefficient. Second, pulsed-laser irradiation can enhance geometric light trapping by increasing surface roughness. Hyperdoping in silicon enables absorption of photons to wavelengths of at least 2.5 μm, while texturing enhances the absorptance to near unity at all absorbing wavelengths. In this paper, we review both effects and comment on outstanding questions and challenges in applying each to increasing the efficiency of photovoltaic devices.

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