Optical hyperdoping: black silicon

Femtosecond laser-assisted microstructuring of silicon for novel detector, sensing, and display technologies, at IEEE/LEOS 2002 Conference on Electro-Optic Sensors and Systems (Glasgow, Scotland), Monday, November 11, 2002:
Arrays of sharp, conical microstructures are obtained by texturing the surface of a silicon wafer using femtosecond laser-assisted chemical etching. The one step, maskless texturing process drastically changes the optical, material and electronic properties of the original silicon wafer. These properties make the textured silicon viable for use in a wide range of commercial devices. First, near-unity absorption of light, from visible to infrared wavelengths, offer opportunities for use in optically active devices such as solar cells and detectors. Significant enhancement of below-band-gap... Read more about Femtosecond laser-assisted microstructuring of silicon for novel detector, sensing, and display technologies
R. A. Myers, R. Farrell, A. Karger, J. E. Carey, and E. Mazur. 2006. “Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring.” Appl. Opt., 45, Pp. 8825–8831. Publisher's VersionAbstract
A processing technique using femtosecond laser pulses to microstructure the surface of a silicon ava- lanche photodiode (APD) has been used to enhance its near-infrared (near-IR) response. Experiments were performed on a series of APDs and APD arrays using various structuring parameters and post- structuring annealing sequences. Following thermal annealing, we were able to fabricate APD arrays with quantum efficiencies as high as 58% at 1064 nm without degradation of their noise or gain performance. Experimental results provided evidence to suggest that the improvement in charge collec- tion is a result of increased absorption in the near-IR.
M. Sher, K. Charles Hammond, L. Christakis, and E. Mazur. 2013. “The photovoltaic potential of femtosecond-laser textured amorphous silicon.” In . SPIE 2013 Photonics West. Publisher's VersionAbstract
Femtosecond laser texturing of silicon yields micrometer scale surface roughness that reduces reflection and enhances light absorption. In this work, we study the potential of using this technique to improve efficiencies of amorphous silicon-based solar cells by laser texturing thin amorphous silicon films. We use a Ti:Sapphire femtosecond laser system to texture amorphous silicon, and we also study the effect of laser texturing the substrate before depositing amorphous silicon. We report on the material properties including surface morphology, light absorption, crystallinity, as well as solar cell efficiencies before and after laser texturing.
F. Fabbri, Y. Lin, G. Bertoni, F. Rossi, M. J. Smith, S. Gradecak, E. Mazur, and G. Salviati. 2015. “Origin of the visible emission of black silicon microstructures.” Appl. Phys. Lett., 107, Pp. 021907-1–021907-4. Publisher's VersionAbstract
Silicon, the mainstay semiconductor in microelectronics, is considered unsuitable for optoelectronic applications due to its indirect electronic band gap that limits its efficiency as light emitter. Here, we univocally determine at the nanoscale the origin of visible emission in microstructured black silicon by cathodoluminescence spectroscopy and imaging. We demonstrate the formation of amorphous silicon oxide microstructures with a white emission. The white emission is composed by four features peaking at 1.98 eV, 2.24 eV, 2.77 eV, and 3.05 eV. The origin of such emissions is related to SiOx intrinsic point defects and to the sulfur doping due to the laser processing. Similar results go in the direction of developing optoelectronic devices suitable for silicon-based circuitry.
M. A. Sheehy, L. Winston, J. E. Carey, C. M. Friend, and E. Mazur. 2005. “Role of the background gas in the morphology and optical properties of laser-microstructured silicon.” Chem. Mater., 17, Pp. 3582–3586. Publisher's VersionAbstract
We irradiate silicon with a train of femtosecond pulses in the presence of SF6, H2S, H2, SiH4, and a mixture of Ar and SF6 in order to analyze the role of the background gas in determining the morphology and the optical properties of the resultant surfaces. We discuss factors that affect the surface morphology created during irradiation and show that the presence of sulfur in these gases is important in creating sharp microstructures. We also show that the presence of sulfur is necessary to create the near-unity absorptance for both above-band and below-band gap radiation (0.25 2.5 micrometer) by silicon; only samples with sulfur concentrations higher than 0.6% absorb 95% for above-band gap radiation and have a flat, featureless absorptance of 90% for below-band gap radiation. KEYWORDS silicon, infrared absorptance, laser materials processing, microstructures, sulfur doping, femtosecond laser irradiation, RBS, elemental semiconductors
B. R. Tull, M. T. Winkler, and E. Mazur. 2009. “The role of diffusion in broadband infrared absorption in chalcogen-doped silicon.” Appl. Phys. A. Publisher's VersionAbstract
Sulfur doping of silicon beyond the solubility limit by femtosecond laser irradiation leads to near-unity broadband absorption of visible and infrared light and the realization of silicon-based infrared photodetectors. The nature of the infrared absorption is not yet well understood. Here we present a study on the reduction of infrared absorptance after various anneals of different temperatures and durations for three chalcogens (sulfur, selenium, and tellurium) dissolved into silicon by femtosecond laser irradiation. For sulfur doping, we irradiate silicon in SF6 gas; for selenium and tellurium, we evaporate a film onto the silicon and irradiate in N2 gas; lastly, as a control, we irradiated untreated silicon in N2 gas. Our analysis shows that the deactivation of infrared absorption after thermal annealing is likely caused by dopant diffusion. We observe that a characteristic diffusion lengthcommon to all three dopantsleads to the reduction of infrared absorption. Using diffusion theory, we suggest a model in which grain size of the re-solidified surface layer can account for this characteristic diffusion length, indicating that deactivation of infrared absorptance may be caused by precipitation of the dopant at the grain boundaries.
T. Baldacchini, J. E. Carey, M. Zhou, and E. Mazur. 2006. “Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser.” Langmuir, 22, Pp. 4917–4919. Publisher's VersionAbstract
Superhydrophobic surfaces exhibit contact angles with water that are larger than 150 and negligible difference in contact angle between the advancing and receding contact angles, the so-called contact angle hysteresis. In this paper, we present a novel and simple structuring process that uses intense femtosecond- laser pulses to create microstructured superhydrophobic surfaces with remarkable wetting characteristics.
B. K. Newman, E. Ertekin, J. Timothy Sullivan, M. T. Winkler, M. A. Marcus, S. Fakra, M. Sher, E. Mazur, J. C. Grossman, and T. Buonassisi. 2013. “Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon.” J. Appl. Phys., 114, Pp. 133507–133507-8. Publisher's VersionAbstract
Silicon doped with an atomic percent of chalcogens exhibits strong, uniform sub-bandgap optical absorptance and is of interest for photovoltaic and infrared detector applications. This sub-bandgap absorptance is reduced with subsequent thermal annealing indicative of a diffusion mediated chemical change. However, the precise atomistic origin of absorptance and its deactivation is unclear. Herein, we apply Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the chemical states of selenium dopants in selenium-hyperdoped silicon annealed to varying degrees. We observe a smooth and continuous selenium chemical state change with increased annealing temperature, highly correlated to the decrease in sub-bandgap optical absorptance. In samples exhibiting strong sub-bandgap absorptance, EXAFS analysis reveals that the atoms nearest to the Se atom are Si at distances consistent with length scales in energetically favorable Se substitutional-type point defect complexes as calculated by density functional theory. As the sub- bandgap absorptance increases, EXAFS data indicate an increase in the Se-Si bond distance. In specimens annealed at 1225 K exhibiting minimal sub- bandgap absorptance, fitting of the EXAFS spectra indicates that Se is predominantly in a silicon diselenide (SiSe2) precipitate state. The EXAFS study supports a model of highly optically absorbing point defects that precipitate during annealing into structures with no sub-bandgap absorptance.
M. Sher and E. Mazur. 2014. “Intermediate Band Conduction in Femtosecond-Laser Hyperdoped Silicon.” Appl. Phys. Lett., 105, Pp. 032103-1–032103-5. Publisher's VersionAbstract
We use femtosecond-laser hyperdoping to introduce non-equilibrium concentrations of sulfur into silicon and study the nature of the resulting intermediate band. With increasing dopant concentration, the sub-bandgap absorption increases. To better understand the dopant energetics, we perform temperature-dependent Hall and resistivity measurements. We analyze the carrier concentration and the energetics of the intermediate band using a two- band model. The temperature-dependence of the carrier concentration and resistivity suggests that the dopant concentration is below the insulator-to-metal transition and that the samples have a localized intermediate band at 70 meV below the conduction band edge.
G. Haberfehlner, M. J. Smith, J. Idrobo, G. Auvert, M. Sher, M. T. Winkler, E. Mazur, N. Gambacorti, S. Gradečak, and P. Bleuet. 2013. “Selenium segregation in femtosecond-laser hyperdoped silicon revealed by electron tomography.” Microscopy and Microanalysis, 19, Pp. 716–725. Publisher's VersionAbstract
Doping of silicon with chalcogens (S, Se, Te) by femtosecond laser irradiation leads to nearunity optical absorptance in the visible and infrared range and is a promising route towards siliconbased infrared optoelectronics. However, open questions remain about the nature of the infrared absorptance and in particular about the impact of the dopant distribution and possible role of dopant diffusion. Here we use electron tomography using a high-angle annular dark field (HAADF) detector in a scanning transmission electron microscope (STEM) to extract information about the threedimensional distribution of selenium dopants in silicon and correlate these findings with the optical properties of selenium- doped silicon. We quantify the tomography results to extract information about the size distribution and density of selenium precipitates. Our results show correlation between nanoscale distribution of dopants and the observed sub- band gap optical absorptance, and demonstrate the feasibility of HAADF-STEM tomography for the investigation of dopant distribution in highly-doped semiconductors.
M. Shen, C. H. Crouch, J. E. Carey, R. J. Younkin, E. Mazur, M. A. Sheehy, and C. M. Friend. 2003. “Formation of regular arrays of silicon microspikes by femtosecond laser irradiation through a mask.” Appl. Phys. Lett., 82, Pp. 1715–1717. Publisher's VersionAbstract
We report fabrication of regular arrays of silicon microspikes by femtosecond laser irradiation of a silicon wafer covered with a periodic mask. Without a mask, microspikes form, but they are less ordered. We believe that the mask imposes order by diffracting the laser beam and providing boundary conditions for capillary waves in the laser-melted silicon.
M. T. Winkler, M. Sher, Y. Lin, M. J. Smith, H. Zhang, S. Gradečak, and E. Mazur. 2012. “Studying femtosecond-laser hyperdoping by controlling surface morphology.” Journal of Applied Physics, 111, Pp. 093511–. Publisher's VersionAbstract
We study the fundamental properties of femtosecond-laser (fs-laser) hyperdoping by developing techniques to control the surface morphology following laser irradiation. By decoupling the formation of surface roughness from the doping process, we study the structural and electronic properties of fs-laser doped silicon. These experiments are a necessary step toward developing predictive models of the doping process. We use a single fs-laser pulse to dope silicon with sulfur, enabling quantitative secondary ion mass spectrometry, transmission electron microscopy, and Hall effect measurements. These measurements indicate that at laser fluences at or above 4 kJ m-2, a single laser pulse yields a sulfur dose > (3 ± 1) x 1013 cm–2 and results in a 45-nm thick amorphous surface layer. Based on these results, we demonstrate a method for hyperdoping large areas of silicon without producing the surface roughness.
J. E. Carey and E. Mazur. 2003. “Femtosecond Laser-Assisted Microstructuring of Silicon for Novel Detector, Sensing and Display Technologies.” In . LEOS 2003. Publisher's VersionAbstract
Arrays of sharp, conical microstructures are obtained by stucturing the surface of a silicon wafer using femtosecond laser-assisted chemical etching. The one step, maskless structuring process drastically changes the optical, material and electronic properties of the original silicon wafer. These properties make microstructured silicon viable for use in a wide range of commercial devices including solar cells, infrared photodetectors, chemical and biological sensors, and field emission devices.
T. Sarnet, J. E. Carey, and E. Mazur. 2012. “From black silicon to photovoltaic cells, using short pulse lasers.” In . International Symposium on High Power Laser Ablation 2012. Publisher's VersionAbstract
Laser created Black Silicon has been developed since 1998 at Harvard University. The unique optical and semiconducting properties of the black silicon first lead to interesting applications for sensors (photodetectors, thermal imaging cameras…). 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 brief review of the use of black silicon for photovoltaic cells.

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