Person: Recht, Daniel
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Recht
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Recht, Daniel
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Publication Energetic Beam Processing of Silicon to Engineer Optoelectronically Active Defects(2012-07-26) Recht, Daniel; Aziz, Michael J.This thesis explores ways to use ion implantation and nanosecond pulsed laser melting, both energetic beam techniques, to engineer defects in silicon. These defects are chosen to facilitate the use of silicon in optoelectronic applications for which its indirect bandgap is not ideal. Chapter 2 develops a kinetic model for the use of point defects as luminescence centers for light-emitting diodes and demonstrates an experimental procedure capable of high-throughput screening of the electroluminescent properties of such defects. Chapter 3 discusses the dramatic change in optical absorption observed in silicon highly supersaturated (i.e., hyperdoped) with the chalcogens sulfur, selenium, and tellurium and reports the first measurements of the optical absorption of such materials for photon energies greater than the bandgap of silicon. Chapter 3 examines the use of silicon hyperdoped with chalcogens in light detectors and concludes that while these devices display strong internal gain that is coupled to a particular type of surface defect, hyperdoping with chalcogens does not lead directly to measurable sub-bandgap photoconductivity. Chapter 4 considers the potential for Silicon to serve as the active material in an intermediate-band solar cell and reports experimental progress on two proposed approaches for hyperdoping silicon for this application. The main results of this chapter are the use of native-oxide etching to control the surface evaporation rate of sulfur from silicon and the first synthesis of monocrystalline silicon hyperdoped with gold.Publication Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon(AIP Publishing, 2014) Sher, Meng-Ju; Simmons, Christie B.; Krich, Jacob Jonathan; Akey, Austin; Winkler, Mark T.; Recht, Daniel; Buonassisi, Tonio; Aziz, Michael; Lindenberg, Aaron M.Intermediate-band materials have the potential to be highly efficient solar cells and can be fabricate by incorporating ultrahigh concentrations of deep-level dopants. Direct measurements of the ultrafast carrier recombination processes under supersaturated dopant concentrations have not been previously conducted. Here, we use optical-pump/terahertz-probe measurements to study carrier recombination dynamics of chalcogen-hyperdoped silicon with sub-picosecond resolution. The recombination dynamics is described by two exponential decay time scales: a fast decay time scale ranges between 1 and 200ps followed by a slow decay on the order of 1 ns. In contrast to the prior theoretical predictions, we find that the carrier lifetime decreases with increasing dopant concentration up to and above the insulator-to-metal transition. Evaluating the material’s figure of merit reveals an optimum doping concentration for maximizing performance.Publication Depth-resolved cathodoluminescence spectroscopy of silicon supersaturated with sulfur(AIP Publishing, 2013) Fabbri, Filippo; Smith, Matthew J.; Recht, Daniel; Aziz, Michael; Gradečak, Silvija; Salviati, GiancarloWe investigate the luminescence of Si supersaturated with S (Si:S) using depth-resolved cathodoluminescence spectroscopy and secondary ion mass spectroscopy as the S concentration is varied over 2 orders of magnitude \((10^{18}–10^{20} cm^{−3})\). In single-crystalline supersaturated Si:S, we identify strong luminescence from intra-gap states related to Si self-interstitials and a S-related luminescence at 0.85 eV, both of which show a strong dependence on S concentration in the supersaturated regime. Sufficiently high S concentrations in Si \((>10^{20} cm^{−3})\) result in complete luminescence quenching, which we propose is a consequence of the overlapping of the defect band and conduction band.Publication Deactivation of metastable single-crystal silicon hyperdoped with sulfur(AIP Publishing, 2013) Simmons, C. B.; Akey, Austin; Krich, Jacob Jonathan; Sullivan, Joseph T.; Recht, Daniel; Aziz, Michael; Buonassisi, TonioSilicon supersaturated with sulfur by ion implantation and pulsed laser melting exhibits broadband optical absorption of photons with energies less than silicon's band gap. However, this metastable, hyperdoped material loses its ability to absorb sub-band gap light after subsequent thermal treatment. We explore this deactivation process through optical absorption and electronic transport measurements of sulfur-hyperdoped silicon subject to anneals at a range of durations and temperatures. The deactivation process is well described by the Johnson-Mehl-Avrami-Kolmogorov framework for the diffusion-mediated transformation of a metastable supersaturated solid solution, and we find that this transformation is characterized by an apparent activation energy of \(E_A=1.7 ± 0.1 eV\). Using this activation energy, the evolution of the optical and electronic properties for all anneal duration-temperature combinations collapse onto distinct curves as a function of the extent of reaction. We provide a mechanistic interpretation of this deactivation based on short-range thermally activated atomic movements of the dopants to form sulfur complexes.Publication Methodology for vetting heavily doped semiconductors for intermediate band photovoltaics: A case study in sulfur-hyperdoped silicon(AIP Publishing, 2013) Sullivan, J. T.; Simmons, C. B.; Krich, J. J.; Akey, Austin; Recht, Daniel; Aziz, Michael; Buonassisi, T.We present a methodology for estimating the efficiency potential for candidate impurity-band photovoltaic materials from empirical measurements. This methodology employs both Fourier transform infrared spectroscopy and low-temperature photoconductivity to calculate a “performance figure of merit” and to determine both the position and bandwidth of the impurity band. We evaluate a candidate impurity-band material, silicon hyperdoped with sulfur; we find that the figure of merit is more than one order of magnitude too low for photovoltaic devices that exceed the thermodynamic efficiency limit for single band gap materials.Publication Enhancing the Infrared Photoresponse of Silicon by Controlling the Fermi Level Location within an Impurity Band(Wiley-Blackwell, 2014) Simmons, Christie B.; Akey, Austin J.; Mailoa, Jonathan P.; Recht, Daniel; Aziz, Michael; Buonassisi, TonioStrong absorption of sub-band gap radiation by an impurity band has recently been demonstrated in silicon supersaturated with chalcogen impurities. However, despite the enhanced absorption in this material, the transformation of infrared radiation into an electrical signal via extrinsic photoconductivity—the critical performance requirement for many optoelectronic applications—has only been reported at low temperature because thermal impurity ionization overwhelms photoionization at room temperature. Here, dopant compensation is used to manipulate the optical and electronic properties and thereby improve the room-temperature infrared photoresponse. Silicon co-doped with boron and sulfur is fabricated using ion implantation and nanosecond pulsed laser melting to achieve supersaturated sulfur concentrations and a matched boron distribution. The location of the Fermi level within the sulfur-induced impurity band is controlled by tuning the acceptor-to-donor ratio, and through this dopant compensation, three orders of magnitude improvement in infrared detection at 1550 nm is demonstrated.Publication Insulator-to-Metal Transition in Sulfur-Doped Silicon(American Physical Society, 2011) Winkler, Mark T.; Recht, Daniel; Sher, Meng-Ju; Said, Aurore J.; Mazur, Eric; Aziz, MichaelWe observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures T>1.7 K both indicate that a transition from insulating to metallic conduction occurs at a 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.Publication Extended infrared photoresponse and gain in chalcogen-supersaturated silicon photodiodes(AIP Publishing, 2011) Said, Aurore J.; Recht, Daniel; Sullivan, Joseph T.; Warrender, Jeffrey M.; Buonassisi, Tonio; Persans, Peter D.; Aziz, MichaelHighly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n+p photodiodesfabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain.Publication Single-Phase Filamentary Cellular Breakdown Via Laser-Induced Solute Segregation(Wiley-Blackwell, 2015) Akey, Austin; Recht, Daniel; Williams, James S.; Aziz, Michael; Buonassisi, TonioNanosecond melting and quenching of materials offers a pathway to novel structures with unusual properties. Impurity-rich silicon processed using nanosecond-pulsed-laser-melting is known to produce nanoscale features in a process referred to as “cellular breakdown” due to destabilization of the planar liquid/solid interface. Here, we apply atom probe tomography combined with electron microscopy to show that the morphology of cellular breakdown in these materials is significantly more complex than previously documented. We observe breakdown into a complex, branching filamentary structure topped by a few nm of a cell-like layer. Singlephase diamond cubic silicon highly supersaturated with at least 10% atomic Co and no detectable silicides is reported within these filaments. In addition, the unprecedented spatio-chemical accuracy of the atom probe allows us to investigate nanosecond formation dynamics of this complex material. Previously-reported properties of these materials can now be reconsidered in light of their true composition, and this class of inhomogeneous metastable alloys in silicon can be explored with confidence.Publication Morphological stability during solidification of silicon incorporating metallic impurities(AIP Publishing, 2014) Warrender, Jeffrey M.; Mathews, Jay; Recht, Daniel; Smith, Matthew; Gradečak, Silvija; Aziz, MichaelWe study the stability of a planar solidification front during pulsed laser melting-induced rapid solidification of silicon containing high concentrations of ion-implated metallic impurities. We calculate the critical impurity concentration for destabilizing plane-front solidification, and introduce the "amplification coefficient", which is an empirical parameter describing the degree of amplification that must accord between the time the planar liquid-solid interface first becomes unstable, and the time of formation of morphological features of interface breakdown that are later observed in the microstructure. By connecting our calculations to experimental observations from the literature we determine this parameter for Au, Co, Cr, Fe, Ga, In, and Zn in (100) Si and Ti in (111) Si, and find that it increases with impurity diffusive speed vd approximately as vd^.56. We present an approximate but simple method of estimating the maximum impurity concentration that may be incorporated in a surface layer of a given thickness without the appearance of cellular breakdown.