Person: Sher, Meng-Ju
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Sher
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Meng-Ju
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Sher, Meng-Ju
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Publication Intermediate Band Properties of Femtosecond-Laser Hyperdoped Silicon(2013-09-23) Sher, Meng-Ju; Mazur, Eric; Aziz, Michael; Kaxiras, EfthimiosThis thesis explores using femtosecond-laser pulses to hyperdope silicon with chalcogen dopants at concentrations above the maximum equilibrium solubility. Hyperdoped silicon is promising for improving efficiencies of solar cells: the material exhibits broad-band light absorption to wavelengths deep below the corresponding bandgap energy of silicon. The high concentration of dopants forms an intermediate band (IB), instead of discrete energy levels, and the IB enables sub-bandgap light absorption. This thesis is divided into two primary studies: the dopant incorporation and the IB properties. First, we study dopant incorporation with a gas-phase dopant precursor (SF6) using secondary ion mass spectrometry. By varying the pressure of SF6, we find that the surface adsorbed molecules are the dominant source of the dopant. Furthermore, we show the hyperdoped layer is single crystalline. The results demonstrate that the dopant incorporation depth, concentration, and crystallinity are controlled respectively by the number of laser pulses, pressure of the dopant precursor, and laser fluence. Second, we study the IB properties of hyperdoped silicon using optical and electronic measurements. We use Fourier transform infrared spectroscopy to study light absorption. The absorption extends to wavelengths as far as 6 μm before thermal annealing and we find the upper bound of the IB location at 0.2 eV below the conduction band edge. For electronic measurements, we anneal the samples to form a diode between the hyperdoped layer and the substrate, allowing us to probe the IB using temperature-dependent electronic transport measurements. The measurement data indicate that these samples form a localized IB at concentrations below the insulator-to-metal transition. Using a two-band model, we obtain the location of the localized IB at >0.07 eV below the conduction band edge. After femtosecond-laser hyperdoping, annealing is necessary to reduce the laser-induced defects; however annealing decreases the sub-bandgap absorption. As we are interested in the IB that contributes to sub-bandgap absorption, we explore methods to reactivate the sub-bandgap absorption. We show that the sub-bandgap absorption is reactivated by annealing at high temperatures between 1350 and 1550 K followed by fast cooling (>50 K/s). Our results demonstrate an ability to control sub-bandgap absorption using thermal processing.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 Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material(AIP Publishing, 2016) Jaramillo, Rafael; Sher, Meng-Ju; Ofori-Okai, Benjamin; Steinmann, V; Yang, Chuanxi; Hartman, Katy; Nelson, Keith; Lindenberg, Aaron; Gordon, Roy; Buonsassisi, TMaterials research with a focus on enhancing the minority-carrier lifetime of the light-absorbing semiconductor is key to advancing solar energy technology for both early stage and mature material platforms alike. Tin sulfide (SnS) is an absorber material with several clear advantages for manufacturing and deployment, but the record power conversion efficiency remains below 5%. We report measurements of bulk and interface minority-carrier recombination rates in SnSthin films using optical-pump, terahertz-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H2S gas increases the minority-carrier lifetime, and oxidation of the surface reduces the surface recombination velocity. However, the minority-carrier lifetime remains below 100 ps for all tested combinations of growth technique and post-growth processing. Significant improvement in SnSsolar cell performance will hinge on finding and mitigating as-yet-unknown recombination-active defects. We describe in detail our methodology for TPC experiments, and we share our data analysis routines in the form freely available software.Publication Carrier Dynamics and Absorption Properties of Gold-Hyperdoped Germanium: Insight Into Tailoring Defect Energetics(American Physical Society (APS), 2021-06-23) Dissanayake, Sashini Senali; Ferdous, Naheed; Gandhi, Hemi H.; Pastor, David; Tran, Tuan T.; Williams, Jim S.; Aziz, Michael; Mazur, Eric; Ertekin, Elif; Sher, Meng-JuHyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; 1.4–3.0 μm) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium (Ge:Au) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that Ge:Au carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of Ge:Au for SWIR photodetection applications.