Meng-Ju Sher

Insulator-to-Metal Transition in Sulfur-Doped Silicon

We 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 4.3×10(20)  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.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites

The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

Pressure-induced phase transformations during femtosecond-laser doping of silicon

Silicon hyperdoped with chalcogens via femtosecond-laser irradiation exhibits unique near-unity sub-bandgap absorptance extending into the infrared region. The intense light-matter interactions that occur during femtosecond-laser doping produce pressure waves sufficient to induce phase transformations in silicon, resulting in the formation of metastable polymorphic phases, but their exact formation mechanism and influence on the doping process are still unknown. We report direct observations of these phases, describe their formation and distribution, and consider their potential impact on sub-bandgap absorptance. Specifically, the transformation from diamond cubic Si-I to pressure-induced polymorphic crystal structures (amorphous Si, Si-XII, and Si-III) during femtosecond-laser irradiation was investigated using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Amorphous Si, Si-XII, and Si-III were found to form in femtosecond-laser doped silicon regardless of the pre...

The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecond laser irradiation

Surface texturing of silicon using femtosecond (fs) laser irradiation is an attractive method for enhancing light trapping, but the laser-induced damage that occurs in parallel with surface texturing can 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, micro-Raman spectroscopy, and transmission electron microscopy. We show that raster scanning a pulsed laser beam with a Gaussian profile enhances the formation of crystalline pressure-induced silicon polymorphs by an order of magnitude compared with stationary pulsed fs-laser irradiation. Based on these observations, we identify resolidification-induced stresses as the mechanism responsible for driving sub-surface phase transformations during the surface texturing of silicon, the understanding of...

Ultrafast Electronic and Structural Response of Monolayer MoS2 under Intense Photoexcitation Conditions

We report on the dynamical response of single layer transition metal dichalcogenide MoS2 to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron-hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions.

Reactivation of sub-bandgap absorption in chalcogen-hyperdoped silicon

Silicon doped with nonequilibrium concentrations of chalcogens using a femtosecond laser exhibits near-unity absorption of sub-bandgap photons to wavelengths of at least 2500 nm. Previous studies have shown that sub-bandgap absorptance decreases with thermal annealing up to 1175 K and that the absorption deactivation correlates with chalcogen diffusivity. In this work, we show that sub-bandgap absorptance can be reactivated by annealing at temperatures between 1350 and 1550 K followed by fast cooling (>50 K/s). Our results suggest that the defects responsible for sub-bandgap absorptance are in equilibrium at high temperatures in hyperdoped Si:chalcogen systems.

Studying femtosecond-laser hyperdoping by controlling surface morphology

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) × 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.

Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material

Materials 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 SnS thin films using optical-pump, terahertz (THz)-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H_2S 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 SnS solar 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 as freely-available software.

Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells

Perovskite solar cells (PSCs) are very promising lab-scale technologies to deliver inexpensive solar electricity. Low-temperature, planar PSCs are of particularly interest for large-scale deployment due to their inherent suitability for flexible substrates and potential for silicon/perovskite tandems. So far, planar PSCs have been prone to large current–voltage hysteresis and low stabilized power output due to a number of issues associated with this kind of device configuration. We find that the suppression of the yellow-phase impurity (∂-FAPbI3) present in formamidium-based perovskites, by RbI addition, contributes to low hysteresis, higher charge carrier mobility, long-lived carrier lifetimes and a champion stabilized power output of 20.3% using SnOx as the electron selective contact. We study the effects of these impurities on the transient behavior that defines hysteresis and its relation to ionic movement. In addition, we find that the formation of a RbPbI3 phase does not significantly affect the charge carrier lifetimes and consequently the performance of the devices. This brings new physical insights onto the role of different impurities in perovskite solar cells, which make these materials so remarkable.

Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon

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 d...