Near-infrared optical properties and proposed phase-change usefulness of transition metal disulfides

The development of photonic integrated circuits would benefit from a wider selection of materials that can strongly-control near-infrared (NIR) light. Transition metal dichalcogenides (TMDs) have been explored extensively for visible spectrum opto-electronics, but the NIR properties of these layered materials have been less-studied. The measurement of optical constants is the foremost step to qualify TMDs for use in NIR photonics. Here we measure the complex optical constants for select sulfide TMDs (bulk crystals of MoS2, TiS2 and ZrS2) via spectroscopic ellipsometry in the visible-to-NIR range. Through Mueller matrix measurements and generalized ellipsometry, we explicitly measure the direction of the ordinary optical axis. We support our measurements with density functional theory (DFT) calculations, which agree with our measurements and predict giant birefringence. We further propose that TMDs could find use as photonic phase-change materials, by designing alloys that are thermodynamically adjacent to phase boundaries between competing crystal structures, to realize martensitic (i.e. displacive, order-order) switching.Comment: supplementary at end of document. 6 main figure

Using Atom-Probe Tomography to Understand ZnO∶Al=SiO2=Si Schottky Diodes

We use electronic transport and atom-probe tomography to study ZnO∶Al/SiO[subscript 2]/Si Schottky diodes on lightly doped n- and p-type Si. We vary the carrier concentration in the ZnO∶Al films by 2 orders of magnitude, but the Schottky barrier height remains nearly constant. Atom-probe tomography shows that Al segregates to the interface, so that the ZnO∶Al at the junction is likely to be metallic even when the bulk of the ZnO∶Al film is semiconducting. We hypothesize that the observed Fermi-level pinning is connected to the insulator-metal transition in doped ZnO. This implies that tuning the band alignment at oxide/Si interfaces may be achieved by controlling the transition between localized and extended states in the oxide, thereby changing the orbital hybridization across the interface.United States. Dept. of Energy (EERE Postdoctoral Research Award)United States. Air Force Office of Scientific Research (Contract FA9550-12-1- 0189)National Science Foundation (U.S.) (Contract DMR-0952794)United States. Dept. of Energy (Bay Area Photovoltaic Consortium. Contract DE-EE0004946)National Science Foundation (U.S.) (Center for Nanoscale Systems. Contract ECS-0335765

Enhancing the Infrared Photoresponse of Silicon by Controlling the Fermi Level Location within an Impurity Band

Strong 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.Engineering and Applied Science

Single-Phase Filamentary Cellular Breakdown Via Laser-Induced Solute Segregation

Nanosecond 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.Engineering and Applied Science

A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm[superscript 2] 2-terminal monolithic perovskite/silicon multijunction solar cell with a V [subscript OC] as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.Bay Area Photovoltaic Consortium (Contract DE-EE0004946)United States. Dept. of Energy (Contract DE-EE0006707

Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination.

Shifting electrochemical oxygen reduction towards 2e- pathway to hydrogen peroxide (H2O2), instead of the traditional 4e- to water, becomes increasingly important as a green method for H2O2 generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H2O2 catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm-2 H2O2 current, and a high H2O2 selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e-/4e- ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e-, are responsible for the H2O2 pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application

Deactivation of metastable single-crystal silicon hyperdoped with sulfur

Silicon 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[subscript 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.Center for Clean Water and Clean Energy at MIT and KFUPMNational Science Foundation (U.S.) (Energy, Power, and Adaptive Systems Grant Contract ECCS-1102050)National Science Foundation (U.S.) (United States. Dept. of Energy Contract EEC-1041895

Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon

Intermediate-band materials have the potential to be highly efficient solar cells and can be fabricated 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 200 ps 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.Center for Clean Water and Clean Energy at MIT and KFUPMNational Science Foundation (U.S.) (Grant Contract ECCS-1102050)National Science Foundation (U.S.) (United States. Dept. of Energy Contract EEC-1041895