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It has been proposed that metal-semiconductor-metal (MSM) structures can be used to tune the absorptivity of a metasurface at infrared wavelengths. Indium arsenide (InAs) is a low-band-gap, high-electron-mobility semiconductor that may enable rapid index tuning for dynamic control over the infrared spectrum. However, direct growth of III-V thin films on top of metals has typically resulted in small-grain, polycrystalline materials that are not amenable to high-quality devices. Previously, epitaxial wafers were used for this purpose. However, the epitaxial constraints required that InAs be used for both the tuning layer and the bottom "metallic" layer, limiting the range of accessible designs. In this work, we show a demonstration of direct growth of single-crystalline InAs on metal to build tunable absorbers/emitters in the infrared regime. The growth was carried out at a temperature of 300 °C by the low temperature templated liquid phase (LT-TLP) method. The size of InAs single-crystalline mesas is ∼2500 μm

Controlled electrical stimulation is essential for evaluating the physiology of cardiac tissues engineered in heart-on-a-chip devices. However, existing stimulation techniques, such as external platinum electrodes or opaque microelectrode arrays patterned on glass substrates, have limited throughput, reproducibility, or compatibility with other desirable features of heart-on-a-chip systems, such as the use of tunable culture substrates, imaging accessibility, or enclosure in a microfluidic device. In this study, indium tin oxide (ITO), a conductive, semi-transparent, and biocompatible material, was deposited onto glass and polydimethylsiloxane (PDMS)-coated coverslips as parallel or point stimulation electrodes using laser-cut tape masks. ITO caused substrate discoloration but did not prevent brightfield imaging. ITO-patterned substrates were microcontact printed with arrayed lines of fibronectin and seeded with neonatal rat ventricular myocytes, which assembled into aligned cardiac tissues. ITO deposited as parallel or point electrodes was connected to an external stimulator and used to successfully stimulate micropatterned cardiac tissues to generate calcium transients or propagating calcium waves, respectively. ITO electrodes were also integrated into the cantilever-based muscular thin film (MTF) assay to stimulate and quantify the contraction of micropatterned cardiac tissues. To demonstrate the potential for multiple ITO electrodes to be integrated into larger, multiplexed systems, two sets of ITO electrodes were deposited onto a single substrate and used to stimulate the contraction of distinct micropatterned cardiac tissues independently. Collectively, these approaches for integrating ITO electrodes into heart-on-a-chip devices are relatively facile, modular, and scalable and could have diverse applications in microphysiological systems of excitable tissues.

There are various mechanisms of light emission in carbon nanotubes (CNTs), which give rise to a wide range of spectral emission characteristics that provide important information regarding the underlying physical processes that lead to photon emission. Here, we report spectra obtained from individual suspended CNT dual-gate field effect transistor (FET) devices under different gate and bias conditions. By applying opposite voltages to the gate electrodes (i.e., Vg1 = −Vg2), we are able to create a pn-junction within the suspended region of the CNT. Under forward bias conditions, the spectra exhibit a peak corresponding to E11 exciton emission via thermal (i.e., blackbody) emission occurring at electrical powers around 8 µW, which corresponds to a power density of approximately 0.5 MW/cm2. On the other hand, the spectra observed under reverse bias correspond to impact ionization and avalanche emission, which occurs at electrical powers of ~10 nW and exhibits a featureless flat spectrum extending from 1,600 nm to shorter wavelengths up to 600 nm. Here, the hot electrons generated by the high electric fields (~0.5 MV/cm) are able to produce high energy photons far above the E11 (ground state) energy. It is somewhat surprising that these devices do not exhibit light emission by the annihilation of electrons and holes under forward bias, as in a light emitting diode (LED). Possible reasons for this are discussed, including Auger recombination.

Here, we demonstrate high electron mobility single crystal InAs mesas monolithically integrated on amorphous dielectric substrates at a growth temperature of 300°C. Critically, a room temperature m...

Here, we show that the turn-on voltage for the hydrogen evolution reaction on a graphene surface can be tuned in a semiconductor-insulator-graphene (SIG) device immersed in a solution. Specifically, it is shown that the hydrogen evolution reaction (HER) onset for the graphene can shift by0.8 V by application of a voltage across a graphene-Al

Photoemission plays a central role in a wide range of fields, from electronic structure measurements to free-electron laser sources. In metallic emitters, single-photon1, multiphoton2–5 or strong-field emission6–10 processes are the primary photoemission mechanisms. Here, using a sub-work-function 3.06 eV continuous-wave laser, photoemission from waveguide-integrated monolayer graphene is observed to occur at peak power densities >5 orders of magnitude lower than reported multiphoton and strong-field emission6,11,12. The behaviour is explained by the emission of hot electrons in graphene. In monolayer graphene, the need for photoelectrons to be transported to an emitting surface is eliminated, dramatically enhancing the probability of emission before thermalization. These results indicate that integrated-photonics-driven hot-electron emission provides a rich new area of exploration for both electron emission and integrated photonics. Unusual photoemission from graphene is explained by the emission of hot electrons. The findings may lead to integrated photonic devices driven by hot-electron emission.

Using hot electrons to drive electrochemical reactions has drawn considerable interest in driving high-barrier reactions and enabling efficient solar to fuel conversion. However, the conversion efficiency from hot electrons to electrochemical products is typically low due to high hot electron scattering rates. Here, it is shown that the hydrogen evolution reaction (HER) in an acidic solution can be efficiently modulated by hot electrons injected into a thin gold film by an Au-Al

Applications such as Ultrafast Electron Diffraction and X-Ray Free Electron Lasers are enabled by high brightness photomitters [1], [2]. High brightness beams require rapid response times, low mean transverse energies (MTE), and high quantum efficiencies. Negative electron affinity (NEA) cathodes, composed of a GaAs wafer and a Cs/O surface layer, have been shown to simultaneously exhibit good external quantum efficiencies (EQE) of ~10-3-10-2 and reasonable MTEs but are rarely used due to (i) the significant complexity associated with fabricating them and (ii) the lack of stability. These include high temperature processing steps, entirely insitu activation, and ultra-high vacuum storage and operating pressures.

Organic molecules with nonlinear optical behavior have advanced a wide range of fields spanning from integrated photonics to biological imaging. With advances in microscopy, an emerging application is multifunctional nonlinear organic imaging agents. Unlike conventional imaging probes which simply emit light through single or multi photon processes, multifunctional materials allow systems to be simultaneously imaged and controlled. In this work, we report a multifunctional molecular probe for modulating and reporting electric fields. The probe molecule consists of two distinct functional modules which are connected by a long alkyl chain. The electric field detector module relies on the two-photon (2p) imaging agent and photo-induced electron transfer (PeT) dye, TPE. Two-photon imaging agents have demonstrated less damage and larger penetration depths in cells and live tissue imaging. The electric field modulator module relies on the organic photoconductor, NAI. To reduce cross-talk and optimize absorption and emission wavelengths, the molecular structure is first studied using density functional theory modeling, and then the multi-functional molecular probe is synthesized. The photophysical, photoconductivity, and biotoxicity of the probe molecule are studied in a range of solvents and solid state, and the results agree with the theoretical predictions. Specifically, 2p excitation in a biocompatible solvent is demonstrated, the photoconductivity is rapid and reversible, and the material has low cytotoxicity. Additionally, the entire system is optically controlled, including signal read-out, and the two modules can be operated simultaneously or individually. This work sets the stage for modulation and detection of bioelectric fields in a range of cell and tissue types., main text and SI, 29 pages

As one of the key neuronal activities associated with memory in the human brain, memory consolidation is the process of the transition of short-term memory (STM) to long-term memory (LTM), which transforms an external stimulus to permanently stored information. Here, we report the emulation of this complex synaptic function, consolidation of STM to LTM, in a single-crystal indium phosphide (InP) field effect transistor (FET)-based artificial synapse. This behavior is achieved via the dielectric band and charge trap lifetime engineering in a dielectric gate heterostructure of aluminum oxide and titanium oxide. We analyze the behavior of these complex synaptic functions by engineering a variety of action potential parameters, and the devices exhibit good endurance, long retention time (>105 s), and high uniformity. Uniquely, this approach utilizes growth and device fabrication techniques which are scalable and back-end CMOS compatible, making this InP synaptic device a potential building block for neuromorphic computing.

Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells., 4 Figures

The growth of crystalline compound semiconductors on amorphous and non-epitaxial substrates is a fundamental challenge for state-of-the-art thin-film epitaxial growth techniques. Direct growth of materials on technologically relevant amorphous surfaces, such as nitrides or oxides results in nanocrystalline thin films or nanowire-type structures, preventing growth and integration of high-performance devices and circuits on these surfaces. Here, we show crystalline compound semiconductors grown directly on technologically relevant amorphous and non-epitaxial substrates in geometries compatible with standard microfabrication technology. Furthermore, by removing the traditional epitaxial constraint, we demonstrate an atomically sharp lateral heterojunction between indium phosphide and tin phosphide, two materials with vastly different crystal structures, a structure that cannot be grown with standard vapor-phase growth approaches. Critically, this approach enables the growth and manufacturing of crystalline materials without requiring a nearly lattice-matched substrate, potentially impacting a wide range of fields, including electronics, photonics, and energy devices.

We observe bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (∼nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4 V) without heating the CNT. Under these conditions, we observe light emission at currents as small as 1 nA and corresponding electrical powers of 4nW, which is 3 orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission is understood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500 kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this f...

Recent developments in nanophotonics have provided a clear roadmap for improving the efficiency of photonic devices through control over absorption and emission of devices. These advances could prove transformative for a wide variety of devices, such as photovoltaics, photoelectrochemical devices, photodetectors, and light-emitting diodes. However, it is often challenging to physically create the nanophotonic designs required to engineer the optical properties of devices. Here, we present a platform based on crystalline indium phosphide that enables thin-film nanophotonic structures with physical morphologies that are impossible to achieve through conventional state-of-the-art material growth techniques. Here, nanostructured InP thin films have been demonstrated on non-epitaxial alumina inverted nanocone (i-cone) substrates via a low-cost and scalable thin-film vapor-liquid-solid growth technique. In this process, indium films are first evaporated onto the i-cone structures in the desired morphology, followed by a high-temperature step that causes a phase transformation of the indium into indium phosphide, preserving the original morphology of the deposited indium. Through this approach, a wide variety of nanostructured film morphologies are accessible using only control over evaporation process variables. Critically, the as-grown nanotextured InP thin films demonstrate excellent optoelectronic properties, suggesting this platform is promising for future high-performance nanophotonic devices.

Hot electron emission from waveguide-integrated graphene has been recently shown to occur at optical power densities multiple orders of magnitude lower than that of metal tips excited by sub-work-function photons. However, the experimentally observed electron emission currents are small, which limits the practical uses of such a mechanism. Here, we explore the performance limits of hot electron emission in graphene through experimentally calibrated simulations. Two regimes of nonequilibrium emission in graphene are identified, (i) single particle hot electron emission, where an electron is excited by a photon and emitted before losing significant energy through scattering; and (ii) ensemble hot electron emission, where the photon source causes nonequilibrium heating of the electron population beyond the electron lattice temperature. It is shown that, through appropriate selection of photon energy, optical power density, and applied electric field, hot electron emission can be used to create ultrahigh current electron emitters with ultrafast temporal responses in both the single particle and ensemble heating regimes. These results suggest that, through appropriate design, hot electron emitters may overcome the limitations of thermionic and field emitters.

There are various mechanisms of light emission in carbon nanotubes (CNTs), which give rise to a wide range of spectral characteristics that provide important information. Here, we report suppression of incandescence via Auger recombination in suspended carbon nanotube pn-junctions generated from dual-gate CNT field effect transistor (FET) devices. By applying equal and opposite voltages to the gate electrodes (i.e., Vg1 = -Vg2), we create a pn-junction within the CNT. Under these gating conditions, we observe a sharp peak in the incandescence intensity around zero applied gate voltage, where the intrinsic region has the largest spatial extent. Here, the emission occurs under high electrical power densities around 0.1MW/cm2 (or 6µW) and arises from thermal emission at elevated temperatures above 800K (i.e., incandescence). It is somewhat surprising that this thermal emission intensity is so sensitive to the gating conditions, and we observe a 1000-fold suppression of light emission between Vg1=0V and Vg1=1...

We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (∼μA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (

Recently, it has been shown that hot-electron photoemission in waveguide-integrated graphene can occur at peak optical power densities many orders of magnitude lower than multiphoton and strong field emission. In this work, we study how the deposition of low-work function lanthanum hexaboride nanoparticles can alter the behavior of hot-electron emission from graphene and thin gold waveguide-integrated hot electron emitters. This approach is promising, as the graphene enables an electrically conductive platform on which to deposit the nanoparticles, while still enabling interaction between the nanoparticles and incident photons. Despite nonideal coatings of LaB6 nanoparticles on the waveguide integrated graphene and gold, there is a nearly order of magnitude improvement over previous graphene-based hot-electron emitters. This hybrid approach demonstrates how a combination of integrated photonics and low-work function coatings can improve the performance of the emerging class of hot-electron emitters.

Here we show that a p-silicon-insulator-graphene photocathode (i) increases the photon-to-hydrogen external quantum efficiency (EQE) above 1, and (ii) reduces the onset potential for the hydrogen evolution reaction via an applied voltage across the graphene-silicon junction. These devices were measured in an acidic solution of 0.5 M H2SO4 using a dual potentiostat setup, allowing independent control over solution potential, graphene potential, and silicon potential. An AM1.5G solar simulator was used to carry out solar illumination measurements. In comparison to a p-Si-insulator control sample, the addition of an insulator/graphene layer increases the saturation photocurrent from ~35 mA/cm2 to ~80 mA/cm2, respectively, indicating that a carrier multiplication process occurs due to the addition of graphene. In this way, the graphene/oxide behaves similarly to an electrocatalyst, where at a given overpotential, the current density is increased. However, in this case, the current density is driven by the incident photon flux and enhanced via a carrier multiplication process. To further understand this effect, a laser excitation source was used to measure power dependent quantum efficiency. The external quantum efficiencies of the SIG devices exhibit a strong dependence on power density, increasing to from EQE~1.5 to EQE~7 for low power densities of ~0.1 mW/cm2. Current component resolved measurements enabled us to separately measure the currents flowing in the graphene, silicon, and solution. During measurements, we show that the measured photoelectrochemical coccurs due to carriers in the silicon directly driving the hydrogen generation, not graphene. This unique result highlights that the graphene/silicon tunnel junction causes carrier multiplication at the silicon surface, which then drives hydrogen generation. This can occur due to electrons tunneling from silicon directly through the graphene and driving the hydrogen evolution reaction. When comparing the control and graphene photocathodes, it is shown that the turn-on voltages are nearly identical, highlighting that the carrier multiplication process does not require additional voltage input and instead is driven by the intrinsic electric field. Finally, we show that applying a bias across the silicon-graphene junction reduces the turn-on voltage for hydrogen evolution reaction by nearly 400 mV for current densities of 40 mA/cm2. It is expected that this applied bias increases the field across the oxide, increasing the total carrier multiplication rate at a given solution bias. Thus p-silicon-insulator-graphene photocathodes are shown to demonstrate: (i) EQE greater than 1, and (ii) a reduction in the voltage needed to drive the electrochemical reaction when a bias is applied across the graphene-silicon junction.

In this Letter, we report the direct growth of single crystal III–V thin film mesas on amorphous SiO2 on Si using templated liquid phase growth. Unlike previous works, where crystal sizes demonstrated have been less than ∼10 μm, here, we show that by tuning the crystal growth conditions, crystals with dimensions greater than 100 μm and of high electron mobility can be directly grown on oxides. Specifically, InAs-on-oxide with mobilities reaching 5100 cm2/V s at 100 K, and ∼3200 cm2/V s at room temperature has been demonstrated. The excellent electronic performance is due to the single crystallinity of the grown material and creates new avenues for the monolithic direct integration of high-performance materials on non-epitaxial substrates, including silicon, and amorphous substrates, such as glasses and metals.

This paper presents the experimental demonstration of control over field enhancement factor and work function of the hybrid field emitter to reduce the threshold field required for electron emission. The field enhancement is engineered through microfabrication of the underlying silicon wafers, and the work function is controlled by the transfer and deposition of monolayer graphene and low work function lanthanum hexaboride (LaB 6 ) nanoparticles. As a result, the threshold field, defined as the electric field required for 10 μΑ/cm2 of emission current density achieved as small as 2.6 V/μm. We also carry out photoemission spectroscopy (PES) to measure the effective work function of our emitters as 3.62eV, significantly lower than work function of the graphene. The measured work function allows us independent extraction of the field enhancement factor of underlying silicon tip array from Fowler-Nordheim (FN) analysis of the experimental I-E curves. Through these measurements and work function characterization, we show that the work function and field enhancement factor can be independently controlled, potentially enabling ultra-low turn on, uniform, and stable emitters.

We introduce the novel approach along with the experimental demonstration of enhanced photon assisted electron emission from monolayer graphene sheet placed over an optical waveguide. A critical challenge on state of the art photon assisted electron emission is low quantum efficiency (QE). One technique that improves QE is enhancing the optical absorption of the incident laser. Here, we specifically introduce utilizing integrated photonic waveguide under electron emitter layer as a mean to transport the photons and evanescently couple them to emitter layer. This evanescent coupling occurs through longer interaction length and photons can be absorbed efficiently compared to free space laser illumination from top on a monolayer of graphene. We also measured the free space photon assisted electron emission from graphene sheet and compared with the performance of the integrated photon assisted emission device.

The authors present a manufacturing cost analysis for producing thin-film indium phosphide modules by combining a novel thin-film vapor–liquid–solid (TF-VLS) growth process with a standard monolithic module platform. The example cell structure is ITO/n-TiO2/p-InP/Mo. For a benchmark scenario of 12% efficient modules, the module cost is estimated to be $0.66/W(DC) and the module cost is calculated to be around $0.36/W(DC) at a long-term potential efficiency of 24%. The manufacturing cost for the TF-VLS growth portion is estimated to be ~$23/m 2 ,as ignificant reduction compared with traditional metalorganic chemical vapor deposition. The analysis here suggests the TF-VLS growth mode could enable lower-cost, high-efficiency III-V photovoltaics compared with manufacturing methods used today and open up possibilities for other optoelectronic applications as well. Copyright © 2016 John Wiley &S ons, Ltd.

We present a comparative study of quasi-metallic (Eg ∼ 100 meV) and semiconducting (Eg ∼ 1 eV) suspended carbon nanotube pn-junctions introduced by electrostatic gating. While the built-in fields of the quasi-metallic carbon nanotubes (CNTs) are 1-2 orders of magnitude smaller than those of the semiconducting CNTs, their photocurrent is 2 orders of magnitude higher than the corresponding semiconducting CNT devices under the same experimental conditions. Here, the large exciton binding energy in semiconducting nanotubes (∼400 meV) makes it difficult for excitons to dissociate into free carriers that can contribute to an externally measured photocurent. As such, semiconducting nanotubes require a phonon to assist in the exciton dissociation process, in order to produce a finite photocurrent, while quasi-metallic nanotubes do not. The quasi-metallic nanotubes have much lower exciton binding energies (∼50 meV) as well as a continuum of electronic states to decay into and, therefore, do not require the absorption of a phonon in order to dissociate, making it much easier for these excitons to produce a photocurrent. We performed detailed simulations of the band energies in quasi-metallic and semiconducting nanotube devices in order to obtain the electric field profiles along the lengths of the nanotubes. These simulations predict maximum built-in electric field strengths of 2.3 V/μm for semiconducting and 0.032-0.22 V/μm for quasi-metallic nanotubes under the applied gate voltages used in this study.

A type-III (broken gap) band alignment heterojunction vertical in-line InAs/AlSb/GaSb tunnel FET, including a 2-nm-thin AlSb tunneling barrier is demonstrated. The impact of overlap and underlap gate is studied experimentally and supported further by quasi-stationary 2-D TCAD Sentaurus device simulations. Hydrogen silsesquioxane is used as a novel mechanical support structure to suspend the 10-nm-thin InAs drain with enough undercut to be able to demonstrate an overlap gate architecture. The overlap gate InAs/AlSb/GaSb TFET shows an ON current density of 22 μA/μm2 at $V_{{\rm GS}} = V_{{\rm DS}} = 0.4$ V and the subthreshold slope is 194 mV/decade at room temperature and 46 mV/decade at 100 K.

Neuromorphic or "brain-like" computation is a leading candidate for efficient, fault-tolerant processing of large-scale data as well as real-time sensing and transduction of complex multivariate systems and networks such as self-driving vehicles or Internet of Things applications. In biology, the synapse serves as an active memory unit in the neural system and is the component responsible for learning and memory. Electronically emulating this element via a compact, scalable technology which can be integrated in a three-dimensional (3-D) architecture is critical for future implementations of neuromorphic processors. However, present day 3-D transistor implementations of synapses are typically based on low-mobility semiconductor channels or technologies that are not scalable. Here, we demonstrate a crystalline indium phosphide (InP)-based artificial synapse for spiking neural networks that exhibits elasticity, short-term plasticity, long-term plasticity, metaplasticity, and spike timing-dependent plasticity, emulating the critical behaviors exhibited by biological synapses. Critically, we show that this crystalline InP device can be directly integrated via back-end processing on a Si wafer using a SiO2 buffer without the need for a crystalline seed, enabling neuromorphic devices that can be implemented in a scalable and 3-D architecture. Specifically, the device is a crystalline InP channel field-effect transistor that interacts with neuron spikes by modification of the population of filled traps in the MOS structure itself. Unlike other transistor-based implementations, we show that it is possible to mimic these biological functions without the use of external factors (e.g., surface adsorption of gas molecules) and without the need for the high electric fields necessary for traditional flash-based implementations. Finally, when exposed to neuronal spikes with a waveform similar to that observed in the brain, these devices exhibit the ability to learn without the need for any external potentiating/depressing circuits, mimicking the biological process of Hebbian learning.

The role of TiO2 thin films deposited by atomic layer deposition on p-InP photocathodes used for solar hydrogen generation was examined. It was found that, in addition to its previously reported corrosion protection role, the large valence band offset between TiO2 and InP creates an energy barrier for holes reaching the surface. Also, the conduction band of TiO2 is well-aligned with that of InP. The combination of these two effects creates an electron- selective contact with low interface recombination. Under simulated solar illumination in HClO4 aqueous electrolyte, an onset potential of >800 mV vs RHE was achieved, which is the highest yet reported for an InP photocathode.

In this study, we develop an approach to characterize the surface and bulk properties for thin films of photovoltaic materials by combining two experimental photoluminescence (PL) techniques with one multiphysics simulation. This contactless, in-line characterization technique allows reliable extraction of key lifetime parameters. In this study, we first discuss the strengths and weaknesses of both steady-state and transient PL techniques (specifically, steady-state PL excitation spectroscopy and time-resolved PL) and show that combining them with numerical simulation can be used to extract surface and bulk lifetimes self consistently. The method is applied to InP thin films grown with a novel vapor-liquid-solid method. The InP thin film tested is found to have a bulk Shockley-Read-Hall (SRH) lifetime of 12 ns and a front surface recombination velocity of 5×10 4 cm/s.

Photocathodes exhibiting simultaneous high quantum efficiency, low mean transverse energy (MTE), and fast temporal response are critical for next generation electron sources. Currently, caesiated negative electron affinity GaAs photocathodes have demonstrated good overall results [Bell and Spicer, Proc. IEEE 58, 1788 (1970); Pierce et al., Appl. Phys. Lett. 26, 670 (1975)]. However, due to the nature of the photoemission process and the details of the Cs surface structure, a tradeoff exists. A low mean transverse energy of ∼25 meV can be obtained by using photons with near bandgap energy, at the cost of an unacceptably high response time, or higher energy photons can be used with a mean transverse energy of ∼60 meV with acceptable response times of 2–5 ps [Karkare et al., J. Appl. Phys. 113, 104904 (2013); Honda et al., Jpn. J. Appl. Phys. 52, 086401 (2013); Pastuszka et al. Appl. Phys. Lett. 71, 2967 (1997)]. Here, it is shown through a calibrated simulation that a thin layer of caesiated GaAs on a waveguide can potentially exhibit photoemission with MTEs ∼30 meV, ultrafast response times of ∼0.2–1 ps, and quantum efficiency of 1%–10%, breaking the traditional tradeoffs associated with bulk negative electron affinity photoemitters.

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TM...

A method for growth of ultralarge grain (>100 μm) semiconductor thin-films on nonepitaxial substrates was developed via the thin-film vapor–liquid–solid growth mode. The resulting polycrystalline films exhibit similar optoelectronic quality as their single-crystal counterparts. Here, deterministic control of nucleation sites is presented by substrate engineering, enabling user-tuned internuclei spacing of up to ∼1 mm. Besides examining the theory associated with the nucleation process, this work presents an important advance toward controlled growth of high quality semiconductor thin films with unprecedented grain sizes on nonepitaxial substrates.

Surface charge transfer is presented as an effective doping technique for III–V nanostructures. We generalize that the technique is applicable to nanoscale semiconductors in the limit where carriers are quantum confined. As a proof-of-concept, potassium surface charge transfer doping is carried out for one-dimensional (1D) and two-dimensional (2D) InAs on Si/SiO2 substrates. Experiments and simulations show that equivalent dopant areal dose of up to ∼2 × 1012 cm–2 is obtained, which is sufficient for degenerate doping of InAs nanostructures. This work presents a new pathway for controllable doping of inorganic semiconductors with limits fundamentally different from those of substitutional doping.

Ballistic transport of electrons at room temperature in top-gated InAs nanowire (NW) transistors is experimentally observed and theoretically examined. From length dependent studies, the low-field mean free path is directly extracted as ~150 nm. The mean free path is found to be independent of temperature due to the dominant role of surface roughness scattering. The mean free path was also theoretically assessed by a method that combines Fermi's golden rule and a numerical Schrödinger-Poisson simulation to determine the surface scattering potential with the theoretical calculations being consistent with experiments. Near ballistic transport (~80% of the ballistic limit) is demonstrated experimentally for transistors with a channel length of ~60 nm, owing to the long mean free path of electrons in InAs NWs.

Here we propose a novel scheme based on recent advances in integrated photonics for highly efficient modulation of the temperature of a thermionic emitter with photons. In the proposed scheme, we evanescently couple thermionic emitters to optical cavities. This method enables efficient coupling between photons in the cavity and thermionic emitter. Specifically, this enables order of magnitude enhancements in absorption as compared to the free space coupling. Furthermore, here, we present device designs and numerical results that demonstrate devices with (i) transient responses in the picosecond time scale, and (ii) steady state devices with ΔT∼1700K utilizing

Water splitting by using sunlight for the production of hydrogen yields a storable product, which can be used as a fuel. There is considerable research into H2 generation, namely the reduction of protons to H2 in aqueous solution using semiconductor photocathodes. To maximize the photoelectrochemical (PEC) performance, the selection of the active materials and device configurations should be carefully considered. First, the short-circuit current density (Jsc) should be maximized by choosing materials with high optical absorption coefficients and low carrier recombination rates, both in the bulk and at the surface. The reflectance should be minimized by using surface nanotexturing to further improve light absorption. The onset potential (Eos) of the PEC device versus the reversible H /H2 redox potential should be maximized. Finally, the surface energy needs to be controlled to minimize the accumulation of gas bubbles on the surface of the photoelectrode. Light absorbers with band gaps in the range of 1.1–1.7 eV provide both a good match to the terrestrial solar spectrum and a significant fraction of the 1.23 eV free energy required to split water. Overpotentials associated with the electron transfer to (solvated) protons in aqueous solution should be minimized by improving carrier transport from semiconductor to electrolyte by decorating the semiconductor with cocatalysts, tuning band edges, and decreasing contact resistance. p-Type Si has been extensively investigated as a photocathode for photochemical hydrogen production. Planar Si has relatively low short-circuit current densities under AM1.5 G illumination, approximately 10 mAcm 2 (reference [9]), compared to what can be achieved in a pn junction solar cell (> 35 mAcm ). Nanostructuring and incorporation of cocatalysts have been used to raise the short-circuit current density to over 30 mAcm . A recent study using np Si radial junction microwires reported an Eos value of 0.54 V and an Jsc value of 15 mA, leading to an overall efficiency near 6%. The onset potential observed to date for p-Si photocathodes is less than half of the value required for overall water splitting (1.23 V). This low onset potential limits the performance of tandem or “Z-scheme” approaches, which would function without external bias, as it limits the potential overlap required for spontaneous water splitting. An ideal photocathode for use in a solar-driven hydrogen production system without bias should have both a high current density and a favorable open-circuit potential versus the reversible H/H2 redox couple. Herein, we employ nanotextured p-InP photocathodes in conjunction with a TiO2 passivation layer and a Ru cocatalyst to increase both Jsc and Eos values under H2 evolution conditions. InP has a number of attractive attributes as a photocathode: 1) Its band gap of 1.3 eV is well-matched to the solar spectrum; InP-based solar cells have achieved AM1.5 G efficiencies of up to 22%. 2) The conduction band edge of InP is slightly above the water reduction potential, thus electron transfer is favorable in this system. 3) The surface-recombination velocity of untreated InP is low (ca. 10 cms 1 for n-type and 10 cms 1 for p-type), which is particularly important for nonplanar devices with high surface areas, such as those explored in this study. For these reasons, InP has been studied previously as a photocathode for both water splitting and CO2 reduction. [18–20] Specifically, Heller and Vadimsky reported attractive PEC performances with current densities up to 28 mAcm 2 and conversion efficiencies of approximately 12% in InP photocathodes. Motivated by these results, we use InP as a model material system to elucidate the role of surface nanotexturing on the PEC device performance. We find that nanotextured InP photocathodes exhibit drastically enhanced performances compared to our planar cells that were processed using identical conditions. We examine the various effects of nanotexturing [*] M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, J. W. Ager, Prof. A. Javey Material Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: jwager@lbl.gov ajavey@berkeley.edu M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, Prof. A. Javey Electrical Engineering and Computer Sciences University of California, Berkeley, CA 94720 (USA) M. H. Lee, T. S. Matthews, J. W. Ager, Prof. A. Javey Joint Center for Artificial Photosynthesis Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)