Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy.

Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

Complications in silane-assisted GaN nanowire growth.

Understanding the growth mechanisms of III-nitride nanowires is of great importance to realise their full potential. We present a systematic study of silane-assisted GaN nanowire growth on c-sapphire substrates by investigating the surface evolution of the sapphire substrates during the high temperature annealing, nitridation and nucleation steps, and the growth of GaN nanowires. The nucleation step - which transforms the AlN layer formed during the nitridation step to AlGaN - is critical for subsequent silane-assisted GaN nanowire growth. Both Ga-polar and N-polar GaN nanowires were grown with N-polar nanowires growing much faster than the Ga-polar nanowires. On the top surface of the N-polar GaN nanowires protuberance structures were found, which relates to the presence of Ga-polar domains within the nanowires. Detailed morphology studies revealed ring-like features concentric with the protuberance structures, indicating energetically favourable nucleation sites at inversion domain boundaries. Cathodoluminescence studies showed quenching of emission intensity at the protuberance structures, but the impact is limited to the protuberance structure area only and does not extend to the surrounding areas. Hence it should minimally affect the performance of devices whose functions are based on radial heterostructures, suggesting that radial heterostructures remain a promising device structure.

Automated Computer Vision-Enabled Manufacturing of Nanowire Devices.

We present a high-throughput method for identifying and characterizing individual nanowires and for automatically designing electrode patterns with high alignment accuracy. Central to our method is an optimized machine-readable, lithographically processable, and multi-scale fiducial marker system─dubbed LithoTag─which provides nanostructure position determination at the nanometer scale. A grid of uniquely defined LithoTag markers patterned across a substrate enables image alignment and mapping in 100% of a set of >9000 scanning electron microscopy (SEM) images (>7 gigapixels). Combining this automated SEM imaging with a computer vision algorithm yields location and property data for individual nanowires. Starting with a random arrangement of individual InAs nanowires with diameters of 30 ± 5 nm on a single chip, we automatically design and fabricate >200 single-nanowire devices. For >75% of devices, the positioning accuracy of the fabricated electrodes is within 2 pixels of the original microscopy image resolution. The presented LithoTag method enables automation of nanodevice processing and is agnostic to microscopy modality and nanostructure type. Such high-throughput experimental methodology coupled with data-extensive science can help overcome the characterization bottleneck and improve the yield of nanodevice fabrication, driving the development and applications of nanostructured materials.

Floating perovskite-BiVO4 devices for scalable solar fuel production.

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability1, whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H2 g-1 h-1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attained a high CO:H2 selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm2 stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm2 counterparts. Bubbles formed under operation further enabled 30-100 mg cm-2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use.

Transparent Quasi-Random Structures for Multimodal Light Trapping in Ultrathin Solar Cells with Broad Engineering Tolerance.

Waveguide modes are well-known to be a valuable light-trapping resource for absorption enhancement in solar cells. However, their scarcity in the thinnest device stacks compromises the multiresonant performance required to reach the highest efficiencies in ultrathin devices. We demonstrate that enriching the modal structure on such reduced length-scales is possible by integrating transparent semiconductor/dielectric scattering structures to the device architecture as opposed to more widely studied metallic textures. This phenomenon allows transparent quasi-random structures to emerge as strong light-trapping candidates for ultrathin solar cells, given that their broad scattering profiles are well-suited to exploit the increased number of waveguide modes for multiresonant absorption enhancement. A thorough study of the design space of quasi-random textures comprising more than 1500 designs confirms the superiority of transparent structures over a metallic embodiment, identifies broad and flexible design requirements to achieve optimal performances, and demonstrates photon harvesting capabilities leading to 20% efficiency with an 80 nm GaAs absorber. Our light-trapping strategy can be applied to a wide range of material systems and device architectures, is compatible with scalable low-cost fabrication techniques, and can assist current trends to reach the highest efficiencies in ever-thinner photovoltaics.

Long-Term Stability and Optoelectronic Performance Enhancement of InAsP Nanowires with an Ultrathin InP Passivation Layer.

The influence of nanowire (NW) surface states increases rapidly with the reduction of diameter and hence severely degrades the optoelectronic performance of narrow-diameter NWs. Surface passivation is therefore critical, but it is challenging to achieve long-term effective passivation without significantly affecting other qualities. Here, we demonstrate that an ultrathin InP passivation layer of 2-3 nm can effectively solve these challenges. For InAsP nanowires with small diameters of 30-40 nm, the ultrathin passivation layer reduces the surface recombination velocity by at least 70% and increases the charge carrier lifetime by a factor of 3. These improvements are maintained even after storing the samples in ambient atmosphere for over 3 years. This passivation also greatly improves the performance thermal tolerance of these thin NWs and extends their operating temperature from <150 K to room temperature. This study provides a new route toward high-performance room-temperature narrow-diameter NW devices with long-term stability.

Giant Magnetoresistance in a Chemical Vapor Deposition Graphene Constriction.

Magnetic field-driven insulating states in graphene are associated with samples of very high quality. Here, this state is shown to exist in monolayer graphene grown by chemical vapor deposition (CVD) and wet transferred on Al2O3 without encapsulation with hexagonal boron nitride (h-BN) or other specialized fabrication techniques associated with superior devices. Two-terminal measurements are performed at low temperature using a GaAs-based multiplexer. During high-throughput testing, insulating properties are found in a 10 µm long graphene device which is 10 µm wide at one contact with an ≈440 nm wide constriction at the other. The low magnetic field mobility is ≈6000 cm2 V-1 s-1. An energy gap induced by the magnetic field opens at charge neutrality, leading to diverging resistance and current switching on the order of 104 with DC bias voltage at an approximate electric field strength of ≈0.04 V µm-1 at high magnetic field. DC source-drain bias measurements show behavior associated with tunneling through a potential barrier and a transition between direct tunneling at low bias to Fowler-Nordheim tunneling at high bias from which the tunneling region is estimated to be on the order of ≈100 nm. Transport becomes activated with temperature from which the gap size is estimated to be 2.4 to 2.8 meV at B = 10 T. Results suggest that a local electronically high quality region exists within the constriction, which dominates transport at high B, causing the device to become insulating and act as a tunnel junction. The use of wet transfer fabrication techniques of CVD material without encapsulation with h-BN and the combination with multiplexing illustrates the convenience of these scalable and reasonably simple methods to find high quality devices for fundamental physics research and with functional properties.

Active Terahertz Modulator and Slow Light Metamaterial Devices with Hybrid Graphene-Superconductor Photonic Integrated Circuits.

Metamaterial photonic integrated circuits with arrays of hybrid graphene-superconductor coupled split-ring resonators (SRR) capable of modulating and slowing down terahertz (THz) light are introduced and proposed. The hybrid device's optical responses, such as electromagnetic-induced transparency (EIT) and group delay, can be modulated in several ways. First, it is modulated electrically by changing the conductivity and carrier concentrations in graphene. Alternatively, the optical response can be modified by acting on the device temperature sensitivity by switching Nb from a lossy normal phase to a low-loss quantum mechanical phase below the transition temperature (Tc) of Nb. Maximum modulation depths of 57.3% and 97.61% are achieved for EIT and group delay at the THz transmission window, respectively. A comparison is carried out between the Nb-graphene-Nb coupled SRR-based devices with those of Au-graphene-Au SRRs, and significant enhancements of the THz transmission, group delay, and EIT responses are observed when Nb is in the quantum mechanical phase. Such hybrid devices with their reasonably large and tunable slow light bandwidth pave the way for the realization of active optoelectronic modulators, filters, phase shifters, and slow light devices for applications in chip-scale future communication and computation systems.

Millimeter-Wave-to-Terahertz Superconducting Plasmonic Waveguides for Integrated Nanophotonics at Cryogenic Temperatures.

Plasmonics, as a rapidly growing research field, provides new pathways to guide and modulate highly confined light in the microwave-to-optical range of frequencies. We demonstrated a plasmonic slot waveguide, at the nanometer scale, based on the high-transition-temperature (Tc) superconductor Bi2Sr2CaCu2O8+δ (BSCCO), to facilitate the manifestation of chip-scale millimeter wave (mm-wave)-to-terahertz (THz) integrated circuitry operating at cryogenic temperatures. We investigated the effect of geometrical parameters on the modal characteristics of the BSCCO plasmonic slot waveguide between 100 and 800 GHz. In addition, we investigated the thermal sensing of the modal characteristics of the nanoscale superconducting slot waveguide and showed that, at a lower frequency, the fundamental mode of the waveguide had a larger propagation length, a lower effective refractive index, and a strongly localized modal energy. Moreover, we found that our device offered a larger SPP propagation length and higher field confinement than the gold plasmonic waveguides at broad temperature ranges below BSCCO's Tc. The proposed device can provide a new route toward realizing cryogenic low-loss photonic integrated circuitry at the nanoscale.

Facet-Related Non-uniform Photoluminescence in Passivated GaAs Nanowires.

The semiconductor nanowire architecture provides opportunities for non-planar electronics and optoelectronics arising from its unique geometry. This structure gives rise to a large surface area-to-volume ratio and therefore understanding the effect of nanowire surfaces on nanowire optoelectronic properties is necessary for engineering related devices. We present a systematic study of the non-uniform optical properties of Au-catalyzed GaAs/AlGaAs core-shell nanowires introduced by changes in the sidewall faceting. Significant variation in intra-wire photoluminescence (PL) intensity and PL lifetime (τ PL ) was observed along the nanowire axis, which was strongly correlated with the variation of sidewall facets from {112} to {110} from base to tip. Faster recombination occurred in the vicinity of {112}-oriented GaAs/AlGaAs interfaces. An alternative nanowire heterostructure, the radial quantum well tube consisting of a GaAs layer sandwiched between two AlGaAs barrier layers, is proposed and demonstrates superior uniformity of PL emission along the entire length of nanowires. The results emphasize the significance of nanowire facets and provide important insights for nanowire device design.

Light management in ultra-thin solar cells: a guided optimisation approach.

The success of ever-thinner photovoltaics relies on the introduction of light management strategies to enhance the absorption of incident illumination. Tailoring these strategies to maximise the absorption of light requires optimising the complex interplay between multiple design parameters. We study this interplay with a transfer matrix method and rigorous coupled-wave analysis, within the context of waveguide modes in an ultra-thin (80 nm) GaAs solar cell. Based on this study, we develop a framework for light management optimisation which is guided by the underlying optical phenomena that determine the most favourable design parameters. In contrast to other optimisation approaches which exhaustively simulate multiple parameter combinations looking for the highest integrated absorption, our framework reduces the parameter space for optimisation, furthers our fundamental understanding of light management and is applicable to multiple length-scales and device architectures. We demonstrate the power of our framework by using it to compare the light trapping performance of photonic crystal gratings to that of engineered quasi-random structures, finding that photonic crystal gratings offer a superior performance in our device of interest.

High-Throughput Electrical Characterization of Nanomaterials from Room to Cryogenic Temperatures.

We present multiplexer methodology and hardware for nanoelectronic device characterization. This high-throughput and scalable approach to testing large arrays of nanodevices operates from room temperature to milli-Kelvin temperatures and is universally compatible with different materials and integration techniques. We demonstrate the applicability of our approach on two archetypal nanomaterials-graphene and semiconductor nanowires-integrated with a GaAs-based multiplexer using wet or dry transfer methods. A graphene film grown by chemical vapor deposition is transferred and patterned into an array of individual devices, achieving 94% yield. Device performance is evaluated using data fitting methods to obtain electrical transport metrics, showing mobilities comparable to nonmultiplexed devices fabricated on oxide substrates using wet transfer techniques. Separate arrays of indium-arsenide nanowires and micromechanically exfoliated monolayer graphene flakes are transferred using pick-and-place techniques. For the nanowire array mean values for mobility µFE = 880/3180 cm2 V-1 s-1 (lower/upper bound), subthreshold swing 430 mV dec-1, and on/off ratio 3.1 decades are extracted, similar to nonmultiplexed devices. In another array, eight mechanically exfoliated graphene flakes are transferred using techniques compatible with fabrication of two-dimensional superlattices, with 75% yield. Our results are a proof-of-concept demonstration of a versatile platform for scalable fabrication and cryogenic characterization of nanomaterial device arrays, which is compatible with a broad range of nanomaterials, transfer techniques, and device integration strategies from the forefront of quantum technology research.

Improving holographic search algorithms using sorted pixel selection.

Traditional search algorithms for computer hologram generation such as Direct Search and Simulated Annealing offer some of the best hologram qualities at convergence when compared to rival approaches. Their slow generation times and high processing power requirements mean, however, that they see little use in performance critical applications. This paper presents the novel sorted pixel selection (SPS) modification for holographic search algorithms that offers mean square error reductions in the range of 14.7-19.2% for the test images used. SPS operates by substituting a weighted search selection procedure for traditional random pixel selection processes. While small, the improvements seen are observed consistently across a wide range of test cases and require limited overhead for implementation.

Scalable Quantum Integrated Circuits on Superconducting Two-Dimensional Electron Gas Platform.

To form a coherent quantum transport in hybrid superconductor-semiconductor (S-Sm) junctions, the formation of a homogeneous and barrier-free interface between two different materials is necessary. The S-Sm junction with high interface transparency will then facilitate the observation of the induced hard superconducting gap, which is the key requirement to access the topological phases (TPs) and observation of exotic quasiparticles such as Majorana zero modes (MZM) in hybrid systems. A material platform that can support observation of TPs and allows the realization of complex and branched geometries is therefore highly demanding in quantum processing and computing science and technology. Here, we introduce a two-dimensional material system and study the proximity induced superconductivity in semiconducting two-dimensional electron gas (2DEG) that is the basis of a hybrid quantum integrated circuit (QIC). The 2DEG is a 30 nm thick In0.75Ga0.25As quantum well that is buried between two In0.75Al0.25As barriers in a heterostructure. Niobium (Nb) films are used as the superconducting electrodes to form Nb- In0.75Ga0.25As -Nb Josephson junctions (JJs) that are symmetric, planar and ballistic. Two different approaches were used to form the JJs and QICs. The long junctions were fabricated photolithographically, but e-beam lithography was used for short junctions' fabrication. The coherent quantum transport measurements as a function of temperature in the presence/absence of magnetic field B are discussed. In both device fabrication approaches, the proximity induced superconducting properties were observed in the In0.75Ga0.25As 2DEG. It was found that e-beam lithographically patterned JJs of shorter lengths result in observation of induced superconducting gap at much higher temperature ranges. The results that are reproducible and clean suggesting that the hybrid 2D JJs and QICs based on In0.75Ga0.25As quantum wells could be a promising material platform to realize the real complex and scalable electronic and photonic quantum circuitry and devices.

Engineering the Photoresponse of InAs Nanowires.

We report on individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either negative or positive photoconductivity (NPC or PPC). The NPC photoresponse time and magnitude is found to be highly tunable by varying the nanowire diameter under controlled growth conditions. Using hysteresis characterization, we decouple the observed photoexcitation-induced hot electron trapping from conventional electric field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Furthermore, we demonstrate surface passivation without chemical etching which both enhances the field-effect mobility of the nanowires by approximately an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an "intrinsic" positive photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications.

On-Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb-In0.75 Ga0.25 As-Quantum-Well-Nb Josephson Junctions.

A superconducting hard gap in hybrid superconductor-semiconductor devices has been found to be necessary to access topological superconductivity that hosts Majorana modes (non-Abelian excitation). This requires the formation of homogeneous and barrier-free interfaces between the superconductor and semiconductor. Here, a new platform is reported for topological superconductivity based on hybrid Nb-In0.75 Ga0.25 As-quantum-well-Nb that results in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions. It is shown that with careful etching, sputtered Nb films can make high-quality and transparent contacts to the In0.75 Ga0.25 As quantum well, and the differential resistance and critical current measurements of these devices are discussed as a function of temperature and magnetic field. It is demonstrated that proximity-induced superconductivity in the In0.75 Ga0.25 As-quantum-well 2D electron gas results in the detection of a hard gap in four out of seven junctions on a chip with critical current values of up to 0.2 µA and transmission probabilities of >0.96. The results, together with the large g-factor and Rashba spin-orbit coupling in In0.75 Ga0.25 As quantum wells, which indeed can be tuned by the indium composition, suggest that the Nb-In0.75 Ga0.25 As-Nb system can be an excellent candidate to achieve topological phase and to realize hybrid topological superconducting devices.

An Ultrafast Switchable Terahertz Polarization Modulator Based on III-V Semiconductor Nanowires.

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.

Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping.

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Single nanowire photoconductive terahertz detectors.

Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening, and nondestructive testing. However, compact optoelectronics suited to the most powerful terahertz technique, time-domain spectroscopy, are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus, nanowires are shown to be well suited for THz device applications and hold particular promise as near-field THz sensors.