Effective Surface Passivation of InP Nanowires by Atomic-Layer-Deposited Al2O3 with POx Interlayer

Low Surface Recombination in Hexagonal SiGe Alloy Nanowires: Implications for SiGe-Based Nanolasers

Monolithic integration of silicon-based electronics and photonics could open the door toward many opportunities including on-chip optical data communication and large-scale application of light-based sensing devices in healthcare and automotive; by some, it is considered the Holy Grail of silicon photonics. The monolithic integration is, however, severely hampered by the inability of Si to efficiently emit light. Recently, important progress has been made by the demonstration of efficient light emission from direct-bandgap hexagonal SiGe (hex-SiGe) alloy nanowires. For this promising material, realized by employing a nanowire structure, many challenges and open questions remain before a large-scale application can be realized. Considering that for other direct-bandgap materials like GaAs, surface recombination can be a true bottleneck, one of the open questions is the importance of surface recombination for the photoluminescence efficiency of this new material. In this work, temperature-dependent photoluminescence measurements were performed on both hex-Ge and hex-SiGe nanowires with and without surface passivation schemes that have been well documented and proven effective on cubic silicon and germanium to elucidate whether and to what extent the internal quantum efficiency (IQE) of the wires can be improved. Additionally, time-resolved photoluminescence (TRPL) measurements were performed on unpassivated hex-SiGe nanowires as a function of their diameter. The dependence of the surface recombination on the SiGe composition could, however, not be yet addressed given the sample-to-sample variations of the state-of-the-art hex-SiGe nanowires. With the aforementioned experiments, we demonstrate that at room temperature, under high excitation conditions (a few kW cm-2), the hex-(Si)Ge surface is most likely not a bottleneck for efficient radiative emission under relatively high excitation conditions. This is an important asset for future hex(Si)Ge optoelectronic devices, specifically for nanolasers.

Vertically Processed GaInP/InP Tandem-Junction Nanowire Solar Cells

We present vertically processed photovoltaic devices based on GaInP/InP tandem-junction III-V nanowires (NWs), contacting approximately 3 million NWs in parallel for each device. The GaInP and InP subcells as well as the connecting Esaki tunnel diode are all realized within the same NW. By processing GaInP/InP tandem-junction NW solar cells with varying compositions of the top junction GaInP material, we investigate the impact of the GaInP composition on the device performance. External quantum efficiency (EQE) measurements on devices with varying GaInP composition provide insights into the performance of the respective subcells, revealing that the GaInP subcell is current-limiting for all devices. I-V measurements under AM1.5G illumination confirm voltage addition of the subcells, resulting in an open-circuit voltage of up to 1.91 V. However, the short-circuit current density is low, ranging between 0.24 and 3.44 mA/cm2, which leads to a resulting solar conversion efficiency of up to 3.60%. Our work shows a path forward toward high-efficiency NW photovoltaics and identifies critical issues that need improvement.

Interface Optimization and Performance Enhancement of Er2O3-Based MOS Devices by ALD-Derived Al2O3 Passivation Layers and Annealing Treatment

In this paper, the effect of atomic layer deposition (ALD)-derived Al₂O₃ passivation layers and annealing temperatures on the interfacial chemistry and transport properties of sputtering-deposited Er₂O₃ high-k gate dielectrics on Si substrate has been investigated. X-ray photoelectron spectroscopy (XPS) analyses have showed that the ALD-derived Al₂O₃ passivation layer remarkably prevents the formation of the low-k hydroxides generated by moisture absorption of the gate oxide and greatly optimizes the gate dielectric properties. Electrical performance measurements of metal oxide semiconductor (MOS) capacitors with different gate stack order have revealed that the lowest leakage current density of 4.57 × 10^-9 A/cm² and the smallest interfacial density of states (Dit) of 2.38 × 10¹² cm^-2 eV^-1 have been achieved in the Al₂O_3/Er₂O₃/Si MOS capacitor, which can be attributed to the optimized interface chemistry. Further electrical measurements of annealed Al₂O_3/Er₂O₃/Si gate stacks at 450 °C have demonstrated superior dielectric properties with a leakage current density of 1.38 × 10^-9 A/cm². At the same, the leakage current conduction mechanism of MOS devices under various stack structures is systematically investigated.

Enhanced Stability and Catalytic Performance of Active Rh Sites on Al2O3 Via Atomic Layer Deposited ZrO2

Modulating the Rh active sites on surfaces of Al₂O₃ is crucial to developing effective three-way catalysts. Herein, an ultralow amount of ZrO₂ (0.0179%) was deposited onto Al₂O₃ nanorods via atomic layer deposition (ALD) to form a catalyst with both thermal stability and low-temperature activity. The results demonstrate that the ALD-ZrO₂ is conducive to improve the catalytic activity of the Rh site and inhibit the formation of irreducible Rh species at high temperature. The obtained catalysts show satisfactory performance for a model NO-CO reaction even after thermal aging at 1050 °C. This strategy shows that a molecularly precise synthesis can lead to the robust promotion of Rh activity under low temperature and provide a promising path toward reducing the deactivation of catalysts at high temperature.

Passivation efficacy study of Al2O3dielectric on self-catalyzed molecular beam epitaxially grown GaAs1-xSbxnanowires

This work evaluates the passivation efficacy of thermal atomic layer deposited (ALD) Al₂O₃dielectric layer on self-catalyzed GaAs_1-xSb_xnanowires (NWs) grown using molecular beam epitaxy. A detailed assessment of surface chemical composition and optical properties of Al₂O₃passivated NWs with and without prior sulfur treatment were studied and compared to as-grown samples using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature photoluminescence (PL) spectroscopy. The XPS measurements reveal that prior sulfur treatment followed by Al₂O₃ALD deposition abates III-V native oxides from the NW surface. However, the degradation in 4K-PL intensity by an order of magnitude observed for NWs with Al₂O₃shell layer compared to the as-grown NWs, irrespective of prior sulfur treatment, suggests the formation of defect states at the NW/dielectric interface contributing to non-radiative recombination centers. This is corroborated by the Raman spectral broadening of LO and TO Raman modes, increased background scattering, and redshift observed for Al₂O₃deposited NWs relative to the as-grown. Thus, our work seems to indicate the unsuitability of ALD deposited Al₂O₃as a passivation layer for GaAsSb NWs.

InAsP Quantum Dot-Embedded InP Nanowires toward Silicon Photonic Applications

Quantum dot (QD) emitters on silicon platforms have been considered as a fascinating approach to building next-generation quantum light sources toward unbreakable secure communications. However, it has been challenging to integrate position-controlled QDs operating at the telecom band, which is a crucial requirement for practical applications. Here, we report monolithically integrated InAsP QDs embedded in InP nanowires on silicon. The positions of QD nanowires are predetermined by the lithography of gold catalysts, and the 3D geometry of nanowire heterostructures is precisely controlled. The InAsP QD forms atomically sharp interfaces with surrounding InP nanowires, which is in situ passivated by InP shells. The linewidths of the excitonic (X) and biexcitonic (XX) emissions from the QD and their power-dependent peak intensities reveal that the proposed QD-in-nanowire structure could be utilized as a non-classical light source that operates at silicon-transparent wavelengths, showing a great potential for diverse quantum optical and silicon photonic applications.

PO x /Al2O3 Stacks for c-Si Surface Passivation: Material and Interface Properties

Phosphorus oxide (PO _x ) capped by aluminum oxide (Al₂O₃) has recently been discovered to provide excellent surface passivation of crystalline silicon (c-Si). In this work, insights into the passivation mechanism of PO _x /Al₂O₃ stacks are gained through a systematic study of the influence of deposition temperature (T _dep = 100-300 °C) and annealing temperature (T _ann = 200-500 °C) on the material and interface properties. It is found that employing lower deposition temperatures enables an improved passivation quality after annealing. Bulk composition, density, and optical properties vary only slightly with deposition temperature, but bonding configurations are found to be sensitive to temperature and correlated with the interface defect density (D _it), which is reduced at lower deposition temperature. The fixed charge density (Q _f) is in the range of + (3-9) × 10¹² cm^-2 and is not significantly altered by annealing, which indicates that the positively charged entities are generated during deposition. In contrast, D _it decreases by 3 orders of magnitude (∼10¹³ to ∼10¹⁰ eV^-1 cm^-2) upon annealing. This excellent chemical passivation is found to be related to surface passivation provided by hydrogen, and mixing of aluminum into the PO _x layer, leading to the formation of AlPO₄ upon annealing.

In situpassivation of GaxIn(1-x)P nanowires using radial AlyIn(1-y)P shells grown by MOVPE

Ga_xIn_(1-x)P nanowires with suitable bandgap (1.35-2.26 eV) ranging from the visible to near-infrared wavelength have great potential in optoelectronic applications. Due to the large surface-to-volume ratio of nanowires, the surface states become a pronounced factor affecting device performance. In this work, we performed a systematic study of Ga_xIn_(1-x)P nanowires' surface passivation, utilizing Al_yIn_(1-y)P shells grownin situby using a metal-organic vapor phase epitaxy system. Time-resolved photoinduced luminescence and time-resolved THz spectroscopy measurements were performed to study the nanowires' carrier recombination processes. Compared to the bare Ga_0.41In_0.59P nanowires without shells, the hole and electron lifetime of the nanowires with the Al_0.36In_0.64P shells are found to be larger by 40 and 1.1 times, respectively, demonstrating effective surface passivation of trap states. When shells with higher Al composition were grown, both lifetimes of free holes and electrons decreased prominently. We attribute the acceleration of PL decay to an increase in the trap states' density due to the formation of defects, including the polycrystalline and oxidized amorphous areas in these samples. Furthermore, in a separate set of samples, we varied the shell thickness. We observed that a certain shell thickness of approximately ∼20 nm is needed for efficient passivation of Ga_0.31In_0.69P nanowires. The photoconductivity of the sample with a shell thickness of 23 nm decays 10 times slower compared with that of the bare core nanowires. We concluded that both the hole and electron trapping and the overall charge recombination in Ga_xIn_(1-x)P nanowires can be substantially passivated through growing an Al_yIn_(1-y)P shell with appropriate Al composition and thickness. Therefore, we have developed an effectivein situsurface passivation of Ga_xIn_(1-x)P nanowires by use of Al_yIn_(1-y)P shells, paving the way to high-performance Ga_xIn_(1-x)P nanowires optoelectronic devices.

Designing a Transparent CdIn2 S4 /In2 S3 Bulk-Heterojunction Photoanode Integrated with a Perovskite Solar Cell for Unbiased Water Splitting

The integration of photoelectrochemical photoanodes and solar cells to build an unbiased solar-to-hydrogen (STH) conversion system provides a promising way to solve the energy crisis. The key point is to develop highly transparent photoanodes, while its bulk separation efficiency (η_sep. ) and surface injection efficiency are as high as possible. To resolve this contradiction, first a novel CdIn₂ S₄ /In₂ S₃ bulk heterojunctions in the interior of nanosheets is designed as a photoanode with high transparency and an ultrahigh η_sep. up to 90%. Furthermore, decorating the ultrathin amorphous SnO₂ layer by atomic layer deposition, the surface oxygen-evolution kinetics of the photoanode are increased significantly. As a result, the onset potential of the photoanode shifts negatively to 0.02 V vs RHE, and the photocurrent density boosts to 2.98 mA cm^-2 at 1.23 V vs RHE, which is ten times higher than that of pristine CdIn₂ S₄ . Such a high-performance photoanode enables the integrated metal sulfide photoanode-perovskite solar cell system to deliver a STH conversion efficiency of 3.3%.

Carrier dynamics and recombination mechanisms in InP twinning superlattice nanowires

Nominal dopant-free zinc blende twinning superlattice InP nanowires have been grown with high crystal-quality and taper-free morphology. Here, we demonstrate its superior optical performance and clarify the different carrier recombination mechanisms at different temperatures using a time resolved photoluminescence study. The existence of regular twin planes and lateral overgrowth do not significantly increase the defect density. At room temperature, the as-grown InP nanowires have a strong emission at 1.348 eV and long minority carrier lifetime (∼3 ns). The carrier recombination dynamics is mainly dominated by nonradiative recombination due to surface trapping states; a wet chemical etch to reduce the surface trapping density thus boosts the emission intensity and increases the carrier lifetime to 7.1 ns. This nonradiative recombination mechanism dominates for temperatures above 155 K, and the carrier lifetime decreases with increasing temperature. However, radiative recombination dominates the carrier dynamics at temperature below ∼75 K, and a strong donor-bound exciton emission with a narrow emission linewidth of 4.5 meV is observed. Consequently, carrier lifetime increases with temperature. By revealing carrier recombination mechanisms over the temperature range 10-300 K, we demonstrate the attraction of using InP nanostructure for photonics and optoelectronic applications.

Detecting Ferric Iron by Microalgal Residue-Derived Fluorescent Nanosensor with an Advanced Kinetic Model

Biomass-derived carbon quantum dots (CQDs) are attractive to serve as fluorescent nanosensors owing to their superior environmental compatibility and biocompatibility. However, the detection range has been limited, only in partial agreement with the experimental data. Thus, an advanced kinetic model for quantifying the fluorescence quenching over a wide range is on demand. Here, we describe a nanosensor for Fe(Ⅲ) detection in real waters, which is developed via microalgal residue-derived CQDs with an advanced kinetic model. The multiple-order kinetic model is established to resolve the incoherence of previous models and unveil the entire quenching kinetics. The results show that the detection range of Fe(Ⅲ) can reach up to 10 mM in the high detection end. The newly obtained kinetic model exhibits satisfactory fittings, clearly elucidating a dynamic quenching mechanism. This work provides a new insight into CQDs-based detection of heavy metals in real water samples by establishing an innovative multiple-order kinetic model.

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Microalgal residue-derived carbon dots synthesized by hydrothermal method are introduced

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An advanced kinetic model with wide concentration applicability is developed

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Waste biomass-derived Fe(Ⅲ) nanosensor is applied in accurate detection of actual water

Strategy toward the fabrication of ultrahigh-resolution micro-LED displays by bonding-interface-engineered vertical stacking and surface passivation

In this study, we proposed a strategy to fabricate vertically stacked subpixel (VSS) micro-light-emitting diodes (μ-LEDs) for future ultrahigh-resolution microdisplays. At first, to vertically stack the LED with different colors, we successfully adopted a bonding-interface-engineered monolithic integration method using SiO2/SiNx distributed Bragg reflectors (DBRs). It was found that an intermediate DBR structure could be used as the bonding layer and color filter, which could reflect and transmit desired wavelengths through the bonding interface. Furthermore, the optically pumped μ-LED array with a pitch of 0.4 μm corresponding to the ultrahigh-resolution of 63 500 PPI could be successfully fabricated using a typical semiconductor process, including electron-beam lithography. Compared with the pick-and-place strategy (limited by machine alignment accuracy), the proposed strategy leads to the fabrication of significantly improved high-density μ-LEDs. Finally, we systematically investigated the effects of surface traps using time-resolved photoluminescence (TRPL) and two-dimensional simulations. The obtained results clearly demonstrated that performance improvements could be possible by employing optimal passivation techniques by diminishing the pixel size for fabricating low-power and highly efficient microdisplays.

Facile Process for Surface Passivation Using (NH4)2S for the InP MOS Capacitor with ALD Al2O3

Ammonium sulfide ((NH₄)₂S) was used for the passivation of an InP (100) substrate and its conditions were optimized. The capacitance–voltage (C–V) characteristics of InP metal-oxide-semiconductor (MOS) capacitors were analyzed by changing the concentration of and treatment time with (NH₄)₂S. It was found that a 10% (NH₄)₂S treatment for 10 min exhibits the best electrical properties in terms of hysteresis and frequency dispersions in the depletion or accumulation mode. After the InP substrate was passivated by the optimized (NH₄)₂S, the results of x-ray photoelectron spectroscopy (XPS) and the extracted interface trap density (D_it) proved that the growth of native oxide was suppressed.

Synthesis and Applications of III-V Nanowires

Low-dimensional semiconductor materials structures, where nanowires are needle-like one-dimensional examples, have developed into one of the most intensely studied fields of science and technology. The subarea described in this review is compound semiconductor nanowires, with the materials covered limited to III-V materials (like GaAs, InAs, GaP, InP,...) and III-nitride materials (GaN, InGaN, AlGaN,...). We review the way in which several innovative synthesis methods constitute the basis for the realization of highly controlled nanowires, and we combine this perspective with one of how the different families of nanowires can contribute to applications. One reason for the very intense research in this field is motivated by what they can offer to main-stream semiconductors, by which ultrahigh performing electronic (e.g., transistors) and photonic (e.g., photovoltaics, photodetectors or LEDs) technologies can be merged with silicon and CMOS. Other important aspects, also covered in the review, deals with synthesis methods that can lead to dramatic reduction of cost of fabrication and opportunities for up-scaling to mass production methods.

Correlated optical and structural analyses of individual GaAsP/GaP core-shell nanowires

We report on the structural and optical properties of GaAs_0.7P_0.3/GaP core-shell nanowires (NWs) for future photovoltaic applications. The NWs are grown by self-catalyzed molecular beam epitaxy. Scanning transmission electron microscopy (STEM) analyses demonstrate that the GaAsP NW core develops an inverse-tapered shape with a formation of an unintentional GaAsP shell having a lower P content. Without surface passivation, this unintentional shell produces no luminescence because of strong surface recombination. However, passivation of the surface with a GaP shell leads to the appearance of a secondary peak in the luminescence spectrum arising from this unintentional shell. The attribution of the luminescence peaks is confirmed by correlated cathodoluminescence and STEM analyses of the same NW.

Uptake of nanowires by human lung adenocarcinoma cells

Semiconductor nanowires are increasingly used in optoelectronic devices. However, their effects on human health have not been assessed fully. Here, we investigate the effects of gallium phosphide nanowires on human lung adenocarcinoma cells. Four different geometries of nanowires were suspended in the cell culture for 48 hours. We show that cells internalize the nanowires and that the nanowires have no effect on cell proliferation rate, motility, viability and intracellular ROS levels. By blocking specific internalization pathways, we demonstrate that the nanowire uptake is the result of a combination of processes, requiring dynamin and actin polymerization, which suggests an internalization through macropinocytosis and phagocytosis.

Unexpected benefits of stacking faults on the electronic structure and optical emission in wurtzite GaAs/GaInP core/shell nanowires

Wurtzite (WZ) GaAs nanowires (NWs) are of considerable interest for novel optoelectronic applications, yet high quality NWs are still under development. Understanding of their polytypic crystal structure and band structure is the key to improving their emission characteristics. In this work we report that the Ga1-xInxP shell provides ideal passivation on polytypic WZ GaAs NWs, producing high quantum efficiency (up to 80%). From optical measurements, we find that the polytypic nature of the NWs which presents itself as planar defects does not reduce the emission efficiency. A hole transferring approach from the valence band of the zinc blende segments to the light hole (LH) band of the WZ phase is proposed to explain the emission enhancement from the conduction band to LH band. The emission intensity does not correlate to the minority carrier lifetime which is usually used to quantify the optical emission quality. Theoretical calculation predicted type-II band transition in polytypic WZ GaAs NWs is confirmed and presents efficient emission at low temperatures. We further demonstrate the performance of single NW photodetectors with a high photocurrent responsivity up to 65 A W-1 operating over the wavelength range from visible to near infrared.

Room-Temperature Midwavelength Infrared InAsSb Nanowire Photodetector Arrays with Al2O3 Passivation

Developing uncooled photodetectors at midwavelength infrared (MWIR) is critical for various applications including remote sensing, heat seeking, spectroscopy, and more. In this study, we demonstrate room-temperature operation of nanowire-based photodetectors at MWIR composed of vertical selective-area InAsSb nanowire photoabsorber arrays on large bandgap InP substrate with nanoscale plasmonic gratings. We accomplish this by significantly suppressing the nonradiative recombination at the InAsSb nanowire surfaces by introducing ex situ conformal Al₂O₃ passivation shells. Transient simulations estimate an extremely low surface recombination velocity on the order of 10³ cm/s. We further achieve room-temperature photoluminescence emission from InAsSb nanowires, spanning the entire MWIR regime from 3 to 5 μm. A dry-etching process is developed to expose only the top nanowire facets for metal contacts, with the sidewalls conformally covered by Al₂O₃ shells, allowing for a higher internal quantum efficiency. Based on these techniques, we fabricate nanowire photodetectors with an optimized pitch and diameter and demonstrate room-temperature spectral response with MWIR detection signatures up to 3.4 μm. The results of this work indicate that uncooled focal plane arrays at MWIR on low-cost InP substrates can be designed with nanostructured absorbers for highly compact and fully integrated detection platforms.

A large-scale, ultrahigh-resolution nanoemitter ordered array with PL brightness enhanced by PEALD-grown AlN coating

III-nitride solid-state microdisplays have significant advantages, including high brightness and high resolution, for the development of advanced displays, high-definition projectors, head-mounted displays, large-capacity optical communication systems, and so forth. Herein, a high-brightness InGaN/GaN multiple-quantum-well (MQW) nanoemitter array with an ultrahigh resolution of 31 750 dpi was achieved by combining a top-down fabrication with surface passivation of plasma-enhanced atomic layer deposition (PEALD)-grown AlN coating. With regard to the nanometer-level top-down etching, the surface damage or defects on the newly-formed sidewall play a significant role in the photoluminescence (PL) quality. Note that these arrays can be effectively passivated by the PEALD-grown AlN coating with an over 345% PL enhancement. In addition, a sharp band bending at the interface of the luminescent InGaN QW and the AlN coating layer can electrically drift away the photogenerated electrons from the surface traps; this leads to enhancement of the bulk PL radiative recombination with a fast PL decay rate. Therefore, we have demonstrated a feasible way for realizing an advanced nanoemitter array with both high brightness and ultrahigh resolution for future smart displays, high-resolution imaging, big-data optical information systems and so on.

A three-dimensional insight into correlation between carrier lifetime and surface recombination velocity for nanowires

The performance of nanowire-based devices is predominantly affected by nonradiative recombination on their surfaces, or sidewalls, due to large surface-to-volume ratios. A common approach to quantitatively characterize surface recombination is to implement time-resolved photoluminescence to correlate surface recombination velocity with measured minority carrier lifetime by a conventional analytical equation. However, after using numerical simulations based on a three-dimensional (3D) transient model, we assert that the correlation between minority carrier lifetime and surface recombination velocity is dependent on a more complex combination of factors, including nanowire geometry, energy-band alignment, and spatial carrier diffusion in 3D. To demonstrate this assertion, we use three cases-GaAs nanowires, InGaAs nanowires, and InGaAs inserts embedded in GaAs nanowires-and numerically calculate the carrier lifetimes by varying the surface recombination velocities. Using this information, we then investigate the intrinsic carrier dynamics within those 3D structures. We argue that the conventional analytical approach to determining surface recombination in nanowires is of limited applicability, and that a comprehensive computation in 3D can provide more accurate analysis. Our study provides a solid theoretical foundation to further understand surface characteristics and carrier dynamics for 3D nanostructured materials.

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.

Balancing the carrier selectivity and extraction by the selective contacts is of vital importance to the performance of the nanowire solar cells. Here Oener et al. employ a permanent local gate to overcome this tradeoff and substantially increase the open-circuit voltage by 335 mV.

Efficient Green Emission from Wurtzite Al xIn1- xP Nanowires

Direct band gap III-V semiconductors, emitting efficiently in the amber-green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the "green gap". Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite Al _xIn_{1- x}P nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the "pure green" 555 nm (2.23 eV). We investigate the band structure of wurtzite Al _xIn_{1- x}P using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires.

Understanding InP Nanowire Array Solar Cell Performance by Nanoprobe-Enabled Single Nanowire Measurements

III-V solar cells in the nanowire geometry might hold significant synthesis-cost and device-design advantages as compared to thin films and have shown impressive performance improvements in recent years. To continue this development there is a need for characterization techniques giving quick and reliable feedback for growth development. Further, characterization techniques which can improve understanding of the link between nanowire growth conditions, subsequent processing, and solar cell performance are desired. Here, we present the use of a nanoprobe system inside a scanning electron microscope to efficiently contact single nanowires and characterize them in terms of key parameters for solar cell performance. Specifically, we study single as-grown InP nanowires and use electron beam induced current characterization to understand the charge carrier collection properties, and dark current-voltage characteristics to understand the diode recombination characteristics. By correlating the single nanowire measurements to performance of fully processed nanowire array solar cells, we identify how the performance limiting parameters are related to growth and/or processing conditions. We use this understanding to achieve a more than 7-fold improvement in efficiency of our InP nanowire solar cells, grown from a different seed particle pattern than previously reported from our group. The best cell shows a certified efficiency of 15.0%; the highest reported value for a bottom-up synthesized InP nanowire solar cell. We believe the presented approach have significant potential to speed-up the development of nanowire solar cells, as well as other nanowire-based electronic/optoelectronic devices.