The Role of Spin-Flip Collisions in a Dark-Exciton Condensate.

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.

InP QDs modified GaAs/PEDOT:PSS hybrid solar cell with efficiency over 15.

GaAs heterojunction solar cells are known as promising substitutions for traditional GaAs solar cells for their low cost and performance potential. Nevertheless, the further performance enhancement is hindered by insufficient spectral absorption and nonradioactive recombination. In this work, an InP quantum dot (QD) modified GaAs/PEDOT:PSS solar cell is designed to enhance spectrum utilization and suppress the nonradioactive carriers loss and the solar cell efficiency at 15.08% is achieved. Furthermore, InP QDs used in this work are synthesized by a novel hydrothermal method. During the synthesis process, ß-cyclodextrin (ß-cyc) was introduced into the reactants and acted as a reaction cell, isolating water and oxygen, enabling the reaction to proceed in ambient air. InP QDs synthesized by this method can achieve band engineering by altering reactant ratios, thereby effectively serving as both a Luminescent Solar Concentrator (LSC) and a Front Surface Field (FSF) in GaAs/PEDOT:PSS solar cells. This work demonstrates an inspiring way to synthesize InP QDs and optimize the performance of GaAs hybrid solar cells.

Unveiling the 3D Morphology of Epitaxial GaAs/AlGaAs Quantum Dots.

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Control of Quantized Spontaneous Emission from Single GaAs Quantum Dots Embedded in Huygens' Metasurfaces.

Advancements in photonic quantum information systems (QIS) have driven the development of high-brightness, on-demand, and indistinguishable semiconductor epitaxial quantum dots (QDs) as single photon sources. Strain-free, monodisperse, and spatially sparse local-droplet-etched (LDE) QDs have recently been demonstrated as a superior alternative to traditional Stranski-Krastanov QDs. However, integration of LDE QDs into nanophotonic architectures with the ability to scale to many interacting QDs is yet to be demonstrated. We present a potential solution by embedding isolated LDE GaAs QDs within an Al0.4Ga0.6As Huygens' metasurface with spectrally overlapping fundamental electric and magnetic dipolar resonances. We demonstrate for the first time a position- and size-independent, 1 order of magnitude increase in the collection efficiency and emission lifetime control for single-photon emission from LDE QDs embedded within the Huygens' metasurfaces. Our results represent a significant step toward leveraging the advantages of LDE QDs within nanophotonic architectures to meet the scalability demands of photonic QIS.

GaAs Mid-IR Electrically Tunable Metasurfaces.

In this work, we explore III-V based metal-semiconductor-metal structures for tunable metasurfaces. We use an epitaxial transfer technique to transfer a III-V thin film directly on metallic surfaces, realizing III-V metal-semiconductor-metal (MSM) structures without heavily doped semiconductors as substitutes for metal layers. The device platform consists of gold metal layers with a p-i-n GaAs junction. The target resonance wavelength can be tuned by modifying the geometry of the top metal grating on the GaAs, while systematic resonance tunability has been shown through the modulation of various carrier concentration injections in the mid-IR range. Electrically tunable metasurfaces with multilevel biasing can serve as a fundamental building block for electrically tunable metasurfaces. We believe that our demonstration can contribute to understanding the optical tuning of III-V under various biased conditions, inducing changes in metasurfaces.

Highly Indistinguishable Single Photons from Droplet-Etched GaAs Quantum Dots Integrated in Single-Mode Waveguides and Beamsplitters.

Integration of on-demand quantum emitters into photonic integrated circuits (PICs) has drawn much attention in recent years, as it promises a scalable implementation of quantum information schemes. A central property for several applications is the indistinguishability of the emitted photons. In this regard, GaAs quantum dots (QDs) obtained by droplet etching epitaxy show excellent performances, making the realization of these QDs into PICs highly appealing. Here, we show the first implementation in this direction, realizing the key passive elements needed in PICs, i.e., single-mode waveguides (WGs) with integrated GaAs-QDs and beamsplitters. We study the statistical distribution of wavelength, linewidth, and decay time of the excitonic line, as well as the quantum optical properties of individual emitters under resonant excitation. We achieve single-photon purities as high as 1 - g(2)(0) = 0.929 ± 0.009 and two-photon interference visibilities of up to VTPI = 0.953 ± 0.032 for consecutively emitted photons.

Anodic Reconstructed p+-GaAs/a-InAsN for Stable and Efficient Photoelectrochemical Hydrogen Evolution.

The extremely poor solution stability and massive carrier recombination have seriously prevented III-V semiconductor nanomaterials from efficient and stable hydrogen production. In this work, an anodic reconstruction strategy based on group III-V active semiconductors is proposed for the first time, resulting in 19-times photo-gain. What matters most is that the device after anodic reconstruction shows very superior stability under the protracted photoelectrochemical (PEC) test over 8100 s, while the final photocurrent density does not decrease but rather increases by 63.15%. Using the experiment and DFT theoretical calculation, the anodic reconstruction mechanism is elucidated: through the oxidation of indium clusters and the migration of arsenic atoms, the reconstruction formed p+-GaAs/a-InAsN. The hole concentration of the former is increased by 10 times (5.64 × 1018 cm-1 increases up to 5.95 × 1019 cm-1) and the band gap of the latter one is reduced to a semi-metallic state, greatly strengthening the driving force of PEC water splitting. This work turns waste into treasure, transferring the solution instability into better efficiency.

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy.

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

Excitation-wavelength-dependent photoluminescence in GaAs nanowires under high-pressure.

GaAs nanowires (NWs) have wide application potential as near-infrared optical devices and the high-pressure strategy has been applied to modulate their crystal and electronic structures. As another typical thermodynamic parameter, temperature can also affect the optical performance of semiconductors. Here we report the excitation-wavelength-dependent photoluminescence (EWDP) in GaAs NWs under high-pressure conditions. The pressure for achieving the maximum photoluminescence (PL) intensity and bandgap transition from direct to indirect of GaAs NWs varies (1.7-2.7 GPa) with the wavelength of the incident lasers (633-473 nm). The Raman peak of GaAs NWs shifts towards higher frequency with increasing excitation wavelengths at the same high-pressure conditions, revealing the stronger heating effect induced by incident laser with the shorter wavelength. The relative temperature difference in GaAs NWs induced by two different lasers can be estimated up to 537 K, and the strong heating effect suppresses the light-emission efficiency in GaAs NWs. With increasing the pressure, the relative temperature difference presents a gradual declining trend and PL intensity presents an opposite trend, which relates to the pressure-induced suppression of nonradiative recombination in GaAs NWs. Our study provides insights into the mechanisms for the EWDP effect and an alternative route to modulate the high-pressure performance of nanodevices.

Self-powered stable high-performance UV-Vis-NIR broadband photodetector based on PVP-Cobalt@Carbon nanofibers/n-GaAs heterojunction.

The self-powered PVP-Co@C nanofibers/n-GaAs heterojunction photodetector (HJPD) was fabricated by electrospinning of the nanofibers onto GaAs. An excellent rectification ratio of 6.60 × 106was obtained fromI-Vmeasurements of the device in the dark. TheI-Vmeasurements of the fabricated device under 365 nm, 395 nm and 850 nm lights, as well asI-Vmeasurements in visible light depending on the light intensity, were performed. The HJPD demonstrated excellent photodetection performance in terms of a good responsivity of âˆ¼225 mA W-1(at -1.72 V) and at zero bias, an impressive detectivity of 6.28 × 1012Jones, and a high on/off ratio of 8.38 × 105, all at 365 nm wavelength. In addition, the maximum external quantum efficiency and NPDR values were 3495% (V = -1.72 V) and 2.60 × 1010W-1(V= 0.0 V), respectively, while the minimum NEP value was ∼10-14W.Hz-1/2for 365 nm atV= 0.V volts. The HJPD also exhibited good long-term stability in air after 30 d without any encapsulation.

Microscopic insights into metal diffusion and ohmic contact formation in delta-doped GaAs/(Al,Ga)As core/shell nanowires.

Semiconductor nanowires (NWs) are promising candidates for use in electronic and optoelectronic applications, offering numerous advantages over their thin film counterparts. Their performance relies heavily on the quality of the contacts to the NW, which should exhibit ohmic behavior with low resistance and should be formed in a reproducible manner. In the case of heterostructure NWs for high-mobility applications that host a two-dimensional electron gas, ohmic contacts are particularly challenging to implement since the NW core constituting the conduction channel is away from the NW surface. We investigated contact formation to modulation-doped GaAs/(Al,Ga)As core/shell NWs using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy and electron tomography to correlate microstructure, diffusion profile and chemical composition of the NW contact region with the current-voltage (I-V) characteristics of the contacted NWs. Our results illustrate how diffusion, alloying and phase formation processes essential to the effective formation of ohmic contacts are more intricate than in planar layers, leading to reproducibility challenges even when the processing conditions are the same. We demonstrate that the NW geometry plays a crucial role in the creation of good contacts. Both ohmic and rectifying contacts were obtained under nominally identical processing conditions. Furthermore, the presence of Ge in the NW core, in the absence of Au and Ni, was found as the key factor leading to ohmic contacts. The analysis contributes to the current understanding of ohmic contact formation to heterostructure core/shell NWs offering pathways to enhance the reproducibility and further optimization of such NW contacts.

Composition and optical properties of (In, Ga)As nanowires grown by group-III-assisted molecular beam epitaxy.

(In, Ga) alloy droplets are used to catalyse the growth of (In, Ga)As nanowires by molecular beam epitaxy on Si(111) substrates. The composition, morphology and optical properties of these nanowires can be tuned by the employed elemental fluxes. To incorporate more than 10% of In, a high In/(In+Ga) flux ratio above 0.7 is required. We report a maximum In content of almost 30% in bulk (In, Ga)As nanowires for an In/(In+Ga) flux ratio of 0.8. However, with increasing In/(In+Ga) flux ratio, the nanowire length and diameter are notably reduced. Using photoluminescence and cathodoluminescence spectroscopy on nanowires covered by a passivating (In, Al)As shell, two luminescence bands are observed. A significant segment of the nanowires shows homogeneous emission, with a wavelength corresponding to the In content in this segment, while the consumption of the catalyst droplet leads to a spectrally-shifted emission band at the top of the nanowires. The (In,Ga)As nanowires studied in this work provide a new approach for the integration of infrared emitters on Si platforms.

Dose-response and efficacy of 904 nm photobiomodulation on diabetic foot ulcers healing: a randomized controlled trial.

PURPOSE: This study aimed to examine the impact of a 904 nm photobiomodulation (PBM) on diabetic ulcers using varying dosages. METHODS: The study was a randomized, double-blind, placebo-controlled clinical trial that compared treatments using PBM (GaAs 904 nm 30w) with three different energy densities (4 J/cm2; 8 J/cm2; 10 J/cm2) in the healing process of non-infected diabetic foot ulcers. Eighty volunteers (48.75% female; 58.5 ± 11.1 years) were randomized into three intervention groups treated with PBM and one control group (PBM placebo). Volunteers performed up 20 interventions with PBM, either placebo or actual, in conjunction with conventional therapy, which involved dressing the wound with Helianthus annuus vegetable oil. The primary variable was the ulcer size reduction rate. RESULTS: GaAs 904 nm PBM yielded a clinically and significant ulcer size rate reduction of diabetic foot ulcers, independently of energy density range (p < 0.05). However, 10 J/cm² had 60% of completely healed ulcers and the highest proportion of patients reaching 50% of ulcer reduction rate after 5 weeks of treatment. In addition, only 10 J/cm² showed a significant difference between control group after a 10-week follow-up (p < 0.05). CONCLUSION: GaAs 904 nm PBM was effective in treating diabetic foot ulcers in this study and a dosage of 10 J/cm², after a 10-week follow-up, proved to be the most effective compared to the other groups. CLINICAL TRIAL REGISTRATION NUMBER: NCT04246814.

On-Chip Circularly Polarized Circular Loop Antennas Utilizing 4H-SiC and GaAs Substrates in the Q/V Band.

This paper presents a comprehensive assessment of the performance of on-chip circularly polarized (CP) circular loop antennas that have been designed and fabricated to operate in the Q/V frequency band. The proposed antenna design incorporates two concentric loops, with the outer loop as the active element and the inner loop enhancing the CP bandwidth. The study utilizes gallium arsenide (GaAs) and silicon carbide (4H-SiC) semiconductor wafer substrates. The measured results highlight the successful achievement of impedance matching at 40 GHz and 44 GHz for the 4H-SiC and GaAs substrates, respectively. Furthermore, both cases yield an axial ratio (AR) of less than 3 dB, with variations in bandwidths and frequency bands contingent upon the dielectric constant of the respective substrate material. Moreover, the outcomes confirm that utilizing 4H-SiC substrates results in a significantly higher radiation efficiency of 95%, owing to lower substrate losses. In pursuit of these findings, a 4-element circularly polarized loop array antenna has been fabricated for operation at 40 GHz, employing a 4H-SiC wafer as a low-loss substrate. The results underscore the antenna's remarkable performance, exemplified by a broadside gain of approximately 9.7 dBic and a total efficiency of circa 92%. A close agreement has been achieved between simulated and measured results.

Direct Band Gap AlGaAs Wurtzite Nanowires.

Wurtzite AlGaAs is a technologically promising yet unexplored material. Here we study it both experimentally and numerically. We develop a complete numerical model based on an 8-band k→·p→ method, including electromechanical fields, and calculate the optoelectronic properties of wurtzite AlGaAs nanowires with different Al content. We then compare them with our experimental data. Our results strongly suggest that wurtzite AlGaAs is a direct band gap material. Moreover, we have also numerically obtained the band gap of wurtzite AlAs and the valence band offset between AlAs and GaAs in the wurtzite symmetry.

Background-Free Near-Infrared Biphoton Emission from Single GaAs Nanowires.

The generation of photon pairs from nanoscale structures with high rates is still a challenge for the integration of quantum devices, as it suffers from parasitic signals from the substrate. In this work, we report type-0 spontaneous parametric down-conversion at 1550 nm from individual bottom-up grown zinc-blende GaAs nanowires with lengths of up to 5 µm and diameters of up to 450 nm. The nanowires were deposited on a transparent ITO substrate, and we measured a background-free coincidence rate of 0.05 Hz in a Hanbury-Brown-Twiss setup. Taking into account transmission losses, the pump fluence, and the nanowire volume, we achieved a biphoton generation of 60 GHz/Wm, which is at least 3 times higher than that of previously reported single nonlinear micro- and nanostructures. We also studied the correlations between the second-harmonic generation and the spontaneous parametric down-conversion intensities with respect to the pump polarization and in different individual nanowires.

A Solid-State Source of Single and Entangled Photons at Diamond SiV-Center Transitions Operating at 80K.

Large-scale quantum networks require the implementation of long-lived quantum memories as stationary nodes interacting with qubits of light. Epitaxially grown quantum dots hold great potential for the on-demand generation of single and entangled photons with high purity and indistinguishability. Coupling these emitters to memories with long coherence times enables the development of hybrid nanophotonic devices that incorporate the advantages of both systems. Here we report the first GaAs/AlGaAs quantum dots grown by the droplet etching and nanohole infilling method, emitting single photons with a narrow wavelength distribution (736.2 ± 1.7 nm) close to the zero-phonon line of silicon-vacancy centers. Polarization entangled photons are generated via the biexciton-exciton cascade with a fidelity of (0.73 ± 0.09). High single photon purity is maintained from 4 K (g(2)(0) = 0.07 ± 0.02) up to 80 K (g(2)(0) = 0.11 ± 0.01), therefore making this hybrid system technologically attractive for real-world quantum photonic applications.

Study on the behavior of clusters in the physical recovery of GaAs scrap.

Gallium arsenide (GaAs) is the most widely used second-generation semiconductor material. However, a large amount of GaAs scrap is generated at various stages of the GaAs wafer production process. Volatile GaAs clusters are inevitably generated during the process of GaAs vacuum thermal decomposition, resulting in lower purity of the recovered arsenic and the loss of gallium. In this study, thermodynamic analysis and dynamics simulation were combined to discuss the possibility of separating GaAs clusters and arsenic from a microscopic perspective. A vacuum thermal decomposition-directional condensation recovery process for GaAs scrap was proposed. By properly adjusting the separation parameters such as heating temperature, holding time and raw material size, high purity of gallium (99.99%) and arsenic (99.5%) were directly recovered under a system pressure of 1 Pa, heating temperature of 1323 K, holding time of 3 h, and GaAs scrap size of 2.5 cm. GaAs clusters were also recovered in powder form. The problem of difficult separation of GaAs clusters from arsenic was effectively solved by this method, and the purity of recovered arsenic was greatly improved. No additives are required and no waste liquid or gas emission in the whole process. The complexity of subsequent arsenic purification operations and the threat of arsenic containing waste to the environment were reduced as well.

Binding Energies and Optical Properties of Power-Exponential and Modified Gaussian Quantum Dots.

We examine the optical and electronic properties of a GaAs spherical quantum dot with a hydrogenic impurity in its center. We study two different confining potentials: (1) a modified Gaussian potential and (2) a power-exponential potential. Using the finite difference method, we solve the radial Schrodinger equation for the 1s and 1p energy levels and their probability densities and subsequently compute the optical absorption coefficient (OAC) for each confining potential using Fermi's golden rule. We discuss the role of different physical quantities influencing the behavior of the OAC, such as the structural parameters of each potential, the dipole matrix elements, and their energy separation. Our results show that modification of the structural physical parameters of each potential can enable new optoelectronic devices that can leverage inter-sub-band optical transitions.

Effect of surface gallium termination on the formation and emission energy of an InGaAs wetting layer during the growth of InGaAs quantum dots by droplet epitaxy.

Self-assembled quantum dots (QDs) based on III-V semiconductors have excellent properties for applications in quantum optics. However, the presence of a 2D wetting layer (WL) which forms during the Stranski-Krastanov growth of QDs can limit their performance. Here, we investigate WL formation during QD growth by the droplet epitaxy technique. We use a combination of photoluminescence excitation spectroscopy, lifetime measurements, and transmission electron microscopy to identify the presence of an InGaAs WL in these droplet epitaxy QDs, even in the absence of distinguishable WL luminescence. We observe that increasing the amount of Ga deposited on a GaAs (100) surface prior to the growth of InGaAs QDs leads to a significant reduction in the emission wavelength of the WL to the point where it can no longer be distinguished from the GaAs acceptor peak emission in photoluminescence measurements. However increasing the amount of Ga deposited does not suppress the formation of a WL under the growth conditions used here.