At the Limit of Interfacial Sharpness in Nanowire Axial Heterostructures.

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor-liquid-solid growth mode. Here, the grading of Al across GaAs/AlxGa1-xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2-3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.

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.

A simulation study of irradiation effect on InAs/GaAsSb type II quantum dot structures.

Particles in space cause irradiation damage to the solar cells (SCs), resulting in the degradation of their performance. Quantum dot solar cells (QDSCs) have higher theoretical efficiency and better irradiation resistance than the conventional GaAs SCs, which makes them highly promising for application in space. In this paper, we study the proton irradiation effect on InAs/GaAs0.8Sb0.2 QDSCs by SRIM program. The simulation result shows that the InAs/GaAs0.8Sb0.2 QDSCs have fewer vacancies than GaAs SCs when irradiated with low-energy proton, which indicates that the InAs/GaAs0.8Sb0.2 QDSCs have better anti-irradiation characteristics. The study about displacements per atom and proton concentration in two SCs shows that protons with low energy and high irradiation fluences will cause more serious damage in InAs/GaAs0.8Sb0.2 QDSCs. In addition, the proton incident angle affects the vacancy distribution, while the number of QD layers has little effect on it.

Room-Temperature Near-Infrared Lasing from GaAs/AlGaAs Core-Shell Nanowires Based on Random Cavity.

Room-temperature lasing based on low-dimensional GaAs nanowires (NWs) is one of the most critical and challenging issues in realizing near-infrared lasers for nanophotonics. In this article, the random lasing characteristics based on GaAs NW arrays have been discussed theoretically. According to the simulation, GaAs/AlGaAs core-shell NWs with an optimal diameter, density, and Al content in the shell have been grown. Systematic morphological and optical characterizations were carried out. It is found that the GaAs NWs with the additional growth of the AlGaAs shell exhibit improved emission by about 2 orders of magnitude at low temperatures, which can be attributed to the suppression of crystal defects. At room temperature, lasing was observed with a threshold around 70.16 mW/cm2, and the random lasing mechanism was discussed in detail. This work is of great significance for the design of random cavities based on semiconductor NWs, which is important for optoelectronic integration.

Plasma-Etched Black GaAs Nanoarrays with Gradient Refractive Index Profile for Broadband, Omnidirectional, and Polarization-Independent Antireflection.

Black GaAs nanotip arrays (NTs) with 3300 nm lengths were fabricated via self-masked plasma etching. We show, both experimentally and numerically, that these NTs, with three gradient refractive index layers, effectively suppress Fresnel reflections at the air-GaAs interface over a broad range of wavelengths. These NTs exhibit exceptional UV-Vis light absorption (up to 99%) and maintain high NIR absorption (33-60%) compared to bare GaAs. Moreover, possessing a graded layer with a low refractive index (n = 1.01 to 1.12), they achieve angular and polarization-independent antireflection properties exceeding 80° at 632.8 nm, aligning with perfect antireflective coating theory predictions. This approach is anticipated to enhance the performance of optoelectronic devices across a wide range of applications.

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.

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.

Molecular dynamics simulation of the scratching process of GaAs with different crystal orientations.

CONTEXT: In order to further improve the manufacturing technology of resonator facet of GaAs (gallium arsenide)-based laser, the scratching process of GaAs was simulated by molecular dynamics. Models of GaAs crystals with different orientations, including GaAs [100], GaAs [110], and GaAs [111], were generated, followed by scratch simulations on these models. The surface characteristics of scratches, damage width, subsurface damage, stack height, and the distribution and activity characteristics of dislocations were analyzed based on the simulation results. The results show that there are obvious anisotropy in the deformation of different crystal orientation during the scratching process of GaAs. Surface features, damage width, subsurface damage, and dislocation dynamics during scraping in GaAs crystals strongly depend on crystal orientation. It was also observed that GaAs exhibits distinct characteristics of dislocation activity during the scratching process, depending on its crystal orientation. In addition, GaAs [110] crystal direction has the smallest maximum damage width and subsurface damage depth. The maximum of maximum damage width is in GaAs [100] crystal direction, and the maximum subsurface damage depth is in GaAs [111] crystal direction. In addition, the stacking height is maximum when GaAs [100] is scraped and minimum when GaAs [110] is scraped. METHODS: The engraving quality of GaAs materials was investigated utilizing the LAMMPS software through molecular dynamics simulations, while observations were facilitated using the OVITO software. The MD simulation was conducted employing the NPT ensemble, with the temperature fixed at 300 K. A time step of 2 fs was utilized, and the total duration of the MD simulation spanned 600 ps.

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.

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.

TCAD simulation study of heavy ion radiation effects on hetero junctionless tunnel field effect transistor.

Semiconductor devices used in radiation environment are more prone to degradation in device performance. Junctionless Tunnel Field Effect Transistor (JLTFET) is one of the most potential candidates which overcomes the short channel effects and fabrication difficulties. In this work, 20 nm JLTFET is proposed with Silicon in the drain/channel region whereas source uses different materials, Silicon Germanium (SiGe), Gallium Nitride (GaN), Gallium Arsenide (GaAs), Indium Arsenide (InAs). The device performance is examined by subjecting it to heavy ion radiation at a lower and higher dose of linear energy transfer (LET) values. It can be seen that the most sensitive location is the source/channel (S/C) interface for SiGe, GaN and GaAs whereas the drain/channel (D/C) interface for InAs. Further analysis is carried out at these vulnerable regions by matching ION of all materials. The parameters, transient peak current (Ipeak), collected charge (QC), threshold voltage shift (ΔVth) and bipolar gain (ß) are extracted using transient simulations. It is observed that for a lower dose of LET, Ipeak of SiGe is 27% lesser than InAs and for higher dose of LET, SiGe shows 56% lesser Ipeak than InAs. SiGe is less sensitive at lower and higher dose of LET due to reduced ΔVth, tunneling and electron density.

Effects ofAs8structure formation on the surface morphology and internal microstructure of GaAs thin films.

Gallium arsenide (GaAs) materials have the advantages of high electron mobility, electron saturation drift rate, and other irreplaceable semiconducting properties. They play an important role in the electronics, solar and other fields. However, during GaAs film sedimentary growth, As atoms can undergo segregation to formAs8clusters because of the influence of external factors, which affect the surface morphology and internal structure of these films. In this study, a series of investigations on the deposition and growth of GaAs crystal films were performed. Additionally, the deposition and growth of GaAs thin films were simulated using molecular dynamics. The influence of As8clusters on the surface morphology and internal structure of GaAs films at different incidence angles, velocities and substrate temperatures was studied by using 'defect analysis technology' and 'diamond structure identification' in open source software, along with surface roughness and radial distribution function. Results show that with increasing incident angle, the number ofAs8clusters decreases and film density increases. Increasing incident velocity increases the irregular movement ofAs8clusters in air, and their deposition on the film surface affects the morphology of the film, and the surface roughness increases first and then decreases. Additionally, we investigated the effect of different substrate temperatures on the film surface. Results show that at a substrate temperature of 1173 K, the number ofAs8clusters in the film decreases or the As8clusters disappear, heterogeneous nucleation occurs in the film, and the crystallization rate increases. Although the dislocation line associated with nucleation may affect the mechanical and optical properties of the film, it considerably reduces the annealing effort after the deposition and growth.

Optical Response of Aged Doped and Undoped GaAs Samples.

We studied epitaxial GaAs samples doped with Ge and Sn up to 1×1019 cm -3, which were stored in a dry and dark environment for 26 years. The optical response of the GaAs samples was determined through the photoluminescence and photoreflectance techniques, taken at different times: just after their fabrication in 1995, 2001 and 2021. The evolution of defects formed by the action of O 2 in the samples and their correlation with doping with Ge and Sn impurities were studied. We obtained the result that aging formed defects of type vacancies, mainly As, which produced energy levels of deep traps linked to the L band. The concentration of vacancies over the 26 years could be as large as 1017 cm -3, and these vacancies form complexes with doping impurities.

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.

Ultra-Thin GaAs Single-Junction Solar Cells for Self-Powered Skin-Compatible Electrocardiogram Sensors.

GaAs thin-film solar cells have high efficiency, reliability, and operational stability, making them a promising solution for self-powered skin-conformal biosensors. However, inherent device thickness limits suitability for such applications, making them uncomfortable and unreliable in flexural environments. Therefore, reducing the flexural rigidity becomes crucial for integration with skin-compatible electronic devices. Herein, this study demonstrated a novel one-step surface modification bonding methodology, allowing a streamlined transfer process of ultra-thin (2.3 µm thick) GaAs solar cells on flexible polymer substrates. This reproducible technique enables strong bonding between dissimilar materials (GaAs-polydimethylsiloxane, PDMS) without high external pressures and temperatures. The fabricated solar cell showed exceptional performance with an open-circuit voltage of 1.018 V, short-circuit current density of 20.641 mA cm-2, fill factor of 79.83%, and power conversion efficiency of 16.77%. To prove the concept, the solar cell is integrated with a skin-compatible organic electrochemical transistor (OECT). Competitive electrical outputs of GaAs solar cells enabled high current levels of OECT under subtle light intensities lower than 50 mW cm-2, which demonstrates a self-powered electrocardiogram sensor with low noise (signal-to-noise ratio of 32.68 dB). Overall, this study presents a promising solution for the development of free-form and comfortable device structures that can continuously power wearable devices and biosensors.

Bias-Tunable Quantum Well Infrared Photodetector.

With the rapid advancement of Artificial Intelligence-driven object recognition, the development of cognitive tunable imaging sensors has become a critically important field. In this paper, we demonstrate an infrared (IR) sensor with spectral tunability controlled by the applied bias between the long-wave and mid-wave IR spectral regions. The sensor is a Quantum Well Infrared Photodetector (QWIP) containing asymmetrically doped double QWs where the external electric field alters the electron population in the wells and hence spectral responsivity. The design rules are obtained by calculating the electronic transition energies for symmetric and antisymmetric double-QW states using a Schrödinger-Poisson solver. The sensor is grown and characterized aiming detection in mid-wave (~5 µm) to long-wave IR (~8 µm) spectral ranges. The structure is grown using molecular beam epitaxy (MBE) and contains 25 periods of coupled double GaAs QWs and Al0.38Ga0.62As barriers. One of the QWs in the pair is modulation-doped to provide asymmetry in potential. The QWIPs are tested with blackbody radiation and FTIR down to 77 K. As a result, the ratio of the responsivities of the two bands at about 5.5 and 8 µm is controlled over an order of magnitude demonstrating tunability between MWIR and LWIR spectral regions. Separate experiments using parameterized image transformations of wideband LWIR imagery are performed to lay the framework for utilizing tunable QWIP sensors in object recognition applications.

Formation of Point Defects Due to Aging under Natural Conditions of Doped GaAs.

The aging dynamics of materials used to build the active part of optoelectronic devices is a topic of current interest. We studied epitaxial samples of GaAs doped with Ge and Sn up to 1×1019 cm-3, which were stored in a dry and dark environment for 26 years. Photoluminescence spectra were taken in three periods: 1995, 2001 and 2021. In the last year, time-resolved photoluminescence, Raman, and X-ray measurements were also performed to study the evolution of defects formed by the action of O2 in the samples and its correlation with the doping with Ge and Sn impurities. We found that oxygen formed oxides that gave off Ga and As atoms, leaving vacancies mainly of As. These vacancies formed complexes with the dopant impurities. The concentration of vacancies over the 26 years could be as large as 1×1018 cm-3.

Modified Broadband Ruthroff-Type Transmission Line Transformer Balun for Isolation-Enhanced Passive Mixer Design.

Generalized broadband operation facilitates multifunction or multiband highly integrated applications, such as modern transceiver systems, where ultra-wideband bidirectional passive mixers are favored to avoid a complex up/down-conversion scheme. In this paper, a modified Ruthroff-type transmission line transformer (TLT) balun is presented to enhance the isolation of the mixer from the local oscillator (LO) to the radio frequency (RF). Compared to the conventional methods, the proposed Ruthroff-type architecture adopts a combination of shunt capacitors and parallel coupled lines to improve the return loss at the LO port, thus effectively avoiding the area consumption for the diode-to-balun impedance transformation while simultaneously providing a suitable point for IF extraction. In addition, a parallel compensation technique consisting of an inductor and resistor is applied to the RF balun to significantly improve the amplitude/phase balance performance over a wide bandwidth. Benefiting from the aforementioned operations, an isolation-enhanced 8-30 GHz passive double-balanced mixer is designed as a proof-of-principle demonstration via 0.15-micrometer GaAs p-HEMT technology. It exhibits ultra-broadband performance with 7 dB average conversion loss and 50 dB LO-to-RF isolation under 15 dBm LO power. The monolithic microwave integrated circuit area is 0.96 × 1.68 mm2 including all pads.

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.

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.