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This study investigates the effects of electron beam irradiation on the lasing characteristics of optically pumped CdSe quantum dot (QD) lasers grown on GaAs substrates using molecular beam epitaxy. The experimental setup allows the creation of two types of CdSe QDs with distinct size differences, with and without electron beam irradiation during the epitaxy. Lasing wavelengths were observed at 534 nm in the non-irradiated region and between 548 and 557 nm in the irradiated regions, depending on the electron beam irradiation conditions, achieving multi-wavelength lasing in the green-yellow ranges on a single chip. Internal laser parameters were evaluated for both irradiated and non-irradiated regions across different cavity lengths. Photoluminescence (PL) spectra revealed significantly higher integrated PL intensity in the irradiated regions compared to the non-irradiated regions, attributed to reduced point defects. However, at the lasing threshold, the impact of these defects is negligible. Lasing spectra exhibited a notable blue shift in the irradiated regions with an increase in threshold intensity due to the state-filling effect, which also causes a decrease in internal quantum efficiency. The modal gain and transparent intensity were evaluated, showing lower gain values in the irradiated regions, consistent with the observed broadening of the PL spectrum. These findings highlight the dynamic nature of the state-filling effect and its impact on lasing performance, providing insights into the mechanisms affecting CdSe QD lasers under electron beam irradiation in green-yellow spectrum range. [ABSTRACT FROM AUTHOR]

The lack of suitable p-type dopant for β-Ga2O3 remains a hurdle for vertical power device applications. Epitaxy of GaN on Ga2O3 substrates was demonstrated as an alternative. (–201)-oriented β-Ga2O3 was converted into (0001)-oriented hexagonal GaN via nitrogen plasma in a plasma-assisted molecular beam epitaxy chamber, as verified by XRD and RHEED. The resulting nitridated GaN layers were characterized by TEM, x-ray reflectivity, and AFM to relate the nitridation conditions to crystallinity, layer thickness, and surface roughness. The crystallinity of subsequently grown epitaxial GaN films was quantified via XRD rocking curves and related to the nitridation layer properties across varying nitridation conditions. Specifically, the effect of the grain size and nitridation layer thickness was investigated to determine their role in threading screw dislocation management. [ABSTRACT FROM AUTHOR]

We investigate the growth of Au/FexNy/MgO trilayers on GaAs(001) substrates by plasma-assisted molecular beam epitaxy. The optimization of the growth conditions made it possible to obtain the compound of stoichiometric Fe4N. Microstructural studies show that Fe4N forms 3D islands at the initial stages of growth. As the Fe4N thickness increases, a columnar growth sets in leading to a strong texturing and to the formation of grains having the same crystallographic orientation. The growth is epitaxial with the relationship GaAs (001) [110]//MgO (001) [110]//Fe4N (001) [110]//Au (012) [0–32]. A chemical analysis at the nanoscale reveals that the interfaces are rather sharp with a limited interdiffusion. Magnetic characterizations show that a trilayer containing a 1-nm-thick Fe4N layer is already ferromagnetic. The easy magnetization axis is in-plane independent of the Fe4N layer thickness (from 1 to 6 nm). This study shows the potential to use Fe4N as a spin injector for spin-optoelectronic applications. [ABSTRACT FROM AUTHOR]

We attain N-polar and Al-polar AlN thin films on Si(111) substrates by plasma-assisted molecular beam epitaxy. The polarity of AlN epilayers has been validated by wet chemical etching using tetramethylammonium hydroxide and by the direct cross-sectional observation of atomic stacking under high-angle annular dark-field scanning transmission electron microscopy. For the 290 nm-thick as-grown N-polar AlN epilayer, x-ray diffraction (XRD) (002) and (102) ω rocking curve peak full width half maximums (FWHMs) are 475 and 1177 arcsec, and the surface mean square roughness (RMS) is 0.30 nm. We flipped the polarity using the metal-flux-modulation-epitaxy (MME) strategy. The MME strategy promotes anti-phase boundaries (APBs) on the { 2 2 ¯ 01 } crystalline planes instead of commonly observed lateral planar APBs in AlN epilayers. Merging of the tilted APBs at ∼50 nm leads to a complete Al-polar surface. For the 180 nm-thick Al-polar AlN epilayer, XRD (002) and (102) peak FWHMs are 1505 and 2380 arcsec, and the surface RMS is 1.41 nm. Strain analysis by XRD and Raman spectroscopy indicates a uniform tensile strain of 0.160% across the N-polar AlN epilayer surface and a strain distribution of 0.113%–1.16% through the epilayer. In contrast, the Al-polar AlN epilayer exhibits a much broader tensile strain distribution of 0.482%–2.406% along the growth direction, potentially due to the interaction of polarity inversion and strain relaxation. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) resonators are essential components of mobile communication technology. Advanced performance SAW resonators are needed to support beyond fifth-generation mobile communication technology, which aims to achieve unprecedentedly high data rates and low latency using wide frequency bands in the RF spectrum. This study theoretically investigates the propagation characteristics of a non-leaky Rayleigh-type SAW (RSAW) propagating in a c-axis-tilted ScAlN film/3C-SiC substrate structure. By appropriately selecting the c-axis tilt angle and the film thickness of the ScAlN film, a high electromechanical coupling coefficient (K2) value was obtained in the second-mode RSAW (Sezawa wave). The maximum K2 value for the Sezawa wave was 8.03%, with a phase velocity of 7456 m/s and a power flow angle of 0° under the structural conditions where the K2 value was maximized. These properties offer significant advantages for achieving wide frequency bands, high operating frequencies, and ease of design for SAW resonators. The structural conditions under which good propagation characteristics were obtained in the Sezawa waves were found to coincide with the conditions that maximize the electromechanical coupling coefficient of the quasi-longitudinal wave in the ScAlN film, and a significant increase in the shear vertical component of Sezawa wave particle displacement was observed. Additionally, ScAlN films with a 40% Sc concentration can be fabricated using sputtering and molecular beam epitaxy. Recent advancements have reported the production of high-quality 3C-SiC wafers and large 3C-SiC film/silicon wafers. Therefore, the c-axis-tilted ScAlN film/3C-SiC substrate structure shows great potential as a candidate for next-generation SAW resonators. [ABSTRACT FROM AUTHOR]

Ridge microstructures were prepared by etching through samples consisting of a series of stacked InAs x P 1 − x quantum wells (QWs) with step graded composition grown on InP by molecular beam epitaxy. Different etching techniques were used: wet etching with HCl/H 2 O and reactive ion etching with CH 4 /H 2. These microstructures were characterized by low-temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each QW was clearly identified. The PL was measured in detail across etched ridge stripes of various widths. Variations of the integrated PL intensities across the etched stripes were observed. The PL intensities for all QWs increase gradually from the edge to the center of the ridge microstructures. The PL intensity measured at the ridge center is systematically reduced for ridges which are 10 or 20 μ m wide as compared to ridges which are 30 μ m wide or larger. On the other hand, the spectral peak position of the PL lines remained constant with high accuracy (0.2–0.4 meV) across the microstructures. These observations are discussed in terms of the different possible mechanisms which determine the PL intensity variations, namely, non-radiative recombination at the etched walls and effects of stray electric fields which result from the etching process. Based on this discussion, we compare quantitatively the different etching processes which we have used. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g., lasers or waveguides) involving etching processes. [ABSTRACT FROM AUTHOR]

Recently, deep-ultraviolet (DUV) light-emitting devices have attracted attention for various applications. GaN/AlN superlattices have emerged as a promising alternative for achieving high-efficiency DUV emission. To fabricate superlattices with high crystal quality and abrupt interfaces, we have utilized face-to-face-annealed sputter-deposited AlN template substrates characterized by a flat surface and low dislocation density. Furthermore, radio-frequency plasma-assisted molecular beam epitaxy with in situ reflection high-energy electron diffraction monitoring was employed for the growth process. The growth of the superlattices follows a specific sequence. Step 1: AlN growth, Step 2: conversion of Al droplets to AlN, Step 3: GaN growth, and Step 4: evaporation of Ga droplets. This study explored the impact of GaN thickness on the GaN/AlN superlattice. The GaN thickness was linearly controlled by changing the duration of Step 3. This approach allowed for the growth of a flat GaN layer up to 1 monolayer (ML) and achieved superlattices with abrupt interfaces. Single-peak cathodoluminescence (CL) emission at 240–245 nm was observed from the superlattices, with the peak shift toward longer wavelengths as the GaN thickness increased. In contrast, quantum dot-like GaN islands were generated with a thickness of over 1 ML, induced by compressive strain. Superlattices with thicker GaN exhibited broad CL emission with multiple peaks. However, the AlN barrier layer reduced the surface roughness and maintained abrupt interfaces within the superlattices. Therefore, to obtain sharp single-peak UV emission from GaN/AlN superlattices, the growth sequence should be controlled to obtain flat GaN layers without dots. [ABSTRACT FROM AUTHOR]

A series of topological insulators (TIs) based saturable absorbers (SAs), e.g., Bi2Se3, Bi2SeTe2, and Bi4Br4, are confirmed to exhibit excellent non-linear optical response due to the topological edge states. Here, we demonstrate how a topological crystalline insulator, SnTe thin films, can be prepared on highly oriented pyrolytic graphite and gold-plated mirrors through molecular beam epitaxy. SnTe-SAs incorporated into Er-doped fiber lasers exhibit a large modulation depth of 27.2% and accomplish mode-locking at 1558 nm with a pulse width of 319 fs, indicating preeminent nonlinear optical performance among the reported TI-based SAs. This work illuminates the preparation of SnTe thin films and demonstrates the great potential of SnTe films in ultrafast optical devices. [ABSTRACT FROM AUTHOR]

Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al (111) ⟨ 2 1 ¯ 1 ¯ ⟩ ∥ sapphire (0001) [ 2 1 ¯ 1 ¯ 0 ]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness. [ABSTRACT FROM AUTHOR]

For many years, Si1−yGey alloys have been applied in the semiconductor industry due to the ability to adjust the performance of Si-based nanoelectronic devices. Following this alloying approach of group-IV semiconductors, adding tin (Sn) into the alloy appears as the obvious next step, which leads to additional possibilities for tailoring the material properties. Adding Sn enables effective bandgap and strain engineering and can improve the carrier mobilities, which makes Si1−x−yGeySnx alloys promising candidates for future opto- and nanoelectronics applications. The bottom-up approach for epitaxial growth of Si1−x−yGeySnx, e.g., by chemical vapor deposition and molecular beam epitaxy, allows tuning the material properties in the growth direction only; the realization of local material modifications to generate lateral heterostructures with such a bottom-up approach is extremely elaborate, since it would require the use of lithography, etching, and either selective epitaxy or epitaxy and chemical–mechanical polishing, giving rise to interface issues, non-planar substrates, etc. This article shows the possibility of fabricating Si1−x−yGeySnx alloys by Sn ion beam implantation into Si1−yGey layers followed by millisecond-range flash lamp annealing (FLA). The materials are investigated by Rutherford backscattering spectrometry, micro-Raman spectroscopy, x-ray diffraction, and transmission electron microscopy. The fabrication approach was adapted to ultra-thin Si1−yGey layers on silicon-on-insulator substrates. The results show the fabrication of single-crystalline Si1−x−yGeySnx with up to 2.3 at. % incorporated Sn without any indication of Sn segregation after recrystallization via FLA. Finally, we exhibit the possibility of implanting Sn locally in ultra-thin Si1−yGey films by masking unstructured regions on the chip, thus demonstrating the realization of vertical as well as lateral Si1−x−yGeySnx heterostructures by Sn ion implantation and flash lamp annealing. [ABSTRACT FROM AUTHOR]

Ge1−xSnx and Si1−x−yGeySnx alloys are promising materials for future opto- and nanoelectronics applications. These alloys enable effective bandgap engineering, broad adjustability of their lattice parameter, exhibit much higher carrier mobility than pure Si, and are compatible with the complementary metal-oxide-semiconductor technology. Unfortunately, the equilibrium solid solubility of Sn in Si1−xGex is less than 1% and the pseudomorphic growth of Si1−x−yGeySnx on Ge or Si can cause in-plane compressive strain in the grown layer, degrading the superior properties of these alloys. Therefore, post-growth strain engineering by ultrafast non-equilibrium thermal treatments like pulse laser annealing (PLA) is needed to improve the layer quality. In this article, Ge0.94Sn0.06 and Si0.14Ge0.8Sn0.06 thin films grown on silicon-on-insulator substrates by molecular beam epitaxy were post-growth thermally treated by PLA. The material is analyzed before and after the thermal treatments by transmission electron microscopy, x-ray diffraction (XRD), Rutherford backscattering spectrometry, secondary ion mass spectrometry, and Hall-effect measurements. It is shown that after annealing, the material is single-crystalline with improved crystallinity than the as-grown layer. This is reflected in a significantly increased XRD reflection intensity, well-ordered atomic pillars, and increased active carrier concentrations up to 4 × 1019 cm−3. [ABSTRACT FROM AUTHOR]

The burgeoning fields of spintronics and topological electronics require materials possessing a unique combination of properties: ferromagnetism, metallicity, and chemical stability. SrRuO3 (SRO) stands out as a compelling candidate due to its exceptional combination of these attributes. However, understanding its behavior under tensile strain, especially its thickness-dependent changes, remains elusive. This study employs machine-learning-assisted molecular beam epitaxy to investigate tensile-strained SRO films with thicknesses from 1 to 10 nm. This work complements the existing focus on compressive-strained SRO, opening a new avenue for exploring its hitherto concealed potential. Using soft x-ray magnetic circular dichroism, we uncover an intriguing interplay between film thickness, electronic structure, and magnetic properties. Our key findings reveal an intensified localization of Ru 4d t2g-O 2p hybridized states at lower thicknesses, attributed to the weakened orbital hybridization. Furthermore, we find a progressive reduction of magnetic moments for both Ru and O ions as film thickness decreases. Notably, a non-ferromagnetic insulating state emerges at a critical thickness of 1 nm, marking a pivotal transition from the metallic ferromagnetic phase. These insights emphasize the importance of considering thickness-dependent properties when tailoring SRO for next-generation spintronic and topological electronic devices. [ABSTRACT FROM AUTHOR]

Scanning spreading resistance microscopy (SSRM) measurements were performed on GaAs thick films grown by hydride vapor phase epitaxy technology under different growth conditions to evaluate their carrier concentrations. For this purpose, a calibration curve was established based on a multilayer staircase structure grown by molecular beam epitaxy. The dopant calibration range measured by secondary ion mass spectrometry is from 5 × 1016 to 1019 cm−3. An abnormal phenomenon in the calibration process was explained by taking into account the parasitic parallel resistance of the calibration samples. Finally, the calibration curve was used to quantitatively analyze the carriers inside the Zn doping p-type GaAs film from 4 × 1016 to 1018 cm−3 range. We demonstrate here the applicability of SSRM to the in-depth analysis of thick epilayers, providing new inputs for the control of thick film technologies. [ABSTRACT FROM AUTHOR]

Four types of barrier detectors based on a type II InAs/InAsSb superlattice with a wide-gap barrier made of a solid AlInAsSb lattice matched to the GaSb buffer were compared. The tested detectors differed in the type of doping of the active layer and the level and type of doping of the contact layer at the barrier. The epitaxial layers were deposited on GaAs (100) substrates using the molecular beam epitaxy method. The spectral and current–voltage characteristics of the analyzed detectors were compared. The highest current responsivities were observed in the structure with a p-type absorber (p+BpN+). Detectors with an n-type absorber (p+Bnn+, n+Bnn+, and nBnn+) show an increase in the current responsivity with an increase in the reverse bias voltage due to the reduction in the undesirable barrier in the valence band. Arrhenius characteristics for the dark current show that only in nBnn+ detectors, it was possible to limit the generation–recombination current. These detectors at 150 K were characterized by the highest normalized detectivity of approximately 3 × 1011 cm · Hz1/2/W. The obtained results were compared with literature data, showing that the parameters of type II superlattice photodetectors are close to those of HgCdTe photodiodes according to the "Rule 07" and "Rule 22" principles. [ABSTRACT FROM AUTHOR]

In this work, we present the investigation of the magnetotransport properties of a capped SnTe film, grown by molecular beam epitaxy, using Shubnikov–de Haas oscillations for the detection of Dirac fermions. The cap layer used was a 10 nm thick Sn0.98Eu0.02Te film, which can also contribute to the transport such that it is mandatory to isolate its contribution from the electrical transport measured in the sample. To separate the contribution from both layers, photoconductivity measurements were performed. A detailed analysis of the Shubnikov–de Haas oscillations is carried out using theoretical expressions and building the Landau-level indexation. We found that Dirac fermions are detected in the SnTe layer, while the cap layer contributes with trivial fermions, protecting SnTe against deterioration due to exposure to the atmosphere. [ABSTRACT FROM AUTHOR]

The surface electric field (F) of β-FeSi2 has been investigated by Franz–Keldysh oscillations (FKOs) observed in photoreflectance spectra. The FKO signals were observed in an undoped β-FeSi2 epitaxial layer (3 nm) grown on p+-type β-FeSi2 layers (UP+ structure) by molecular beam epitaxy. The surface electric field obtained from the FKO at 11 K was F = 220 kV/cm. The surface electric field was almost independent of the excitation light power and temperature, showing that the surface electric field was not affected by the surface photovoltage effect. In the dependence of F on the thickness of the undoped layer (d), F was nearly constant at d = 2–56 nm. The result showed that the surface electric field of β-FeSi2 was applied to a thin region (<2 nm) at the surface. [ABSTRACT FROM AUTHOR]

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition. [ABSTRACT FROM AUTHOR]

Emerging quantum materials as well as novel traditional electronic and photonic materials may enable a new generation of information science devices if they can be synthesized on suitable substrates. Additionally, material and device designs may benefit from tunable properties through engineered epitaxial strain for the manipulation of the electronic character. In this work, three series of III–V ternary alloys, GaInSb, AlInSb, and InAsSb, are grown via molecular beam epitaxy on GaAs (001) substrates to explore the flexibility of abrupt metamorphic epitaxial layers with tunable lattice parameters between 6.135 and 6.479 Å. Their deposition on both homomorphic GaAs and pseudomorphic AlAs buffers is also explored. The structures of these alloys are characterized via reflection high-energy electron diffraction, high-resolution x-ray diffraction, atomic force microscopy, and transmission electron microscopy to assess their suitability as stable buffer layers with wide variability of accessible lattice parameters. [ABSTRACT FROM AUTHOR]

The Sn-doping effects on the electrical conduction and optical properties of GaAs(631)A epilayers grown by molecular beam epitaxy were investigated. We found that the conduction type conversion, frequently observed in the doping of layers grown on high-index substrates, is avoided when tin-doping is implemented. The maximum free-carrier concentration (n) obtained in GaAs(631):Sn was 2 × 1019 cm−3, an order of magnitude higher than previously reported for GaAs(631):Si, and within the same order of magnitude for the growth of GaAs(100):Si. The electron mobility was suitable for many optoelectronic applications. Raman spectroscopy showed low lattice disorder in (631) oriented samples, compared with singular (100) samples. The photoluminescence characterization of the samples revealed the blueshift of the optical transitions close to E0 associated with the Moss–Burstein effect for Sn doping. Photoreflectance spectroscopy was used to study the doping properties at the critical points E1 and E1 + Δ1, where the major affectation with n was perceived in the broadening parameter Γ. [ABSTRACT FROM AUTHOR]

Wurtzite ScxAl1−xN/GaN (x = 0.13–0.18) multi-quantum wells grown by molecular beam epitaxy on c-plane GaN are found to exhibit remarkably strong and narrow near-infrared intersubband absorption in the technologically important 1.8–2.4 μm range. Band structure simulations reveal that, for GaN wells wider than 3 nm, the quantized energies are set by the steep triangular profile of the conduction band caused by intrinsic polarization fields. As a result, the intersubband transition energies provide unique and direct access to essential ScAlN polarization parameters. Measured infrared absorption indicates that the spontaneous polarization difference of the presumed lattice-matched Sc0.18Al0.82N/GaN heterostructure is smaller than the theoretically calculated value. The intersubband transition energies are relatively insensitive to the barrier alloy composition indicating negligible variation of the net polarization field in the probed 0.13–0.18 Sc composition range. [ABSTRACT FROM AUTHOR]

MnO(001) thin films were grown on commercial MgO(001) substrates at 520 °C by reactive molecular beam epitaxy (MBE) using Mn vapor and O2-seeded supersonic molecular beams (SMBs) both with and without radio frequency (RF) plasma excitation. For comparison, MnO(001) films were grown by reactive MBE using O2 from a leak valve. X-ray photoelectron spectroscopy confirmed the Mn2+ oxidation state and 10%–15% excess oxygen near the growth surface. Reflection high-energy electron diffraction and x-ray diffraction evidenced that the films were rock salt cubic MnO with very strong (001) orientation. High-angle annular dark field scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy demonstrated abrupt MnO/MgO interfaces and indicated [(001)MnO||(001)MgO] epitaxial growth. Ex situ atomic force microscopy of films deposited without RF excitation revealed smooth growth surfaces. An SMB-grown MnO(001) film was converted to Mn3O4 with strong (110) orientation by post-growth exposure to an RF-discharge (RFD) SMB source providing O atoms; the surface of the resultant film contained elongated pits aligned with the MgO 110 directions. In contrast, using the RFD-SMB source for growth resulted in MnO(001) films with elongated growth pits and square pyramidal hillocks aligned along the MgO 110 and 100 directions, respectively. [ABSTRACT FROM AUTHOR]

The giant spin–orbit torque (SOT) generated by topological surface states in topological insulators (TIs) provides an energy-efficient writing method for magnetic memory. In this study, we demonstrate a topological insulator/spin valve (TI/SV) device that operates at room temperature. An ultrathin, high-quality TI (Bi0.5Sb0.5)2Te3 (BST) thin film is epitaxially grown as a functional layer on a (0001)-Al2O3 substrate via molecular beam epitaxy in ultrahigh vacuum. Subsequently, Ta/CoFe/Cu/CoFe/IrMn layers are grown on BST/Al2O3 thin films using magnetron sputtering to form TI/SV devices via a subsequent standard lithography process. The resulting TI/SV devices exhibit a giant magnetoresistance of up to ∼1.1% at room temperature. Additionally, a low switching current density of approximately 1.25 × 105 A cm−2 is achieved, which implies high potential for further reducing the energy consumption of SOT-based devices. The SOT conversion efficiency and charge-spin conversion efficiency of the TI layer are approximately 4.74 × 10−6 Oe A−1 cm2 and 1.33, respectively, as extracted from the SOT-induced shift of the magnetic switching field. Moreover, the switching current density reduces steadily with the device size scaling down. This study can facilitate the realization of energy-efficient magnetic memory devices in the future. [ABSTRACT FROM AUTHOR]

In this work, we show how diffraction-based chemically sensitive dark-field transmission electron microscopy (DFTEM) reveals the presence of Bi hetero-antisites (BiGa) at the interface of Ga(As,Bi)/(Al,Ga)As quantum well (QW) structures grown by molecular beam epitaxy on GaAs(001). The presence of BiGa is demonstrated by the striking appearance of "dark-lines" at the interfaces under two-beam DFTEM imaging conditions using the (002) diffraction spot. Additional analytical scanning (S)TEM procedures reveal Ga depletion and Bi accumulation at the exact position of the dark-lines, consistent with BiGa at this location. The precise location of the dark-lines agrees with the position of growth interruptions made to adjust substrate temperature and the As/Ga flux ratio and, most importantly, the realization of a Bi pre-treatment before QW growth. We believe the Bi pre-treatment may have favored formation of BiGa hetero-antisites. We validate the use of g002 DFTEM for further investigations of the intricate bismuth incorporation into the lattice and its dependence on the growth conditions. Finally, g002 DFTEM imaging is positioned as a very powerful technique for the detection of point defects in general in materials with the zinc-blende crystal structure, beyond dilute bismide alloys. [ABSTRACT FROM AUTHOR]

β-Ga2O3, an emerging wide bandgap semiconductor material, holds significant potential for various applications. However, challenges persist in improving the crystal quality and achieving controllable doping of β-Ga2O3. In particular, the relationship between these factors and the mechanisms behind them are not fully understood. Molecular beam epitaxy (MBE) is viewed as one of the most sophisticated techniques for growing high-quality crystalline films. It also provides a platform for studying the effects of doping and defects in heteroepitaxial β-Ga2O3. In our study, we tackled the issue of Si source passivation during the MBE growth of Si-doped β-Ga2O3. We did this by using an electron beam vaporize module, a departure from the traditional Si effusion cell. Our research extensively explores the correlation between Si doping concentration and film properties. These properties include microstructure, morphology, defects, carrier conductivity, and mobility. The results from these investigations are mutually supportive and indicate that a high density of defects in heteroepitaxial β-Ga2O3 is the primary reason for the challenges in controllable doping and conductivity. These insights are valuable for the ongoing development and enhancement of β-Ga2O3-based device techniques. [ABSTRACT FROM AUTHOR]

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids. [ABSTRACT FROM AUTHOR]

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices. [ABSTRACT FROM AUTHOR]

The spin control possibility and its application in optoelectronic devices began an intensive research into its utilization, in particular, in the wide-gap semiconductors such as GaN doped with transition metal ions. Due to a strong p–d hybridization in Ga1−xMnxN, the Curie temperature above 300 K was already expected for x = 5%, providing that the free hole concentration necessary for the hole-mediated ferromagnetism exceeds 1020 cm−3. In this context, the development of non-equilibrium techniques enabled the engineering high-quality epitaxial layers of (Ga,Mn)N exhibiting uniform ferromagnetism at low-end cryogenic temperatures. The Tutorial is focused on the molecular beam epitaxy growth method of the Mn-enriched GaN magnetic semiconductors, summarizes the (Ga,Mn)N structural and electronic studies, and explains fundamental ferromagnetic properties, including the determination of the Mn concentration and the Curie temperature based on magnetic measurements. Most studies reveal the homogenous substitution of Mn3+ ions in the GaN matrix. Nevertheless, achieving room-temperature ferromagnetism still remains a challenge. Therefore, in the Tutorial, future research is suggested that can help obtain the homogenous ferromagnetism in (Ga,Mn)N at much elevated temperatures. [ABSTRACT FROM AUTHOR]

AgCrS e 2 exhibits remarkably high ionic conduction, an inversion symmetry-breaking structural transition, and is host to complex non-colinear magnetic orders. Despite its attractive physical and chemical properties and its potential for technological applications, studies of this compound to date are focused almost exclusively on bulk samples. Here, we report the growth of AgCrS e 2 thin films via molecular beam epitaxy. Single-orientated epitaxial growth was confirmed by x-ray diffraction, while resonant photoemission spectroscopy measurements indicate a consistent electronic structure as compared to bulk single crystals. We further demonstrate significant flexibility of the grain morphology and cation stoichiometry of this compound via control of the growth parameters, paving the way for the targeted engineering of the electronic and chemical properties of AgCrS e 2 in thin-film form. [ABSTRACT FROM AUTHOR]

The effect of the GaAsP strain-compensating layer on type-II GaAs1−xSbx/InyGa1−yAs was investigated. GaAsSb/InGaAs multiple quantum wells (MQWs) without and with GaAsP strain-compensating layers were grown by molecular beam epitaxy. Increasing Sb or In compositions can extend photoluminescence (PL) emission at longer wavelength along with the highly induced compressive strain in the QWs. The power-dependent PL measured at low temperature reveals the type-II band characteristics of the GaAs1−xSbx/InyGa1−yAs system. A detailed analysis of the experimental data reveals that the GaAsP layers compensate the compressive strain of GaAsSb/InGaAs. The type-II QWs with GaAsP layers, (8 nm) GaAs0.84Sb0.16/(2.5 nm) In0.3Ga0.7As/(10 nm) GaAs0.85P0.15, emits PL at ∼1.1 μm, up to 210 K, while the PL of those strained sample without GaAsP vanishes at lower temperature. In view of the described sample, x-ray diffraction (XRD) analysis along with the simulation shows the validity of the procedure, resulting in nearly matched parameters of QW thicknesses and material compositions—(8.9 nm) GaAs0.835Sb0.165/(2.3 nm) In0.3Ga0.7As/(10.3 nm) GaAs0.85P0.15, with those of the designed QW. The thicknesses of QW from the TEM image, (8.6 nm) GaAsSb/(3.1 nm) InGaAs/(10.1 nm) GaAsP, agree well with the XRD results. [ABSTRACT FROM AUTHOR]

Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 1019 cm−3 and occurs rapidly at 1 × 1020 cm−3. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase. [ABSTRACT FROM AUTHOR]

We study epitaxial growth, crystal structures, and magnetic properties of Fe–As compound thin films grown on GaAs (111)B substrates at various values of the As4:Fe flux ratio γ, using molecular beam epitaxy. The samples grown at low As4 flux (γ = 0.3, sample A) show mainly a body-centered-cubic (bcc) crystal structure, exhibiting ferromagnetic properties similar to bcc Fe. Meanwhile, the Fe–As samples grown at medium γ (2.7–4.5, sample group B) comprise regions of Ni2In-type FeAs (a hexagonal crystal with lattice constants of a = 0.399 nm and c = 0.536 nm), which are grown at the bottom and interface with the GaAs buffer layer, and a layer of non-stoichiometric FeAs with a DO3 structure (a = 0.522 nm) formed on the top. The DO3-structure FeAs phase contains partially transformed regions, which are characterized by thin stripes in a scanning transmission electron microscopy image. Furthermore, in the sample grown with high γ = 8.5 (sample C), a hexagonal Fe–As crystal with a large in-plane lattice constant (a = 0.691 nm and c = 0.542 nm) and threefold screw axes are observed. None of these crystal structures of Fe–As compounds has ever been reported. While sample C shows no ferromagnetism, the samples in group B exhibit strong ferromagnetism with high Curie temperature TC above 400 K. These new ferromagnetic Fe–As compounds are promising for spintronic device applications. [ABSTRACT FROM AUTHOR]

Using a molecular beam epitaxy technique, we experimentally demonstrate a multiferroic heterostructure consisting of metastable ferromagnetic Co 3 Mn on piezoelectric Pb(Mg 1 / 3 Nb 2 / 3 )O 3 –PbTiO 3 (PMN-PT). Inserting a 2-nm-thick Fe layer between Co 3 Mn and PMN-PT(001) allows the formation of bcc Co 3 Mn layers even at an extremely low growth temperature of ∼ 80 ° C. Upon increasing this temperature to 200 ° C, a bcc Co 3 Mn/Fe/PMN-PT(001) multiferroic heterostructure with a relatively large saturation magnetization of ∼ 1680 kA/m and an atomically flat interface is obtained, resulting in an obvious converse magnetoelectric (CME) effect. The large CME effect originates mainly from the strain-induced modulation of the magnetic anisotropy energy, supported by the first-principles calculations. [ABSTRACT FROM AUTHOR]

Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures.

GeSnC alloys offer a route to direct bandgap semiconductors for CMOS-compatible lasers, but the use of CBr4 as a carbon source was shown to reduce Sn incorporation by 83%–92%. We report on the role of thermally cracked H in increasing Sn incorporation by 6x–9.5x, restoring up to 71% of the lost Sn, and attribute this increase to removal of Br from the growth surface as HBr prior to formation of volatile groups such as SnBr4. Furthermore, as the H flux is increased, Rutherford backscattering spectroscopy reveals a monotonic increase in both Sn and carbon incorporation. X-ray diffraction reveals tensile-strained films that are pseudomorphic with the substrate. Raman spectroscopy suggests substitutional C incorporation; both x-ray photoelectron spectroscopy and Raman suggest a lack of graphitic carbon or its other phases. For the lowest growth temperatures, scanning transmission electron microscopy reveals nanovoids that may account for the low Sn substitutional fraction in those layers. Conversely, the sample grown at high temperatures displayed abrupt interfaces, notably devoid of any voids, tin, or carbon-rich clusters. Finally, the surface roughness decreases with increasing growth temperature. These results show that atomic hydrogen provides a highly promising route to increase both Sn and C to achieve a strongly direct bandgap for optical gain and active silicon photonics. [ABSTRACT FROM AUTHOR]

Phosphorus (P) doping in ZnTe grown by molecular beam epitaxy (MBE) under alternating source supply method was investigated to achieve p-type P-doped ZnTe (ZnTe:P) thin films using InP as a P dopant source, and the result was compared with those grown under a simultaneous MBE growth where the source beams were supplied simultaneously. As a result, P concentration in ZnTe thin films was found to increase with increasing the InP flux, and high P concentration up to 6.6 × 1019 cm−3 was confirmed by secondary ion mass spectroscopy (SIMS) analyses. However, In incorporation was also observed in the ZnTe:P thin films, despite that the detected In concentration by SIMS was more than one order of magnitude lower than the P concentration and almost two order of magnitude lower than those grown by the simultaneous MBE. Photoluminescence measurement of ZnTe:P thin film grown under alternating source supply showed a P-related acceptor bound exciton (Ia) peak at 2.37 eV, and the intensity of Ia emission increased after the annealing treatment, indicating the activation of P acceptor. The annealing also decreases the resistivity of the film. The results clearly indicate that the alternating source supply growth is effective to obtain ZnTe:P thin films with better P doping properties. [ABSTRACT FROM AUTHOR]

Direct bandgap group IV materials could provide intimate integration of lasers, amplifiers, and compact modulators within complementary metal–oxide–semiconductor for smaller, active silicon photonics. Dilute germanium carbides (GeC) with ∼1 at. % C offer a direct bandgap and strong optical emission, but energetic carbon sources such as plasmas and e-beam evaporation produce defective materials. In this work, we used CBr4 as a low-damage source of carbon in molecular beam epitaxy of tin-free GeC, with smooth surfaces and narrow x-ray diffraction peaks. Raman spectroscopy showed substitutional incorporation of C and no detectable sp2 bonding from amorphous or graphitic carbon, even without surfactants. Photoluminescence shows strong emission compared with Ge. [ABSTRACT FROM AUTHOR]

Direct bandgap group IV materials could provide intimate integration of lasers, amplifiers, and compact modulators within complementary metal–oxide–semiconductor for smaller, active silicon photonics. Dilute germanium carbides (GeC) with ∼1 at. % C offer a direct bandgap and strong optical emission, but energetic carbon sources such as plasmas and e-beam evaporation produce defective materials. In this work, we used CBr4 as a low-damage source of carbon in molecular beam epitaxy of tin-free GeC, with smooth surfaces and narrow x-ray diffraction peaks. Raman spectroscopy showed substitutional incorporation of C and no detectable sp2 bonding from amorphous or graphitic carbon, even without surfactants. Photoluminescence shows strong emission compared with Ge. [ABSTRACT FROM AUTHOR]

InAs nanowires (NWs) were grown on SiOx pinholes formed on Si(111) substrates by molecular beam epitaxy. Influences of electron-beam (EB) irradiation on the SiOx layer on the pinhole formation and the subsequent InAs NW growth were studied. As the EB irradiation dose increased, the pinhole density in the SiOx layer decreased. From atomic force microscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy results, it was found that the pinhole etching of the SiOx layer by Ga droplets was suppressed by carbon adsorption due to the EB irradiation. By forming high-density pinholes on the SiOx layer without the EB irradiation, high-density InAs NWs with 1–2 × 1010 cm−2 were grown successfully, and the uniformity in the NW diameter improved. The standard deviation of the NW diameter was 1.8 nm (8.8%) for high-density NWs. In addition, the NW diameter decreased with decreasing EB dose, and the NW diameter was controlled by adjusting the diameter of Ga droplets forming the pinholes. As the NW diameter decreased, photoluminescence spectra of the NWs shifted to higher energies than the bandgap energy of the wurtzite InAs bulk. From these results, we successfully fabricated high-density and high-uniformity InAs NWs with quantum size effects on EB-unirradiated SiOx/Si(111). [ABSTRACT FROM AUTHOR]

This work investigates the structural properties of HgCdTe thin films grown on InSb (211)B substrates using molecular beam epitaxy (MBE). The Cd composition of thin films is accurately determined using non-destructive approaches based on x-ray diffraction (XRD) and reflectance infrared spectroscopy. The as-grown HgCdTe thin films exhibit characteristic surface defects with a size of 7–10 μm and density of ∼105 cm−2, resulting in an additional spread in XRD full width at half maximum. Cross-sectional transmission electron microscopy results indicate that these defects are caused by surface In droplet formation during the oxide removal process of InSb substrate, which subsequently results in the formation of In4Te3 inclusions and extended defects in MBE-grown HgCdTe. Our findings provide additional confirmation that suppressing thermally induced damage of the InSb substrate is necessary for fabricating high-performance infrared detectors using HgCdTe grown on InSb substrates. [ABSTRACT FROM AUTHOR]

High ScN fraction ScxAl1−xN has promise in important application areas including wide bandwidth RF resonators and filters, and ferroelectric devices such as non-volatile memory, but demands high crystal quality. In this work, the role of the nucleation layer (NL), ScxAl1−xN growth temperature, and strain management to preserve the wurtzite crystal structure are investigated to maximize both acoustoelectric and ferroelectric material properties for high ScN fraction ScxAl1−xN grown on SiC substrates. A 5 nm AlN nucleation layer reduces the x-ray diffraction 0002 reflection full width at half maximum (FWHM) for a Sc0.32Al0.68N film by almost a factor of 2, and reducing the growth temperature to 430 °C enables a Sc0.40Al0.60N film with a FWHM of 4100 arcsec (1.1°) while being only 150 nm thick. Grading the initial ScxAl1−xN layer from x = 0.32 to 0.40 suppresses the formation of rock-salt grain nucleation at the Sc0.40Al0.60N lower interface and reduces the anomalously oriented grain density by an order of magnitude. Increasing the total ScxAl1−xN growth thickness to 500 nm produces an average x = 0.39 ScxAl1−xN layer with a FWHM of 3190 arcsec (0.89°) and an anomalously oriented grain areal fill factor of 1.0%. These methods enable the lowest heteroepitaxial ScxAl1−xN FWHM reported for x ∼ 0.4, with layer thicknesses and defect densities appropriate for high frequency (>10 GHz) filter applications. [ABSTRACT FROM AUTHOR]

Tunability of interfacial effects between two-dimensional (2D) crystals is crucial not only for understanding the intrinsic properties of each system, but also for designing electronic devices based on ultra-thin heterostructures. A prerequisite of such heterostructure engineering is the availability of 2D crystals with different degrees of interfacial interactions. In this work, we report a controlled epitaxial growth of monolayer TaSe2 with different structural phases, 1H and 1 T, on a bilayer graphene (BLG) substrate using molecular beam epitaxy, and its impact on the electronic properties of the heterostructures using angle-resolved photoemission spectroscopy. 1H-TaSe2 exhibits significant charge transfer and band hybridization at the interface, whereas 1 T-TaSe2 shows weak interactions with the substrate. The distinct interfacial interactions are attributed to the dual effects from the differences of the work functions as well as the relative interlayer distance between TaSe2 films and BLG substrate. The method demonstrated here provides a viable route towards interface engineering in a variety of transition-metal dichalcogenides that can be applied to future nano-devices with designed electronic properties.

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope concentration depth profiles in a SiGe/28Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time‐of‐flight secondary‐ion mass spectrometry down to their respective limits of isotope concentrations and depth resolution. Spin‐echo dephasing times T2echo=128μs$T_2^\mathbf {echo}=128 \,\umu\mathrm{s}$ and valley energy splittings EVS around 200μeV$200 \,\umu\mathrm{e\mathrm{V}}$ have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing toward the suppression of qubit decoherence through hyperfine interaction with crystal host nuclear spins or via scattering between valley states. The concentration of nuclear spin‐carrying 29Si is 50 ± 20ppm in the 28Si QW. The resolution limits of APT allow to uncover that both the SiGe/28Si and the 28Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment, representative of the thermal budget experienced by the heterostructure during qubit device processing, broadens the top SiGe/28Si QW interface by about two monolayers, while the width of the bottom 28Si/SiGe interface remains unchanged. Using a tight‐binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of 0.3$0.3$% in the QW, resulting from segregation, is instrumental for the observed large EVS=200μeV$E_\mathrm{VS}=200 \,\umu\mathrm{e\mathrm{V}}$. Minimal Ge additions <1%, which get more likely in thin QWs, will hence support high EVS without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubit properties. [ABSTRACT FROM AUTHOR]

The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (TN ≈ 310 K) and semiconducting properties. The results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films are presented. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle‐resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first‐principles calculations for altermagnetic spin‐splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn‐richness that is intrinsic to the MBE‐grown samples. When combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications. [ABSTRACT FROM AUTHOR]

A significant (up to 4 times) photoluminescence enhancement of single InP/InAsP/InP nanowires transferred onto a silicon oxide‐covered silver layer on silicon substrate with a metal surface roughness level of less than 1 nm and a dielectric thickness of 5 nm has been demonstrated. This phenomenon is explained by the interaction of electron–hole pairs in the semiconductor with surface plasmon polaritons. The photoluminescence kinetics and results of modeling confirm the indicated enhancement mechanism. [ABSTRACT FROM AUTHOR]

Sc-alloyed AlN epilayers were grown on commercial GaN/sapphire templates by molecular beam epitaxy (MBE) to validate the Sc composition of 18% and in-plane lattice matching with GaN. As-grown Sc0.18Al0.82N/GaN heterostructure exhibits a two-dimensional electron gas (2DEG) density of 3.58 × 1013 (2.96 × 1013) cm−2 and a mobility of 241 (295) cm2⋅V−1⋅s−1 at 300 (77) K, demonstrating the feasibility of employing its high spontaneous polarization toward high 2DEG density and highlighting detrimental impurity scattering due to the regrowth interface. Implementation of AlN impurity blocking layers boosts 2DEG mobility to 1290 (8730) cm2 V−1⋅s−1at 300 (77) K. In addition, we have engineered a surface-treatment strategy to selectively decompose the 2.5-nm-thick GaN cap layer of commercial GaN(2.5 nm)/Al0.20Ga0.80N(22 nm)/GaN heterostructure epi-wafers at high temperature prior to MBE regrowth of Sc0.18Al0.82N to eliminate impurity incorporation at the regrowth interface. Regrowth of a Sc0.18Al0.82N layer on the pristine Al0.20Ga0.80N surface increases 2DEG density from 7.89 × 1012 to 9.57 × 1012 cm−2, together with a slight reduction in mobility from 2160 to 1970 cm2⋅V−1⋅s−1 at 300 K, reducing the sheet resistance by 10%. Our calculation implies that it is practical to boost 2DEG density to over 2.0 × 1013 cm−2 by thinning Al0.20Ga0.80N down to 4 nm. [ABSTRACT FROM AUTHOR]

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 × 1020 cm−3 nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications. [ABSTRACT FROM AUTHOR]

Due to the outstanding physical properties and the integrated characteristics, such as the gapless band structure, high state density, high conductivity, dangling‐bond‐free surface, and tunable Fermi level, two‐dimensional layered topological semimetals (2D LTSMs) exhibit a promising application prospect for including electrode contact and terahertz detection. Despite the surging in attention, a comprehensive strategy is still crucial to meet the necessary conditions for the practical application of 2D LTSMs. According to the fermion types and the band structures, 2D LTSMs can be divided into Dirac semimetals, Weyl semimetals, and nodal‐line semimetals. This review comprehensively encapsulates the intrinsic properties, typical synthesis methods, and applications in electronics and optoelectronics about these 2D LTSMs. To establish the fundamental theory, typical crystal structures, and physical properties of different 2D LTSMs are summarized at first. The effective synthesis strategies including exfoliation, molecular beam epitaxy, and vapor deposition are analyzed systematically. Finally, the article emphasizes the exploration of applications, significant challenges, and promising development directions of 2D LTSMs, which will drive 2D LTSMs to become the promisingly leading technology for next‐generation electronic and optoelectronic systems. [ABSTRACT FROM AUTHOR]

Photoelectrolysis of water is a sustainable option for the production of hydrogen fuel. GaN nano‐ or microstructures are considered for water‐splitting due to their general high chemical stability and high surface‐to‐volume ratio enhancing the process effectiveness. In this study GaN structures with dodecagonal microrods are used as a working electrode for the water‐splitting process. Microrods are grown using a plasma‐assisted molecular beam epitaxy process allowing tailoring of microrod height and density. Their unique property is the of presence twelve sidewalls with alternating a‐ and m‐plane orientations. This enables a simultaneous study of the chemical stability of c‐, a‐, and m‐plane walls of GaN. The water‐splitting process is performed using a 1 mol l−1 NaOH electrolyte solution. Non‐zero current measured at zero bias under illumination indicates that the process takes place. A degradation of the GaN structure is observed after a prolonged process time. In short‐term exposures, etching of microrod sidewalls is observed. Roughening of the a‐plane walls studied by transmission electron microscopy indicates that this orientation is etched with a fastest rate. The internal crystalline structure is not influenced by the etching and remains stable as shown by the X‐ray absorption spectroscopy study. [ABSTRACT FROM AUTHOR]

In nitride‐based laser diodes, the poor conductivity of the p‐type region, higher light absorption, and the difficult processing of an ohmic p‐contact with low resistance are major challenges. Recently interband tunnel junctions (TJs) have been used to replace much of the hole transport layers by electron transport layers, and also to allow for two n‐type contacts. In this work, GaN‐based laser diode stacks with TJs are analyzed. The devices are grown with plasma‐assisted molecular beam epitaxy and include two waveguides with active regions consisting of 25 nm wide InGaN quantum well (QW), each, which are connected through a TJ. A second TJ is placed on top to eliminate the p‐type contact. In this study, the time‐dependent spectral behavior of such structures is analyzed by streak camera measurements. The two QWs have different lasing thresholds and emission wavelength. Using the temporal resolution of the streak camera, a lasing onset of the second QW a few nanoseconds after the first one is observed. Single shot measurements show unusual intensity oscillations from both QWs, however, one is stable and the other laser diode (LD) has irregular oscillations. [ABSTRACT FROM AUTHOR]

Recently, N‐polar GaN/AlGaN/AlN high‐electron‐mobility transistors (HEMTs) have been demonstrated using molecular beam epitaxy (MBE). However, the MBE method is not suitable for mass production because of the need for ultrahigh vacuum, small chamber size, and long deposition time. This study fabricates N‐polar GaN‐ or AlGaN‐channel HEMTs on AlN with polarization directions opposite to those of Ga‐polar GaN‐based HEMTs without any cap layer and demonstrates field‐effect transistor static characteristics. In this article, the focus is on the crystal quality of the underlying layer and the effect of crystalline quality on electrical properties is investigated. The crystalline quality of the twist component of the N‐polar AlN layer is improved by combining the Al‐polar tiny‐pit layer and polarity inversion from Al‐ to N‐polar. Finally, it is shown that the crystalline quality of the twist component is a crucial factor for improving the electron mobility of the N‐polar GaN channel and simultaneously increasing IDS. [ABSTRACT FROM AUTHOR]