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The formation of AlN/AlGaN short period superlattices (SPSLs) was investigated though the introduction of a constant Ga overpressure during the metal modulated epitaxy (MME) growth of AlN. A combination of x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) analyses found that control over the Al composition in the AlGaN layer was achieved through modulating the Ga beam equivalent pressure (BEP), with a minimum partial pressure of 3 × 10−7 Torr needed for Ga to incorporate at a growth temperature of 825 °C. A minimum Al composition in the AlGaN layer of 72% was achieved for a Ga BEP of 1 × 10−6 Torr using this method. An apparent limit of the AlGaN layer thickness of 3–4 ML indicated that the incorporation of Ga was confined to the consumption region of the MME growth process. Determination of this behavior made clear the requirements of having both XRD and STEM in order to be able to fully characterize the SPSL layer structure. Finally, AFM imaging highlighted that the presence of Ga on the surface behaved as a surfactant, with a minimum RMS roughness of 0.46 nm achieved at the maximum Ga BEP of 1 × 10−6 Torr. [ABSTRACT FROM AUTHOR]

In this work, the impact of Al doping and Mg alloying on the conduction band misalignment (Δ E C) between ZnO and (100) Si with a SiO x interlayer was studied by combining capacitance vs voltage, Hall and x-ray diffraction measurements, energy-dispersive x-ray spectroscopy, secondary mass spectrometry, and high-resolution scanning transmission electron microscopy. To decouple the effect of the high carrier density in the ZnO-based layers due to the Al introduction, the measured Δ E C was corrected for the conduction band lowering effect taking into account the conduction band non-parabolicity of ZnO. Then, from the Mg content dependence, using the interface-induced gap states approach, branch point energies referred to the valence band maximum equal to (2.7 ± 0.2) and (3.6 ± 0.4) eV were extracted for ZnO and MgO, respectively. These branch point energies were obtained under the assumption of a linear variation between the respective values of the corresponding two binary compound semiconductors, ZnO and MgO, and taking into account the presence of the SiO x interlayer. Furthermore, in the case of the undoped Zn 0.96 Mg 0.04 O layers, a ∼ 0.27 eV reduced Δ E C was found, with the difference with respect to Zn 0.94 Mg 0.06 O:Al attributed to the presence of a downward band bending toward the interface with SiO x. Full 1 × 1 cm test solar cells based on Zn 0.8 Mg 0.2 O:Al layers exhibited short circuit currents, open circuit voltages, fill factors, and efficiencies that varied in the (28 ± 1) mA / cm 2 , (430 ± 20) mV, (61 ± 2) %, and (7.2 ± 0.3) % ranges with the residual Δ E C ∼ 0.6 eV being among the main causes of the reduced device performances. [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]

Hafnium diboride (HfB2) is a promising candidate as a seed layer for GaN growth on Si substrates due to its excellent lattice and thermal coefficients matching with both materials. This work investigates the epitaxial growth of AlB2-type non-stoichiometric HfB2 (HfB2+δ with −0.1 < δ < 0.6) thin films on Si(111) using magnetron co-sputtering. We demonstrated that the process temperature significantly affected the surface roughness (RRMS ∼ 0.5–4 nm), film composition, and the nucleation of secondary impurity phases. Films deposited between 700 and 900 °C exhibit epitaxial growth on the Si substrate with a well-defined relationship of (0001) HfB 2 ‖ (111) Si and [ 11 2 ¯ 0 ] HfB 2 ‖ [ 1 1 ¯ 0 ] Si . Detailed x-ray diffraction and scanning transmission electron microscopy analyses reveal that impurity phases detected at high temperatures are primarily carbon-rich phases, identified as HfCx or HfCxBy. Interestingly, this secondary phase's crystal orientation follows the orientation of its surroundings. The different findings in terms of contamination (C and O) and deposition temperature offer valuable insights for further growth optimizing of high-quality epitaxial HfB2 thin films on Si(111) for future GaN-on-Si integration. [ABSTRACT FROM AUTHOR]

Ag-induced crystallization and layer exchange (AgILE) in a stack of amorphous Si/Ag/quartz substrate has been investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), scanning TEM-high angle annular dark field (STEM-HAADF) imaging, and electron tomography, covering length scales from a few tens of micrometers to a few tens of nanometers. The size of Ag grains in the underlying as-deposited Ag film varied from ∼10 to 500 nm. The following processes could be discerned using in situ heating of plan-view samples at 500 °C in STEM: (i) AgILE propagation preferentially along regions of small Ag grains, (ii) formation of pushed-up Ag in the vicinity of AgILE reaction cells, (iii) migration and agglomeration of pushed-up Ag on small and large Ag grains, which tend to inhibit AgILE and promote dendricity, and (iv) dispersion of pushed-up Ag, which tend to reduce dendricity. The resulting dendricity was largely confined to the peripheral regions of the impinging reaction cells and decreased with annealing time. In contrast, dendricity due to AgILE and crystallization at 550 °C is stable and extends right from the center to the periphery of the reaction cells. The microscopic mechanism of AgILE and, in particular, the effect of annealing temperature is investigated. The results are discussed in the light of existing literature and compared with Al-induced layer exchange. Annealing at temperature equal to or slightly less than 500 °C is found to be necessary in the case of AgILE to avoid dendricity and to obtain a continuous Si layer with large Si grains. [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]

Bragg interferometry (BI) is an imaging technique based on four-dimensional scanning transmission electron microscopy (4D-STEM) wherein the intensities of select overlapping Bragg disks are fit or more qualitatively analyzed in the context of simple trigonometric equations to determine local stacking order. In 4D-STEM based approaches, the collection of full diffraction patterns at each real-space position of the scanning probe allows the use of precise virtual apertures much smaller and more variable in shape than those used in conventional dark field imaging such that even buried interfaces marginally twisted from other layers can be targeted. With a coarse-grained form of dark field ptychography, BI uses simple physically derived fitting functions to extract the average structure within the illumination region and is, therefore, viable over large fields of view. BI has shown a particular advantage for selectively investigating the interlayer stacking and associated moiré reconstruction of bilayer interfaces within complex multi-layered structures. This has enabled investigation of reconstruction and substrate effects in bilayers through encapsulating hexagonal boron nitride and of select bilayer interfaces within trilayer stacks. However, the technique can be improved to provide a greater spatial resolution and probe a wider range of twisted structures, for which current limitations on acquisition parameters can lead to large illumination regions and the computationally involved post-processing can fail. Here, we analyze these limitations and the computational processing in greater depth, presenting a few methods for improvement over previous works, discussing potential areas for further expansion, and illustrating the current capabilities of this approach for extracting moiré-scale strain. [ABSTRACT FROM AUTHOR]

An area-selectable Mg doping via ion implantation (I/I) is essential to realize gallium nitride (GaN) based power switching devices. Conventional post-implantation annealing forms considerable defects in the GaN, resulting in extremely low activation efficiency. The recent invention of ultrahigh-pressure annealing (UHPA) has substantially improved the p-type activation efficiency; however, the UHPA causes an unexpected Mg diffusion. Thus, both annealing processes resulted in a much lower Mg concentration in the GaN matrix than the Mg dose. In this study, the effect of a sequential N I/I for p-type Mg-implanted GaN was investigated by the correlative cathodoluminescence, transfer length method, scanning transmission electron microscopy, and atom probe tomography (APT) analyses. APT results have revealed that the sequential N I/I can successfully maintain the Mg concentration in the GaN matrix in the higher range of 1018 cm−3 or more. Our investigation suggests that sequential N I/I is a promising technique to maintain the Mg concentration higher and improve the p-type activation efficiency. [ABSTRACT FROM AUTHOR]

Dislocations have been proposed to affect the performance and reliability of GaN power semiconductors by being conductive pathways for leakage current. However, no direct evidence of a link between their electrical behavior and physical nature in carbon-doped semi-insulating GaN buffer layers has been obtained. Therefore, we investigate the electrical activity of dislocations by conductive atomic force microscopy and electron beam induced current to distinguish electrically active dislocations from non-active ones. We investigated six electrically active dislocations and discovered distinct carbon enrichment in the vicinity of all six dislocations, based on cross-sectional scanning transmission electron microscopy using electron energy loss spectrometry. Electrically non-active dislocations, which are the vast majority, sometimes also showed carbon enrichment, however, in only two out of seven cases. Consequently, carbon segregation seems to be a requirement for electrical activity, but a carbon surplus is not sufficient for electrical activity. We also performed first-principles total-energy calculations for mixed type threading dislocations, which validates carbon accumulation in the dislocation vicinity. The electrical and physical characterization results, complemented by density functional theory simulations, support the previously hypothesized existence of a carbon defect band and add new details. [ABSTRACT FROM AUTHOR]

Energy-dispersive x-ray spectroscopy and high-angle annular dark-field in a Cs-corrected scanning transmission electron microscope are employed to characterize the atomic-scale strain distribution in the active region of the InAs-based interband cascade laser. For the first time, energy-dispersive x-ray spectroscopy is utilized for the quantitative calculation of the zero-strain region, by which the geometric phase analysis of high-angle annular dark-field imaging has been carried out. The strain distribution of the active region with high accuracy has been obtained. The analysis of the out-of-plane strain shows that the active region in the InAs-based interband cascade laser is strain-compensated, while a certain degree of elemental intermixing still exists in the active region. This detailed strain distribution can provide valuable insights into the optimization of the growth conditions for the active region such as growth temperature, V/III flux ratio, and growth process to minimize the elemental intermixing and obtain a better performance interface while maintaining the strain-compensated state. [ABSTRACT FROM AUTHOR]

Domain boundaries in perovskite oxides often exhibit abundant physical properties and phenomena. Here, epitaxial LaTiO3 thin films on (100) SrTiO3 substrates are prepared by pulsed-laser deposition. X-ray diffraction and transmission electron microscopy investigations reveal that the epitaxial LaTiO3 thin films have good crystallinity but a high density of domain boundaries. Atomic-scale scanning transmission electron microscopy observations reveal that two types of domain boundaries are formed in the LaTiO3 thin films. The type I domain boundaries are formed on the {100} crystal planes, while the type II domain boundaries on the {110} crystal planes. Electron energy-loss spectroscopy analyses suggest that the valence states of Ti ions at the type I domain boundaries are +3, while those at the type II domain boundaries are +4. First-principles calculations reveal that the bandgap decreases at both domain boundaries compared to the bulk. The carrier concentration at the type I domain boundaries is significantly higher than that of the bulk, while the carrier concentration at the type II domain boundaries is lower. These findings suggest that domain boundaries play an important role in tailoring the electrical properties of the LaTiO3 thin films, thereby promoting the potential applications and property modulation of related materials and devices. [ABSTRACT FROM AUTHOR]

We report on the magnetic coupling behavior in all-ferromagnetic La0.7Sr0.3MnO3/SrRuO3 (LSMO/SRO) heterostructures deposited on the SrTiO3 (111) substrate, where the interface leads to an enhanced exchange field as compared to the one deposited on SrTiO3 (001). Importantly, high-resolution scanning transmission electron microscopy reveals distinct interface structures of the (111) heterostructures depending on the growth sequence. The heterostructure with an SRO bottom layer shows an atomically flat and abrupt interface, while the one with an LSMO bottom layer shows a deformed interface due to the surface roughening of LSMO deposited directly on SrTiO3 (111). As a result, the heterostructure with an abrupt interface exhibits a robust antiferromagnetic coupling between LSMO and SRO, while the one with a rough interface shows negligible magnetic coupling. Our results demonstrate the key role of an abrupt interface in determining the magnetic properties of oxide heterostructures. [ABSTRACT FROM AUTHOR]

This paper explores the fundamental structural origins of the 0.7 eV band emission peak, known as D07. The increased attention on these d-band emission lines originates mainly from the correlation between crystal defect and the intensified recombination of less dominant charge carriers. This association holds substantial importance, impacting not just the electronics sector but also raising concerns about reduced efficiency in silicon solar cells. By employing hyperspectral photoluminescence imaging, we pinpointed regions manifesting high D07 peak emissions on a microscopic scale. Subsequently, we conducted a structural investigation utilizing scanning electron microscopy and electron backscatter diffraction. Following this, we used a focused ion beam to extract areas of interest, allowing for a detailed characterization of the sample using high-resolution scanning transmission electron microscopy at the atomic scale. This approach aids in identifying defects and determining grain boundary orientation. In areas of high D07 band emission, we found Σ 3 { 114 } { 101 } grain boundaries decorated with two-layer fault twin and/or an extrinsic two-layer stacking faults. In addition, density functional theory calculations suggest oxygen impurities as a possibility for substitutional segregation to these types of defects. It is therefore plausible that the D07 line might be attributed to stacking faults featuring oxygen agglomerates. [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]

Load-dependent and angle-dependent nanoindentation tests were performed on a ( 2 ¯ 01) -oriented single-crystal β-Ga2O3 substrate to study the mechanical properties of the material. The anisotropy of the mechanical properties was examined, especially with regard to the dependence of the elastic modulus (E), hardness (H), and form of plastic deformation on the rotation angle of a Berkovich indenter with respect to the monoclinic structure. E reached a maximum value and H a minimum value when the sample was rotated to such an angle that one of the three facets of the Berkovich indenter was parallel to the [010] direction. To compare our experimental results with the theoretical calculation based on the monoclinic structure, the elastic surface of β-Ga2O3 was calculated using fourth-rank stiffness and compliance tensors and visualized in three-dimensional space. Two-dimensional sectional maps of the elastic surface were obtained for a range of crystal planes, and good agreement was obtained between the experimental observations and calculations. Plastic deformation at the indenter impressions was evaluated using scanning and transmission electron microscopy, and dislocations, cleavage, and cracks were observed. Their structure and density depended on the rotation angle. Our results clearly show a strong anisotropy of the mechanical properties of β-Ga2O3, which is very different from what is found in other compound semiconductors for power-device applications, such as GaN and 4H-SiC. This emphasizes the importance of customizing the machining process for β-Ga2O3, rather than simply using "machining recipes" for other materials. [ABSTRACT FROM AUTHOR]

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]

High quality, uni-polar, epitaxial AlN with minimum oxygen content promises excellent surface acoustic wave and bulk acoustic wave resonator characteristics such as high electromechanical coupling coefficient and power handling capabilities, which is particularly useful for RF filter applications. By systematically varying the growth temperature, the study investigates its impact on the oxygen levels, defect states, and crystallographic texture of the AlN thin films using a combination of atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, spectroscopic ellipsometry, scanning transmission electron microscopy, as well as room temperature and temperature dependent I–V measurements. The research demonstrates that the films grown at a temperature of 700 ° C exhibit the most favorable results. These films exhibit the lowest oxygen levels, possess epitaxial growth, and display the highest crystalline quality (XRD AlN 0002 ω − FWHM = 1.3 °). Additionally, these films demonstrate a significant reduction in sub-bandgap absorption. By comparing the cathode current measured during deposition, we suggest that the presence of an impurity layer formed during idle time between depositions as a possible source of oxygen in the sputter chamber. In addition, the study presents a possible model to explain the mixed polarity observed in AlN and proposes various ways to achieve uni-polar AlN on silicon substrates. [ABSTRACT FROM AUTHOR]

Here, we describe a fully in situ method of fabricating light-scattering structures on III-V materials that generates a rough morphology via vapor phase etching and redeposition. Fully in situ methods support higher industrial throughput by utilizing the growth reactor to generate the light-trapping structures after device growth without removal from the reactor. We use HCl and PH3 to etch and redeposit scattering morphologies on Ga0.5In0.5P in a dynamic hydride vapor phase epitaxy (D-HVPE) reactor. We show that the addition of PH3 leads to redeposition during the vapor phase HCl etching of Ga0.5In0.5P and that HCl flow rate and time exposed to HCl-PH3 each independently cause a linear increase in the redeposited feature size, indicating that redeposition proceeds by island growth in a III-Cl-limited, hydride-enhanced HVPE regime. Auger electron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) reveal redeposition to be highly Ga-rich GaInP, i.e., Ga(In)P. The Ga-rich nature of the redeposition results from the higher thermodynamic driving force for Ga incorporation than for In during HVPE growth and the difference in the volatility of the III-Cl etch products. The resulting morphologies have high broadband scattering, as determined by normal specular reflectance and integrating sphere measurements, indicating effectiveness as light-scattering structures. In a 270-nm-thick GaAs photovoltaic device with a textured back surface, we achieve a 4.9% increase in short circuit current density (JSC) without any loss in open-circuit voltage (VOC) relative to a planar control using only a 60 s in situ texturing treatment. [ABSTRACT FROM AUTHOR]

In this work, we report quantitative morphometry of freeze-dried graphene-based aerogels (i.e., graphene aerogel-GA, nitrogenated GA-NGA, graphene-carbon nanotube hybrid-Gr-MWCNTs, carbon foam-CF, and CF-GA hybrid-CF-GA) and monoliths, prepared by hydrothermal and organic sol-gel methods, respectively. X-ray micro-computed tomography (XMCT) in combination with scanning and transmission electron microscopy allowed visualization of internal microstructures in three-dimensional space. Quantitative morphometry analysis through the reconstructed volume renderings from two-dimensional sliced images revealed hierarchical structures possessing interlaced thin sheets, honeycomb organization, and topological interconnected pore background domains. The influence of small-diameter functionalized multi-walled carbon nanotubes (MWCNTs) inclusions to graphene-like sheets and integration with CF is assessed through quantitative morphometry analysis in terms of volume-weighted pore size, wall thickness, and porosity levels. Hybrid composite porous solids elucidated cross-linking reinforced by a homogeneous distribution of CNTs into complex sheets of GA and CF matrices. A consistent trend impacting porosity and interconnectedness was found following NGA ≥ GA > CF > Gr-MWCNT2:1 > CF-GA > Gr-MWCNT3:1 > Gr-MWCNT5:1, from XMCT image processing and analyses in corroboration with physical properties and reliability. The experimental results provide insights and guide the design of characteristic porous carbonaceous and graphene-based functional nanomaterials for energy sciences, environmental engineering, and fundamental reactive transport of fluids. [ABSTRACT FROM AUTHOR]

The structure, composition, and spatial distribution heterogeneity of artificial pinning centers affect the critical current density of REBa2Cu3O7−y (RE: rare earth) coated conductors. Nanoscale structures and compositions have been analyzed with transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). However, microscale heterogeneity has been difficult to characterize. Here, YBa2Cu3O7−y thin films doped with double-perovskite Ba2YbNbO6 were prepared via pulsed-laser deposition and characterized with TEM, STEM, and scanning electron microscopy (SEM). Cross-sectional and plan-view TEM/STEM imaging revealed hybrid pinning structures consisting of nanorods, nanoparticles, and planar defects that were formed spontaneously. Nanorods were imaged with high spatial resolution via field-emission SEM of thin-foil specimens. Focused-ion-beam (FIB) micro-sectioning enables SEM imaging of microscale heterogeneity in nanorod spatial distributions. By using TEM/STEM in conjunction with FIB-SEM, the coated conductor inhomogeneity was directly evaluated from the nano- to micrometer scales. [ABSTRACT FROM AUTHOR]

Epitaxial lift-off (ELO) of high-quality GaN layers allows for integration with a variety of materials enabling improved performance, reduced costs, and development of new electronics. Of the ELO technologies, two-dimensional (2D) material-based lift-off offers great promise but is still in the early stages of development and has yet to demonstrate the scale and yield of other ELO technologies. Here, we demonstrate the potential of this process's scalability, speed, and yield through epitaxial growth and lift-off of 4-in. GaN films using a 2D boron nitride (BN) van der Waals (vdW) buffer layer. Since the BN layer acts as the growth template and the mechanical release layer, both the quality and adhesion of the GaN layer are correlated with the BN morphology and uniformity. Detailed spectroscopic mapping demonstrates excellent BN uniformity, which translates into growth of high-quality GaN as shown in mapping of the x-ray rock curves (XRCs), atomic force microscopy, and photoluminescence. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal abrupt chemically distinct interfaces between the sapphire, BN, and AlN/GaN layers essential for efficient lift-off. Combined with the BN/GaN vdW heterostructure, Ni spalling is used to efficiently lift-off and transfer a full 4-in. GaN layer. Post transfer characterization of a 1.9 μm thick GaN layer transferred to a SiO2/Si wafer shows a very minimal change in the XRC and photoluminescence. Strain measurements before and after transfer show that the process fully relaxes residual strain formed in the GaN during high-temperature growth. This work highlights the potential for industry scalability of an exciting 2D material-based lift-off technology, which can facilitate higher power and more efficient radio frequency devices. [ABSTRACT FROM AUTHOR]

We have investigated the impact of high-temperature Mg-implantation in GaN layers on distribution of Mg-enriched defects using scanning transmission electron microscopy and atom probe tomography. For this, 1 × 1019 cm−3 Mg ions have been implanted in GaN layers at room temperature (RT) and 1000 °C, followed by annealing at 1300 °C. A smaller number of Mg-enriched defects were observed in the sample implanted at 1000 °C in comparison to the sample implanted at RT. The implantation of Mg ions at 1000 °C resulted in a higher amount of randomly distributed Mg in the GaN matrix, which, in turn, leads to more uniform and enhanced donor–acceptor pair emission, leading to higher Mg activation. [ABSTRACT FROM AUTHOR]

Trench defects, resulting in low emission efficiency in green and longer spectrum ranges, are widely observed in III-nitride alloy multiple quantum wells (MQWs), particularly in those with high indium content. There is a lack of understanding of the atomic formation mechanism of trench defects; however, it is crucial to the efficiency of devices. Here, we provided a thermodynamic analysis through first-principles calculations based on the density functional theory combined with experimental confirmation to reveal the atomic formation mechanism of trench defects in the InGaN MQWs system. The In-rich region is easy to form and induces basal plane stacking faults (BSFs) at the interface between the InGaN quantum well and the GaN quantum barrier (QB). The boundary between BSF and non-BSF regions exhibits a much slower growth rate due to the formation of homoelementary bonds, resulting in a V-shaped groove shape. Based on high-angle annular dark field scanning transmission electron microscopy, we observe the trench defects originating from the thick GaN QB layer due to the formation of closed-loop V-shaped grooves and the BSF. Besides, the cathodoluminescence measurements show that the InGaN QW within the defect has excess indium and poor crystal quality. [ABSTRACT FROM AUTHOR]

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨ 110 ⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨ 111 ⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions. [ABSTRACT FROM AUTHOR]

Metallic delafossites, ABO2 (A = Pd or Pt), are layered oxides that are as conductive as elemental metals. The high conductivity and surface polarity make metallic delafossites fascinating electrode materials for heterostructure devices. Here, we report the successful growth of c-axis-oriented PdCoO2 thin films on Al2O3 (001) substrates by magnetron sputtering that is widely used in industries. The observation of the PdCoO2 thin films through scanning transmission electron microscopy revealed layered crystal structures. A sharp interface exhibiting a layer stacking sequence of Pd/CoO2/Al2O3 was observed clearly, similar to the interfaces obtained with other growth methods such as pulsed laser deposition and molecular beam epitaxy. This layer stacking is particularly interesting because it can induce a high work function at the interface. The in-plane resistivity of the as-grown PdCoO2 thin film was 73 μΩ cm at room temperature, which decreased to 11 μΩ cm after post-annealing. The residual resistivity ratio of the annealed thin films was approximately 2.9. The impurity phases of PdOx were observed using x-ray diffraction and scanning transmission electron microscopy. The sputtering deposition of c-axis-oriented thin films could lead to the practical application of the polar surface of PdCoO2 in semiconductor devices. [ABSTRACT FROM AUTHOR]

Epitaxial growth of a wide bandgap semiconductor β-Ga2O3 thin film with high crystal quality plays a decisive role in constructing optical and electronic devices. However, except for the native substrate, the scarcity of appropriate non-native substrates or the poor crystallization of the deposit in thin film growth severely limits the fabrication and applicability of the final heterostructures and devices. Here, by taking the consistent symmetry and closely matched atomic spacing between β-Ga2O3 (−201) and the cubic perovskite (111)-oriented plane of SrTiO3, we realize the epitaxial growth of single crystal β-Ga2O3 (−201) thin films on the SrTiO3 (111) substrate by the pulsed laser deposition method, as confirmed by wide-range reciprocal-space mapping and high-resolution scanning transmission electron microscopy. The fabricated β-Ga2O3 (−201) photodetector device on the SrTiO3 (111) substrate exhibits excellent ultraviolet optical detection performance with large on/off switching ratios and a fast response speed. Moreover, the β-Ga2O3/SrTiO3 (111) heterojunction shows type-II heterostructure characteristics for energy band alignment, which displays superior ability for electron–hole pairs separation with large conduction and small valance band offsets of 1.68 and 0.09 eV, respectively. The results offer us a new way to obtain high-quality β-Ga2O3 (−201) thin film heterostructures on cubic SrTiO3 (111) substrates and fabricate β-Ga2O3-based optical and electronic devices. [ABSTRACT FROM AUTHOR]

The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990s. Strategic use of V-defects in pre-quantum well superlattices or equivalent preparation layers has enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semi-polar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The "large" V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector b = ± a ± c) character threading dislocations. In addition, "small" V-defects often form in the multi-quantum well region and are believed to be deleterious to high-efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast transmission electron microscopy that during V-defect filling, the threading dislocation, which runs up the center of the V-defect, will "bend" onto one of the six { 10 1 ¯ 1 } semi-polar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs. [ABSTRACT FROM AUTHOR]

We report the effects of Si doping on the growth dynamics and size distribution of entirely catalyst-free GaAs nanowire (NW) arrays grown by selective area molecular beam epitaxy on SiO2-masked Si (111) substrates. Surprising improvements in the NW-array uniformity are found with increasing Si doping, while the growth of undoped NWs appears in a metastable regime, evidenced by large size and shape distributions, and the simultaneous presence of crystallites with tetrahedral termination. Correlating scanning electron microscopy and transmission electron microscopy investigations, we propose that the size and shape distributions are strongly linked to the underlying twin defect formation probabilities that govern the growth. Under the present growth conditions, Si-doping of GaAs NWs leads to a very high twin defect formation probability (∼0.4), while undoped NWs exhibit a nearly threefold decreased probability (∼0.15). By adopting a model for facet-mediated growth, we describe how the altered twin formation probabilities impact the competing growth of the relevant low-index NW facets, and hence, NW size and shape. Our model is further supported by a generic Monte Carlo simulation approach to highlight the role of twin defects in reproducing the experimentally observed size distributions. [ABSTRACT FROM AUTHOR]

The structural properties of CVD-grown (Si)GeSn heterostructures were assessed thanks to scanning transmission electron microscopy at the nanometer scale. Quantitative energy dispersive x-ray (EDX) spectroscopy together with precession electron diffraction and geometrical phase analysis (GPA) were performed to probe the chemical and structural properties of the different layers. Results presented in this paper demonstrated the advantages of a multilayer structure, with successive layers grown at decreasing temperatures in order to gradually accommodate the in-plane lattice parameter and incorporate more and more Sn into the stack. It was shown how the GeSn emissive layer could be manufactured with low plastic deformation and a high relaxation rate, necessary for better light emission performances. SiGeSn alloys used as confinement barriers around the emissive layer were also investigated. For such thin layers, we showed the importance of the starting lattice parameter (SLP) prior to the growth on their composition. Indeed, higher SLPs resulted, for the very same process conditions, into higher Sn contents and lower Si contents. The interest in combining EDX, which was accurate enough to detect slight chemical concentration variations, and GPA, for local strain analyses, was clearly demonstrated. Present results will be very useful to predict and control the bandgap and structural quality of (Si)GeSn materials and, in turn, device properties. [ABSTRACT FROM AUTHOR]

A magnetocaloric effect (MCE) with sizable isothermal entropy change (ΔS) maintained over a broad range of temperatures above the blocking temperature is reported for a rare earth-free superparamagnetic nanoparticle system comprising of Fe–TiN heterostructure. Superparamagnetic iron (Fe) particles were embedded in a titanium nitride (TiN) thin film matrix in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition method. High angle annular dark-field images in conjunction with dispersive energy analysis, recorded using scanning transmission electron microscopy, show a clear presence of alternating layers of Fe and TiN with a distinct atomic number contrast between Fe particles and TiN. Quantitative information about the isothermal entropy change (ΔS) and the magnetocaloric effect in the multilayer Fe–TiN system has been obtained by applying Maxwell relation to the magnetization vs temperature data at various fields. With the absence of a dynamic magnetic hysteresis above the blocking temperature, the negative ΔS as high as 4.18 × 103 J/Km3 (normal or forward MCE) is obtained at 3 T at 300 K. [ABSTRACT FROM AUTHOR]

A process sequence enabling the large-area fabrication of nanopillar-patterned semiconductor templates for selective-area heteroepitaxy is developed. Herein, the nanopillar tops surrounded by a SiNx mask film serve as nanoscale growth areas. The molecular beam epitaxial growth of InAs on such patterned GaAs (111) A templates is investigated by means of electron microscopy. It is found that defect-free nanoscale InAs islands grow selectively on the nanopillar tops at a substrate temperature of 425 °C. High-angle annular dark-field scanning transmission electron microscopy imaging reveals that for a growth temperature of 400 °C, the InAs islands show a tendency to form wurtzite phase arms extending along the lateral ⟨ 11 2 ¯ ⟩ directions from the central zinc blende region of the islands. This is ascribed to a temporary self-catalyzed vapor–liquid–solid growth on { 11 1 ¯ } B facets, which leads to a kinetically induced preference for the nucleation of the wurtzite phase driven by the local, instantaneous V/III ratio, and to a concomitant reduction of surface energy of the nanoscale diameter arms. [ABSTRACT FROM AUTHOR]

Bournonite (CuPbSbS3) and enargite (Cu3AsS4) have recently been used as absorber layers in thin-film photovoltaic devices due to their ideal bandgap and ferroelectric properties. An understanding of the ferroelectric domain structure in these materials is required so that the benefits of the internal depolarizing electric fields can be fully exploited. Here, the atomic structure and electronic properties of domain walls (DWs) are elucidated through a combined aberration-corrected scanning transmission electron microscopy and density functional theory study. ∼90° and 180° DWs are observed in bournonite. As the 180° DW is charge neutral, it cannot contribute to the anomalous photovoltaic effect that leads to high open circuit voltages. The ∼90° DW shows a slight offset across the boundary, but the contributions of this to the anomalous photovoltaic effect are negligible. The DWs are also electrically passive, i.e., they do not result in significant recombination and do not block charge carrier transport. A high density of stacking faults (SF) was, however, observed in enargite. The SFs have a large number of defect states within the bandgap, which would lower the device efficiency through Shockley–Read–Hall recombination. [ABSTRACT FROM AUTHOR]

The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al2O3 dielectrics on β-Ga2O3 and β-(AlxGa1−x)2O3 films is investigated as a function of crystal orientations and Al compositions of β-(AlxGa1−x)2O3 films. The interface and film qualities of Al2O3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al2O3 dielectrics with abrupt interfaces on (010), (100), and ( 2 ¯ 01) oriented β-(AlxGa1−x)2O3 films. The surface stoichiometries of Al2O3 deposited on all orientations of β-(AlxGa1−x)2O3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al2O3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al2O3/β-(AlxGa1−x)2O3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(AlxGa1−x)2O3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al2O3/β-(AlxGa1−x)2O3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the ( 2 ¯ 01) orientation. The results from this study on MOCVD growth and band offsets of amorphous Al2O3 deposited on differently oriented β-Ga2O3 and β-(AlxGa1−x)2O3 films will potentially contribute to the design and fabrication of future high-performance β-Ga2O3 and β-(AlxGa1−x)2O3 based transistors using MOCVD in situ deposited Al2O3 as a gate dielectric. [ABSTRACT FROM AUTHOR]

Swift heavy ion irradiation was carried out to examine the ionization effects on structural changes of δ-Sc4Hf3O12 in which oxygen vacancies are regularly arranged. The specimens were irradiated at room temperature with 92 MeV xenon ions to fluences ranging from 3 × 1012 to 1 × 1014/cm2 and characterized by grazing (glancing) incidence x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. It was found that the pristine long-range ordered rhombohedral δ-phase undergoes a reconstructive transformation toward a long-range disordered cubic oxygen-deficient fluorite phase promoted by ionization effects. In addition, an ordered phase with a short-range structure different from the δ-type was formed in a layer going from the surface to a depth of ∼4.5 μm in the specimen irradiated to a fluence of 1 × 1014/cm2. It was found that the ordered phase is formed from the disordered cubic fluorite phase. This structural change is anomalous, because it is the opposite process of the usual irradiation-induced structural change, the order-to-disorder phase transformation. Electron diffraction experiments revealed that short-range ordered regions in this layer possess an oxygen-excess bixbyite organization (C-type heavy rare-earth oxides) with randomly filled anion vacant sites to account for the different stoichiometry and a long-range average oxygen-deficient fluorite phase. [ABSTRACT FROM AUTHOR]

Swift heavy ion irradiation was carried out to examine the ionization effects on structural changes of δ-Sc4Hf3O12 in which oxygen vacancies are regularly arranged. The specimens were irradiated at room temperature with 92 MeV xenon ions to fluences ranging from 3 × 1012 to 1 × 1014/cm2 and characterized by grazing (glancing) incidence x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. It was found that the pristine long-range ordered rhombohedral δ-phase undergoes a reconstructive transformation toward a long-range disordered cubic oxygen-deficient fluorite phase promoted by ionization effects. In addition, an ordered phase with a short-range structure different from the δ-type was formed in a layer going from the surface to a depth of ∼4.5 μm in the specimen irradiated to a fluence of 1 × 1014/cm2. It was found that the ordered phase is formed from the disordered cubic fluorite phase. This structural change is anomalous, because it is the opposite process of the usual irradiation-induced structural change, the order-to-disorder phase transformation. Electron diffraction experiments revealed that short-range ordered regions in this layer possess an oxygen-excess bixbyite organization (C-type heavy rare-earth oxides) with randomly filled anion vacant sites to account for the different stoichiometry and a long-range average oxygen-deficient fluorite phase. [ABSTRACT FROM AUTHOR]

Beta-gallium oxide (β-Ga2O3) has recently attracted significant attention as an outstanding candidate for ultra-wide bandgap applications due to its unique advantages. Point and extended defects in β-Ga2O3 can significantly reduce the net doping and play an essential role with their functionality in advancing β-Ga2O3 device performance. It is, therefore, critical to gain an atomic level understanding of the structure of the defects and how they correlate to important properties of defects in β-Ga2O3. In this Perspective, we provide an overview of the recent characterization works involving scanning transmission electron microscopy and related techniques revealing the detailed structure of various point and extended defects in β-Ga2O3 and β-(AlxGa1−x)2O3 heterostructures. This article aims to offer insight into how defects determine important aspects of the material, such as in crystal growth, dopant incorporation and activation, and phase stability. The new information that we summarize here is expected to help achieve atomic scale control of defects in β-Ga2O3 materials and devices for development of the next generation power electronics applications. [ABSTRACT FROM AUTHOR]

Defects in high performance multi-crystalline silicon wafers can be detrimental to the lifetime of the solar cell. It is, therefore, important to study and understand the underlying structure and chemical elements present at these defective areas in order to suppress them. The underlying cause of the D-band emission line " v e r y i n t e n s e D 3 " (VID3) has not yet been understood, although many theories have been proposed. In this paper, we have investigated the underlying causes of the d-band emission peak VID3 by hyperspectral photoluminescence imaging, scanning electron microscopy, electron backscatter diffraction, scanning transmission electron microscopy, and density functional theory (DFT) to understand the defect structure in areas of a VID3 emission peak in more detail. We found a high VID3 peak intensity at sub-grain and Σ 3 twin boundaries bordering to grains with a small misorientation, which suggests higher stress in these regions. Defects close to the twin boundary indicate a light element dopant in the area, such as oxygen. DFT calculations show that oxygen is prone to segregate to this boundary. [ABSTRACT FROM AUTHOR]

Dislocations introduced by Vickers tip microindentation of an a-plane free-standing semi-insulating Fe-doped GaN halide vapor phase epitaxy (HVPE) crystal were investigated by means of cathodoluminescence and scanning transmission electron microscopy techniques. Detailed combined analyses of both spectral properties and the core structure of the introduced a-screw dislocations revealed that Fe-doped GaN exhibit not only dislocation-bound emission at ∼3.35 eV of perfect a-screw dislocations previously found in such kind of samples but also luminescent bands at 3.1–3.2 and 3.3 eV due to dissociated a-screw dislocations and extended dislocation nodes previously observed only in low-resistance n-GaN. For the first time, all these luminescent bands were observed together in the same sample. Structural studies revealed the coexistence of the dislocations with the dissociated and the perfect core as well as with extended dislocation nodes, thus establishing a correlation between previously observed luminescence bands and a fine dislocation core structure. [ABSTRACT FROM AUTHOR]

Novel physical phenomena arising from the complex interplay between the spin, charge, orbital, and lattice orders can emerge in perovskite-type oxide heterostructures. Here, we investigate the lattice and electronic structures on the interface of the YBa2Cu3O7/LaAlO3 heterostructure prepared by pulsed laser deposition with the combination of spherical aberration-corrected scanning transmission electron microscopy and density functional theory. Both reconstructed and normal interfaces are observed at sub-angstrom resolution, and these superconducting planes closest to the corresponding interface known as CuO2(a) and CuO2(c) planes, respectively. Due to the lattice reconstruction and charge density redistribution, CuO2(a) plane moves closer to the reconstructed interface with bond length of in-plane Cu and apical oxygen reduced. Consequently, the d3z2−r2 orbital loss electrons further results in two orbitals (dx2−y2 and d3z2−r2) crossing the Fermi level, showing two-band superconductivity behavior at the reconstructed interface. However, the CuO2(c) plane remains unaffected due to the protection of the charge reservoir layer at the normal interface and exhibits electronic structures similar to bulk cuprate superconductors, where only dx2−y2 orbitals contribute to the states at the Fermi level. These results suggest that the interfacial reconstruction might be a possible pathway to manipulate the electronic structures of the superconducting oxide heterostructures. [ABSTRACT FROM AUTHOR]

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an Al0.95Ga0.05N nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an Al0.29Ga0.71N nucleation layer, wurtzite inclusions tend to form at the GaN/ Al0.29Ga0.71N interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morphology. [ABSTRACT FROM AUTHOR]

We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN–SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the ⟨ 40–41 ⟩ facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods. [ABSTRACT FROM AUTHOR]

Phase and microstructural evolution of gallium oxide (Ga2O3) films grown on vicinal (0001) sapphire substrates was investigated using a suite of analytical tools. High-resolution transmission electron microscopy and scanning transmission electron microscopy of a film grown at 530 °C revealed the initial pseudomorphic growth of three to four monolayers of α-Ga2O3, a 20–60 nm transition layer that contained both β- and γ-Ga2O3, and a top ∼700 nm-thick layer of phase-pure κ-Ga2O3. Explanations for the occurrence of these phases and their sequence of formation are presented. Additional growths of Ga2O3 films in tandem with x-ray diffraction and scanning electron microscopy investigations revealed that the top layer varied in phase composition between ∼100% κ-Ga2O3 and ∼100% β-Ga2O3; the surface microstructure ranged from poorly coalesced to completely coalesced grains as a function of growth temperature, growth rate, or diluent gas flow rate. In general, it was found that the κ-phase is favored at lower growth temperatures and higher triethylgallium flow rates (low VI/III ratios). The growth of nominally single-phase κ-Ga2O3 within the top layer was observed in a temperature range between 500 and 530 °C. Below 470 °C, only amorphous Ga2O3 was obtained; above 570 °C, only the β-phase was deposited. [ABSTRACT FROM AUTHOR]

Austenitic stainless steels are subject to the precipitation of chromium carbides (M23C6) during exposure to high temperatures, causing these alloys to be susceptible to intergranular corrosion due to chromium depletion along grain boundaries. The acoustic nonlinearity parameter, β, shows sensitivity to the formation of carbides in these alloys. The Thermo-Calc TC-PRISMA module was used to model the nucleation and growth of grain boundary M23C6 carbides. The model was verified with scanning transmission electron microscopy analysis that allowed measurements of the grain boundary precipitates. The paper introduces a reduced-order model of the acoustic nonlinearity based on the formation of misfit dislocations at the interface of the grain boundary precipitate and matrix to explain the change in β during isothermal aging. A direct relationship between the radius of the M23C6 grain boundary carbides and β was observed and verified with nonlinear ultrasound measurements on 304L and 316L stainless steels. [ABSTRACT FROM AUTHOR]

Metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are two versatile growth techniques that can readily produce multilayer structures with atomic-level precision control, which have found broad applications in technology. However, compared to MBE, MOCVD growth involves the surface reaction of metal-organic precursor compounds, which changes during film deposition. Consequently, a thorough investigation on the chemical profile layer-by-layer is critical for optimizing MOCVD film performance. Here, we examine Sb segregation in an MOCVD-grown InAs/InAs1−xSbx superlattice by analyzing composition and lattice strain at atomic resolution using scanning transmission electron microscopy and compare with the previously reported MBE growth results. Our findings show a different Sb profile along the growth direction in MOCVD, with the segregation coefficient being higher at the InAsSb-on-InAs interface (0.807 ± 0.021) than at the InAs-on-InAsSb interface (0.695 ± 0.009), giving rise to asymmetric composition and lattice strain profiles unlike those obtained with MBE. Furthermore, we obtain direct evidence of Sb clusters with size of ∼1–3 nm and Sb ordering within the InAs1−xSbx layer, which is largely absent in the reported MBE growth. These findings demonstrate the concurrent interplay between surface segregation, surface reconstruction, and surface reaction that is unique to MOCVD growth with broad implications on preparing Sb-containing quantum materials. [ABSTRACT FROM AUTHOR]

Conversion of Shockley partial dislocation pairs from unexpandable to expandable combinations has been considered possible during epitaxial growth. The step-flow model was proposed to explain the conversion, in which an unexpandable 30° C-core partial dislocation in the substrate changes into an expandable 30° Si-core partial dislocation in the epitaxial grown layer. We observed this conversion experimentally by a transmission electron microscope and confirmed the core-species change by high-angle annular dark-field scanning transmission electron microscopy. In addition, other unexpandable combinations of partial dislocations were examined for the possibility of converting to expandable. As a result, the unexpandable basal plane dislocations with a Burgers vector of ±(1/3)[ 11 2 ¯ 0 ] in the substrate were confirmed to be a necessary condition for forming expandable 30° Si-core partial dislocations after epitaxial growth that could expand single Shockley-type stacking faults and degrade reliability of 4H-SiC power devices. [ABSTRACT FROM AUTHOR]

The core structures of dislocations are crucial for understanding the plastic deformation mechanisms and the functional properties of materials. Here, we use the scanning transmission electron microscopy imaging techniques of high-resolution high angle annular dark field and integrated differential phase contrast to investigate the atomic structure of a pair of climb-dissociated 1 2 a [ 01 1 ¯ ] dislocations in a bending-deformed relaxor ferroelectric Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb1/3)O3–PbTiO3 single crystal. Cations at one dislocation core are found to arrange in the same way as the climb-dissociated 1 2 a [ 01 1 ¯ ] dislocation core in SrTiO3, while the other one is different. Oxygen depletion was observed at both dislocation cores. Geometric phase analysis of the lattice rotation shows opposite signs at both sides of the dislocations, demonstrating the strain gradient, which is known to give rise to flexoelectric polarization. Using the peak finding method, the polarization (a combination of ferroelectric and flexoelectric) around dislocations was mapped at the unit-cell scale. The polarization direction obtained is consistent with that predicted based on the flexoelectric effect in a perovskite oxide with [011] geometry. Head-to-head positively charged and tail-to-tail negatively charged domain walls were revealed based on the polarization map, suggesting a new way to stabilize charged domain walls via dislocations. A distinct dislocation core configuration has been observed, and a unit-cell scale polarization map helps understand the flexoelectric effects (coupling between strain gradient and polarization) around dislocations in a relaxor ferroelectric. [ABSTRACT FROM AUTHOR]

We studied the effect of the (Ni0.8Fe0.2)100−xCrx seed layer on the current-in-plane giant magnetoresistance (CIP-GMR) in Fe16Co66Ni18/Cu multilayer films. Both the GMR ratio (ΔR/R) and interlayer exchange coupling (IEC) between the FeCoNi layers showed strong dependences on the Cr concentration x in the seed layer. The highest ΔR/R of ∼30% was obtained for x = 38 (at. %) due to a comparatively strong antiparallel IEC resulting from a strong [111]-out-of-plane-texture of the [FeCoNi/Cu] multilayer. This confirms that a strong [111] texture is essential for the GMR in antiferromagnetically coupled multilayers as reported in previous works. However, we found that the strong [111] texture is not sufficient to obtain large ΔR/R. For x = 48, the [FeCoNi/Cu] multilayer showed a strong [111] texture as that for x = 38; however, the observed ΔR/R and antiparallel IEC for x = 48 were much smaller than those for x = 38. Detailed microstructural and chemical analyses by scanning transmission electron microscopy revealed that the fluctuation of the thickness of the Cu spacer layer can explain the reduction of antiparallel IEC that gave rise to the decrease in ΔR/R in the case of x = 48. We also discuss the effect of IEC energies on the sensitivity of GMR sensors. [ABSTRACT FROM AUTHOR]

Properties of AlN/GaN surfaces are important for realizing the tunability of devices, as the presence of surface states contributes to Fermi level pinning. This pinning can influence the performance of high electron mobility transistors and is also important for passivation of the surface when developing high-power electronic devices. It is widely understood that both AlN and GaN surfaces oxidize. Since there are many possible reconstructions for each surface, it is a challenge to identify the relevant surface reconstructions in advance of a detailed simulation. Because of this, different approaches are often employed to down select initial structures to reduce the computational load. These approaches usually rely on either electron counting rules or oxide stoichiometry, as both of these models tend to lead to structures that are energetically favorable. Here we explore models from these approaches but also explore a reconstruction of the (0001) surface directly observed using scanning transmission electron microscopy with predictive density functional theory simulations. Two compositions of the observed surface reconstruction—one which obeys oxide stoichiometry and one which is cation deficient and obeys electron counting—are compared to reconstructions from the previous work. Furthermore, surface states are directly calculated using hybrid exchange-correlation functionals that correct for the underestimation of the bandgaps in AlN and GaN and improve the predicted positions of surface states within the gap. It is found that cation deficiency in the observed reconstruction yields surface states consistent with the experiment. Based on all of these results, we provide insight into the observed properties of oxidized AlGaN surfaces. [ABSTRACT FROM AUTHOR]

Double perovskite Ba2LuNbO6 (BLNO)-doped YBa2Cu3O7−y (YBCO) thin films are fabricated on a SrTiO3 (001) substrate by pulsed laser deposition, and their nanostructures are characterized by transmission electron microscopy and scanning transmission electron microscopy. Cross-sectional observations and elemental mapping reveal that BLNO self-assembles during thin film deposition, and consequently, nanorods extending straight from the substrate to the surface are formed in the YBCO thin films. It is confirmed that stacking faults perpendicular to the growth direction disturb the formation of BLNO nanorods. Strain maps extracted by geometric phase analysis reveal that the tensile strain occurs in the YBCO matrix around the BLNO nanorods. Misfit dislocations are periodically introduced at the interface between the nanorod and the matrix, which results in the inhomogeneous strain of YBCO around the BLNO nanorods. The superconducting properties of the YBCO + BLNO thin films are compared with those of other previously reported YBCO thin films with normal perovskite and double perovskite nanorods. [ABSTRACT FROM AUTHOR]