Adatom Migration Research Articles

Atomically self-healing of structural defects in monolayer WSe2

Abstract Minimizing disorder and defects is crucial for realizing the full potential of two-dimensional transition metal dichalcogenides (TMDs) materials and improving device performance to desired properties. However, the methods in defect control currently face challenges with overly large operational areas and a lack of precision in targeting specific defects. Therefore, we propose a new method for the precise and universal defect healing of TMD materials, integrating real-time imaging with scanning transmission electron microscopy (STEM). This method employs electron beam irradiation to stimulate the diffusion migration of surface-adsorbed adatoms on TMD materials grown by low-temperature molecular beam epitaxy (MBE), and heal defects within the diffusion range. This approach covers defect repairs ranging from zero-dimensional vacancy defects to two-dimensional grain orientation alignment, demonstrating its universality in terms of the types of samples and defects. These findings offer insights into the use of atomic-level focused electron beams at appropriate voltages in STEM for defect healing, providing valuable experience for achieving atomic-level precise fabrication of TMD materials.

Thermal Stability of High-Entropy Alloy Nanoparticles Evaluated by In Situ TEM Observations.

High-entropy alloy (HEA) nanoparticles (NPs) have attracted attention in several fields because of their fascinating properties. The high mechanical strength, good thermal stability, and superior corrosion resistance of HEAs, which are derived from their high configurational entropy, are attractive features. Herein, we investigated the thermal stability of FeCoNiCuPd HEA NPs on reduced graphene oxide via in situ transmission electron microscopy observations at elevated temperatures. The HEA NPs maintained their structure, size, and composition at 700 °C, and their size gradually decreased accompanied by the preferential sublimation of Cu. On the contrary, the deterioration of the monometallic Pd NPs begins at temperatures greater than 700 °C according to Ostwald ripening, which involves the migration of adatoms or mobile molecular species. Theoretical calculations revealed that the detachment of adatoms from clusters (i.e., the first step of Ostwald ripening) was suppressed in the case of HEA NPs because of the high-configuration-entropy effect.

Cone array formation on Si surfaces by low-energy He plasma irradiation with magnetron sputtering pre-deposited Ta

Low-energy ion beam irradiation, combined with the introduction of impurities, presents a promising approach for nanopatterning silicon (Si) surfaces. In this research, we investigate the surface evolution of Si (100) surfaces irradiated by 75 eV helium (He) plasma, in the presence of tantalum (Ta), traditionally regarded as an impurity incapable of initiating pattern formation, as evidenced in prior studies. The Ta impurities are pre-deposited onto the Si surfaces using the magnetron sputtering method, which offers a more controlled and quantifiable approach compared with the conventional co-deposition route. After irradiation at 800 K, dense cone arrays are produced on the Si surface. The growth of the cones is explored for fluence spanning (1–10) × 1025 m−2. The cross-sectional scanning electron microscope images indicate that the cone lengths and base width are well characterized by t1/2 dependence. The kinetics of the cone growth follow Fick’s law, characterized by an effective diffusive mechanism with the coefficients of diffusion: D = 6.49 ± 0.83 × 10−16 m−2 s−1. Transmission electron microscope observations reveal that the cone has distinct inner and outer parts with different microstructures and a clear interface. The growth mechanism of these cones is elucidated as a composite process involving preferential sputtering, the destabilizing influence of He bubbles, and the migration of adatoms.

Sn-doped n-type GaN layer with high electron density of 1020 cm−3 grown by halide vapor phase epitaxy

A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE.

Surface stress release in AlCrN coatings determined by synchrotron radiation multi-reflection grazing-incidence X-ray diffraction

In this work, the residual stress distribution of c. 3.8 μm arc-evaporated AlCrN coatings was investigated by the synchrotron radiation multi-reflection grazing-incidence X-ray diffraction method. Ar ion bombardment post-treatment was also used to tune the stresses in the AlCrN coating to elucidate the evolution of stress therein. The result showed that the as-deposited AlCrN coating showed large surface stress of −1500 MPa, this sharply decreased in magnitude to −800 MPa in the internal coating. Ar ion bombardment (post-treatment) could significantly reduce the surface stresses by around −1000 MPa and the interior stresses by around −400 MPa for an AlCrN coating, the “shot peening” effect arising therefrom could promote migration and diffusion of the adatoms, which contributed to the reduction of the micro-strain and interior stress. Furthermore, Ar ion bombardment also promoted the growth of surface grains, which also contributed to the stress reduction in the near-surface region of the AlCrN coating. In addition, Ar ion bombardment could increase the substrate stresses from −32 MPa to −210 MPa. The results suggested that internal stress reduction could be achieved by promoting the migration and diffusion of adatoms.

Assessing factors that determine adatom migration and clustering on a thin film oxide; Pt1 and Rh1 on the “29” CuxO/Cu(1 1 1) surface

The dynamical nature of single-site catalysts under non-equilibrium conditions poses significant challenges in the full characterization of their active sites. The “29” oxide is a thin CuxO film grown on Cu(111) which provides a well-defined surface on which to study the structure and chemistry of atomically dispersed precious metals. A combination of experimental and first-principles approaches is used to investigate the factors that influence the mobility of metal adatoms and their clustering tendencies. Our work elucidates how a homogenous coordination environment of neutral single-atoms can be obtained when its tendency to break oxide bonds within its vicinity is low. We demonstrate that this in turn affects the chemical activity of atomically dispersed species on thin-film oxides, as defects on the oxide would in fact allow such species to be more accessible to CO. We thus highlight the importance of understanding the relationship between defects and atomically dispersed active sites upon designing single-site catalysts.

Migration-Enhanced Metal–Organic Chemical Vapor Deposition of Wafer-Scale Fully Coalesced WS2 and WSe2 Monolayers

Metal–organic chemical vapor deposition (MOCVD) is widely employed for the wafer-scale synthesis of transition metal dichalcogenide (TMDC) monolayers (MLs). Despite large efforts devoted to understanding the intricate nucleation and lateral growth mechanisms of TMDCs, little attention has been paid to the migration of adatoms on the top of an ML and its influence on parasitic/premature bilayer (BL) nucleation. In this work, using a commercial multi-wafer MOCVD platform, a novel two-stage migration-enhanced MOCVD process is introduced to realize the deposition of wafer-scale fully coalesced tungsten disulfide (WS2) and tungsten diselenide (WSe2) MLs with only sparse BL nucleation in a reasonable deposition time. With the WS2 ML coverage exceeding 99% on 2 in. sapphire substrates within 3 h, BL coverage is suppressed to ∼15%. Following the same migration enhancement approach, WSe2 MLs are synthesized in 90 min with <20% BL coverage. The migration of W adatoms on the already formed stable WS2 (or WSe2) ML domains is promoted by ramping down the delivery of the tungsten precursor. From the qualitative analysis of the nanomorphology, the migration length of W adatoms is estimated to be ≤100 nm. This approach can be seen as a reliable and solid basis for the development of future large-scale TMDC ML deposition techniques.

Controlling metal adatoms on InGaN growing front for defect suppression and high-stability visible-light photodetection

Despite the successful commercialization of InGaN-based light emitting diodes, the further application of this material system in photosensitive devices remains hindered by complex crystal defects and lacking understanding of them. Herein, the InGaN epilayers are grown through the synchronous control of In and Ga adatoms on the growing front. Reducing excess In adatoms lowers the surface strain and suppresses the formation of misfit dislocation (MD) networks, while enhancing Ga adatom migration reduces the trench defects as well as the related In-rich hillock regions. Therefore, the nonradiative centers (NRCs) and the localized states played by these surface-visible defects, are both effectively reduced, leading to superior optical performance of the InGaN alloy. Furthermore, optoelectronic characterization using coplanar Schottky-contact photodetectors reveals that the localized states and the deep-level traps associated with these surface defects cause the persistent photoconductive (PPC) effect, resulting in unstable and slow photo-response. Therefore, the suppression of these complex defects contributes to the realization of high-performance InGaN visible-light photodetectors (VLPDs) with high speed and high-temperature stability, exhibiting great potential in wide visible-light applications.

An ab initio approach to anisotropic alloying into the Si(001) surface.

By employing density functional theory calculations, we explore the initial stage of competitive alloying of co-deposited silver and indium atoms into a silicon surface. In particular, we identify respective adsorption positions and activation barriers governing their diffusion on a dimer-reconstructed silicon surface. Furthermore, we develop a growth model that appropriately describes diffusion mechanisms and silicon morphology with the account of silicon dimerization and the presence of C-type defects. Based on the surface kinetic Monte Carlo simulations, we examine the dynamics of bimetallic adsorption and elaborate on the temperature effects on the submonolayer growth of an Ag-In alloy. A close inspection of adatom migration clearly indicates effective nucleation of Ag and In atoms, followed by the formation of orthogonal atomic chains. We show that the epitaxial bimetallic growth might potentially lead to exotic ordering of adatoms in the form of anisotropic two-dimensional lattices via orthogonally oriented single-metal rows. We argue that this scenario becomes favorable provided above room temperature, while our numerical results are shown to be in agreement with the experimental findings.

Structural characteristics of 3C–SiC thin films grown on Si-face and C-face 4H–SiC substrates by high temperature chemical vapor deposition

Hetero-epitaxy of 3C–SiC on Si-face and C-face 4H–SiC is experimentally investigated and corresponding growth model is identified. This study of polytypes of SiC will be useful for band-gap engineering, optical, electrical, and electronic applications of SiC. 3C–SiC was epitaxially deposited on 4H–SiC using a high temperature chemical vapor deposition. The primary factor affecting the epi-quality on Si-face SiC is double positioning boundary (DPB) defect. An adatom migration model elucidates the formation of the V-shaped DPB defect structures. On C-terminated 4H–SiC, super-V-shaped-structure (SVSS) defects and polycrystalline complexes in 3C–SiC are formed. These characteristics of 3C–SiC are measured and analyzed using transmission electron microscopy, X-ray diffraction, atomic force microscopy, and scanning electron microscopy techniques. A model considering adatoms' migration in step-flow direction and anti-step-flow direction explains the SVSS defects. Effects of both step-flow and anti-step-flow growth mode on the atom adsorption near the step edge are discussed.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (&amp;gt;108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.

Tuning the p-type doping of GaN over three orders of magnitude via efficient Mg doping during halide vapor phase epitaxy

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors.

Asymmetric Interfacet Adatom Migration as a Mode of Anisotropic Nanocrystal Growth.

Crystals are known to grow nonclassically or via four classical modes (the layer-by-layer, dislocation-driven, dendritic, and normal modes, which generally involve minimal interfacet surface diffusion). The field of nanoscience considers this framework to interpret how nanocrystals grow; yet, the growth of many anisotropic nanocrystals remains enigmatic, suggesting that the framework may be incomplete. Here, we study the solution-phase growth of pentatwinned Au nanorods without Br, Ag, or surfactants. Lower supersaturation conditions favored anisotropic growth, which appeared at variance with the known modes. Temporal electron microscopy revealed kinetically limited adatom funneling, as adatoms diffused asymmetrically along the vicinal facets (situated inbetween the {100} side-facets and {111} end-facets) of our nanorods. These vicinal facets were perpetuated throughout the synthesis and, especially at lower supersaturation, facilitated {100}-to-vicinal-to-{111} adatom diffusion. We derived a growth model from classical theory in view of our findings, which showed that our experimental growth kinetics were consistent with nanorods growing via two modes simultaneously: radial growth occurred via the layer-by-layer mode on {100} side-facets, whereas the asymmetric interfacet diffusion of adatoms to {111} end-facets mediated longitudinal growth. Thus, shape anisotropy was not driven by modulating the relative rates of monomer deposition on different facets, as conventionally thought, but rather by modulating the relative rates of monomer integration via interfacet diffusion. This work shows how controlling supersaturation, a thermodynamic parameter, can uncover distinct kinetic phenomena on nanocrystals, such as asymmetric interfacet surface diffusion and a fundamental growth mode for which monomer deposition and integration occur on different facets.

Indirect measurement of the carbon adatom migration barrier on graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300–1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 ± 0.03) eV.

Wake-up free Hf0.5Zr0.5O2 thin film with enhanced ferroelectricity and reliability synthesized by atomic layer crystallization induced by substrate biasing

Attentions on HfO2-based ferroelectric thin films have been rapidly raised due to the promising non-volatile memory applications in advanced artificial intelligence. In this study, the novel “atomic layer substrate bias induced crystallization (ALCISB)” technique is proposed to prepare wake-up free and back-end-of-line compatible Hf0.5Zr0.5O2 (HZO) thin films. The adatom migration boosted by the energy transfer from the plasma in the ALCISB process contributes to the film densification and the crystallinity enhancement of the HZO layer. The ferroelectric remnant polarization (2Pr) and the dielectric constant are increased from 3 to 48 μC/cm2 and 21.4 to 27.7, respectively, which are attributed to the crystallization into orthorhombic phase by the ALCISB technique. Furthermore, the ALCISB treatment suppresses the amount of oxygen vacancies, leading to the extension of the endurance up to 109 cycles and the wake-up free operation of the HZO thin film. The retention characteristic is almost free of degradation after extrapolating to 10 years of thermal treatment. The exceptional ferroelectric properties of the HZO thin films synthesized by ALCISB are highly favorable to non-volatile ferroelectric memory devices in the near future.

Indirect Measurement of the Carbon Adatom Migration Barrier on Graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300-1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 \pm 0.03) eV.

Thermodynamic Regulation of Dendrite-Free Li Plating on Li3Bi for Stable Lithium Metal Batteries.

Rechargeable batteries with metallic lithium (Li) anodes are attracting ever-increasing interests because of their high theoretical specific capacity and energy density. However, the dendrite growth of the Li anode during cycling leads to poor stability and severe safety issues. Here, Li3Bi alloy coated carbon cloth is rationally chosen as the substrate of the Li anode to suppress the dendrite growth from a thermodynamic aspect. The adsorption energy of a Li atom on Li3Bi is larger than the cohesive energy of bulk Li, enabling uniform Li nucleation and deposition, while the high diffusion barrier of the Li atom on Li3Bi blocks the migration of adatoms from adsorption sites to the regions of fast growth, which further ensures uniform Li deposition. With the dendrite-free Li deposition, the composite Li/Li3Bi anode enables over 250 cycles at an ultrahigh current density of 20 mA cm-2 in a symmetrical cell and delivers superior electrochemical performance in full batteries.

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Growth evolution of polar-plane-free faceted GaN structures on (112¯2) and (1¯1¯22¯) GaN substrates

We compare the growth evolution of polar-plane-free faceted GaN structures on (112¯2) and (1¯1¯22¯) planes. The crystal morphologies of the three-dimensional (3D) GaN structures depend on surface orientations. To discuss the underlying mechanism, the temporal developments of the cross-sectional shapes during growth are visualized by periodically inserting AlGaN markers. Quantitative analyses using these markers reveal that as the growth proceeds, the growth rates of the top and inclined facets of the 3D GaN on (112¯2) monotonically decrease, whereas those of the 3D GaN on (1¯1¯22¯) monotonically increase. The opposite tendencies are attributed to the difference in the surface diffusion of adatoms between the top and inclined facets. Furthermore, it is suggested that the surface bond configuration of each crystallographic plane strongly affects the adatom migration, resulting in distinct 3D GaN morphologies on (112¯2) and (1¯1¯22¯).

Surface morphology evolution and underlying defects in homoepitaxial growth of GaAs (110)

The evolution of the surface morphology and underlying pyramidal defects in homoepitaxial GaAs (110) layers was investigated with respect to the layer thickness up to 1 µm. Ga and As atoms coexisting in a one-to-one atomic ratio on the (110)-oriented GaAs surface exhibit different incorporation rates and adatom migration rates, giving rise to local As atomic disordering in the epitaxial layer. Accordingly, this disordering triggers distinctive shapes of three-dimensional surface islands composed of specific facets and underlying microtwins. Single triangular-shaped islands initially appear at an early stage of epitaxial growth. With increasing layer thickness, all of the single triangular islands suddenly become paired triangular islands by forming secondary single triangular islands nearby, like decalcomania. A further increase of the layer thickness induces a complete transformation into four-legged starfish shapes containing directional side edges. At this stage, the underlying pyramidal defects of the surface islands are filled with {111} microtwins. Surprisingly, the evolutions of island shapes and underlying twin formation are closely associated with the strain relaxation in the thick GaAs (110) layers, which is unexpected in any homoepitaxial system. We found that a higher growth temperature retards the shape evolution of surface islands, leading to suppression of compressive strain.