Adatom Diffusion Research Articles

Role of the shadowing effect in the growth kinetics of III-V nanowires by molecular beam epitaxy

A model for III-V nanowire (NW) growth in molecular beam epitaxy (MBE) is developed, which describes the NW growth by surface diffusion of adatoms influenced by the shadowing effect. It is shown that the shadowing effect strongly influences the growth kinetics in dense ensembles of NWs. A new solution for the NW length as a function of its radius and deposition thickness is obtained. A comparison is given for theoretical and experimental lengths of InP NWs grown on either adsorbing or reflecting substrates. Keywords: nanowires, adatom diffusion, shadowing effect.

Роль эффекта затенения в кинетике роста III-V нитевидных нанокристаллов методом молекулярно-пучковой эпитаксии

A model for III-V nanowire (NW) growth in molecular beam epitaxy (MBE) is developed which describes the NW growth by surface diffusion of adatoms influenced by the shadowing effect. It is shown that the shadowing effect strongly influences the growth kinetics in dense ensembles of NWs. A new solution for the NW length as a function of its radius and deposition thickness is obtained. A comparison is given for theoretical and experimental lengths of InP NWs grown on either adsorbing or reflecting substrates.

A generalizable machine learning potential of Ag–Au nanoalloys and its application to surface reconstruction, segregation and diffusion

Owing to the excellent catalytic properties of Ag–Au binary nanoalloys, nanostructured Ag–Au, such as Ag–Au nanoparticles and nanopillars, has been under intense investigation. To achieve high accuracy in molecular simulations of Ag–Au nanoalloys, the surface properties must be modeled with first-principles precision. In this work, we constructed a generalizable machine learning interatomic potential for Ag–Au nanoalloys based on deep neural networks trained from a database constructed with first-principles calculations. This potential is highlighted by the accurate prediction of Au (111) surface reconstruction and the segregation of Au toward the Ag–Au nanoalloy surface, where the empirical force field (EFF) failed in both cases. Moreover, regarding the adsorption and diffusion of adatoms on surfaces, the overall performance of our potential is better than the EFFs. We stress that the reported surface properties are blind to the potential modeling in the sense that none of the surface configurations is explicitly included in the training database; therefore, the reported potential is expected to have a strong generalization ability to a wide range of properties and to play a key role in investigating nanostructured Ag–Au evolution, where accurate descriptions of free surfaces are necessary.

Growth of flat-topped, mound-shaped grains with voids when depositing silver thin films at high substrate temperatures using direct-current magnetron sputtering

Thin films of silver were deposited on nonalkali glass substrates at substrate temperatures ranging from room temperature (28 °C) to 150, 200, 300, 400, and 500 °C at discharge pressures of 0.40, 1.20, and 2.00 Pa using direct current magnetron sputtering. On the basis of the measured cross-sectional and surface morphologies, crystallographic structures, and film properties, I discuss the dependence of the film structure and properties on the substrate temperature. The x-ray diffraction measurements showed that the <111> orientation was preferred for all deposition conditions. Scanning electron microscope observations revealed a microstructure of convex-shaped fine grains for a substrate at room temperature, while laterally growing, mound-shaped grains with flat-topped surfaces appeared at substrate temperatures of 400 and 500 °C. Atomic force microscopy also showed an increase in the lateral size and height of the mound-shaped structures with increasing substrate temperature. The lateral grain size evaluated from the areal particle density obtained from atomic force microscopy increased significantly with increasing substrate temperature, reaching 600–800 nm at a substrate temperature of 500 °C. The film stress also changed from compressive to tensile with increasing substrate temperature. The relative density, defined as the ratio of the deposited amount-of-substance to the physical-thickness, decreased significantly with increasing substrate temperature and, at a substrate temperature of 500 °C, was approximately 0.6 times as large as that obtained for thin films deposited at room temperature. The high surface diffusivity of the Ag adatoms induces the growth of laterally growing, mound-shaped grains. Besides, the energy accumulated in the thin films during sputter deposition induces the void formation to increase the efficiency of energy release in the form of heat.

Bulk and surface diffusion of neodymium in alpha-uranium: Ab initio calculations and kinetic Monte Carlo simulations

• A Nd interstitial is found to be intrinsically unstable in alpha- U . • The binding between the substitutional Nd and the U vacancy in strong in alpha- U . • Nd atoms migrate in bulk alpha- U via the vacancy mechanism. • Radiation can significantly accelerate the bulk diffusivity of Nd in alpha- U . • Diffusion of Nd adatoms via pore surfaces is much faster than bulk diffusion. A fundamental understanding of lanthanide transport in metallic fuels is critical for high fidelity modeling of the fuel-cladding chemical interaction (FCCI) phenomenon, which can lead to the formation of brittle intermetallic compounds and premature failure of the cladding. Here we report a combined ab initio density functional theory (DFT) and kinetic Monte Carlo (KMC) study of the bulk diffusivity of Nd in α-U , fully taking into account the effect of radiation enhanced diffusion. The vacancy mechanism is considered to be the dominant mechanism for the bulk diffusion of Nd since a Nd interstitial is found to be intrinsically unstable in α-U . The surface diffusivity of a Nd adatom on α-U (001) surface has been further predicted using KMC simulations parameterized by DFT calculations. The present study suggests that Nd transport via the surface diffusion mechanism can be many orders of magnitude faster than bulk diffusion. Furthermore, the results from the present lower length scale study can be used to inform mesoscale phase-field simulations to determine the effective diffusion coefficient of Nd through α-U with a porous microstructure.

Influences of gallium and nitrogen partial pressure on step-bunching and step-meandering morphology of InGaN quantum barrier layer

As the templates for the growth of the quantum well layer, the morphology of the quantum barrier layer is crucial. Through a cross-comparison among samples grown under different conditions, the effect of TMGa flux, NH3 flux, indium content, and temperature on morphology of InGaN quantum barrier grown by Metal-Organic Chemical Vapor Deposition (MOCVD) is studied. It is found that TMGa and NH3 flux increments are counterbalancing each other in terms of adatom diffusion. Without balanced V/III fluxes, the surface roughens either by spiral hillocks or by strong step meandering. The morphology can be quite different even under a similar V/III ratio. LED samples using the V/III unbalanced quantum barriers, compared to the V-III balanced ones, present inferior optical properties.

Current-Induced One-Dimensional Diffusion of Co Adatoms on Graphene Nanoribbons.

One-dimensional diffusion of Co adatoms on graphene nanoribbons has been induced and investigated by means of scanning tunnelling microscopy (STM). To this end, the nanoribbons and the Co adatoms have been imaged before and after injecting current pulses into the nanoribbons, with the STM tip in direct contact with the ribbon. We observe current-induced motion of the Co atoms along the nanoribbons, which is approximately described by a distribution expected for a thermally activated one-dimensional random walk. This indicates that the nanoribbons reach temperatures far beyond 100 K, which is well above the temperature of the underlying Au substrate. This model system can be developed further for the study of electromigration at the single-atom level.

Abnormal diffusion profile of adatoms on extremely wide terraces of the Si(111) surface

The authors experimentally studied the distribution of the adatom concentration on extremely wide terraces of the Si(111) surface whose dimensions are comparable with the diffusion length of adatoms. The extremely wide terraces were created during in situ experiments carried out by ultrahigh vacuum reflection electron microscopy by high-temperature annealing of Si(111) samples (more than 1000 °C) followed by rapid cooling to 750 °C to form 7 × 7 superstructure domains. A detailed analysis of the surface morphology of the terraces was carried out by ex situ atomic force microscopy under ambient conditions. Based on high-resolution (1.2 nm/pixel) atomic force microscopy images, panoramic topographic images of the terraces were formed. Digital processing of the panoramic images visualized the distribution of the adatom concentration n. For a terrace cooled from 1070 °C, central terrace regions show minimum n values around 0.13 BL (1 bilayer (BL) = 1.56×1015 cm−2); close to the monatomic step bordering the terrace, n increases to about 0.14 BL. The authors determined that this radial distribution n(r) at 1070 °C corresponds to the adatom diffusion coefficient D = 59 ± 12 μm2/s. It was found that, for a terrace cooled from 1090 °C, the approach assuming the same adatom diffusion length over the entire terrace does not describe the experimental n(r) distribution. For its analysis, the authors used the solution of the stationary diffusion equation under the assumption that D is not constant. Based on a numerical solution, the dependence of D on the experimentally measured n values was obtained. Under the assumption that adatom lifetime does not depend on n at 1090 °C, the adatom diffusion coefficient was found to decrease from 140 μm2/s at n = 0.093 BL (in the central terrace regions) to 5 μm2/s at n = 0.118 BL (near the step). The results of this work experimentally demonstrated that the control over adatom concentration can be used to significantly vary the diffusion properties of the adsorption layer on the crystal surface.

Scaling laws for step bunching on vicinal surfaces: Role of the dynamical and chemical effects.

We study the evolution of step bunches on vicinal surfaces using a thermodynamically consistent step-flow model. By accounting for the dynamics of adatom diffusion on terraces and attachment-detachment at steps (referred to collectively as the dynamical effect), this model circumvents the quasistatic approximation that prevails in the literature. Furthermore, it generalizes the expression of the step chemical potential by incorporating the necessary coupling between the diffusion fields on adjacent terraces (referred to as the chemical effect). Having previously shown that these dynamical and chemical effects can explain the onset of step bunching without recourse to the inverse Ehrlich-Schwoebel (iES) barrier or other extraneous mechanisms, we are here interested in the evolution of step bunches beyond the linear-stability regime. In particular, the numerical resolution of the step-flow free boundary problem yields a robust power-law coarsening of the surface profile, with the bunch height growing in time as H∼t^{1/2} and the minimal interstep distance as a function of the number of steps in the bunch cell obeying ℓ_{min}∼N^{-2/3}. Although these exponents have previously been reported, the novelty of the present approach is that these scaling laws are obtained in the absence of an iES barrier or adatom electromigration. In order to validate our simulations, we take the continuum limit of the discrete step-flow system via Taylor expansions with respect to the terrace size, leading to a novel nonlinear evolution equation for the surface height. We investigate the existence of self-similar solutions of this equation and confirm the 1/2 coarsening exponent obtained numerically for H. We highlight the influence of the combined dynamical-chemical effect and show that it can be interpreted as an effective iES barrier in the setting of the standard Burton-Cabrera-Frank theory. Finally, we use a Padé approximant to derive an analytical expression for the velocity of steadily moving step bunches and compare it to numerical simulations.

Modulating effect of evanescent waves on thin film growth

Atomic-scale smooth thin films are keys to successful integration and proper function of many multilayer-structured devices. However, the intrinsic island-like growth mode prevents human being from realizing such ultrasmooth films of many important functional materials. To solve this problem, we propose a negative entropy-infusing method that employs evanescent waves to enhance the downward interlayer diffusion of adatoms and thus transform the island-like growth mode to the layer-by-layer growth mode. The formulas of the optical force and the lowered diffusion barrier were derived, and the application of this theory on a simplified example demonstrates significantly improved surface morphologies through numerical simulations.

Atomistic simulation of the surface configuration of the Ni–Re cluster

The atomistic simulation of the diffusion and growth processes of the Re adatom on the Wulff1289 Ni cluster was preformed via molecular dynamics methods. The nudged elastic band method was used to calculate the diffusion activation energies. All possible diffusion paths and diffusion mechanisms were studied. Results showed that the diffusion process of Re adatoms on the (111) facet was preferred among all the diffusion paths. As expected, a cluster with Nicore–Reshell structure was obtained through one-by-one atom deposition at 300 K. This work will help improve effectively the utilization rate and catalytic performance of the rare metal Re. Moreover, it can provide a theoretical foundation for artificially controlling the configuration of alloy clusters.

Determination of Activation Energy of Surface Diffusion Based on Thermal Oscillations of Atoms

This paper covers calculations of the activation energy of surface diffusion of ad-atoms on the substrate surface from the point of view of thermal oscillations of substrate atoms and ad-atoms. The main characteristic of oscillations of atoms and geometric mean frequency was calculated based on statistical approximation of the Debye model using the reference values of entropy and heat capacity of metals. The basic principle of the model of activation energy calculation presented in the paper is the formation of potential wells and barriers during oscillations of atoms localized in the sites of the lattice. Oscillations of atoms were considered in the framework of quasiclassical quantum approximation as the oscillations of harmonic oscillators in the potential parabolic wells. Dimensions of the negative part of values of the potential well energy were determined by the amplitude of thermal oscillations of atoms. Positive values constituted a significant part of the potential well energy values. Barriers were formed owing to interaction of positive values of the energy of parabolic wells of adjacent atoms. Therefore, in order to make the ad-atom jump, it is necessary to get out of the potential well having the negative values, and to overcome the potential barrier. The energy required for the ad-atom jump on the substrate surface was the activation energy of surface diffusion. The results obtained in this paper agree satisfactorily with the results of another method, which is based on determining the energy of ad-atom binding with the substrate atoms.

Selective area growth rates of III-V nanowires

Selective area growth (SAG) of semiconductors is a scalable method for fabricating gate-controlled quantum platforms. This letter reports on the adatom diffusion, incorporation, and desorption mechanisms that govern the growth rates of SAG nanowire (NW) arrays. We propose a model for the crystal growth rates that considers two parameter groups: the crystal growth control parameters and the design parameters. Using GaAs and InGaAs SAG NWs as a platform, we show how the design parameters such as NW pitch, width, and orientation have an impact on the growth rates. We demonstrate that by varying the control parameters (i.e., substrate temperature and beam fluxes) source, balance, and sink growth modes may exist in the SAG selectivity window. Using this model, we show that inhomogeneous growth rates can be compensated by tuning the design parameters.

A molecular dynamics simulation of Ti–TiN multilayer deposition on FeCrNi(001) alloy substrate

In this work, to investigate the physical vapor deposition (PVD)-deposited Ti–TiN multilayer coating on the FeCrNi (Fe 78.4%- Cr 15.17%- Ni 6.43%) substrate, made of GTD 450 compressor blade material, the process was simulated using molecular dynamics (MD) method with the 2NN-MEAM (nearest-neighbor modified embedded-atom-method) potential. The results revealed that although the Ti film growth mode is epitaxy, the N adatoms diffusion on the Ti surface prevents the epitaxial growth of the TiN film. Comparing the film's structure showed that the surface morphology and crystallinity of Ti are better than those of TiN. The average mean biaxial stress is compressive in both the substrate and deposited layers, while it becomes tensile in the substrate-Ti interface layers. On the other hand, in spite of being compressive the average normal stress of the substrate, the deposited layers are tensile. Increasing the film's thickness increases the average mean biaxial stress of the substrate-Ti interface and the Ti–TiN film. The results showed that the enhancement of substrate temperature from 600 to 800 K causes to reduce the average mean biaxial stress of the substrate, the substrate-Ti interface, and the TiN film surface. However, growing the number of layers with a constant film thickness reduces stresses in the substrate, the interface, and the film region. Enhancing incident energy increases the average mean biaxial stress of both substrate and film. In contrast, it decreases the surface roughness of the film.

Selective area epitaxy of high quality Wurtzite-InAs heterostructure on InGaAs nanopillars at indium-rich region using MOCVD

We report synthesis of Au-free selective area epitaxy of InGaAs/InAs heterostructure nanopillars on patterned Si (111) substrate by metal-organic chemical vapor deposition. Effect of trimethylindium supply on InGaAs/InAs heterostructure nanopillars morphology and crystal structure is studied. Transmission electron microscopy images reveal subtle and consistent differences at the growth front of the InGaAs nanopillars for different indium-flow rate fraction. Notably, the nanopillar growth front exhibits zincblende-InAs cap for the low indium-flow rate fraction, whereas a sharp wurtzite-InAs segment is found at high indium-flow rate fraction. Nanopillar geometry profile fitting attributes that the wurtzite-InAs segment formation is ascribed to indium adatom diffusion on the sidewall surface along with direct impingement of indium growth species on the top facet of the nanopillar. Additionally, X-ray photoelectron spectroscopy confirms that indium segregation is more pronounced at the sidewall of the nanopillar under arsenic-limited region.

On the influence of the micropulse on Nb thin films deposited by MPPMS and DOMS: A comparative study

Nb thin films deposited by modulated pulsed power magnetron sputtering (MPPMS) and deep oscillation magnetron sputtering (DOMS) were comparatively studied under a similar average power by controlling the micropulse duty cycle. It was found that DOMS discharge both showed higher discharge peak current and peak voltage, time delay between the current and voltage was much smaller compared with a MPPMS discharge. All Nb thin films were observed with Nb(110) preferred orientation and compact columnar structure. The increase of micropulse duty cycle led the gradual movement of Nb(110) to high scattering angle direction, meanwhile the DOMS Nb(100) diffraction peaks were all on the left of MPPMS Nb(110). An increase for compressive residual stress σ could be observed for both techniques, and all DOMS Nb thin films showed higher residual stress, hardness and elastic modulus. The anomalous increase in σ also led to the deterioration of DOMS Nb thin film scratch adhesion. Despite the grain sizes of DOMS Nb thin films were all smaller than MPPMS Nb thin films, σ in DOMS Nb thin films generated by the adatom diffusion and ion irradiation still overwhelmed the tensile stress generated by the volume shrinkage of the growing grains. The special afterglow of DOMS in the microsecond scale gave a new controlling dimension for thin film growth.

ТЕОРЕТИЧНА МОДЕЛЬ ФОРМУВАННЯ ДВОВИМІРНИХ НАНОСТРУКТУР ВЕРТИКАЛЬНОГО ГРАФЕНУ ПІД ДІЄЮ ПЛАЗМИ

Vertically oriented graphene nanostructures have been grown for more than decade, but the mechanisms of their formation are still unclear. A multifactor model is proposed, which is verified by comparison with experimental data and describes the processes of growth of the structure of vertical graphene in plasma. The role of chemical and physical processes that cannot be directly characterized by available experimental methods, such as surface diffusion of adatoms and radicals under the action of ions, has been studied. Ion bombardment is a key factor that significantly accelerates the growth rate through the formation of surface defects and, consequently, increases the energy of surface adsorption. Hydrocarbon radicals formed on the substrate under the bombardment diffuse to the graphene nanosheets and serve as the main source of the construction material. Thus, the leading role in the formation of vertical graphene belongs to surface diffusion, rather than direct deposition from the gas phase. The temperature of the sample is also an important parameter, which affects the growth process according to the following mechanism: at low temperatures the adsorption from the gas phase is more intense, but the diffusion processes are slowed down; elevated temperatures have the opposite effect. The surface density of graphene nanosheets, which can be controlled at the stage of nucleation, strongly affects the height of the structure due to the redistribution of ion fluxes during the growth: as the nanosheets grow, the ion current density decreases to the side edge of the sheet and increases to the upper edge. This process leads to a decrease in the ion current density at the side edge of the nanosheet, and, as a consequence, to a change in the dependence of the graphene sheet length on time: from a saturated curve or a quasilinear time dependence to a parabolic dependence. The assumption of surface diffusion of hydrocarbon radicals as the dominant growth mechanism is consistent with existing experimental data; these results confirm the physical model, and also bring a deeper understanding of the physics of growth of vertical graphene.

Surface Diffusion of Underpotential‐Deposited Lead Adatoms on Gold Nanoelectrodes

AbstractA simple and readily applicable voltammetric approach is described to characterize and measure the site‐hopping surface diffusion of underpotential‐deposited (UPD) metal adatoms at nanoelectrodes. UPD refers to the deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulk deposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomic nanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro‐ and macroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by other electrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass, only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidly loaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immune to Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore, surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect these diffusional surface processes so that the integral charges vary linearly with the inverse of the square root of the scan rate. This is easily observable and measurable with usual bench‐level electrochemical instrumentation. In this work, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the major involvement of this site‐hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterize it quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be ∼4.4×10−11 cm2 s−1 at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol−1 (i. e., 0.19 eV per Pb adatom) for the reaction of inter‐sites exchange of Pb adatoms on a polycrystalline Au surface.

Material migration in W and Mo during bubble growth and fuzz formation

Growth of helium (He) induced bubbles and fuzz in tungsten (W) and molybdenum (Mo) is investigated using samples of W films on Mo substrates and Mo films on W substrates exposed to He-containing plasma in the temperature range of 340 to 1075 K, fluence range of 1.0–14 × 1025 He·m−2, and incident ion energy of <50 eV. No fuzz (only up to 2 nm diameter bubbles) and no material transport occur in W films at ⩽750 K, while precursors-of or fully-developed fuzz and material mixing occur in W and Mo films at ⩾800 K. This suggests that fuzz forms in multi-material systems as long as one material meets the conditions for fuzz formation, namely T s/T m ∼ 0.27–0.5 where T s and T m are the sample exposure and material melting temperatures, respectively. Larger He bubbles, more material mixing, and further-developed fuzz occur at higher temperature due to increased mobility of He atoms and small He clusters. Accumulation of substrate material at the surface of fuzzy W and Mo thin-film (<80 nm) samples suggests fuzz growth by material transport from the bubble layer in the bulk up to the fiber tip, likely by a two-step process: (i) diffusion of punched dislocation loops in the bulk toward the fuzz base and (ii) diffusion of adatoms along the fuzz base and fiber surface (with effective transport of adatoms upwards due to trapping of adatoms at curved surfaces of fiber tips and/or due to the continuous generation of adatoms at the fuzz base). While the bubble size and fuzz thickness increase with reduced W concentration in Mo thin-film samples at 838 K likely due to an increase in trap mutation and dislocation loop punching in Mo compared to W, the fuzz thickness decreases with reduced W concentration at 1075 K despite an increase in the bubble size likely due to slower diffusion of interstitial loops in Mo.

Microscopic mechanism of adatom diffusion on stepped SiC surfaces revealed by first-principles calculations

We report first-principles total-energy calculations based on real-space density-functional theory that unveil the atom-scale mechanisms of surface diffusion of adatoms on the Si-faced 3C-SiC(111) stepped surface. The quality of the epitaxial layer of SiC affects the device performance. Therefore, fundamental knowledge of the microscopic mechanisms of epitaxial growth is crucial for the improvement of the quality of SiC power devices. However, adatom diffusion on the growing stepped SiC surfaces is still unknown. We identify diffusion pathways for three important adatom species of SiC chemical vapor deposition (CVD), Si, C, and H, and obtain the corresponding energy profiles for both on the surface terraces and near the surface steps, providing a complete picture of the adatom diffusion. We find that the Si adatom is most mobile on the terrace and shows prominent Ehrlich-Schwoebel (ES) effect in the inter-terrace diffusion, whereas the C and H adatoms show less ES effect and in an ascending diffusion even the inverse ES effect appears for the C adatom. The results obtained are fundamentals to explore the microscopic mechanism of the epitaxial growth on 3C-SiC(111) surfaces and equivalently hexagonal SiC(0001) surfaces.