Layer Growth Mode Research Articles

MD simulation of effect of crystal orientations and substrate temperature on growth of Cu/Ni bilayer films

We prepare Cu/Ni bilayer films by depositing the incident atoms on Cu substrates with various surface orientations and under different temperatures and investigate interfacial structure, surface roughness, radial distribution function and hardness of the films. We find that the incident atoms can penetrate (001) substrate more easily than other surfaces, resulting in a transitional layer consisting of two kinds of atoms. Stacking faults are generated in the bilayer films deposited on the (111) substrate, which can reduce misfit strain and thus account for the layer growth mode of the films. The surface roughness decreases with the increase in deposition temperature. Moreover, we also find that a certain degree of roughness benefits the formation of coherent interface due to the tilted-layer epitaxial growth. The hardness differs for the films deposited at different temperatures.

Development of Long Coated Conductors with High In-field Ic Performance by PLD Method at High Production Rate

We fabricated short samples and a 93 m long coated conductor (C. C.) of EuBa2Cu3O7-δ (EuBCO) with BaHfO3 (BHO) by the IBAD and the PLD methods, which exhibited the high in-field minimum Ic value, (Ic(min)), performance of 141.2 (77K in 3 T) and 411.3 (65K in 3 T) A/cm-w for a short sample, and 133.9 (77K in 3 T) A/cm-w for 93 m long C. C. with 3.6μm in thickness, respectively. Moreover, this long EuBCO with BHO coated conductor also showed high uniform longitudinal Ic distributions and n-value in magnetic fields. However, the deposition rate for obtaining the high in-field Ic performance was comparatively slow down to 10μm/h. To realize the low production cost for EuBCO with BHO coated conductors, improvement of the deposition rate of the EuBCO with BHO layer with high Ic is required. To solve this problem, we optimized growth conditions including deposition conditions. One of the objectives of this work was changing the layer growth mode from the vapor-solid (VS) mode to the vapor-liquid-solid (VLS) one to fabricate EuBCO with BHO layers for achievement of high production rate and maintaining the high in-field Ic and Jc performance of the films deposited at slow deposition rates. As a result, we fabricated EuBCO with BHO coated conductors at a high deposition rate of about 40μm/h and production rate of about 10 m/h, which revealed the Ic(min) value of 48.7 A/cm-w at 77K in 3 T for 1.35μm in thickness.

Surface ligand-mediated isolated growth of Pt on Pd nanocubes for enhanced hydrogen evolution activity

Br− ions can affect the growth mode of Pt on a Pd substrate. When Br− ions adsorb on some sites of the Pd cube, the newly formed Pt atoms tended to grow on the Br− free area, facilitating island growth mode. Otherwise, without Br− ions, because of the relatively high diffusion rate between Pt and Pd, Pt atoms can epitaxially deposit on the whole surface of the Pd nanocubes, forming a layered growth mode structure.

Effects of the surface stoichiometry of seeds on GaN layer growth by hydride vapour phase epitaxy

The effect of the atmosphere in a reactor prior to hydride vapour phase epitaxy on the surface stoichiometry of both the GaN template and layer growth was studied. The surface stoichiometry of metallic Ga layers was clarified by X-ray photoelectron spectroscopy using templates without NH3 protection. The metallic Ga layer acted as a mask and exerted a significant effect on the subsequent epitaxial layer growth mode. GaN grown on the template without protection followed island growth in the initial growth stage. In contrast, GaN epitaxy on the template with NH3 protection quickly converts to pseudo-2D growth. The images of CL illustrate that the GaN epilayer on the template without protection has a lower dislocation density than the GaN epilayer grown on the template with NH3 protection. Reasons behind this effect have been discussed.

Coarse-Grained W p C-Fe Hardmetals with High Hardness Fabricated by Direct Current Arc In Situ Metallurgy

Coarse-grained WpC-Fe (P = 1,2) hardmetals were fabricated with tungsten and graphite powders by the direct current arc in situ metallurgy technique. Investigations on the microstructures, morphology evolution, and performance for the resultant materials were also carried out. It is found that with decreasing the applied current, the content of W2C decreases, but the content and grain size of WC increase. The largest WC grain can reach around 150 μm. The shape of WC changes from hexangular prism to truncated triangle prism, and ultimately transforms into triangular prism due to the different growth rates of the prismatic planes. Furthermore, a layer by layer growth mode of the WC was observed. The performance tests indicate that the average microhardnesses of the WC and W2C are 2559 and 1916 HV0.2, respectively. For the WC grain, the microhardness of 2761 HV0.2 of the triangular basal plane is much higher than the 2357 HV0.2 of the prismatic plane.

Electronic structure of PTCDA on Sn/Si(1 1 1)-[formula omitted

The electronic structures of PTCDA on the Sn/Si(111)-23×23 surface have been thoroughly studied by high-resolution photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS). Upon deposition of PTCDA, there is an unusual charge transfer from the Sn/Si(111)-23×23 surface to the molecules. This is clearly shown by a new component in the Sn 4d core-level spectra that shifts towards higher binding energy. In contrast to the literature, the charge provided by Sn is donated to the carbonyl C instead of the O atoms. This is revealed by a new component in the C 1s core-level spectra that shifts towards lower binding energy. The charge transfer causes a splitting of the HOMO level in the valence band spectra. As indicated in the NEXAFS spectra, it also induces a splitting of the LUMO level of the molecules. For thick films the NEXAFS results suggest a layer by layer growth mode.

Confining the Nucleation and Overgrowth of Rh to the {111} Facets of Pd Nanocrystal Seeds: The Roles of Capping Agent and Surface Diffusion

This article describes a systematic study of the spatially confined growth of Rh atoms on Pd nanocrystal seeds, with a focus on the blocking effect of a surface capping agent and the surface diffusion of adatoms. We initially used Pd cuboctahedrons as the seeds to illustrate the concept and to demonstrate the capabilities of our approach. Because the Pd{100} facets were selectively capped by a layer of chemisorbed Br(–) or I(–) ions, we were able to confine the nucleation and deposition of Rh atoms solely on the {111} facets of a Pd seed. When the synthesis was conducted at a relatively low temperature, the deposition of Rh atoms followed an island growth mode because of the high Rh–Rh interatomic binding energy. We also facilitated the surface diffusion of deposited Rh atoms by increasing the reaction temperature and decreasing the injection rate for the Rh precursor. Under these conditions, the deposition of Rh on the Pd{111} facets was switched to a layered growth mode. We further successfully extended this approach to a variety of other types of Pd polyhedral seeds that contained Pd{111} and Pd{100} facets in different proportions on the surface. As expected, a series of Pd–Rh bimetallic nanocrystals with distinctive elemental distributions were obtained. We could remove the Pd cores through selective chemical etching to generate Rh hollow nanoframes with different types and degrees of porosity. This study clearly demonstrates the importance of facet capping, surface diffusion, and reaction kinetics in controlling the morphologies of bimetallic nanocrystals during a seed-mediated process. It also provides a new direction for the rational design and synthesis of nanocrystals with spatially controlled distributions of elements for a variety of applications.

Origin of Different Growth Modes for Epitaxial Manganite Films

The microstructures of the Bi0.4Ca0.6MnO3 (BCMO) and La0.67Ca0.33MnO3 (LCMO) epitaxial films are investigated by transmission electron microscopy in detail. BCMO epitaxial films (~ 10 and ~ 40 nm) exhibit an island growth mode whereas the LCMO films (~ 6 and ~ 30 nm) follow a layer by layer growth mode. Combined with the critical thickness models for the expected onset of the misfit dislocations in epitaxial films, an atomic collapse model is introduced to explain their mechanism of formation in manganite films. At the beginning of deposition, the strain caused by the lattice mismatch between the epitaxial film and substrate can be accommodated by elastic deformation. With the increase of film thickness, the strain becomes larger and larger. When the film thickness reaches the critical thickness, the strain can only be relaxed by the formation of misfit dislocations. Meanwhile, the atomic configuration of the epitaxial film will reorganize and some atoms begin to collapse, thus an island morphology will be formed. Once the collapse morphology is formed, maintenance of this wave‐like morphology depends on atomic diffusion length of the deposited atoms. If the diffusion length of the deposited atoms is long, the island morphology will not be maintained. If the diffusion length of the deposited atoms is short, the island morphology will keep until the epitaxial film is thick enough. The results could shed light on the growth modes for other perovskite epitaxial films.

Effect of compressive strain relaxation on surface morphology in GaAsP growth on GaP substrate

Surface morphology change of GaAs0.6P0.4 and GaAs layer grown on GaP substrate during growth was investigated by an Atomic Force Microscope. The compressive strain in the GaAsP layer was relaxed by {111} plane slips while that in the GaAs layer was relaxed by formation of island structures. During growth, the GaAs layer showed much flatter surface morphology. It was found that the mechanism of the strain relaxation largely affects the surface morphology. Finally, we compared the critical thickness for the {111} plane slip and the transition thickness in which the layer growth mode transits from a layer growth to an island growth to explain the different strain relaxation mechanisms.

Laterally Resolved Orientation and Film Thickness of Polar Metal Chlorine Phthalocyanines on Au and ITO

The molecular orientation and homogeneity of gallium(III) and aluminum(III) chlorine phthalocyanine (GaClPc and AlClPc) are studied on flat (gold single crystal, Au(100)) and rough surfaces (indium tin oxide, ITO) by Raman spectroscopy in combination with X-ray absorption spectroscopy (XAS) in detail. In the case of Raman spectroscopy, Euler angles are used to describe the geometry while Raman imaging provides insight into homogeneity. For XAS, synchrotron radiation is applied to study the angle dependence of the excitation structure. On Au(100) the molecules are highly ordered and tend to align parallel to the surface (lying) with a layer by layer growth mode. Contrarily, on technically relevant substrates, as in the case of ITO, less ordered films due to the roughness of the surface nature are present. The results are discussed in relation to the substrate's nature and the influence of dipole moments.

Ga-induced restructuring of Si(5 5 12) − 2 × 1 reconstructed surface at room temperature

Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.

Role of growth mode in the formation of magnetic properties of InMnAs grown by MOVPE

The magnetic and structural properties of InMnAs self-assembled dots and epitaxial layers prepared from the identical composition of the gas phase were investigated. All structures under study were grown by low pressure metal organic vapor phase epitaxy at the growth temperature of 500°C. Magnetization measurements indicated that the Curie temperature of InMnAs dots prepared under the Stranski–Krastanov growth mode varied from 7 to 235K in relation to the material composition. The Curie temperature of epitaxial layers prepared under layer by layer growth mode was found to be independent from the material composition. Transmission electron microscopy and microstructure analysis revealed that the dots have a zinc blende structure and they are plastically relaxed.

Magnetism of ultra-thin iron films seen by the nuclear resonant scattering of synchrotron radiation

Conversion electron Mössbauer spectroscopy proved in the past to be very useful in studying surface and ultrathin film magnetism with monolayer resolution. Twenty years later, its time-domain analogue, the nuclear resonant scattering (NRS) of synchrotron radiation, showed up to be by orders of magnitude faster and more efficient. The evolution of the spin structure in epitaxial 57Fe films on a tungsten W(110) was studied via the accumulation of the NRS time spectra directly during Fe film deposition. In the 0.5 – 4 monolayers Fe thickness range, the complex non-collinear magnetic structure was derived from the NRS data, resulting from the deviation from the layer by layer growth mode. For thicker Fe films, the in-plane thickness induced spin reorientation transition could be clearly identified. Based on the NRS analysis it is shown that SRT process originates at the Fe/W(110) interface and proceeds through a transient fan-like magnetization structure.

Pt Nano-Layer Formation by Redox Replacement of Cu Adlayer on Au(111) Surface

Platinum is one of the most important catalysts for many chemical and electrochemical reactions. 1 Deposition of small amount of platinum on metal surfaces has been shown to improve the electrocatalytic properties of the metal electrodes. 2 Platinum does not exhibit underpotential deposition (UPD) behavior and studies showed that electrochemical deposition of platinum on Au(111) surface from platinum complex solutions proceeded with a layer by layer growth mode to form a multilayer of Pt, 3

Effect of growth conditions on the compositional homogeneity of Cd1 − x Zn x Te crystals

A crystal growth unit has been designed for Cd1 − x Zn x Te growth by the axial-heat-flux-close-to-the-phase-interface (AHP) method at argon overpressures of up to 12 MPa. The influence of melt superheating, growth rate, melt layer thickness, argon pressure, and growth mode on the Zn distribution in 21- to 45-mm-diameter crystals and defect formation in Cd1 − x Zn x Te ingots has been investigated. The results demonstrate that the AHP method offers the possibility of effectively controlling the shape of the solid-liquid interface and ensures low fluid velocity and high radial compositional homogeneity. The feeding method used markedly improves the axial Zn profile and ensures low dislocation density (5 × 103 cm−2) in the grown crystal.

Valence changes of Sm atoms adsorbed on Mo (2 1 1) surface

Adsorption of Sm thin layers on the Mo (2 1 1) face was investigated by using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and changes of the work function measurements (Δ ϕ). The layer growth mode and the valence change of Sm atoms were determined. The Sm layer growth mode is layer-by-layer, at least to 3 ML, followed by the crystallite growth. It was found out that for θ < 0.3 ML Sm atoms are adsorbed only in divalent state. At coverage about 0.5 ML the number of both trivalent and divalent atoms is comparable. At θ = 1 ML, 11% of Sm atoms are divalent, and 89% trivalent. The second layer is composed of only divalent atoms. The first two layers have different surface densities, namely 6.15 × 10 14 atoms/cm 2 for the first layer, and 5.56 × 10 14 atoms/cm 2 for the second. Divalent Sm atoms creating the second layer form islands with the p(3 × 1) structure observed by LEED and STM.

Surface Morphology and Defect Formation Mechanisms for HgCdTe (211)B Grown by Molecular Beam Epitaxy

The surface morphology and crystallinity of HgCdTe films grown by molecular beam epitaxy (MBE) on both CdZnTe and CdTe/Si (211)B substrates were characterized using atomic force microscopy (AFM), as well as scanning (SEM) and transmission (TEM) electron microscopy. Crosshatch patterns and sandy-beach-like morphologies were commonly found on MBE (211) HgCdTe epilayers grown on both CdZnTe and CdTe/Si substrates. The patterns were oriented along the $$ {\left[ {\overline{2} 13} \right]} $$ , $$ {\left[ {\overline{2} 31} \right]} $$ , and $$ {\left[ {0\overline{1} 1} \right]} $$ directions, which were associated with the intersection between the (211) growth plane and each of the eight equivalent HgCdTe slip planes. This was caused by strain-driven operation of slip in these systems with relative large Schmid factor, and was accompanied by dislocation formation as well as surface strain relief. Surface crater defects were associated with relatively high growth temperature and/or low Hg flux, whereas microtwins were associated with relatively low growth temperature and/or high Hg flux. AFM and electron microscopy were used to reveal the formation mechanisms of these defects. HgCdTe/HgCdTe superlattices with layer composition differences of less than 2% were grown by MBE on CdZnTe substrates in order to clarify the formation mechanisms of void defects. The micrographs directly revealed the spiral nature of growth, hence demonstrating that the formation of void defects could be associated with the Burton, Cabrera, and Frank (BCF) growth mode. Void defects, including microvoids and craters, were caused by screw defect clusters, which could be triggered by Te precipitates, impurities, dust, other contamination or flakes. Needle defects originated from screw defect clusters linearly aligned along the $$ {\left[ {0\overline{1} 1} \right]} $$ directions with opposite Burgers vector directions. They were visible in HgCdTe epilayers grown on interfacial superlattices. Hillocks were generated owing to twin growth of void or needle defects on (111) planes due to low growth temperature and the corresponding insufficient Hg movement on the growth surface. Therefore, in addition to nucleation and growth of HgCdTe in the normal two-dimensional layer growth mode, the BCF growth mode played an important role and should be taken into account during investigation of HgCdTe MBE growth mechanisms.

Epitaxial Growth of Heterogeneous Metal Nanocrystals: From Gold Nano-octahedra to Palladium and Silver Nanocubes

With octahedral Au nanocrystals as seeds, highly monodisperse Au@Pd and Au@Ag core-shell nanocubes were synthesized by a two-step seed-mediated method in aqueous solution. Accordingly, we have preliminarily proposed a general rule that the atomic radius, bond dissociation energy, and electronegativity of the core and shell metals play key roles in determining the conformal epitaxial layered growth mode.

Pulsed layer growth of AlInGaN nanostructures

AbstractA pulsed layer growth mode in the metal‐organic vapor phase epitaxy (MOVPE) was used to fabricate excellent quality AlInGaN nanostructures. The amount of material was varied, resulting in AlInGaN layer thicknesses between nominally 1.5 nm and 6 nm, respectively. We have analyzed the material properties by X‐ray diffraction (XRD) as well as photoluminescence (PL) spectroscopy. The observed XRD‐spectra and the PL intensity show the high quality of the deposited material.By analyzing the PL spectra we have found an energetic shift of the resonance lines from 2.65 eV to 3.33 eV with decreasing well thickness. We attribute this shift mainly to the presence of internal electric fields at the AlIn‐GaN/GaN interface. Power‐dependent and time resolved PL experiments confirm this observation. Comparing the luminescence at elevated temperatures, the pulsed layer epitaxy structures reveal a much higher intensity as the conventional grown samples. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Experimental investigation of different configurations for the seeded growth of SiC crystals via a VLS mechanism

AbstractThe growth of SiC crystals or epilayers from the liquid phase has already been reported for many years. Even if the resulting material can be of very high structural quality and the possibility to close micropipes was demonstrated, handling the liquid phase still is a challenge. Moreover, it is highly difficult to stabilize the C dissolution front and then to stabilize the growth front over a long growth time. Based on the Vapour‐Liquid‐Solid (VLS) mechanism, we present a new configuration for the growth of SiC single crystal which should allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. The process consists in a modified top and bottom seeded solution growth method, in which the liquid is held under electromagnetic levitation and fed from the gas phase. 3C‐SiC crystals exhibiting well‐faceted morphology were successfully obtained at 1100‐1200 °C with exceptional growth rates, varying from 1 to 1.5 mm/h in Ti‐Si melt. It was shown that the nucleation density decreases simultaneously with increasing propane partial pressure. At 1200‐1400 °C, thick homoepitaxial 6H‐SiC layers were successfully obtained in Co‐Si and Ti‐Si melts, with growth rate up to 200 µm/h. Large terraces with smooth surfaces are observed suggesting a layer by layer growth mode, and the influence of the system pressure was demonstrated. It was shown that the terrace size decrease simultaneously with increasing propane partial pressure which suggests the beginning of a two dimensional to three dimensional growth mode transition. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)