Simulation of surfactant effect on growth of metal homoexpitaxial Sb－Ag/Ag(111)

A kinetic Monte Carlo simulation is performed in order to study the effect of Sb atoms as a surfactant on the growth of Ag on Ag(111). In our model the repulsive mechanism in which the surfactant Sb atoms repel diffusing Ag adatoms, and the exchange mechanism between Ag and Sb atoms, are considered. Our simulations show that the effects of Sb atoms for Ag/Ag(111) growth system are mainly to increase the chances for Ag atoms to overcome the Ehrlich--Schwoebel barrier both in the interlayer growth and along the edge diffusion. The influence of the coverage of Sb atoms and substrate temperature on the growth of Ag/Sb/Ag(111) is discussed.     

A Study of Surfactant-Induced Growth of Ag on Ag （111）

A new growth model is introduced to describe surfactant-induced growth of Ag on Ag （111） with realistic physical parameters. In this model, the A-S exchange mechanism is considered for the first time. Using the Monte Carlo simulations, the influence of exchange mechanism, surface temperature T, the exchange barrier Eεx, and the coverage of surfactant θM on the growth mode and morphology during multilayer film growth of Ag/Ag （111） are studied in detail Both the referenced value of surfactant coverage and the method to obtain perfect layer-by-layer film in surfactantinduced Ag/Ag （111） system are provided. Our simulation results are consistent with many experimental observations for surfactant-induced growth of Ag on Ag （111）.     

Adsorptions and diffusions of carbon atoms on the surface and in the subsurface of Co （200）： A first-principles density-functional study

First-principles calculations based on density functional theory are used to investigate the adsorptions and diffusions of carbon atoms on the surface and in the subsurface of Co(200). The preferred site for the carbon atom on the surface is the hollow site, and the preferred site in the subsurface is the octahedral site. There is charge transfer from the surface to the adsorbed carbon atom, and for the most favorable adsorbed structure the charge transfer is largest. Moreover, the energy barriers for the diffusions of carbon atoms on the surface and from the surface into the subsurface and then back to the surface are calculated in detail. The results indicate that the energy barrier for the diffusion of carbon atoms on the surface is comparable to that from the subsurface to the surface. The results imply that both the direct surface nucleation and the surface segregation from Co bulk can be observed in the chemical vapor deposition growth of graphene on Co(200)substrate, which can gain a new insight into the growth mechanism of graphene.     

Dynamical Behaviour of Ag and Cu Double-Layer Islands on fcc （111） Surfaces

Molecular dynamics simulations are preformed to investigate the diffusion behaviour of Ag/Cu double-layer islands on Ag(111)/Cu(111) surfaces, showing that atoms of the top island usually diffuse into the lower island by exchange mechanism when they move to the verge of the lower island via a concerted motion. Compared to the Cu adatoms, the Ag adatoms can easily form a kind of stable compact cluster of hexagonal island during their diffusion processes, and this cluster make it possible that the decay rate of the top Ag island is considerably slower than that of the Cu island. In addition, we find that the exchange mechanism is predominant compared to the jumping process in the simple potential calculation. These simulation results are in agreement with previous experimental observations and theoretical calculations.     

Growth and Characterization of InSb Thin Films on GaAs(001) without Any Buffer Layers by MBE

We report the growth of InSb layers directly on GaAs(001) substrates without any buffer layers by molecular beam epitaxy(MBE). Influences of growth temperature and Ⅴ/Ⅲ flux ratios on the crystal quality, the surface morphology and the electrical properties of InSb thin films are investigated. The InSb samples with roomtemperature mobility of 44600 cm~2/Vs are grown under optimized growth conditions. The effect of defects in InSb epitaxial on the electrical properties is researched, and we infer that the formation of In vacancy(V_(In))and Sb anti-site(Sb_(In)) defects is the main reason for concentrations changing with growth temperature and Sb_2/In flux ratios. The mobility of the InSb sample as a function of temperature ranging from 90 K to 360 K is demonstrated and the dislocation scattering mechanism and phonon scattering mechanism are discussed.     

Nucleation Behaviour in the Initial Stage of Surfactant-Mediated Epitaxial Growth

The nucleation kinetics in the early stage of epitaxial growth mediated by a monolayer of surfactant is studied by using kinetic Monte Carlo simulations. Our simulation model includes three main kinetic parameters: a small barrier for adatom diffusion on the surfactant terrace, a higher barrier for the exchange of adatoms with their underneath surfactant atoms, and a highest barrier for the recovery exchange in which an exchanged adatom resurfaces to the top of the surfactant layer. The simulations reveal a distinct transition of nucleation behaviour as the different atomic processes are activated successively with increasing temperature. The total nucleus density as a function of temperature exhibits a complex N-shape with a minimum and a maximum, which define the transition temperatures. The characteristic behaviour of nucleation density is helpful to rationalize the experimental observations on the temperature dependence of growth mode in some surfactant-mediated epitaxial systems.     

STM study of selenium adsorption on Au(111) surface

Adsorption of chalcogen atoms on metal surfaces has attracted increasing interest for both the fundamental research and industrial applications. Here, we report a systematic study of selenium(Se) adsorption on Au(111) at varies substrate temperatures by scanning tunneling microscopy. At room temperature, small Se clusters are randomly dispersed on the surface. Increasing the temperature up to 200℃, a well-ordered lattice of Se molecules consisting of 8 Se atoms in ringlike structure is formed. Further increasing the temperature to 250℃ gives rise to the formation of Se monolayer with Au(111)-3~(1/2) ×3~(1/2) lattices superimposed with a quasi-hexagonal lattice. Desorption of Se atoms rather than the reaction between the Se atoms and the Au substrate occurs if further increasing the temperature. The ordered structures of selenium monolayers could serve as templates for self-assemblies and our findings in this work might provide insightful guild for the epitaxial growth of the two-dimensional transition metal dichalcogenides.     

Growth Mechanism of Vertically Aligned Ag（TCNQ） Nanowires

Highly oriented Ag(TCNQ) nanowires have been prepared on Si(111) wafer at 1O0℃ by the vapour-transport reaction between silver and TCNQ without any other catalyst. X-ray diffraction analysis shows that the composition and crystal structure of the obtained nanostructure were Ag(TCNQ) crystalline. Most Ag(TCNQ) nanowires were grown uniformly and vertically on the substrate with diameters ranging from 50 to 30Onto and the lengths measuring from 2 to 50μm by scanning electron microscopy. Ag particles were observed on the substrate from pure thin Ag film heated under the same conditions as used in synthesizing the nanowires. Nucleation and short Ag(TCNQ) nanowires were prepared by controlling the reaction time, providing direct evidence of the growth mechanism in a nanometre scale. The growth process was explained according to the vapour-liquid-solid model. The gradient of temperature and the densely distributed Ag particles may contribute to the vertically aligned growth. These results will be helpful for the controllable synthesis of Ag(TCNQ) nanowires.     

Physical properties of FePt nanocomposite doped with Ag atoms:First-principles study

L10FePt nanocomposite with high magnetocrystalline anisotropy energy has been extensively investigated in the fields of ultra-high density magnetic recording media. However, the order–disorder transition temperature of the nanocomposite is higher than 600℃, which is a disadvantage for the use of the material due to the sustained growth of FePt grain under the temperature. To address the problem, addition of Ag atoms has been proposed, but the magnetic properties of the doped system are still unclear so far. Here in this paper, we use first-principles method to study the lattice parameters,formation energy, electronic structure, atomic magnetic moment and order–disorder transition temperature of L10FePt with Ag atom doping. The results show that the formation energy of a Ag atom substituting for a Pt site is 1.309 eV, which is lower than that of substituting for an Fe site 1.346 eV. The formation energy of substituting for the two nearest Pt sites is2.560 eV lower than that of substituting for the further sites 2.621 eV, which indicates that Ag dopants tend to segregate L10FePt. The special quasirandom structures(SQSs) for the pure FePt and the FePt doped with two Ag atoms at the stable Pt sites show that the order–disorder transition temperatures are 1377℃ and 600℃, respectively, suggesting that the transition temperature can be reduced with Ag atom, and therefore the FePt grain growth is suppressed. The saturation magnetizations of the pure FePt and the two Ag atoms doped FePt are 1083 emu/cc and 1062 emu/cc, respectively,indicating that the magnetic property of the doped system is almost unchanged.     

Ternary Eutectic Growth in a Highly Undercooled Liquid Alloy

The crystal nucleation and growth mechanism during the formation of Ag42.4 Cu21.6 Sb36 ternary eutectic are investigated under substantial undercoo]ing conditions. The x-ray diffraction analysis shows that the solidified eutectic phases are not invo]ved (Ag) within a wide undercooling range of 6-114 K. This indicates that under high undercooling condition, the phase constitution of Ag-Cu-Sb eutectic is different from that in the equilibrium phase diagram. With the increase of undercoo]ing, the crystalline morpho]ogy of Ag42.4 Cu21.6 Sb36 alloy transforms from the mixed structure of primary θ(Cu2Sb), two pseudobinary eutectics and (ε θ Sb) ternary eutectic into a unique (ε θ Sb) ternary eutectic. The calculated results indicate that θ(Cu2Sb) is the leading nucleating phase among the three eutectic phases. In addition, the growth morpho]ogy of primary ε(Ag3Sb) compound in Ag60Cu6Sb34 alloy exhibits the characteristics of solid solution and its orthorhombic dendrite grows along the (111) directions.     

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

Simulation of Surfactant Effects on Growth of Semiconductor Hetero-Epitaxial Sb-Ge/Si(111)

A kinetic Monte Carlo simulation is performed in order to study the effect of Sb as a surfactant on the growth of Ge/Si(111). In our model the exchange mechanism between Ge and Sb atoms and the re-exchange mechanism in which the exchanged Ge adatom re-exchange with the lifted Sb atom to return to the surfactant layer, are considered. Our simulation shows the re-exchange process plays an important role on the growth mode transition in Ge/Sb/Si(111)
system. The influences of the substrate temperature and the deposition rate on the growth of Ge/Sb/Si(111) system is discussed.     

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

We address the role of surfactant adsorbates in determining changes in the homoepitaxial growth mode of metals, discussing the case of Sb on Ag (111). From ab initio calculations, we extract evidence that the mechanism operative in this system is that Sb induces an irregular shape and an increase in density of the growing Ag islands, and an ensuing increase of the number of attempts for an adatom to descend to a lower terrace. This results from a combination of peculiar properties of this system: Sb is adsorbed insubstitutional surface sites, leading to the formation of a Sb–Ag surface alloy; deposited Ag has reduced mobility on Sb-covered Ag (111), from which follows a higher nucleation probability. The island shape is irregular since the surface alloy is disordered. Surface seggregation of Sb once the growing layer is completed furthers the phenomenon for many deposited Ag layers. Our explanation of the surfactant action of Sb on Ag (111) does not require a reduction of the downstep diffusion barrier, which may, however, be a concurrent factor helpful to interlayer mass transport and layerby-layer growth.     

Ag(111)和Au(111)上铋的初始生长行为

半金属铋(Bi)的表面合金具有的Rashba效应,和其具体结构性质有重要关联.本文结合扫描隧道显微镜(STM)和密度泛函理论(DFT),系统地研究了Bi原子在Ag(111)和Au(111)上的不同初始生长行为.在室温Ag(111)上,连续的Ag2Bi合金薄膜会优先在Ag台阶边缘形成;在570 K Ag(111)上,随着...     

Initial growth processes of Ag on polar and non-polar semiconductor substrates

Initial growth processes of Ag on both InSb(111)A and α-Sn(111) substrates at room temperature have been investigated by using reflection high-energy electron diffraction and Auger electron spectroscopy. The results show that the growth features are quite different for each system: Ag grows in the Stranski-Krastanov mode on InSb(111)A, while for Ag/α-Sn(111) the majority of the deposited Ag atoms are consumed in forming an Ag-Sn alloy. Discrete variational-Xα calculations showed that an onset of such growth modes is closely related to interfacial chemical bonding features for both systems.     

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si(111)√3-Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si(111)√3-Ag surface.     

Nature of contrast in Ge/Si(111) layers in scanning tunneling microscopy in the presence of Bi and Sb surfactants

It is known that the use of Bi surfactant (unlike Sb) upon the growth of Ge layers on Si(111) increases the contrast between Ge and Si atoms in a scanning tunneling microscope. This makes it possible to distinguish the Ge and Si surfaces. This effect is studied using computer simulation based on the density functional theory. To explain the observed difference between the Ge and Si layers, both structural and electronic effects are considered. The local density of electronic states, as well as the corresponding decay length to vacuum, has been calculated for each of the surfaces. The simulation results have been compared to the previous scanning tunneling microscopy data.     

Fabrication and optical properties of self-assembled InAsSb/InP nanostructures on InP (001) substrate

This article presents a study on the growth and optical properties of self-assembled InAsSb/InP nanostructures on (001) InP substrate, which are potential candidate materials for making mid-infrared lasers. The surfactant effect of Sb atoms is found to play a crucial role in the formation of flat InAsSb quantum dashes with almost identical island width no matter the change of InAsSb deposition thickness. The critical thickness for the transition from two-dimensional plane growth to three-dimensional island growth is observed to be less than two monolayer. And the photoluminescence measurements on InAsSb quantum dashes with different nominal Sb composition well demonstrate the band-gap bowing effect induced by the incorporation of Sb atoms into InAs quantum dots. The photoluminescence linewidth of InAsSb quantum dashes also present unusual temperature behavior, which can be attributed to the narrow size distribution of InAsSb quantum dashes.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.