Formation of copper islands on a native SiO2 surface at elevated temperatures

A combination of in situ X-ray photoelectron spectroscopy analysis and ex situ scanning electron- and atomic force microscopy has been used to study the formation of copper islands upon Cu deposition at elevated temperatures as a basis for the guided growth of copper islands. Two different temperature regions have been found: (I) up to 250 °C only close packed islands are formed due to low diffusion length of copper atoms on the surface. The SiO₂ film acts as a barrier protecting the silicon substrate from diffusion of Cu atoms from oxide surface. (II) The deposition at temperatures above 300 °C leads to the formation of separate islands which are (primarily at higher temperatures) crystalline. At these temperatures, copper atoms diffuse through the SiO₂ layer. However, they are not entirely dissolved in the bulk but a fraction of them forms a Cu rich layer in the vicinity of SiO₂/Si interface. The high copper concentration in this layer lowers the concentration gradient between the surface and the substrate and, consequently, inhibits the diffusion of Cu atoms into the substrate. Hence, the Cu islands remain on the surface even at temperatures as high as 450 °C.     

Epitaxial growth of a graphene single crystal on the Ni(111) surface

The thermally controlled synthesis of graphene from propylene molecules on the Ni(111) surface in ultrahigh vacuum is studied by scanning tunneling microscopy and density functional theory. It is established that the adsorption of propylene on Ni(111) atomic terraces at room temperature results in the dehydration of propylene molecules with the formation of single-atomic carbon chains and in the complete dissociation of propylene at the edges of atomic steps with the subsequent diffusion of carbon atoms below the surface. The annealing of such a sample at 500°С leads to the formation of multilayer graphene islands both from surface atomic chains and by the segregation of carbon atoms collected in the upper nickel atomic layers. The process of formation of an epitaxial graphene monolayer until the complete filling of the nickel surface is controllably observed. Atomic defects seen on the graphene surface are interpreted as individual nickel atoms incorporated into graphene mono- or bivacancies.     

Preferred structures of the atomic Ag islands on silicone oil surfaces

Varying the substrate temperature T(s) from 285 to 353 K, both the aggregation behavior of Ag atoms and the preferred structures of the atomic Ag islands on silicone oil surfaces are investigated. After deposition, the deposited Ag atoms form isolated islands with a preferred height. Our observations reveal that, as T(s) increases, the preferred island height increases from 20.0 to 33.0 nm, which results in the decrease of the Ag apparent coverage, from 9.6 ± 0.1% to 6.5 ± 0.3%. Further, the crystal structure of the Ag islands changes from amorphous to polycrystalline as the substrate temperature T(s) goes up. Subsequently a 3D aggregation mechanism of the Ag atoms on the liquid substrates is proposed.     

STM study of structural changes on Si(100)2×1-Sb surface induced by atomic hydrogen

Using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have studied the structural changes of the Si(100)2×1-Sb surface caused by hydrogen adsorption at both room temperature (RT) and 300°C. We have found that the ordering of a 2×1-Sb surface is more stable against atomic hydrogen exposure at 300°C than at RT, and that some Sb atoms desorb during atomic hydrogen exposure at 300°C. However, upon hydrogen exposure at both temperatures, we have observed neither three-dimensional islands nor the hydrogen terminated Si substrate which were reported for hydrogen interaction with the other metal/Si systems. On the 2×1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 550°C annealing, long bright lines similar to those reported for the Bi/Si(100) system have also been found.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

薄膜外延生长的计算机模拟

以Cu膜为例，用Monte-Carlo算法模拟了薄膜生长的随机过程，并提出了更加完善的模型.在合理选择原子间相互作用计算方法的基础上，考虑了原子的吸附、在生长表面的迁移及迁移所引起的近邻原子连带效应、从生长表面的脱附等过程.模拟计算了薄膜的早期成核情况以及表面粗糙度和相对密度.结果表明，随着衬底温度的升高或入射率的降低，沉积在衬底上的原子逐步由离散型分布向聚集状态过渡形成一些岛核，并且逐步由二维岛核向三维岛核过渡.在一定的原子入射率下，存在三个优化温度，成核率最高时的最大成核温度Tn、薄膜的表面粗糙度最低 关键词： Monte-Carlo算法 计算机模拟 薄膜生长     

Effects of O defects on adsorption of small Ag clusterson a MgO(001) surface

The interaction of Ag atoms with a defective MgO(001) surface is systematically studied based on density functional theory. The Ag clusters are deposited on neutral and charged oxygen vacancies of the MgO(001) surface. The structures of Ag clusters take the shape of simple models of two- or three-dimensional (2D and 3D) metal particles deposited on the MgO surface. When the nucleation of the metal clusters occurs in the F_s (missing neutral O) centre, the interaction with the substrate is considerably stronger than that in the F_s⁺ (missing O^- ) centre. The results show that the adsorption of Ag atoms on the MgO surface with oxygen vacancy is stronger than on a clear MgO surface, thereby attracting more Ag atoms to cluster together, and forming atomic islands.     

Graphene-graphite phase transition at the surface of a carbonized metal

A phase transition leading to the transformation of a graphene layer into a multilayer graphite film at the surface of a carbonized metal has been experimentally studied on the atomic level under ultrahigh-vacuum conditions. It has been shown that this process is governed by dynamic equilibrium between edge atoms of graphene islands and a chemisorbed carbon phase, two-dimensional carbon “gas,” and is observed in the temperature range of 1000–1800 K. The features of the phase transition at the surfaces Ni(111), Rh(111), and Re(10-10) are similar, although the specific kinetic characteristics of the process depend on the properties of the substrate. It has been shown that change in the emissivity of the substrate after the formation of a multilayer graphite film increases the rate of the phase transition and leads to a temperature hysteresis.     

Comparison of electronic structures of mass-selected Ag clusters and thermally grown Ag islands on sputter-damaged graphite surfaces

Ag cluster anions consisting of 3–16 atoms were deposited on sputter-damaged HOPG surfaces using a soft-landing technique (mean deposition energy less than 0.2 eV/atom) at room temperature. For investigations of the structures of deposited clusters, X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used. In addition, the chemical properties of deposited clusters were studied using atomic oxygen and CO. Comparison of the properties of deposited Ag clusters and Ag islands with similar sizes grown by evaporating Ag atoms on the same substrate shows different results, implying that two different preparation methods give either different shapes of Ag clusters and islands, or dissimilar metal–support interactions. PACS 73.22.-f     

Effect of the substrate pretreatment on the epitaxial growth of indium nitride

The influence of the substrate pretreatment on crystallinity of indium nitride films grown on (1 1 1)GaAs by radio frequency sputtering were investigated. It was shown that the crystalline quality of InN layers grown on GaAs can be improved by presputtering the substrate in nitrogen plasma prior to the growth. By Auger electron spectroscopy and atomic force microscopy analysis we revealed that GaN islands form on the surface of GaAs substrate due to the presputtering. The optimum presputtering time for growing InN single crystal was assessed to be the time at which GaN islands cover the substrate surface entirely.     

Mechanism and kinetics of early growth stages of a GaN film

The growth of GaN islands on the substrate surface covered with an AlN buffer layer is theoretically investigated at the stages of nucleation and Ostwald ripening in the temperature range 480–1000°C. The following inferences are made from analyzing the results obtained. (1) At temperatures T>650°C, the growth of islands is controlled by the chemical reaction of formation of GaN molecules around the periphery of the island surface. Islands nucleated at these temperatures are characterized by a large spread in their sizes. (2) At temperatures T<600°C, the island growth is governed by the surface diffusion of nitrogen atoms. Islands nucleated at these temperatures are virtually identical in size. (3) In the temperature range 600–650°C, the mechanism of island growth gradually changes over from the mechanism associated with the surface diffusion of nitrogen atoms with a large mean free path to the mechanism determined by the diffusion of gallium atoms with a smaller mean free path. The supersaturation, flux, and size distribution functions of GaN nuclei are calculated at different substrate temperatures. The phase diagrams describing the evolution in the phase composition of GaN islands with variations in temperature are constructed.     

Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

Effect of dislocations on the shape of islands during silicon growth on the oxidized Si(111) surface

The formation of dislocation-rich and dislocation-free silicon islands during growth in the absence of mechanical stresses has been studied by scanning tunneling microscopy. The rounded shape of islands obtained at growth temperatures of 400–500°C on the oxidized Si(111) surface is associated with the presence of dislocations within them. The transfer of atoms from the oxidized surface to the islands occurs due to the barrier of the potential energy at the SiO₂/Si boundary. The {111} and {311} facets dominate in the shape of the islands grown at 500–550°C. Their appearance indicates the absence of the threading dislocations in the islands and that the growth is limited by the stage of the nucleation of a new atomic layer.     

Changes in morphology and optical properties of silver island films on transparent dielectric substrates under exposure to laser radiation

The influence of a single nanosecond laser pulse on the morphology of an island silver film was studied. It is shown that, during the pulse time span, the islands can change their shape from an arbitrary one to nearly spheroidal. The islands change their shape due to diffusion of silver atoms on the islands’ surface (self-diffusion), as well as on the substrate surface.     

Admetal-induced substrate surface restructuring during metal-on-metal electrochemical deposition studied by in situ scanning tunneling microscopy

Based on time-dependent in situ scanning tunneling microscopy (STM) studies, we demonstrate that for Ni on Ag(111) and Ru on Au(111), electrochemical metal-on-metal deposition can result in pronounced substrate surface restructuring. For Ni/Ag(111), we observe that at low deposition flux and low coverage, Ni submonolayer islands at steps are partly embedded in the Ag terraces, whereas at higher deposition flux and higher coverage, substrate restructuring results in the formation of monolayer bays in the Ag terraces. We suggest that this restructuring process proceeds predominantly via step edge diffusion of Ag atoms. For Ru/Au(111), the formation of fjords and monolayer holes in the Au terraces is observed at low and high Ru coverage, respectively. The importance of the Au surface mobility for the restructuring process is demonstrated by comparing experiments in H₂SO₄ and HCl solutions, in which Au exhibits strongly different surface mobilities. For this system, restructuring involves Au diffusion along Au steps, Au atom detachment from the Au steps, and upward exchange diffusion. According to these observations and their comparison with similar findings for vacuum deposition, we conclude that this restructuring requires (i) a high substrate surface mobility and (ii) a stronger bonding of substrate atoms to deposit islands than to the substrate.     

An atomic force microscope study of vanadium-benzene sandwich clusters soft-landed on self-assembled monolayers

Multiple-decker vanadium-benzene sandwich clusters V_n(benzene)_n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C₁₈H-SAM) and fluorinated alkanethiol (C₁₀F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface.     

Vertical friedel oscillations in interface-induced surface charge modulations of ultrathin quantum islands

Two-dimensional Pb islands of a few atomic layers are grown on the incommensurate Si(111)-Pb surface at low temperatures. Among them, two types of islands having different stacking with the substrate are observed. These islands, respectively, display an alternating image contrast with their thickness. Besides, the contrasts of the islands of different types are complementary to each other layer by layer. These intriguing behaviors do not show significant bias dependence throughout the range from -3 to +3 V and can be explained by the vertical charge oscillation with the growth of a new layer. The charge oscillation in the out-of-plane direction originates from electron scattering by the in-plane potential variation at the Pb/Si interface.     

Adsorption of nitrogen oxide molecules to the surface of nanosized nickel clusters formed on the (111) surface of a magnesium oxide film

The properties of the systems formed on deposition of Ni atoms on the (111) surface of a MgO film of thickness equal to six monomolecular layers grown on a Mo(110) crystal face and the adsorption of NO nitrogen oxide molecules to the system surface have been studied by methods of electron spectroscopy (AES, XPES, LEED, LEIBSS) and reflective infrared absorption spectroscopy. On deposition of Ni atoms on the surface of MgO at a substrate temperature of 600 K, three-dimensional islands of Ni are formed. The subsequent adsorption of NO results in molecule dissociation even at 110 K. The efficiency of this process depends on the morphology of the Ni layer.     

Scanning tunneling microscopy investigation of CoO/Fe(001) and Fe/CoO/Fe(001) layered structures

We report on the nanometer scale morphology of CoO thin films grown on top of Fe(001) substrates from the early stages of interface formation (few atomic layers), and on the surface topography of Fe/CoO/Fe(001) layered structures. The growth of the CoO films is dominated by formation of islands up to about 5 nominal atomic layers, then it proceeds in the layer-plus-island regime. The surface topography of thin Fe films grown on top of the CoO/Fe systems is strongly influenced by the morphology of the latter. Moreover, we observe a strong relationship between the growth mode and the chemical interactions at the CoO/Fe interface, since thick layers of iron oxides develop only below the CoO islands, as an effect of the proximity between Fe and Co atoms. We finally discuss possible implications of our observations on the magnetic properties of these layered magnetic structures.     

Structure and phonon spectrum of a submonolayer Ni film on the surface of Cu(100)

The equilibrium atomic structure and the phonon spectra of a submonolayer (θ = 0.5 monolayer) Ni film deposited on the surface of Cu(100) are calculated using the potentials obtained by the embedded atom method. We consider atomic relaxation, the vibrational state density distribution on Ni and substrate atoms, and polarization of vibrational modes. Variation of the phonon spectrum upon segregation of Cu atoms on the film surface is considered. It is shown that mixing of vibrations of Ni adatoms with vibrations of substrate atoms occurs in the entire frequency range, leading to a frequency shift of the vibrational modes of the substrate and to the occurrence of new vibrational states atypical of a clean surface. The Cu(100)–c(2 × 2)–Ni structure is dynamically stabler when placed in the subsurface layer of the substrate.