Effect of substrate on the nucleation and growth of aluminum films deposited from methylpyrrolidine alane

Methylpyrrolidine alane complex was used to deposit aluminum films on various types of substrates in a low pressure chemical vapor deposition reactor. The films grow easily on metallic and transition metal oxide surfaces, but not on any other tested semiconductor and dielectric substrates below 200 °C, showing strong substrate dependency. The free energies of precursor adsorption, surface dissociation reaction and product desorption, as well as the film wettability to substrate are among the key factors which affect the energy barrier for nucleation or deposition selectivity. In general, a metal substrate can enhance nucleation because it catalyzes the surface reactions and bonds strongly with aluminum. The oxidation-reduction reaction may occur between the precursor and substrate on a metal oxide surface. The reduced metal sites can be the seed nuclei and are possibly responsible for Al growth on the surfaces of transition metal oxides.     

Effect of substrate morphology on the nucleation and growth of ZnO nanorods prepared by spray pyrolysis

ZnO nanorods were prepared by a spray pyrolysis technique on both as-received and etched Indium Tin Oxide (ITO)/glass substrates. The morphologies of the ITO substrates, the ZnO nucleation mechanism and the development of ZnO nanorods on both types of ITO substrates were investigated by Atomic Force Microscopy and Scanning Electron Microscopy methods. It was found that the amount of nucleation sites on as-received ITO is significantly higher compared to that on the etched ITO. As a result, well-shaped, elongated, strongly c-axis-oriented ZnO nanorods were obtained on the etched ITO/glass substrates. In contrast, randomly oriented ZnO nanocrystals with different shapes and sizes, as well as low aspect ratios, were obtained on the as-received substrates. It was found that ZnO nucleation follows the grain-boundary nucleation mechanism.     

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

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Pathway into the silicon nucleation on silicene substrate at nanoscale

The solidification process of silicon atoms on the heterogeneous surface of silicene in different shapes, ranging from plane, curved to tubular substrates, is studied by means of molecular dynamics (MD) simulations. The shape of nucleus determines the stacking sequence of silicon atoms. Silicene plate induces strong ordered liquid layers while the silicene nanotube (SNT) makes the silicon imprint its cylindrical structure. In the confined nanospace between SNTs, the growth competition has been observed, which causes structural changes at the shared interface. The internal potential field around SNT is responsible for the formation of spiral structures and the growth competition. The ordering degree decays with increasing distances from the SNT, which is the result of the decreasing acting force from nucleus. This study provides an opportunity for comprehensive and satisfactory understanding of the heterogeneous nucleation at nanoscale.     

Nucleation kinetics in thin film growth III transient nucleation and nucleation on preferred substrate sites

An analytical method is described by which the initial (transient) stage of thin film nucleation may be generally treated. The method is extended to cover simultaneous nucleation at substrate defect sites and at normal lattice sites. Post-transient nucleation and growth under these latter conditions is also considered in detail. The analysis is applied to defect-controlled nucleation under complete condensation conditions and the results are compared with those previously derived for nucleation on a perfect defect-free substrate. The application of this analysis to computer simulation of thin film nucleation and growth is discussed.     

Theory of nucleation in binary systems

A method of calculating the rate of nucleation in phase transformations in binary mixtures is proposed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 2, pp. 201–204, August, 1984.     

The effect of substrate inhomogeneity on the kinetics of heterogeneous nucleation from vapours

An equation expressing the time dependence of the number of nuclei is derived on the basis of the following supposition. The substrate is considered as a system of active centres stimulating the nucleation process and prossessing varied potency with respect to nucleus formation. A nucleation exclusion zone with reduced adatom concentration is formed around every growing nucleus, within which the probability of new nucleus formation is practically zero. As they spread with time the nucleation exclusion zones may overlap adjacent active centres on which nuclei have not yet formed. In this way a part of the active centres can be deactivated and will not participate in the nucleation process. As a consequence, the saturation nucleus density is determined by the combined influence of both the finite number of centres active at the given super-saturation and the nucleation exclusion zones. It is shown that the nucleation rate, the spreading rate of the nucleation exclusion zones, as well as the centre activity distribution, introduced as parameters in the theory, can be evaluated if experimental data for the variation of nucleus density with time are available. The effect of coalescence on the saturation nucleus density derived according to the present model is discussed.     

Fabrication of discrete gallium nanoislands on the surface of a Si(001) substrate using a focused ion beam

A gallium focused ion beam (FIB) has been used to implant Ga at specific sites on the surface of undoped Si(001) substrates. Upon annealing at 600?°C, discrete nanoscale surface islands form within the FIB patterned regions when the total Ga ion dose, or fluence, is greater than 1.0 × 10(16) ions cm( - 2). The number of islands depends on the size of the irradiated region and a single island can be achieved for a FIB milled region that is 100 nm × 100 nm. The average sizes of the islands were found to range from 24.5 nm when exposed to a total ion dose of 1.2 × 10(16) ions cm( - 2) to 45 nm for a dose of 3.0 × 10(16) ions cm( - 2). We have confirmed that these surface islands are metallic Ga by performing a selective chemical etch that removes the islands and by transmission electron microscopy characterization. These patterned Ga surface templates could serve as nucleation sites for the lateral arrangement of discrete quantum dot structures.     

Microcrack nucleation in thermal barrier coating systems

Crack nucleation in thermal-barrier coating (TBC) systems subjected to a monotonic cooling process is studied. The TBC system is modeled using the finite element method, where cracks are represented as discrete discontinuities across continuum elements using the partition-of-unity method. The numerical implementation used for crack nucleation is based on an algorithm where, at insertion of a discontinuity, the traction response is derived from a cohesive zone model that has been modified to (i) behave like an initially rigid cohesive model, and to (ii) ensure smoothness of the traction-separation law at zero crack opening. Accordingly, an adequate convergence behavior of the numerical formulation can be warranted in boundary value problems of systems with relatively complex geometries. In the present numerical study, a comparison is made between TBC systems composed of different constitutive models. The fracture patterns and evolutions of the overall crack growth resulting from the simulations clearly illustrate the importance of accounting for the effects of plasticity in the bond coating and anisotropy in the top coating. The computed fracture profile is in good correspondence with experimental observations reported in the literature.     

Effect of nucleation on the crystalline structure of nanowhiskers

An exactly solvable model of nucleation during the growth of nanowhiskers has been constructed. Activation barriers for the formation of nuclei in various positions are determined as dependent on the chemical potentials and surface energies. It is shown that the nucleation at the triple contact line is energetically favorable. In a broad range of system parameters, the formation of a hexagonal vurtzite-like crystalline phase takes place (particularly for GaAs nanowhiskers growing on an GaAs(111)B substrate surface activated by gold drops). Dependences of the probability of the hexagonal phase formation on the degree of supersaturation in the liquid catalyst and the NW radius are determined.

     

Effect of temperature on the formation of silicon nanoislands on noncrystalline substrates in microwave low-pressure gas discharge plasma

Conditions for the synthesis of nanodimensional silicon islands on noncrystalline substrates in microwave low-pressure gas discharge plasma have been studied in the case of weak interactions at the deposit-substrate interface. It is established that the formation of silicon nanoislands proceeds via the overgrowth (healing) of depressions on the initial substrate surface. The effect of temperature on the kinetics of nanoisland growth and the possibilities of controlling the parameters of nanoisland morphology are determined.     

Domain nucleation in thermomagnetic memory systems near the Curie point

The approximation of Landau second-order phase transitions for ferromagnetics near the Curie point is used for a numerical determination of the two-dimensional microstructure which forms the basis of a data-storage domain. This structure and its lower level of the spectrum of small oscillations are strongly delocalized. Depending on the recording regimes, this may be responsible for the severe irregularity of the domain wall shape sometimes observed or the formation of ring domains. Pis’ma Zh. Tekh. Fiz. 24, 13–17 (August 26, 1998)     

On the kinetics of nucleation and growth reactions in inhomogeneous systems

Nucleation and growth kinetics in systems with a small degree of inhomogeneity are usually modeled through the KJMA (Kolmogorov–Johnson–Mehl–Avrami) theory, that is by using the local values of the nucleation and growth rates which are proper to the region where the transition takes place. In this study, a general expression for the kinetics is derived which applies, in principle, to any degree of inhomogeneity and conforms to previous approaches. The model is employed to study, analytically, first order corrections to the KJMA formula in the case of simultaneous nucleation and interface-limited growth. It is shown that under these circumstances, the nucleus shape is a circle (two-dimensional) whose center is displaced with respect to the point where the nucleation event occurs. The displacement of the center and the radius of the nucleus are both functions of time. The behavior of the Avrami exponent and the impingement factor as a function of the fraction of transformed volume is investigated and discussed.     

Analysis of buried heterointerfacial hydrogen in highly lattice-mismatched epitaxy on silicon

We realized the epitaxial growth of a Sr layer on Si(111) with an atomically abrupt heterointerface - in spite of its large lattice mismatch (12%) with Si - by introducing a monoatomic layer of H on Si. In order to identify the buried H, we carried out a combination analysis involving neutron reflectometry and resonant nuclear reaction of ¹H(¹⁵N,αγ)¹²C analysis. We found different neutron reflectivity profiles resulting from a contrast variation between the H and D atoms at the buried heterointerface. Furthermore, the depth γ-ray intensity profiles revealed that the H at the heterointerface acts as an effective buffer layer that enables it to manage the highly mismatched epitaxy on Si.     

Effect of three-stage isothermal annealing on the nucleation process in GeSe2 glasses

We studied the mechanism of crystallization of GeSe₂ bulk glass into the so-called low-temperature GeSe₂ crystalline phase. Initial isothermal annealing at a temperature T₁ below 400 °C and subsequent isothermal crystallization at 440 °C were carried out in a differential scanning calorimeter (DSC). The crystallization time (t_c), defined by the interval from the onset of annealing at 440 °C to the observation of a DSC peak, became shorter with increasing T₁. Nucleation occurs during this initial annealing. Next, T₁ was fixed at 330 °C and specimens were subsequently annealed at T₂<T₁. It was found that t_c increased as T₂ decreased from 330 °C. The nuclei generated at T₁ were reconverted to the glassy state.     

Estimating substrate temperature from the character of the field-ion image of an adsorbed molecule

Separate molecules possessing high dipole moments (water, naphthalene) were deposited from the gas phase onto a conducting sample under the action of an inhomogeneous electric field perpendicular to the substrate surface. It was established that the character of the image of adsorbed molecules observed with a field-ion microscope depends on the substrate temperature and the electric field strength. Using this phenomenon, it is possible to estimate the temperature of adsorbed molecules (and of the substrate) by examining their images or to solve the reverse problem of identifying molecules adsorbed at the known substrate temperature and electric field strength. It is suggested to explain the observed effects by considering an adsorbed molecule as a gyroscope exhibiting nutation. The unique local character of this method makes it a promising tool for nanotechnology and for the investigation of fine thermal effects on solid surfaces.     

Prediction of substrate temperature in dc ion-plating systems

The dominant control variables of a dc-biased ion-plating system have been described in terms of a generalized parameter U_s. U_s is related to the experimentally-determined temperatures of various non-cooled metal substrates (Ta, Mo, and mild-steel) by a linear expression T_s = 38.0 U_s + 19.0. This expression allows the substrate temperature to be predicted in degrees centigrade for different values of the control variables as an aid in the production of high-quality thin films.