Enhanced Surface Diffusion Research Articles

FOCUSED THERMAL BEAM DIRECT PATTERNING ON INGAN DURING MOLECULAR BEAM EPITAXY

During InGaN Molecular Beam Epitaxy (MBE) growth, the material surface is exposed to a small diameter pulse laser beam that is controlled by scanning mirrors. Local heating effects are observed at the points of exposure. The materials are characterized by Wavelength Dispersive Spectroscopy (WDS), Scanning Electron Microscopy (SEM), and Photoluminescence (PL). Indium mole fraction of materials is reduced where laser exposure takes place. The effect of local thermal heating appears to enhance surface diffusion while not causing ablation or evaporation under the conditions studied. PL efficiency is significantly increased by focused thermal beam exposure. Laser written regions have 7 times higher PL intensity compared to non-written areas, which might be due to surface texturing that causes higher extraction efficiency.

Investigation into the influence of direct current (DC) power in the magnetron sputtering process on the copper crystallite size

This paper discusses the influence of direct current (DC) power in the magnetron sputtering process on the crystallite size of the copper (Cu) thin films deposited on p-type silicon substrate at room temperature. X-ray diffraction (XRD) and Karl Suss four-point probe were employed to study the film crystallinity and conductivity, respectively. From the analysis on the XRD patterns, high DC power enhances the Cu film crystallinity with larger crystallite size, which is deduced using Sherrer’s formula. The behavior of the electrical property of the Cu films complies with the trend of the film crystallinity with DC power, in which the film conductivity increases with increasing DC power. We attribute these phenomena to the enhanced surface diffusion mechanism of the adatom during the sputtering deposition process, which improves the microstructure of the Cu film.

Supported Ni catalysts from nominal monolayer grow single-walled carbon nanotubes

Fe, Co, and Ni are catalytically effective for growing single-walled carbon nanotubes (SWNTs). On substrates, however, Ni tends to yield only multi-walled carbon nanotubes. Because enhanced surface diffusion at the elevated growth temperature required for deposition might cause coarsening of Ni catalyst nanoparticles, adjusting the nominal Ni thickness should be crucial for controlling the particle size. Using our previously developed combinatorial method, we prepared a thickness profile of Ni on a quartz glass (SiO 2) substrate and found that Ni nanoparticles catalyzed the growth of SWNTs by chemical vapor deposition only when nominal Ni thickness was in the monolayer range.

On the equilibrium and dynamic behavior of alcohol vapors in activated carbon

As alcohol molecules such as methanol and ethanol have both polar and non-polar groups, their adsorption behavior is governed by the contributions of dispersion interaction (alkyl group) and hydrogen bonding (OH group). In this paper, the adsorption behavior of alcohol molecules and its effect on transport processes are elucidated. From the total permeability ( B T ) of alcohol molecules in activated carbon, an adsorption mechanism is proposed, describing well the experimental data, by taking combination effects of clustering, entering micropores, layering and pore filling processes. Unlike the case of non-polar compounds, it was found that at low pressures there are two rises in the B T of alcohol molecules in activated carbon. The first rise is due to the major contribution of surface diffusion to the transport (which is the case of non-polar molecules) and the second one may be associated with cluster formation at the edge of micropores and entering micropores when the clusters are sufficiently large enough to induce a dispersive energy. In addition the clusters formed may enhance surface diffusion at low pressures and hinder gas phase diffusion and flow in meso/macropores.

Energetic Surface Smoothing of Complex Metal-Oxide Thin Films

A novel energetic smoothing mechanism in the growth of complex metal-oxide thin films is reported from in situ kinetic studies of pulsed laser deposition of on , using x-ray reflectivity. Below 50% monolayer coverage, prompt insertion of energetic impinging species into small-diameter islands causes them to break up to form daughter islands. This smoothing mechanism therefore inhibits the formation of large-diameter 2D islands and the seeding of 3D growth. Above 50% coverage, islands begin to coalesce and their breakup is thereby suppressed. The energy of the incident flux is instead rechanneled into enhanced surface diffusion, which leads to an increase in the effective surface temperature of DeltaT approximately 500 K. These results have important implications on optimal conditions for nanoscale device fabrication using these materials.

Analysis of Nanocrystalline and Microcrystalline ZnO Sintering Using Master Sintering Curves

Master sintering curves were constructed for dry‐pressed compacts composed of either a nanocrystalline or a microcrystalline ZnO powder using constant heating rate dilatometry data and an experimentally determined apparent activation energy for densification of 268±25 and 296±21 kJ/mol, respectively. The calculated activation energies for densification are consistent with one another, and with values reported in the literature for ZnO densification by grain boundary diffusion. Grain boundary diffusion appears to be the single dominant mechanism controlling intermediate‐stage densification in both the nanocrystalline and the microcrystalline ZnO during sintering from 65% to 90% of the theoretical density (TD). Based on both the consistency of the calculated activation energy as a function of density and the narrow dispersion of the sintering data about the master sintering curve (MSC) for the nanocrystalline ZnO, there is no evidence of either significantly enhanced surface diffusion or grain growth during sintering relative to the microcrystalline ZnO. The MSC constructed for the nanocrystalline ZnO was used to design time–temperature profiles to successfully achieve four different target sintered densities on the MSC, demonstrating the applicability of the MSC theory to nanocrystalline ceramic sintering. The most significant difference in sintering behavior between the two ZnO powders is the enhanced densification in the nanocrystalline ZnO powder at shorter times and lower temperatures. This difference is attributed to a scaling (i.e., particle size) effect.

Deposition phase diagrams for Si1−xGex:H from real time spectroscopic ellipsometry

Phase diagrams have been developed to describe rf plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated silicon–germanium alloys (Si1−xGex:H) on crystalline Si substrates held at 200°C. For a series of three such overlapping diagrams established at different flow ratios G=[GeH4]/{[SiH4]+[GeH4]}, the boundary describing the thickness at which the amorphous-to-(mixed-phase microcrystalline) transition occurs [designated a→(a+μc)] shifts rapidly to higher H2-dilution flow ratio R=[H2]/{[SiH4]+[GeH4]} with increasing G. This demonstrates that Ge incorporation strongly suppresses microcrystallite nucleation from the amorphous phase. For each individual phase diagram of this series, the thickness at which an amorphous regime roughening transition occurs [designated a→a] increases with increasing R right up to the a→(a+μc) boundary. Under wide ranges of deposition conditions, however, the maximum a→a transition thickness decreases rapidly with increasing G for the series of phase diagrams with different ratios G. This decrease is weakest for cathodic PECVD of Si1−xGex:H with a self-bias near −20V, suggesting precursor surface diffusion enhancement under such conditions.

Encapsulation of metallic nanoclusters in carbon and boron nitride thin films prepared by ion-beam sputtering

Dual ion-beam sputtering has been used to achieve the growth of silver and iron nanoclusters embedded within graphite-like carbon and boron nitride thin films. Encapsulation of the nanoclusters in carbon nanocages has been realized when the deposition was performed at 500 °C whereas a temperature of 200 °C was sufficient to form hexagonal BN layers surrounding the nanoclusters. We show that, thanks to the enhanced surface diffusion of the deposited species, self-organized arrays of nanoclusters elongated along the growth direction can be produced by argon-assisted co-deposition of the metal and the matrix material. Vertical ordering of the nanoclusters, which develops from a topology-induced self-organization, can also be obtained by alternate deposition of very thin metallic layers separated by BN layers, as shown by high-resolution transmission electron microscopy and grazing incidence small-angle X-ray scattering.

Nanometer‐Scale Modification and Welding of Silicon and Metallic Nanowires with a High‐Intensity Electron Beam

We demonstrate that a high-intensity electron beam can be applied to create holes, gaps, and other patterns of atomic and nanometer dimensions on a single nanowire, to weld individual nanowires to form metal-metal or metal-semiconductor junctions, and to remove the oxide shell from a crystalline nanowire. In single-crystalline Si nanowires, the beam induces instant local vaporization and local amorphization. In metallic Au, Ag, Cu, and Sn nanowires, the beam induces rapid local surface melting and enhanced surface diffusion, in addition to local vaporization. These studies open up a novel approach for patterning and connecting nanomaterials in devices and circuits at the nanometer scale.

Plasma influence on the properties and structure of indium tin oxide films produced by reactive middle frequency pulsed magnetron sputtering

Abstract Reactive pulsed magnetron sputtering was used to produce conductive and transparent tin-doped indium oxide (ITO) films with low thickness inhomogeneity. Due to the parallel operation of two magnetrons, the deposition system allows in situ investigations of the plasma influence on the film properties. The distribution of the film resistivity, refractive index, structure and stoichiometry along the substrate are presented and related to the spatial distribution of the plasma flow escaping the magnetrons, and the substrate temperature. A higher plasma flow likely causes a localized relaxation of the distorted In–O bonds in amorphous phase which prevails in ITO films prepared at unheated substrates. This leads to a decrease of the film resistivity due to free electrons density and mobility enhancement. The free electron density increase is caused likely by generation of oxygen vacancies. Deposition on a heated substrate ( T s / T m = 0.3) leads to a change of the film growth mode due to enhanced surface diffusion of the adatoms which results in a textured low resistivity film. This also causes significant improvements of the homogeneity of the film properties that is important for ITO applications.

Combinatorial method to prepare metal nanoparticles that catalyze the growth of single-walled carbon nanotubes

Enhanced surface diffusion at the growth temperature of single-walled carbon nanotubes (SWNTs) can cause coarsening of metal catalysts. By balancing the nominal thickness and surface diffusion length of metals, metal nanoparticles of desirable size are expected to form spontaneously under the SWNTs growth conditions. Our combinatorial method, using a library of nominally 0.001 to 1 nm thick sputter-deposited cobalt patterns, identified in a single experimental run that cobalt nanoparticles from submonolayers can catalyze the growth of high-quality SWNTs.

Suppression of Anomalous Interface Effects by Localization of Solute Redistribution in Thin Interface Phase-Field Modeling of Solidification

A phase field model for alloy solidification was developed to suppress the anomalous interface effects such as enhanced surface diffusion, chemical potential jump and surface stretching by localizing the solute redistribution into a narrow region within the phase-field interface. Application of this model to a free dendritic growth in an undercooled liquid yields quantitatively the same results as previously reported anti-trapping model. By localization of the solute redistribution into a region of single grid spacing the anomalous interfacial effects can be effectively suppressed. This model can be used for quantitative phase field calculation with an enhanced computational efficiency.

Growth kinetics of epitaxial Y-stabilizedZrO2 films deposited on InP

The nucleation and growth kinetics of Y-stabilizedZrO2 (YSZ) films have been studied by atomic force microscopy, scanning electron microscopyand x-ray diffraction. The investigated films are cube-on-cube epitaxially deposited onannealed (100)InP by pulsed laser deposition (PLD) exhibiting a two-dimensional growthhabit in spite of the large system misfit ( of tensile strain). Beyond the nucleation regime, the YSZ growth kinetics is analysedwithin the frame of the dynamic scaling theory. Such an analysis suggests the existence of aunique growth mechanism operating during a wide range of deposition times, i.e.,50–104 s. This mechanism corresponds to the diffusion ruled by the surface local curvature andlimited by the irreversible aggregation to kinks. On the basis of the achieved results,models accounting for the surface diffusion enhancement induced by PLD are discussed.

Ge/Si quantum dot nanostructures grown with low-energy ion beam-assisted epitaxy

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge + ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density, and narrowing of island size distribution, as compared with conventional MBE experiments. Moreover, it was found that ion beam assists the transition from hut- to dome-shaped Ge islands on Si(100). The crystal perfection of Ge/Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The studies of Si/Ge/Si(100) structures indicated defect-free Ge nanopaticles and Si layers for the initial stage of heteroepitaxy (five monolayers of Ge) in pulsed ion beam action growth mode at 350 °C. Continuous ion beam irradiation was found to induce dislocations around Ge clusters. The results of kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D–3D transition: (1) surface defect generation by ion impacts, and (2) enhancement of surface diffusion.

MODIFICATION OF GROWTH MODE OF Ge ON Si BY PULSED LOW-ENERGY ION-BEAM IRRADIATION

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge + ion action during molecular beam epitaxy (MBE) of Ge on Si (100). The continuous and pulsed ion beams were used. These studies have shown that ion-beam bombardment during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density and narrowing of island size distribution, as compared with conventional MBE experiments. The crystal perfection of Ge / Si structures with Ge islands embedded in Si was analyzed by Rutherford backscattering/channeling technique (RBS) and transmission electron microscopy (TEM). The results of Kinetic Monte Carlo (KMC) simulation have shown that two mechanisms of ion beam action can be responsible for stimulation of 2D–3D transition. They are: (1) surface defect generation by ion impacts and (2) enhancement of surface diffusion.

Enhanced surface diffusion through termination conversion during epitaxial SrRuO3 growth

During the initial growth of the ferromagnetic oxide SrRuO3 on TiO2-terminated SrTiO3, we observe a self-organized conversion of the terminating atomic layer from RuO2 to SrO. This conversion induces an abrupt change in growth mode from layer by layer to growth by step advancement, indicating a large enhancement of the surface diffusivity. This growth mode enables the growth of single-crystalline and single-domain thin films. Both conversion and the resulting growth mode enable the control of the interface properties in heteroepitaxial multilayer structures on an atomic level.

Ion-beam-induced surface modification and nanostructuring of AIIIBV semiconductors

Ion-beam-induced modification of InSb (0 0 1) semiconductor surface has been studied by means of atomic force microscopy and Kelvin probe force microscopy. It was found that non-stoichiometric sputtering of the compound surface and beam enhanced surface diffusion led to unusual development of surface structures in the form of dots and wires with nanometer scale dimensions. The shape, the size and the surface density of nanostructures were investigated as a function of the beam flux and fluence, and as a function of the crystal orientation with respect to the ion-beam direction. The mechanisms involved in the surface modification were compared with results of Monte-Carlo computer simulations of anisotropic surface diffusion.

CO 2 selectivity and lifetimes of high silica ZSM-5 membranes

High silica ZSM-5 membranes have been grown on porous α-alumina supports via hydrothermal synthesis for CO 2 sequestration from methane reforming processes. The optimized synthesis parameters for high silica ZSM-5 membranes, including gel composition and ageing, seed composition and deposition techniques, temperature, time, and membrane calcination parameters are discussed. The permeation characteristics of several membranes for gases present during methane reforming processes (H 2, CO 2, O 2, CH 4, N 2 and CO) were measured, as well as for the test gases He and SF 6. The permeance of CO 2 is higher than that of the other gases and is between two and three times higher than that of the smaller He atoms or H 2 molecules. The higher permeance of CO 2 is likely due to enhanced surface diffusion. Membranes showed a decrease in CO 2 permeation over time, which was partially regenerated after a heating cycle.

Ion-beam-induced surface modification and nanostructuring of A IIIB V semiconductors

Ion-beam-induced modification of InSb (0 0 1) semiconductor surface has been studied by means of atomic force microscopy and Kelvin probe force microscopy. It was found that non-stoichiometric sputtering of the compound surface and beam enhanced surface diffusion led to unusual development of surface structures in the form of dots and wires with nanometer scale dimensions. The shape, the size and the surface density of nanostructures were investigated as a function of the beam flux and fluence, and as a function of the crystal orientation with respect to the ion-beam direction. The mechanisms involved in the surface modification were compared with results of Monte-Carlo computer simulations of anisotropic surface diffusion.

Modified Structure Zone Model to Describe the Morphological Evolution of ZnO Thin Films Deposited by Reactive Sputtering

The morphological evolution of zinc oxide (ZnO) thin films deposited by magnetron sputtering is described by the use of a Structure Zone Model. A modified Structure Zone Model was revealed, in which the boundaries between zones with specific features are shifted toward lower homologous temperatures (T/Tm) than in the classical models. The range of homologous temperatures for this study were 0.13 < T/Tm < 0.43. The promotion of the formation of high temperature structures at relatively low temperatures is a consequence of the energetic species generated during the sputtering process that bombard the growing film. The reduction of the shadowing effect, along with the substrate heating that increases the surface diffusion, led to the suppression of zone T. Two new subzones of zone II were identified, IIa and IIb. The film in subzone IIa displays pronounced faceting. The film in subzone IIb has a characteristic smooth surface due to enhanced surface diffusion at higher substrate temperatures during deposition, although pitted due to still incomplete surface diffusion.