Rough Growth Research Articles

The effect of high-flux H plasma exposure with simultaneous transient heat loads on tungsten surface damage and power handling

The performance of the full-W ITER divertor may be significantly affected by the interplay between steady-state plasma exposure and transient events. To address this issue, the effect of a high-flux H plasma on the thermal shock response of W to ELM-like transients has been investigated. Transient heating of W targets is performed by means of a high-power Nd:YAG laser with simultaneous exposure to H plasma in the linear device Magnum-PSI. The effects of simultaneous exposure to laser and plasma have been compared to those sequentially and to laser only. Transient melting is found to be aggravated during plasma exposure and to occur at lower heat flux parameters. Roughness and grain growth are observed to be driven by peak temperature, rather than by the loading conditions. The temperature evolution of the W surface under a series of transients is recorded by fast infrared thermography. By accounting for changes in the reflectivity at the damaged surface as measured by ellipsometry, a reduction in power handling capabilities of the laser/plasma affected W is concluded. The evidence of reduced power handling of the W surface under conditions as described here is of great concern with respect to the durability of W PFCs for application in fusion devices.

Preparation of AlN Thin Film Deposited on SiO<sub>2</sub> for Improving Temperature Frequency Coefficient of FBAR Used in CSACs

AlN films were deposited by RF magnetron sputtering on the SiO2 films with different thickness. Preferred orientation, grain size, surface roughness and grain growth of the AlN films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM). It's found that when the thickness of SiO2 films are 100 nm and 200 nm, the AlN films have characteristics of good C-axis preferred orientation, good surface roughness and vertical grain growth, which are suitable for application in thin film bulk acoustic wave resonators (FBARs). When the thickness of SiO2 film is 300 nm, although the AlN film has characteristics of good C-axis preferred orientation and surface roughness, the grain growth is tilted, which is not suitable for application in the FBARs.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(β), with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.

Volumetric flame synthesis of one-dimensional molybdenum oxide nanostructures

A laminar counter-flow diffusion flame formed with methane/acetylene and oxygen enriched-air was used for the controlled synthesis of 1-D molybdenum oxide nanostructures directly in the gas phase. Raw material was introduced into the oxidizer side of the flame in the form of solid molybdenum wires with ∼99% purity. Molybdenum oxide vapors formed in the gas phase were transported by the gas flow in the flame environment possessing strong thermal and chemical gradients. The generated nanostructures were collected thermophoretically from the flame volume. Essential morphological variations of generated nanomaterials were observed depending on sampling position within the flame volume and probe parameters. The mechanism behind the synthesis of the spherical and 1-D nanoforms is analyzed and modeled numerically. The nanorod growth model involves monomer transport, nucleation and growth. The monomer formation is through the oxidation and vaporization of the probe material. The nucleation model is based on the classical nucleation theory. The model predicts the trajectory and growth of the formed nuclei as they are transported in the flame volume. It is considered that the ends of the cylindrical 1-D nanorods grow by the phenomenon of rough growth while the lateral faces exhibit layered growth. The growth model also considers the contribution of the monomers diffusing on the nanoparticle surface as well as the effect of the atoms impinging directly onto the growth sites and compares their relative contributions. The model qualitatively predicts the variation of aspect ratio of the formed nanomaterials with increase of monomer concentration as observed in the experiments.

Surface Roughness and Critical Exponent Analyses of Boron-Doped Diamond Films Using Atomic Force Microscopy Imaging: Application of Autocorrelation and Power Spectral Density Functions

The evolution of the surface roughness of growing metal or semiconductor thin films provides much needed information about their growth kinetics and corresponding mechanism. While some systems show stages of nucleation, coalescence, and growth, others exhibit varying microstructures for different process conditions. In view of these classifications, we report herein detailed analyses based on atomic force microscopy (AFM) characterization to extract the surface roughness and growth kinetics exponents of relatively low boron-doped diamond (BDD) films by utilizing the analytical power spectral density (PSD) and autocorrelation function (ACF) as mathematical tools. The machining industry has applied PSD for a number of years for tool design and analysis of wear and machined surface quality. Herein, we present similar analyses at the mesoscale to study the surface morphology as well as quality of BDD films grown using the microwave plasma-assisted chemical vapor deposition technique. PSD spectra as a function of boron concentration (in gaseous phase) are compared with those for samples grown without boron. We find that relatively higher boron concentration yields higher amplitudes of the longer-wavelength power spectral lines, with amplitudes decreasing in an exponential or power-law fashion towards shorter wavelengths, determining the roughness exponent (α ≈ 0.16 ± 0.03) and growth exponent (β ≈ 0.54), albeit indirectly. A unique application of the ACF, which is widely used in signal processing, was also applied to one-dimensional or line analyses (i.e., along the x- and y-axes) of AFM images, revealing surface topology datasets with varying boron concentration. Here, the ACF was used to cancel random surface “noise” and identify any spatial periodicity via repetitive ACF peaks or spatially correlated noise. Periodicity at shorter spatial wavelengths was observed for no doping and low doping levels, while smaller correlations were observed for relatively higher boron concentration. These semiquantitative spatial analyses may prove useful in comparing synthesis techniques and varying compositional makeups of diamond films and other technologically important electronic materials. These findings in terms of critical exponents are also correlated with traditional Raman spectroscopy and x-ray diffraction structural properties, thus helping to provide insight into the growth kinetics, albeit in reverse manner.

HVPE-GaN growth on misoriented ammonothermal GaN seeds

The results of the Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal GaN seeds (Am-GaN) with various misorientations: 0.3, 0.5 and 1 degree to the [10−10] and [11−20] directions are presented. Growth rate and structural quality of the HVPE-GaN layers are analyzed. Morphology of the crystal growing surface at the beginning of the crystallization process (after one and two hours of growth) and at the end of it (after eight hours) is presented as a function of the Am-GaN seed misorientation. Based on these results a rough growth model is proposed and discussed in detail.

Physical and Electrical Characterization of ZnO Thin Films Prepared by Sol-Gel Method

In this paper, Zinc Oxide (ZnO) thin films are deposited on glass substrate by sol-gel method. The ZnO thin films with well defined orientation were acquired by spin coating of zinc acetate dehydrate monoethanolamine (MEA), de-ionized water and isopropanol alcohol. These films were pre-heated at 275°C for 10 min and then annealed at 350°C, 450°C and 550°C for 80 min. The effect of annealing temperature and different thickness on structure and surface morphology of the thin films were verified by Atomic Force Microscopy (AFM). It was found that there was a significant effect of annealing temperature on the structural parameters of the films such as roughness exponent, fractal dimension and interface width. Thin films also were characterizied by X-ray Diffractometery (XRD) method. XRD analysis revealed that the annealed ZnO thin films consist of single phase ZnO with wurtzite structure and show the c-axis grain orientation. Increasing annealing temperature increased the crystallite size and the c-axis orientation of the film after 450°C. Also In this study, ZnO thin films in different thickness have been prepared by sol-gel method on the glass substrate at room temperature. The thicknesses of films are 100, 150 and 250 nm. Using fractal analysis, morphological characteristics of surface films thickness in amorphous state were investigated. The results show that with increasing thickness, surface roughness (RMS) and lateral correlation length (ξ) are decreased. Also, the roughness exponent (α) and growth exponent (β) were determined to be 0.74±0.02 and 0.11±0.02, respectively. Keywords—ZnO, Thin film, Fractal analysis, Morphology, AFM, annealing temperature, different thickness, XRD.

Pore-scale study of diffusion–reaction processes involving dissolution and precipitation using the lattice Boltzmann method

A pore-scale model combining the lattice Boltzmann method (LBM) and a fluid–solid interface tracking method is employed to simulate the diffusion–reaction processes involving dissolution and precipitation. Coupled sub-processes including mass transport, chemical reactions, and solid structure evolution are considered. Effects of the precipitation of the secondary solid phase on the dissolution of the primary solid phase are investigated under different dissolution–precipitation reaction kinetics, molar volumes of the primary and secondary solid phases, powder size, surface roughness, and nucleation and crystal growth mechanisms. Different morphologies of the precipitates are predicted by the pore-scale simulations. It is found that the precipitation has opposite effects on the underlying dissolution processes. The favorable effect is that the precipitation reaction consumes the product of the dissolution reaction, thus facilitating the dissolution; while the adverse effect is that the generated precipitates cover the surface of the primary solid phase, thus separating the reactive surface from the reactive components. Based on the extent to which the precipitates affect the dissolution, four types of coupled dissolution–precipitation processes are identified and discussed.

Preparation of free-standing GaN substrates from GaN layers crystallized by hydride vapor phase epitaxy on ammonothermal GaN seeds

Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is overviewed. Morphology of the crystal growing surface at the beginning of the crystallization process and at the end of it is presented. Based on these results a rough growth model is proposed. Smooth GaN layers up to 1 mm thick and of a high purity, excellent crystalline quality, without any cracks, and with a low dislocation density are grown. Preparation of the free-standing HVPE-GaN crystals by slicing as well as structural, electrical and optical qualities of the resulting wafers are reported and discussed.

Automated self seeding of batch crystallizations via plug flow seed generation

In this study, seed slurry from a single addition anti-solvent plug flow crystallization of benzoic acid was used to seed the equivalent batch cooling crystallization. The experimental conditions were carried out to simulate automated self-seeding. This involves withdrawal of solution from a batch crystallizer, which is then mixed with anti-solvent within a plug flow crystallizer, in order to generate a seed slurry which is fed directly back to the batch crystallizer. This seeding strategy allowed the final CSD of the batch crystallization to be controlled by variation of the crystal size from the plug flow seeding device at a constant seed loading. The ability to use unequal feed/anti-solvent inlet flowrates (in the Roughton vortex mixer) proved effective in controlling the batch CSD at 2% seed loading and constant feed composition.The morphology of batch product from plug flow seeding was found to have a much lower aspect ratio than the more needle like dry seed and primary nucleated product. Unlike dry seed, which was found to have rough irregular growth on the crystal surface, the plug flow product had flat regular growth as per primary nucleated batch crystallization.

Characterization of optical coatings using a multisource table-top scatterometer

Light scattering measurement and analysis is a powerful tool for the characterization of optical and nonoptical surfaces. To enable a more comprehensive postmeasurement characterization, three visible laser sources were recently implemented in a highly sensitive table-top scatterometer with 3D spherical detection capability. Based on wavelength scaling, the instrument is utilized to characterize thin-film coatings and their substrates with respect to surface roughness, roughness growth, and contamination. Topographic measurement techniques are used to verify the results. The spectral sensitivity to contamination (scatter loss) is demonstrated to be significantly different for single surfaces and interference coatings. In addition, power losses of a highly reflective coating are analyzed.

Regime II–III transition in isotactic polybutene-1 tetragonal crystal growth

The lateral growth rate and growth shape morphology of isotactic polybutene-1 tetragonal crystals were investigated for crystallization from the melt at temperatures of 68–101 °C. The growth rate of tetragonal crystals shows supercooling dependence derived from the nucleation theory, and a regime II–III transition is observed at temperatures of 77–82.2 °C. The morphology of single crystals is rounded below temperatures of 77–85 °C, while the growth shape has a faceted morphology at a higher crystallization temperature of 100 °C. The kinetic roughening transition occurs between 77 and 85 °C. The regime II growth mode is observed above 82.2 °C and proceeds by multiple nucleation on the faceted growth front, while the regime III growth mode is observed below 77 °C and proceeds by rough surface growth on the kinetically roughened growth front. The observed regime II–III transition can therefore be explained by the morphology transition of crystal growth shape.

Nanorough silica coatings by chemical vapor deposition

Control over the growth of nanoscale roughness on surfaces and interfaces is critical for applications ranging from tunable wettability to anti-reflective coatings. Herein, we investigate the growth of nanoscale roughness on silicon substrates. Nanoscale roughness is introduced by chemical vapor deposition (CVD) of tetraethoxysilane under mild conditions and quantified by scanning force microscopy. We study the dependence of nanoroughness on the surface properties of the substrate before CVD, in particular on annealing and plasma cleaning, while taking an untreated substrate as reference. Nanoroughness is highest on annealed samples and always increases over longer CVD time. Furthermore, we report the time-dependent growth of the thickness of the silica layer. These results contribute to understanding of the role of surface pretreatment in the context of growing silica on a substrate.

Loop punching and bubble rupture causing surface roughening —A model for W fuzz growth

We develop a multi-scale computational model for studying tungsten fuzz formation under low-energy He irradiation. The molecular dynamics and kinetic Monte Carlo results show that the W fuzz growth mechanism is the following: the He atoms are trapped in W, forming bubbles and causing growth by loop punching. The bubbles close to the surface rupture. The balance between these processes leads to a stochastic surface growth, causing the surface roughness and fuzz thickness growth to scale as . The growth rates agree with the experimental results.

Simulation of rail roughness growth on small radius curves using a non-Hertzian and non-steady wheel–rail contact model

A time-domain model for the prediction of long-term growth of rail roughness (corrugation) on small radius curves is presented. Both low-frequency vehicle dynamics due to curving and high-frequency vehicle–track dynamics excited by short-wavelength rail irregularities are accounted for. The influence of non-Hertzian and non-steady effects in the wheel–rail contact model on rail wear is studied. The model features a contact detection method that accounts for wheelset yaw angle as well as surface irregularities and structural flexibilities of wheelset and rail. The development of corrugation on a small radius curve is found to be highly influenced by the wheel–rail friction coefficient. For vehicle speed 25km/h and friction coefficient 0.3, predictions of long-term roughness growth on the low rail show decreasing magnitudes in the entire studied wavelength interval. For friction coefficient 0.6, roughness growth is found at several wavelengths. The corresponding calculation for the high rail contact of the trailing wheelset indicates no roughness growth independent of friction coefficient. The importance of accounting for the phase between the calculated wear and the present rail irregularity is demonstrated.

Tuning of ripple patterns and wetting dynamics of Si (100) surface using ion beam irradiation

Ripple patterns on Si (100) surface have been fabricated using 200 keV Ar+ oblique ion beam irradiation. Dynamical evolution of patterns is studied for the fluences ranging from 3 × 1017 ions/cm2 to 3 × 1018 ions/cm2. AFM study reveals that the exponential growth of roughness with stable wavelength of ripples up to higher fluence values is lying in the linear regime of Continuum models. Stylus Profilometer measurement was carried out to emphasize the role of sputtering induced surface etching in ripple formation. Rutherford Backscattering Spectroscopy shows the incorporation of Ar in the near surface region. Observed growth of ripples is discussed in the framework of existing models of surface patterning. Role of ion beam sputtering induced surface etching is emphasized in formation of ripples. In addition, the wetting study is performed to demonstrate the possibility of engineering the hydrophilicity of ripple patterned Si (100) surface.

Validation of a prediction model for tangent rail roughness and noise growth

Railways can inevitably cause railway rolling noise, which is induced by both wheel and rail roughness. Due to the deformation and wear between the wheel/rail at the contact patch, the rail roughness may grow in amplitude after a number of wheelset passages. This results in increasing railway rolling noise. Rail roughness is a common aspect in all transit systems and much research has been pursued to predict and mitigate its growth on track but prediction of resultant noise growth has not been a focus. This paper provides the experimental validation of the modified Railway Rolling Noise Prediction Software (RRNPS) model for the prediction of rail roughness growth and corresponding noise growth. The model is a new framework to enable noise growth predictions due to rail roughness growth mechanics. It is validated by means of several experiments that have been performed along a straight railway line. Through comparisons between predictions and measurements, it is shown that the RRNPS model gives reliable predictions on rail roughness growth and corresponding noise growth. Subsequently this model is used to predict how speed, normal force, wheelset traffic and ballast vertical stiffness affect rail roughness growth and corresponding noise growth.

Competing Crystal Growth in Ge–Sb Phase‐Change Films

Analysis of crystal growth in thin films of phase‐change materials can provide deeper insights in the extraordinary phase transformation kinetics of these materials excellently suited for data storage applications. In the present work crystal growth in GexSb100‐x thin films with x = 6, 7, 8, 9, and 10 is studied in detail, demonstrating that the crystallization temperature increases from ∼80 °C for Ge6Sb94 to ∼200 °C for Ge10Sb90 and simultaneously the activation energy for crystal growth also significantly increases from 1.7 eV to 5.5 eV. The most interesting new finding is that in the thin films containing 8, 9, and 10 at% Ge two competing growth modes occur which can have several orders of magnitude difference in growth rate at a single external temperature: an initial mode with isotropic slow growth producing circular crystals with smooth surfaces and growth fronts and a fast growth mode producing crystals with triangular shape having rough surfaces and growth fronts indicative of dendritic‐like growth. The slow‐growth mode becomes increasingly dominant for crystallization at low temperatures when the Ge concentration is increased from 8 to 10 at% Ge. For a certain Ge concentration, the slow growth mode becomes increasingly dominant at lower temperatures and the fast growth mode at higher temperatures. Latent heat produced during crystallization is considered a principal factor explaining the observations. The fast growth mode is associated with (eutectic) decomposition generating more latent heat and instable growth fronts and the slow growth mode is associated with thermodynamically less stable homogeneously alloyed crystals generating less latent heat, but stable growth fronts.

Simulation of in situ reflectance-time during MOVPE of GaN on sapphire substrate

An optical model incorporating time-dependent surface roughness and time-dependent growth rate was used to simulate the in situ reflectance-time during the metalorganic vapor phase epitaxy (MOVPE) of GaN on sapphire substrate. Excellent fits to experimental data have been obtained and leaded to determine the high temperature GaN optical constants and both profiles of surface roughness and growth rate during the transient from three-dimensional (3D) to two-dimensional (2D) growth modes. From simulation, recovery time (τr), thickness and surface roughness are extracted. The results show that the growth rate increases to reach a limit value when 2D growth mode is established. At the same time, surface roughness decreases from a high value (65–75nm) to lower than 10nm if the growth is ended by 2D mode. In the case of surface smoothing is followed by surface degradation, a sum of two surface roughness profiles was suggested and given good simulation results. In this case, the reflectivity damping is characterized by new time constant so called damping time (τd) that also considered as fit parameter. Several in situ reflectivity traces corresponding to different growth conditions are chosen for these simulations. High temperature GaN refractive index extracted from simulations are close to those reported in the literature. The output simulations are correlated with ex situ analyses by UV–Visible transmission and Hall Effect that showed good accordance.

Linear surface roughness growth and flow smoothening in a three-dimensional biofilm model

The sessile microbial communities known as biofilms exhibit varying architectures as environmental factors are varied, which for immersed biofilms includes the shear rate of the surrounding flow. Here we modify an established agent-based biofilm model to include affine flow and employ it to analyze the growth of surface roughness of single-species, three-dimensional biofilms. We find linear growth laws for surface geometry in both horizontal and vertical directions and measure the thickness of the active surface layer, which is shown to anticorrelate with roughness. Flow is shown to monotonically reduce surface roughness without affecting the thickness of the active layer. We argue that the rapid roughening is due to nonlocal surface interactions mediated by the nutrient field, which are curtailed when advection competes with diffusion. We further argue the need for simplified models to elucidate the underlying mechanisms coupling flow to growth.