Rough Growth Research Articles

Initial stage of GaN nucleation on [formula omitted]30°-Ga reconstructed 4H-SiC( [formula omitted]) Si by molecular-beam epitaxy

Gallium nitride (GaN) initial nucleation kinetics by the influence of SiC(0 0 0 1) Si surface structure has been investigated. The Ga induced √3×√3 R 30° and 3×3 surface reconstructions are found to be more efficacious for GaN growth. During the initial stage of GaN/4H-SiC(0 0 0 1) Si growth by molecular-beam epitaxy, coherent polygon islands grow wide (∼40 nm) along the lateral directions. These coherent islands develop 2D growth through early coalescence as evidenced by the in situ reflection high-energy electron diffraction observation. The control of Ga-adatom migration and the adsorption of 1/3 and 1 monolayer (ML) of Ga-adatom govern the surface morphology of the GaN layers. The bulk surfaces of (1×1) and (2×1) introduces incoherent nuclei, resulting in a delay of GaN coalescence, exhibits rough growth front and poor surface morphology.

Rapid Roughening in Thin Film Growth of an Organic Semiconductor (Diindenoperylene)

The scaling exponents alpha, beta, and 1/z in thin films of the organic molecule diindenoperylene deposited on SiO2 under UHV conditions are determined. Atomic-force microscopy, x-ray reflectivity, and diffuse x-ray scattering were employed. The surface width displays power law scaling over more than 2 orders of magnitude in film thickness. We obtained alpha = 0.684+/-0.06, beta = 0.748+/-0.05, and 1/zeta = 0.92+/-0.20. The derived exponents point to an unusually rapid growth of vertical roughness and lateral correlations. We suggest that they could be related to lateral inhomogeneities arising from the formation of grain boundaries between tilt domains in the early stages of growth.

Surface Roughness Study of Low-temperature PECVD a-Si:H.

AbstractSurface topography of a-Si:H thin films, deposited at 75°C by Plasma-enhanced Chemical Vapor Deposition (PECVD) has been examined using helium/silane feedstock mixtures under different substrate bias conditions. Notable differences in the surface roughness evolution are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Smooth and apparently featureless surfaces result from deposition on RF powered surfaces, upon which a self-bias induces high-energy ion bombardment. Rougher surfaces result from films deposited on electrically grounded surfaces. These anodic films show that after a transition period, surface roughness grows linearly with processing time, exhibiting mounded type growth as evidenced by 2-D power spectral density functions of surface height measurements. Linear growth in roughness has been predicted for shadow growth models assuming film precursor sticking coefficients of one and random angle approach of film precursor species. Growth of this nature has not been reported before in a-Si:H studies, which usually assume directional deposition conditions and sticking coefficients less than unity – occurring even at low processing temperatures.

Nucleation and growth of atomic layer deposited HfO2 gate dielectric layers on chemical oxide (Si–O–H) and thermal oxide (SiO2 or Si–O–N) underlayers

A study was undertaken to determine the efficacy of various underlayers for the nucleation and growth of atomic layer deposited HfO2 films. These were compared to films grown on hydrogen terminated Si. The use of a chemical oxide underlayer results in almost no barrier to film nucleation, enables linear and predictable growth at constant film density, and the most two-dimensionally continuous HfO2 films. The ease of nucleation is due to the large concentration of OH groups in the hydrous, chemical oxide. HfO2 grows on chemical oxide at a coverage rate of about 14% of a monolayer per cycle, and films are about 90% of the theoretical density of crystalline HfO2. Growth on hydrogen terminated Si is characterized by a large barrier to nucleation and growth, resulting in three-dimensional, rough, and nonlinear growth. Thermal oxide/oxynitride underlayers result in a small nucleation barrier, and nonlinear growth at low HfO2 coverages. The use of chemical oxide underlayers clearly results in the best HfO2 layers. Further, the potential to minimize the chemical oxide thickness provides an important research opportunity for high-κ gate dielectric scaling below 1.0 nm effective oxide thickness.

Layer-by-layer epitaxy in limited mobility nonequilibrium models of surface growth.

We study, using noise-reduction techniques, layer-by-layer epitaxial growth in limited mobility solid-on-solid nonequilibrium surface growth models, which have been introduced in the context of kinetic surface roughening in ideal molecular beam epitaxy. Multiple hit noise reduction and long surface diffusion length lead to qualitatively similar layer-by-layer epitaxy in (1+1)- and (2+1)-dimensional limited mobility growth simulations. We discuss the dynamic scaling characteristics connecting the transient layer-by-layer growth regime with the asymptotic kinetically rough growth regime.

Bole Epiphytes on Three Conifer Species from Queen Charlotte Islands, Canada

To compare effects of aspect, height on tree, and host species on species richness and cover of epiphytic lichens and bryophytes on conifers, we sampled Chamaecyparis nootkatensis, Picea sitchensis, and Tsuga heterophylla on the north and south sides in the first, second, and third meters of tree trunks in Vladimir J. Krajina Ecological Reserve. A total of 11 taxa of mosses, 26 of liverworts, and 4 of lichens were recognized. Bryophytes, especially liverworts, dominated in both cover and number of occurrences, with the most common being Dicranum scoparium, Isothecium myosuroides, Bazzania flaccida, and Scapania bolanderi. Rhytidiadelphus loreus was abundant at the tree bases but not at other heights. There was no host specificity. The rough mat growth form comprised both the highest cover and greatest number of species of bryophytes on each of the three tree species. This study demonstrates that under the right bark conditions liverworts can be more abundant than mosses and both more abundant than lichens on conifer bark.

Rail corrugation in The Netherlands—measurements and simulations

Rail roughness growth on Dutch railway tracks has been monitored since the early 1990s. On the track between Amersfoort and Baarn, significantly different levels of roughness have been detected on adjacent tangent track sections. The only obvious difference between two selected test sites located 250 m apart, one with a smooth rail and one with a corrugated rail, is the rail manufacturer. Detailed metallurgical investigations on samples taken from the test sites are discussed. Roughness growth is also studied through numerical simulation of dynamic train/track interaction. Traffic data and track properties collected from databases and field measurements are adopted in the simulation. The occurrence of a corrugated and a non-corrugated rail on the same track is explained by differences in wear resistance. The material quality of the two investigated rail samples is similar, but a clear difference in interlamellar spacing and cementite content at the surface is observed. The non-corrugated rail proved to be less wear resistant. Good agreement between simulated and measured roughness growth at the corrugated site is obtained with respect to wavelength content and wear rate. From simulations, the large growth rate is explained by a low track receptance near the sleepers around 1200 Hz.

In vitro MC3T3 osteoblast adhesion with respect to surface roughness of Ti6Al4V substrates

This work investigates the role of the surface roughness of Ti6Al4V on the cell morphology, proliferation and adhesion, and in particular on the variation of the expression of cell adhesion proteins. Standardised test samples with five different surface preparations are used: sandblasted, 80, 1200, and 4000 grade polished, mirror polished. Surface roughness is analysed by Scanning Electron Microscopy and LASER Confocal Microscopy. Cell culture experiments are performed with MC3T3-E1 mouse osteoblasts after 3 days culture: proliferation rate, morphology and adhesion are assessed. The variations of expression of cell adhesion proteins are evidenced by indirect immune fluorescence method: actin from the cytoskeleton, vinculin from the focal adhesion complex, fibronectin and collagen I from the extracellular matrix. The results reveal a clear influence of surface roughness of Ti6Al4V on cell proliferation, morphology and adhesion. A significant correlation is established between surface roughness and cell growth. More the surface is smooth more the osteoblasts proliferate and appear spread out on the test samples. In addition, the expression of adhesion proteins varies with respect to the surface roughness. These results indicate a direct relationship between the decrease of cell adhesion and the increase of cell proliferation on mirror polished materials.

Development and ordering of mounds during metal(100) homoepitaxy

Scanning-tunneling microscopy studies combined with atomistic modeling for Ag/Ag(100) homoepitaxy reveal complex growth behavior at 300 K: initial smooth growth up to \ensuremath{\sim}25 ML, where three-dimensional (3D) mounds develop from 2D islands; then an extended regime of mound steepening for \ensuremath{\sim}1000 ML producing unexpected rough growth; and finally an asymptotic regime with cooperative mound ordering and coalescence dynamics quite distinct from that in systems with up-down symmetry. The steepening regime is compressed upon lowering temperature, so while initial growth is rougher, asymptotic growth is actually smoother.

Surface Roughness Evolution of PECVD Cathodic and Anodic a-Si:H.

AbstractSurface or interface roughness can impact optical, electronic, and MEMS applications of thin a-Si:H films. Deposition at lower temperatures can be advantageous for some applications of a-Si:H, but lower temperature deposition generally leads to rougher films. We have found that the evolution of surface roughness growth can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD). Notable differences in the surface roughness evolution and deposition rate are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Suppression of surface roughness growth of a-Si:H can be achieved under conditions of relatively high ion bombardment even at deposition temperatures as low as 75 C. Atomic force microscopy (AFM) was used to measure the relative surface roughness profile as a function of deposition time. Analysis of the power spectral density of the roughness yielded important statistical surface parameter information. Based on these observations, insight is given into growth mechanisms under the two deposition conditions.

From two-dimensional heterogeneous nucleation-induced kinetic roughening to needle-like pattern formation

We report in this letter a special type of kinetic roughening at a growing crystal face induced by foreign particles. At relatively low supercoolings, the faceted growth of the {110} faces of naphthalene crystals from its melt reveals a transition from smooth to rough growth when foreign particles are in contact with the crystal surface, and followed by an increase in the growth rate. This is due to the vanishing of the two-dimensional nucleation barrier at the particle contact area, which gives rise to the formation of a needle-like pattern formation at the contact point. At high supercoolings, the particle-induced kinetic roughening is suppressed. The particle-induced kinetic roughening was also observed at the {110} faces of n-C21H44 crystals.

Epitaxial mounding in limited-mobility models of surface growth

We study, through large-scale stochastic simulations using the noise reduction technique, surface growth via vapor deposition, e.g., molecular beam epitaxy (MBE), for simple nonequilibrium limited mobility solid-on-solid growth models, such as the Family model, the Das Sarma--Tamborenea model, the Wolf-Villain (WV) model, the larger-curvature (LC) model, and other related models. We find that $(d=2+1)$-dimensional surface growth in several noise reduced models (most notably the WV and LC models) exhibits spectacular quasiregular mound formation with slope selection in their dynamical surface morphology in contrast to the standard statistically scale-invariant kinetically rough surface growth expected (and earlier reported in the literature) for such growth models. The mounding instability in these epitaxial growth models does not involve the Ehrlich-Schwoebel step-edge diffusion barrier. The mounded morphology in these growth models arises from the interplay between the line tension along step edges in the plane parallel to the average surface and the suppression of noise and island nucleation. The line tension tends to stabilize some of the step orientations that coincide with in-plane high-symmetry crystalline directions, and thus the mounds that are formed assume quasiregular structures. The noise reduction technique developed originally for Eden-type models can be used to control the stochastic noise and enhance diffusion along the step edge, which ultimately leads to the formation of quasiregular mounds during growth. We show that by increasing the diffusion surface length together with supression of nucleation and deposition noise, one can obtain a self-organization of the pyramids in quasiregular patterns. The mounding instability in these simple epitaxial growth models is closely related to the cluster-edge diffusion (as opposed to step-edge barrier) driven mounding in MBE growth, which has been recently discussed in the literature. The epitaxial mound formation studied here is a kinetic-topological instability [which can happen only in $(d=2+1)$-dimensional, or higher dimensional, growth, but not in $(d=1+1)$-dimensional growth because no cluster diffusion around a closed surface loop is possible in ``one-dimensional'' surfaces], which is likely to be quite generic in real MBE-type surface growth. Our extensive numerical simulations produce mounded (and slope-selected) surface growth morphologies which are strikingly visually similar to many recently reported experimental MBE growth morphologies.

Foreign body induced kinetic roughening: Kinetics and observations

A new and novel phenomenon: kinetic roughening induced by foreign particles at a growing crystal face was observed for the first time. Beginning with relatively low supercoolings, when foreign particles are in contact with the crystal surface, the faceted growth of the {110} faces of naphthalene crystals from its melt reveals a transition from smooth growth to rough growth, followed by an increase in the growth rate. This is due to the vanishing of the two-dimensional (2-D) nucleation barrier at the particle contact area. At high supercoolings, the particle-induced kinetic roughening will be suppressed. To describe this unusual phenomenon, the thermodynamic and kinetic criteria for this type of kinetic roughening were derived. The effect of particles on kinetic roughening is determined in terms of the step correlation parameter f, which is 0.035 for the naphthalene crystallization system. This is the first quantitative measurement for the interaction (or contact) parameters between the crystal steps and foreign particles.

Universality class of fluctuating pulled fronts.

It has recently been proposed that fluctuating "pulled" fronts propagating into an unstable state should not be in the standard Kardar-Parisi-Zhang (KPZ) universality class for rough interface growth. We introduce an effective field equation for this class of problems, and show on the basis of it that noisy pulled fronts in d+1 bulk dimensions should be in the universality class of the ((d+1)+1)D KPZ equation rather than of the (d+1)D KPZ equation. Our scenario ties together a number of heretofore unexplained observations in the literature, and is supported by previous numerical results.

Rough growth fronts of evaporated gold films, compared with self-affine and mound growth models

The power spectra, S( q, t), of evaporated gold films at room temperature are quantitatively compared with the Das Sarma Tamborenea model. The experimental curves fit the theory. However, the noise parameter, deduced from the fitting, disagrees with the predicted value. The two-dimensional (2D) experimental height–height correlation functions of scanning tunnelling microscope (STM) images present a hexagonal anisotropy. This fact agrees with the existence of a mound structure in the three-dimensional STM images. In addition the 2D Fourier transform module shows the predicted maxima for a mound model.

Layer Growth and Alloy Formation of Ir on Fe(100)

The growth mode and chemical structure of Ir deposited on Fe(100) are studied by specular reflection of fast ions and ion-induced Auger-electron spectroscopy. Temperatures below 500 K lead to a rough growth front, which is in agreement with a statistical growth of Ir atoms. For higher temperatures, we observe a layer-by-layer growth mode, where the Ir film is completely terminated by Fe atoms.

LAYER GROWTH AND ALLOY FORMATION OF Ir ON Fe(100)

The growth mode and chemical structure of Ir deposited on Fe(100) are studied by specular reflection of fast ions and ion-induced Auger-electron spectroscopy. Temperatures below 500 K lead to a rough growth front, which is in agreement with a statistical growth of Ir atoms. For higher temperatures, we observe a layer-by-layer growth mode, where the Ir film is completely terminated by Fe atoms.

The Development of Surface Roughness and Implications for Cellular Attachment in Biomedical Applications

AbstractThe application of a microscopic surface texture produced by ion beam sputter texturing to the surfaces of polymer implants has been shown to result in significant increases in cellular attachment compared to smooth surface implants in animal studies. A collaborative program between NASA Glenn Research Center and the Cleveland Clinic Foundation has been established to evaluate the potential for improving osteoblast attachment to surfaces that have been microscopically roughened by atomic oxygen texturing. The range of surface textures that is feasible depends upon both the texturing process and the duration of treatment. To determine whether surface texture saturates or continues to increase with treatment duration, an effort was conducted to examine the development of surface textures produced by various physical and chemical erosion processes. Both experimental tests and computational modeling were performed to explore the growth of surface texture with treatment time. Surface texturing by means of abrasive grit blasting of glass, stainless steel and polymethylmethacrylate surfaces was examined to measure the growth in roughness with grit blasting duration by surface profilometry measurements. Laboratory tests and computational modeling was also conducted to examine the development of texture on Aclar® (chlorotrifluoroethylene) and Kapton® polyimide, respectively. For the atomic oxygen texturing tests of Aclar®, atomic force microscopy was used to measure the development of texture with atomic oxygen fluence. The results of all the testing and computational modeling support the premise that development of surface roughness obeys Poisson statistics. The results indicate that surface roughness does not saturate but increases as the square root of the treatment time.

Suppressing the surface roughness and columnar growth of silicon nitride films

The growth and structure evolution of silicon nitride films prepared by radio frequency magnetron sputtering at low pressure were investigated for thickness changes. Grazing-incidence X-ray reflectivity, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy were used to study the structure, surface roughness and composition, respectively, of the films. It was found that with an increase in thickness from 5 to 830 nm, the silicon nitride films have a similar surface structure and their surface roughness only slightly increases. The surface roughness of all the silicon nitride films is only a little more than 0.2 nm, much less than previously reported results. AFM measurement of the fractured film surface indicated that thin silicon nitride films with a thickness below approximately 300 nm have a dense, smooth and uniform structure, and columnar structures gradually appear only in thick films. These results are mainly attributed to the special design of the sputtering system and the energetic particle bombardment of the growing surface, by which the surface roughness and columnar growth were suppressed.

Amorphous thin film growth: minimal deposition equation

A nonlinear stochastic growth equation is derived from (i) the symmetry principles relevant for the growth of vapor deposited amorphous films, (ii) no excess velocity, and (iii) a low-order expansion in the gradients of the surface profile. A growth instability in the equation is attributed to the deflection of the initially perpendicular incident particles due to attractive forces between the surface atoms and the incident particles. The stationary solutions of the deterministic limit of the equation and their stability are analyzed. The growth of the surface roughness and the correlation length of the moundlike surface structure arising from the stochastic growth equation is investigated.