Surface Growth Kinetics Research Articles

Effect of off-orientation of seed crystal on silicon carbide (SiC) single-crystal growth on the (11 $$\bar 2$$ 0) surface0) surface

The effect of off-orientation growth has been investigated in terms of stacking fault formation during physical vapor transport (PVT) growth of silicon carbide (SiC) single crystals on the (11\(\bar 2\)0) seed crystal surface. Occurrence of stacking fault formation is largely dependent on the direction of off-orientation, and basal plane stacking fault density is significantly reduced by growing the crystals on a (11\(\bar 2\)0) seed crystal off-oriented toward 〈0001〉. The density of the basal plane stacking faults rapidly decreases from 100–150 cm−1 to ∼10 cm−1 as the degree of off-orientation is increased from 0 to 10 deg. The results are interpreted in the framework of microscopic facet formation during PVT growth, and the introduction of off-orientation of seed crystal is assumed to prevent (01\(\bar 1\)0) and (10\(\bar 1\)0) microfacet formation on the (11\(\bar 2\)0) growing surface through modification of the surface growth kinetics and to suppress the stacking fault formation.

Murine marrow-derived mesenchymal stem cell: isolation, in vitro expansion, and characterization.

In spite of the attention given to the study of mesenchymal stem cells (MSCs) derived from the bone marrow (BM) of humans and other species, there is a lack of information about murine MSCs. We describe the establishment of conditions for the in vitro expansion of plastic-adherent cells from murine BM for over 50 passages, and provide their characterization regarding morphology, surface marker profile and growth kinetics. These cells were shown to differentiate along osteogenic and adipogenic pathways, and to support the growth and differentiation of haematopoietic stem cells, and were thus operationally defined as murine mesenchymal stem cells (mMSCs). mMSCs were positive for the surface markers CD44, CD49e, CD29 and Sca-1, and exhibited a homogeneous, distinctive morphology. Their frequency in the BM of adult BALB/c and C57Bl/6 mice, normal or knockout for the alpha-L-iduronidase (IDUA) gene, was preliminarily estimated to be 1 per 11,300-27,000 nucleated cells. The emergence of a defined methodology for the culture of mMSCs, as well as a comprehensive understanding of their biology, will make the development of cellular and genetic therapy protocols in murine models possible, and provide new perspectives in the field of adult stem cells research.

Direct condensation modelling for a two-particle growth system: application to GaAs grown by hydride vapour phase epitaxy

A new phenomenological model for the growth of GaAs in the GaCl/AsH 3/HCl/H 2 vapour phase system is developed. The surface growth kinetics are modelled by taking into account the mechanisms of As and GaCl adsorption and chlorine desorption by H 2 into HCl. Two ad-species AsGaCl and AsGa interact on the surface through a reversible reaction, which is described through a modified two-particle Burton, Cabrera and Frank model. Kinetics data are determined by synthesising experimental and computed results. It is shown that when surface diffusion limitations can be neglected, the growth rate is reduced to a one-particle-like direct condensation expression, weighted by a sticking coefficient which takes into account the desorption frequency of the precursor AsGaCl and its reversible transformation into the crystal particle AsGa. Variations of the growth rate are discussed as a function of the ad-species surface coverage ratios and of the supersaturation of the vapour phase.

Si 1−y C y / Si (001) gas-source molecular beam epitaxy from Si2H6 and CH3SiH3: Surface reaction paths and growth kinetics

In situ surface probes and postdeposition analyses were used to follow surface reaction paths and growth kinetics of Si1−yCy alloys grown on Si(001) by gas-source molecular-beam epitaxy from Si2H6/CH3SiH3 mixtures as a function of C concentration y (0–2.6 at %) and temperature Ts (500–600 °C). High-resolution x-ray diffraction reciprocal lattice maps show that all layers are in tension and fully coherent with their substrates. Film growth rates R decrease with both y and Ts, and the rate of decrease in R as a function of y increases rapidly with Ts. In situ isotopically tagged D2 temperature-programmed desorption (TPD) measurements reveal that C segregation during steady-state Si1−yCy(001) growth results in charge transfer from Si surface dangling bonds to second-layer C atoms, which have a higher electronegativity than Si. From the TPD results, we obtain the coverage θSi*(y,Ts) of Si* surface sites with C backbonds as well as H2 desorption energies Ed from both Si and Si* surface sites. θSi* increases with increasing y and Ts in the kinetically limited segregation regime while Ed decreases from 2.52 eV for H2 desorption from Si surface sites with Si back bonds to 2.22 eV from Si* surface sites. This leads to an increase in the H2 desorption rate, and hence should yield higher film deposition rates, with increasing y and/or Ts during Si1−yCy(001) growth. The effect, however, is more than offset by the decrease in Si2H6 reactive sticking probabilities at Si* surface sites. Film growth rates R(Ts,JSi2H6,JCH3SiH3) calculated using a simple transition-state kinetic model, together with measured kinetic parameters, were found to be in excellent agreement with the experimental data.

In-situ monitoring of surface stoichiometry and growth kinetics study of GaN (0001) in MOVPE by spectroscopic ellipsometry

GaN surface stoichiometry and growth kinetics in MOVPE were studied by in-situ spectroscopic ellipsometry. The effect of MOVPE conditions on both the surface stoichiometry and growth kinetics was investigated. The surface stoichiometry, such as N-rich, Ga-rich and Ga-excess surfaces, was monitored, and was drastically changed by the variation of the NH3 partial pressure. When the TMG supply was interrupted during the growth, the layer-by-layer decomposition/revaporation was observed in H2/NH3 ambient. The decomposition rate was measured as a function of the NH3 flow rate at the conventional epilayer growth temperatures (1050–1140 C). The decomposition rate was decreased with the increase in the N coverage on the GaN surface. it was found that the surface stoichiometry is a very important parameter for the control of the MOVPE growth kinetics.

Kinetic Monte Carlo study of crystal growth from solution

A study of both surface micromorphology and growth kinetics of the (001) face of a crystal growing from solution is performed on the basis of Monte Carlo simulations taking into account surface diffusion. Special attention is paid to growth simulations performed for layer-by-layer growth mechanism. An influence of the size of two-dimensional array describing crystal surface on simulation results is also discussed. The presented results show that the size of the surface array affects both growth mechanism and growth rate. The simulation results are confronted with predictions of crystal growth theories. The influence of surface diffusion on growth process is compared with some results reported for vapor growth, and it is found that for solution growth, the influence is much weaker. An implementation of fast simulation algorithm with explicit growth time is presented. It is revealed that calculations of the average crystal growth rate using time proportional to the number of events fail in the case of the perfect face growing at low supersaturations.

Surface Processes and Growth Kinetics in the Crystallization of Virus and Protein Crystals

Surface Processes and Growth Kinetics in the Crystallization of Virus and Protein Crystals

Surface structures and growth kinetics of InGaN(0001) grown by molecular beam epitaxy

The surface structures and growth kinetics of InGaN(0001) are studied. It is well known that during molecular beam epitaxy GaN surfaces undergo a smooth to rough transition when the growth condition is switched from Ga rich to N rich. It is found here that indium atoms have only a small effect on this transition when deposited on GaN(0001̄), but when deposited on GaN(0001) the indium acts as a surfactant and greatly extends the regime of smooth growth. Near the smooth/rough transition of InGaN(0001) growth, a bright ∛×∛ reconstruction is observed at growth temperature. The formation kinetics of this reconstruction are studied in detail. Scanning tunneling microscopy and total energy computations are used to study the structure of InGaN(0001) surfaces under metal rich conditions. Indium is found to occupy the top two atomic layers of the crystal; its incorporation in the second layer produces significant strain, leading to the formation of small pits on the surface and increased indium concentration inside and around the pits.

A rate equation model for the growth of GaN on GaN(0001̄) by molecular beam epitaxy

GaN(0001̄) films were grown by molecular beam epitaxy using ammonia and elemental Ga. The surface reactivity and growth kinetics of GaN(0001̄) were investigated as a function of growth parameters using desorption mass spectroscopy. Growth proceeds either by island nucleation or by step flow, depending on the steady state surface coverage of Ga. Three Ga adsorption states were found on the surface, one chemisorption and two weak states. One of the weak states corresponds to Ga adsorbed on a gallided surface, while the other corresponded to an intrinsic physisorption state on a hydrogen-passivated, nitrided surface. An abrupt growth mode transition between excess Ga and excess nitrogen was found as a function of growth parameters. The transition was modeled by rate equations based on growth at step edges and the three types of adsorption states.

Effects of pressure on growth kinetics of tetragonal lysozyme crystals

Growth kinetics of tetragonal hen egg-white lysozyme crystals under high pressures was studied by in situ measurements. Normal growth rates of {1 1 0} and {1 0 1} faces were measured under 0.1, 50, and 100 MPa. The solubilities under high pressure enabled us to evaluate supersaturation under high pressure. We found that both growth rates decreased with pressure under the same supersaturation. This means that the surface growth kinetics significantly depends on pressure. The birth and spread model was applied to understand the pressure effects on the growth kinetics. It was found that the increase in the average ledge surface energy of the two-dimensional nuclei with pressure explained the decrease in the growth rate.

In situ atomic force microscopy studies of surface morphology, growth kinetics, defect structure and dissolution in macromolecular crystallization

Surface morphologies of thaumatin, catalase, lysozyme and xylanase crystals were investigated using in situ atomic force microscopy. For thaumatin, lysozyme and xylanase crystals, growth steps having a height equal to the unit cell parameter were produced both by screw dislocations and two-dimensional nuclei. Growth of catalase crystals proceeded in alternating patterns exclusively by two-dimensional nucleation and the successive deposition of distinctive growth layers, each having a step height equal to half the unit cell parameter. The shapes of islands on successive layers were related by 2-fold rotation axes along the 〈0 0 1〉 direction. Experiments revealed that step bunching on crystalline surfaces occurred either due to two- or three-dimensional nucleation on the terraces of vicinal slopes or as a result of uneven step generation by complex dislocation sources. Growth kinetics for thaumatin and catalase crystals were investigated over wide supersaturation ranges. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was seen. From the supersaturation dependencies of tangential step rates and the rates of two-dimensional nucleation, the kinetic coefficients of the steps and the surface free energy of the step edge were calculated. Adsorption of impurities which formed filaments on the surfaces of catalase and thaumatin crystals was recorded. Cessation of growth of xylanase and lysozyme crystals was also observed and appeared to be a consequence of the formation of dense impurity adsorption layers. Crystal growth resumed upon scarring of the impurity adsorption layer and clearing of the crystal surface with the AFM tip. Adsorption of three-dimensional clusters, which consequently developed into either properly aligned multilayer stacks or misaligned microcrystals was recorded. For catalase crystals, incorporation of misoriented microcrystals as large as 50×3×0.1 μm 3 produced elastic deformations in growth layers of ≈0.6%, but did not result in the defect formation. Etching experiments on catalase crystals revealed high defect densities.

Low-temperature growth properties of Si 1− xGe x by disilane and solid-Ge molecular beam epitaxy

Low temperature (⩽500°C) growth properties of Si 1− x Ge x by disilane and solid-Ge molecular beam epitaxy have been studied with an emphasis on surface morphology and growth kinetics. It is found that low-temperature growth (<500°C) is in layer-by-layer mode and atomically-smooth surfaces have been obtained in as-grown samples with large Ge composition (>0.5). Ge composition dependence on substrate temperature, Ge cell temperature and disilane flow rate have been investigated. It is found that in low-temperature growth (⩽500°C) and under large disilane flux, Ge composition increases with the increase of Ge flux and further increase of Ge flux leads to the saturation of Ge composition. Similar compositional dependence has been found at different growth temperatures. The saturated composition increases with the decrease of substrate temperature. The results can be explained if H desorption is assumed to occur from both Si and Ge monohydrides without diffusional exchange and the presence of Ge enhances H desorption on a Si site.

Relation between surface reconstruction transitions and growth kinetics of zincblende (0 0 1) GaN

Abstract We study the relation between surface reconstruction transitions and surface kinetics for cubic GaN grown by plasma-assisted molecular beam epitaxy on (0 0 1) GaAs. Reflection high-energy electron diffraction is used to monitor the transient behavior of the surface reconstruction upon the pulsed supply of either Ga or N at a given substrate temperature. A quantitative analysis of the dynamics of the surface reconstruction transitions yields the desorption fluxes of Ga and N. The exceptionally high thermal stability of the (2 × 2)-reconstructed surface in vacuum is examined by ab initio total-energy calculations.

In Situ Optical Interferometric Studies of the Growth and Dissolution Behavior of Paracetamol (Acetaminophen). 1. Growth Kinetics

Paracetamol (acetaminophen, 4-hydroxyacetanilide) is a typical representative of over-the-counter analgesic and antipyretic drugs which are commonly administered as tablets. For these materials, crystallization from solution is an important stage of pharmaceutical production which defines many physicochemical properties of the solid dosage forms. The approach developed in this work involves investigation of the surface growth kinetics of single paracetamol crystals using in situ laser interferometry. All the major crystal faces of paracetamol (e.g. {110}, {001} and {201}) have shown a dislocation growth mechanism. The structure and the effective Burgers vector of the dislocations have been obtained and the specific step free energy, α, and the step kinetic coefficient, β, determined. The dissolution behavior of paracetamol crystals is affected by dislocation core instability and, to a lesser extent, by stress in the uppermost crystal layer for which the critical undersaturations have been determined. The...

Surface Growth Kinetics and Directed Percolation in a Ballistic Deposition-Reaction Model

The surface growth kinetics and directed percolation processes in a (2 + 1)-dimensional ballistic deposition-reaction model are studied in this paper. As the probability P increases a transition of the morphologic structure is found, which results from the diffusion and the directed percolating processes of the particles.

Surface growth kinetics and morphological structural transition in a (2 + 1)-dimensional deposition model

A new ballistic-like surface growth model of two kinds of particle (A and C) depositing on a two-dimensional substrate is proposed. The scaling behaviour of the surface width W is obtained for different values of the deposition probability P of particle C and system sizes. As the probability P increases a transition of the morphological structure is found, which is defined by the exponent varying with the probability P. The physical origin of the transition is discussed in terms of the diffusion of particles and the directed percolating processes.

Atomic Force Microscopy Studies of Surface Morphology and Growth Kinetics in Thaumatin Crystallization

Surface morphology and growth kinetics of thaumatin crystals in the supersaturation range σ = 0−1.65 were investigated using in situ atomic force microscopy (AFM). At σ < 0.7 growth steps were produced both by screw dislocations and two-dimensional nuclei. At higher supersaturations growth proceeded exclusively by two-dimensional nucleation. Adsorption of three-dimensional clusters on the surfaces of growing crystals that resulted in the formation of multilayer stacks which then grew tangentially was observed. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was also seen. From the supersaturation dependence of tangential step rates, the kinetic coefficient of the steps β〈011〉 was determined to be 2 × 10-4 cm/s. The supersaturation dependence of the rate of two-dimensional nucleation was measured, and this yielded the supersaturation dependence of the normal growth rate. From these data the surface free energy of the step edge was calculated to be α ≈ 0.4 erg/cm2 for σ > 1.2 and 0.2 erg/cm2 for σ < 1.2. It was suggested that, at σ < 1.2, two-dimensional nucleation was not homogeneous and occurred at sites more active than normal, e.g., at defects or sites of impurity incorporation. Values of α in the range of 0.4−0.7 erg/cm2 were also obtained on the basis of the sizes of critical nuclei, both for growth on dislocations and for growth by two-dimensional nucleation.

Random-Like Deposition Model of Surface Growth Kinetics

The surface growth kinetics of random-like deposition model with two kinds of particles (A and C) is studied. The scaling behavior of the surface width is obtained for various deposition probabilities of particle C and system sizes. We also found a change of different morphologic structures which results from the diffusion of particle C.

Ion-Assisted Deposition of Lead Titanate Thin Films by Controlled-Magnetic Field Sputtering

Ferroelectric thin films of lead titanate were deposited on glass and MgO(100) of 15 mm×15 mm in size by dc magnetron sputtering. The discharge characteristics were studied through the observation of the target erosion pattern and by the probe current technique at the substrate site with different magnetic field configurations. The radius of the target erosion pattern corresponded to the magnetic field component vertical to the target surface, which in turn affected the plasma density distribution over the substrate holder. Low-energy ion bombardment onto a growing film on the glass enhanced the surface reaction rate and growth kinetics, and hence the crystallinity and the orientation were also affected. In the case of the film on MgO(100), the c-axis lattice parameter was expanded by the induced defect structure.

CdTe and HgTe surface growth kinetics for molecular and metalorganic molecular beam epitaxy

The surface growth kinetics of CdTe and HgTe have been investigated during molecular and metalorganic molecular beam epitaxy. The surface growth kinetics was studied through in-situ measurements of the growth rate as a function of flux ratio and substrate temperature on the (001), (111)B, and (211)B CdTe surface orientations. For the (001) and (111)B CdTe growth kinetics, the existence of low binding energy surface precursor sites was proposed for both molecular and atomic growth species before lattice incorporation. Intensity oscillations were observed during HgTe growth on misoriented (111)B surfaces and during CdTe growth on the (211)B orientation. The (211)B surface reconstructions displayed both vicinal and singular surface characteristics, depending on the growth flux ratio.