Solid-vapor Interface Research Articles

Lattice density-functional theory of surface melting: the effect of a square-gradientcorrection

I use the method of classical density-functional theory in the weighted-densityapproximation of Tarazona to investigate the phase diagram and the interface structure ofa two-dimensional lattice-gas model with three phases—vapour, liquid and triangular solid.While a straightforward mean-field treatment of the interparticle attraction is unable togive a stable liquid phase, the correct phase diagram is obtained when including a suitablychosen square-gradient term in the system grand potential. Taking this theory for granted,I further examine the structure of the solid–vapour interface as the triple point isapproached from low temperature. Surprisingly, a novel phase (rather than the liquid) isfound to grow at the interface, exhibiting an unusually long modulation alongthe interface normal. The conventional surface-melting behaviour is recoveredonly by artificially restricting the symmetries being available to the density field.

Comprehensive perspective on the mechanism of preferred orientation in reactive-sputter-deposited nitrides

Texture control of sputter-deposited nitride films has provoked a great deal of interest due to its technological importance. Despite extensive research, however, the reported results are scattered and discussions about the origin of preferred orientation (PO) are sometimes conflicting, and therefore controversial. The aim of this study is to acquire a clear perspective in order to discuss the origin of PO of sputter-deposited nitrides. Among nitrides, we focus on titanium nitride (TiN), aluminum nitride (AlN), and tantalum nitride (TaN), which are three commonly used nitrides. First, we collected reported experimental results about the relation between operating conditions and PO, because PO is considered to be determined by film formation processes, such as surface diffusion or grain growth, which is affected by operating conditions. We also collected reported results about such PO-determining processes. Then, we categorized the PO-determining processes into an initial stage and a growth stage of film deposition, and further categorized each stage into a vapor–solid interface and a solid–solid interface. Then, we related each stage and interface to film morphology and to PO-determining processes. Finally, based on existing results, previous models, and proposed schema, we discuss the origin of PO. Based on previous experimental results on film morphology, PO of nitride films occurred in the growth stage at the vapor–solid interface, where the sticking process of the precursor and the surface diffusion process determine PO, rather than in the initial stage and in the growth stage at the solid–solid interface. TiN (002) PO, however, seems to be caused in the initial stage at the solid–solid interface.

Droplet stability in a finite system: Consideration of the solid–vapor interface

The thermodynamic stability of a single, one-component droplet in a finite system with adsorbing walls is investigated. The conditions under which a stable equilibrium state of the droplet is predicted to exist depend critically on the adsorption isotherm of the confining walls. If the amount absorbed remains finite when the pressure in the vapor is greater than the fluid’s saturation pressure, then a stable equilibrium state is possible. When the model is extended to a system of multiple droplets, the stable equilibrium state is predicted to always correspond to a single droplet.

Wetting of nanopatterned surfaces: The striped surface

We report an extensive set of results for the wetting of striped nanopatterned surfaces by simple fluids. Metropolis Monte Carlo simulations are used to investigate the validity of Cassie’s law for the surface tension of heterogeneous surfaces, for the case where the surface properties vary periodically on a nanometre scale. A variety of statistical mechanical sum rules is employed to measure the interfacial free energies and a set of interfacial order parameters associated with patterned inhomogeneous fluids. We directly observe two classes of interfacial phase transitions: (i) an unbending transition at the solid–vapor interface, which must precede complete wetting in systems where low-energy regions are not completely wet; (ii) a surface crystallization–layering transition associated with a hemicylindrical region of enhanced liquid structure at the substrate–liquid boundary. For a vapor phase in contact with a striped surface we observe hemicylindrical drops pinned to the stripe boundaries, with a mechanical contact angle unrelated to that defined by Young’s equation. The magnitude and variation of the mechanical contact angle with system parameters can be understood from minimal models of adsorption on patterned surfaces.

Some approaches to producing microporous materials and coatings by EB PVD

Techniques to produce bulk microporous structures as well as coatings, are examined in metal and ceramic systems based on the role of second source additives to the main component during their simultaneous evaporation and condensation in an electron beam apparatus. An arbitrary grouping of additives is proposed to differentiate the extent of their interaction in the vapor and solid phases of the main component. Additives are chosen from components that essentially do not interact (group 1), that are essentially removed (group 2), and those that react and are accompanied by the creation and removal of gaseous products during formation of the main condensate phase (group 3). These additives are shown to have a marked influence on the vapor solid interface shape and the dimensions of the deposited columnar crystals, their boundary zones and their internal porous structure. The chemical nature and content of the additives to the vapor phase opens up a broad spectrum of possibilities for construction of stable porous materials with micron and sub-micron size pores and large percentages (30–40%) of voids. Three characteristic shapes of microporous structures were distinguished within columnar and dendritic crystallites containing additives: gaps between dense and porous columns, micropores inside columnar cells and interdendritic (branch) porosity.

Molecular mechanisms of hydrophobic transitions

This review article provides new insights into the molecular mechanisms of hydrophobicity that emerge within the phenomenological framework of classical capillarity. The thermodynamic analysis of surface phenomena via the Gibbs adsorption equation that forms the basis of the experimental physical chemistry of liquid-fluid interfaces extends with its rigorous implications towards the wetting of solid surfaces. Observations on equilibrium contact angles, contact angle hysteresis, and the dynamics of wetting hitherto based on qualitative results that received fragmentary and eclectic interpretations are accounted for in terms of the values of adsorption pressures that act at the three-phase line. Hydrophobic surfaces and hydrophobic transitions induced by reversible solute adsorption qualify as two distinct limiting cases in this thermodynamic context. The equilibrium and kinetic aspects of the wetting of uniform and chemically heterogeneous surfaces by pure liquids and surfactant solutions is discussed with reference to reversible liquid-fluid interfaces. The importance of the solvent and solute adsorption at the solid-vapor interface for proper assessment of static and dynamic dewetting transitions is particularly addressed.

Dynamic Wetting of Silica by Aqueous Triblock Copolymer Solutions at Low Concentrations

We report on the wetting of silica by aqueous solutions of triblock copoly(ethylene oxide-tetrahydrofuran-ethylene oxide), PEO-PTHF-PEO. The wetting behavior was studied by means of a Wilhelmy force balance and direct images of the contact angle. The results show that the three-phase contact line (tcl) advances in jumps over the surface when this is immersed with constant rate into copolymer solutions. This stick-slip behavior was studied with respect to both the concentration of copolymers in the solution and the immersion rate of the silica plate into the solution. The wetting results are compared with adsorption data obtained by ellipsometry and dynamic surface tension measurements. It is clear that the stick-slip spreading behavior results from the same basic mechanisms as have previously been proposed for short chain cationic surfactants. The pinning of the contact line is due to the formation (through adsorption) of an autophobic copolymer layer at the solid-vapor interface. At low copolymer concentrations, transport of copolymers to the solid-vapor interface seems to occur mainly through surface diffusion from the liquid-vapor interface to the tcl. Depinning and slip occurs when the dynamic contact angle has increased above a critical value which exceeds that stipulated by the Young's equation in the presence of the autophobic layer at the solid-vapor interface.

Surface Roughness Statistics and Temperature Step Stress Effects for D-T Solid Layers Equilibrated inside a 2 mm Beryllium Torus

Solid D-T layers are equilibrated inside a 2 mm diameter beryllium toroidal cell at temperatures ranging from 19.0 K to 19.6 K, using the beta-layering process. Each experimental run consisted of multiple cycles of rapid- or slow-freezing of the initially liquid D-T charge. Each of these freeze cycles was followed by a lengthy period of beta-layering equilibration, which was terminated by melting the layer. The temperature was changed in discrete steps at the end of some equilibration cycles in an attempt to simulate actual ICF target conditions. High-precision images of the D-T solid-vapor interface were analyzed to yield the surface roughness σrms as a sum of modal contributions. Results show an average σrms. of 1.3 ± 0.3 μm for layers equilibrated at 19.0 K and show an inverse dependence of σrms on equilibration temperature up to 19.525 K. Inducing sudden temperature perturbations lowered σrms to 1.0 ± 0.05 μm.

Design, Synthesis and Characterization of Precursors for Chemical Vapor Deposition of Oxide-Based Electronic Materials

AbstractFerroelectric and other high dielectric constant metal oxides currently are sought-after for a variety of applications in the electronics industry. To meet the demand of preparation of these interesting materials in a manner compatible with traditional silicon-based fabrication procedures, chemical vapor deposition routes are desired for film growth. Compounds displaying high vapor phase stability are necessary as precursors for these applications. Additionally, in general, it is preferred to utilize compounds in a liquid state, due to the more rapid re-establishment of equilibrium at a liquid-vapor interface, compared to that present at a solid-vapor interface. This combination of desired molecular properties, in turn, presents a great challenge to the coordination chemist. Several of the metals of interest for these uses reside in groups 2–5. Common design features are emerging for the ligands best suited for attachment to these metals for subsequent utilization in the deposition of metal oxides. In order to achieve coordinative saturation of the relatively high ionic radii exhibited by most of these elements, multidentate, monoanionic ligands are relied upon. In the past, most often, homoleptic ligand sets have been employed, thereby reducing the chance for ligand scrambling to occur during the growth process. Such disproportionation processes have been credited, in previous work, with the observation of a temporal decay in vapor pressure of heteroleptic compounds. In some interesting new developments, it has been found that heteroleptic compounds possess sufficient vapor phase integrity to permit their evaluation as CVD precursors. These, and related, results are presented herein.

Model to analyse compositional distribution on the substrate surface for MOVPE grown ternary alloys

The lateral compositional distribution of indium on InGaAs sample surface, which was grown by metalorganic vapor phase epitaxy (MOVPE), has been investigated by high resolution X-ray diffractometry (HRXRD). It was found that the In content increases exponentially along the gas flow direction. Based on the X-ray results, a general simplified model, which analyses the lateral compositional distribution for a ternary alloy grown in a horizontal MOVPE reactor, is proposed according to the chemical chain reaction. This model takes into account the thermal decomposition and vapor–solid interface reaction in the MOVPE process. Furthermore, according to this model, some proposals were suggested in order to improve the compositional uniformity in a ternary alloy.

Effects of microgravity on Hg 1− xCd xTe/( [formula omitted])CdTe epitaxy by CVT under transient growth conditions

Transient epitaxial growth experiments of Hg 1− x Cd x Te on ( 1 0 0 )CdTe substrates by chemical vapor transport (CVT), using HgI 2 as a transport agent, were performed at normal and reduced gravity during the USML-2 flight. The carrier mobility at 77 K of the epitaxial layer grown in microgravity is about twice that of the ground specimen. This result is related to the improved compositional uniformity (IR mapping) and structural quality (X-ray rocking curves) of the space-grown epitaxial layer and islands relative to ground samples. These observations and the lower dislocation density of the growth interface in microgravity demonstrate the effects of residual convective flow on the transport and deposition processes on ground near the solid–vapor interface of this system in the transient growth regime.

HRXRD study of compositional uniformity of MOVPE grown InGaAs

The lateral compositional distribution of indium content on InGaAs sample surface, which was grown on InP substrate by metal organic vapor phase epitaxy (MOVPE), was investigated by high resolution X-ray diffractometry (HRXRD). It was found that the In content increases exponentially in the gas flow direction. Based on the X-ray results, a general simplified model, which analyses the lateral composition distribution on the sample surface for a ternary alloy grown in a horizontal MOVPE reactor, is proposed according to a chemical chain reaction and mass transport theory. This model takes into account the thermal decomposition and vapor–solid interface reaction in the MOVPE process. Furthermore, according to this model, some proposals were suggested in order to improve the compositional uniformity in a ternary alloy.

Layering transitions, disordered flat phases, reconstruction, and roughening

We study in light of recent ellipsometry, vapor pressure isotherm and specific-heat measurements on the thermodynamics of adsorbed thin films on graphite, the connection between the layering phase diagrams of thin films on periodic substrates and the thermodynamics of the solid-vapor interface of a semi-infinite crystal. The latter is the limit of the former when the film becomes infinitely thick, and we are interested in connecting this limiting behavior to the thermodynamics of films of finite thickness. We argue that the concepts of surface roughening, preroughening, and reconstruction provide a quantitatively useful framework within which to discuss this connection. Through general renormalization-group arguments and, in more detail, through a self-consistent mean-field treatment that explicitly accounts for all relevant phases, we show that the same types of interactions that lead to these different surface phases lead also to the reentrant layering transitions seen in the recent experiments. By appropriate tuning of the mean-field parameters we can semiquantitatively reconstruct all the observed experimental phase diagrams. It turns out that certain experimental phase diagrams with “zippers” require that the preroughening transition become first order. Our renormalization-group arguments predict such behavior in certain parameter ranges. In addition, for different parameters we predict the existence of an, as yet unobserved, θ disordered flat phase with spontaneously broken particle-hole symmetry and continuously varying surface height with an accompanying intermeshing layering phase diagram. The underlying lattice in the experiments is triangular, and this actually enhances the stability of the disordered flat phase and the corresponding reentrant layering transitions in the films.

Microstructural pathways for the sintering of alumina ceramics

This study used surface area density measurements and microstructural paths to show differences in the microstructural evolution of two densifying alumina ceramics. There were significant differences in comparison to previous studies of this nature, which have been conducted mostly on powder metals. The study showed that the alumina ceramic derived from high surface area powders, lost 66% of its solid-vapor interface on heating to 1,000 C at 5 C/min. The results were consistent with pure coarsening in this temperature range. The measurements of surface area density of solid-solid boundaries suggested that most of the solid-solid interface was formed at densities below 70% or during the calcination of the powders. It is apparent that more work is required to determine which microstructure paths are most useful in the study of densifying ceramics.

Effect of baffles on orbital accelerations — Induced bubble oscillations in microgravity

The behavior of sloshing dynamics modulated fluid systems driven by the orbital accelerations including gravity gradient and jitter accelerations with partially-filled rotating fluids has been studied. Present study is applicable to a full-scale Gravity Probe-B Spacecraft dewar tank with and without baffle. Results of slosh wave excitation along the liquid—vapor interface induced by jitter acceleration-dominated orbital accelerations provide a torsional moment with an up-and-down movement of bubble oscillations in the rotating dewar. The results show rightward and leftward movement of bubble oscillations transverse to the rotating axis, and up-and-down movement of bubble oscillations longitudinal to the rotating axis of dewar container. The orbital accelerations also induce an eccentric contour of bubble oscillations in a horizontal r—θ plane. As viscous force between liquid—solid interface, and surface tension force between liquid—vapor—solid interface can greatly contribute to the damping effect of slosh wave excitation, the rotating dewar with baffle provides more areas of liquid—solid and liquid—vapor—solid interfaces than that of a rotating dewar without baffle. Results show that the damping effect provide by a baffle reduce the amplitude of slosh wave excitation and reduce the degree of asymmetry in liquid—vapor distribution. Computation of bubble (helium vapor) mass center fluctuations also verifies that a rotating dewar with baffle produces less fluctuations than that of a rotating dewar without baffle.

Quasi-Thermodynamic Analysis of Metalorganic Vapor Phase Epitaxy of GaN

ABSTRACTA thermodynamic analysis of GaN grown by MOVPE has been proposed based on quasi-thermodynamic equilibrium established on the solid-vapor interface. Phase diagrams for the MOVPE growth of GaN using TEGa and NH3 has been calculated. The phase diagram is consists of four phases regions: the region for single condensed phase of GaN, the region for double condensed phase of GaN (s) +Ga(l), the etching region with Ga droplets and the etching region without Ga droplets. The effect of growth temperature, reactor pressure, content of carrier gas, deposition ratio of NH3 and V/III ratio upon growth of GaN using MOVPE has been studied.

Phase diagrams for the MOVPE growth of ZnTe and ZnSeTe

The phase diagrams for the MOVPE growth of ZnTe and ZnSeTe have been proposed for the first time, based on the thermodynamic equilibrium established at the solid-vapor interface. The regions for the single condensed phase of ZnTe(s) and of ZnSeTe(s) have been investigated, respectively. Additionally, the growth conditions of appearance for the double condensed phase of ZnTe(s) + Zn(s or l) and ZnTe(s) + Te(s or l) for the ZnTe system, of ZnSeTe(s) + Zn(s or l) and ZnSeTe(s) + Te(s or l) for the ZnSeTe system are discussed.

Growth stability in high temperature vapour growth

Growth stability is studied in a system for high temperature vapour growth of PbSnTe. The transition from unstable to stable growth conditions is observed. The observation compared with theoretical analysis of the concentration and temperature field in the crystal and vapour at the growth interface confirms the recently published theory of Louchev [1] [J. Crystal Growth 140 (1994) 219] concerning diffusion, heat transfer, equilibrium molecular density and kinetics mechanism of morphological instability in physical vapour deposition and our conception of constitutional supersaturation criterion [2] [Grasza, in: Mass and Heat Transfer, Elementary Crystal Growth]. It is shown, that, additionally to the dependence of the morphological stability on the kinetic coefficient, the relation of the temperature gradient across the depositing layer to the gradient of concentration of the deposited substance in front of the growth interface, and the dependence of equilibrium molecular density (pressure) of the depositing substance on temperature, are the most important factors influencing the stability of the solid-vapour interface.

Melting as a first‐order phase transition in a system of defects

AbstractThe self‐consistent quasiharmonic model is used to describe the equilibrium atomic structure of a crystal and the thermodynamical properties of vacancies at high temperature. It is shown that an increase in the amplitude of the thermal atomic vibrations with temperature leads to a considerable reduction of the vacancy formation energy in the high‐temperature region, which results in the generation of a large number of defects near the melting point and, as a consequence, in the significant softening of the acoustic vibration modes of the crystal. The effective attraction between the vacancies through the phonon subsystem of the crystal is found to lead to the first‐order phase transition with a discontinuous change in the defect concentration at the transition point. The melting lines are calculated for the rare gas crystals. The effects of the solid–vapour interface on the kinetics of the solid‐liquid transition are studied.

A surface acoustic wave technique for the observation of dynamic wetting

The spreading of small quantities of viscous oils has traditionally been studied using side views and plan views to obtain contact angles and profiles. The techniques employed have included optical profiling, interferometry, x-ray reflectivity and ellipsometry. In this work we report an alternative method, based on surface acoustic waves, which has the advantage of working directly within the spreading plane. In these experiments 150 ns pulses of 250 MHz surface acoustic waves are emitted along the solid-vapour interface towards a stripe of viscous oil. Both reflections from the advancing front edge of the oil and from a double transit signal which passes through the oil are observed. As the stripe becomes wider, changes in surface acoustic wave path lengths occur and these are measured using a phase comparison method. Results for the spreading of a small stripe of 100 000 cS polydimethylsiloxane oil are presented. The results for the time dependence of the width of the stripe agree with both theory and optical experiments.