Surface Roughening Transition Research Articles

Surface roughening transition induced by phase transformation in hafnium nitride films

Although surface roughening of films holds an important role in determining the final surface morphologies and properties, how to control it is not yet well explored. Here, we report that the surface roughening of hafnium nitride films varies significantly as a stoichiometry-driven phase transformation takes place. Through a combination of surface morphological measurements and simulations, we demonstrate that the variation of roughening mechanisms arises from the changes in diffusion behavior during growth. This study shows that the phase transformation can induce surface roughening transition, thus providing a new handle in controlling the morphologies and physical properties of thin films.

Halogen and hydrogen bonds in compressed pentachloroethane

In pentachloroethane, C2HCl5, high pressure initially strongly compresses the C–H⋯Cl bonds in phase I; however, in phase II which is stable above 0.62 GPa the role of hydrogen bonds is diminished and molecular aggregation is dominated by halogen bonds Cl⋯Cl. Both phases have been determined by X-ray diffraction and the phase diagram of C2HCl5 has been outlined. The transition between phases I and II retains some relation between their structures and reduces the symmetry from class mmm (space group Pnma) to 2/m (space group P21/n11). The discontinuous transition, with the shear strain exceeding 21°, is so strong that its progress can be visually observed even for powdered samples. The single-crystal growth suggests a surface-roughening transition, as phase I grows in the form of dendrites combining into oval plates, while phase II forms prisms with sharp edges.

Consequences of High Adatom Energy during Pulsed Laser Deposition of La0.7Sr0.3MnO3

The impact of the adatom energy on the stoichiometry, surface morphology, and crystalline twinning during pulsed laser deposition of La0.7Sr0.3MnO3 is studied. We show that although nonthermal growth using highly energetic adatoms results in very smooth ultrathin films, it also causes preferential resputtering of Mn and a surface roughening transition with increasing film thickness. This can be circumvented by carefully tuning the adatom energy into thermal growth, resulting in more Mn rich samples and a delayed roughening transition. Furthermore, we demonstrate that the crystalline twinning can be controlled by controlling the adatom energy. Hence, a detailed control of the adatom energy during growth opens for better stoichiometry control as well as surface quality.

Quantum oscillations in surface properties

Many properties of metallic thin films have been shown to oscillate with film thickness due to quantum size effects, i.e. the confinement of electrons inside epitaxial metal overlayers which causes quantization of the electronic states. This is a very general phenomenon and it affects both bulk properties of the films, such as resistivity or superconducting transition temperatures, and surface properties, such as chemical reactivity, diffusivity, thermal stability, i.e surface roughening transitions. In this paper we describe some of these thickness-dependent properties which affect the stability of nanostructures and allow us to tailor their properties. We shall concentrate in the paradigmatic example of thin films of Pb grown on metallic and semiconducting substrates and how one can achieve the growth of highly perfect, atomically flat, epitaxial films on different substrates, due to the kinetic constrains imposed by the presence of QSE, a topic increasingly important in the production of nanoscale quantum devices.

Surface roughening and growth mode transition of polycrystalline thin films

Surface roughening of Cu and Ta polycrystalline thin films deposited by magnetron sputtering at different homologous temperature Ts/Tm (Ts and Tm are the substrate temperature and the material melting point, respectively) have been studied using atomic force microscopy. With increasing Ts/Tm, the surface roughness of the films increased and growth exponent first increased and then decreased. The observed temperature dependence of growth exponent indicates a transition of growth modes from random growth at lower Ts/Tm through surface diffusion-dominated growth at intermediate Ts/Tm to anomalous scaling growth at higher Ts/Tm. We also found that the surface roughening of the films reflects the anomalous scaling behavior when the value of Ts/Tm is lower than 0.41, which implies that the boundary diffusion mechanism plays an important role in the scaling behavior of surface roughening.

Faceted single crystals of mesoporous silica SBA-16 from a ternary surfactant system: surface roughening model

We have synthesized mesoporous silica SBA-16 at room temperature and low pH condition by using the surfactant system C 8TMAB/SDS/F127 as template. Faceted single crystals of rhombdodecahedron form can be obtained under well-controlled conditions. Synthetic factors in controlling the morphology, including stirring, water content, the ionic surfactants and acidity are explored. A schematic theory for the formation of the equilibrium crystal structure based on surface roughening transition is proposed to explain the formation of faceted single crystal.

Roughening transition of grain boundaries in metals and oxides

Extensive theoretical analysis and experimental observations show surface roughening transitions of crystals. The surface roughening is characterized by step free energy, which gradually decreases to 0 at the roughening transition temperature. For a crystal of finite size, the surface roughening transition is manifested by gradual increase of the curved edge and corner areas. In alloys, the interfaces between the solid and the liquid phases can be either singular, partially rough, or completely rough at different temperatures. Their thermally induced roughening transitions are similar to those of the solid-vapor interfaces. The interface roughening and the reverse transition to singular structures can also be induced by additives. The grain boundaries of any misorientation angles in oxides and metals also show roughening transitions. The singular grain boundaries have either flat, hill-and-valley, or kinked shapes, and with temperature increase or composition changes, they become defaceted to curved shapes. These defaceted grain boundaries are rough. It is thus possible to produce either singular or rough grain boundaries by heat-treatment or additives to vary their properties.

Surface Roughening Transition and Coarsening of NbC Grains in Liquid Cobalt‐Rich Matrix

When NbC–30 wt% Co powder compact is sintered at various temperatures where NbC grains (with small amounts of Co) coexist with a liquid Co–NbC matrix, the NbC grains undergo a surface roughening transition with temperature increase and the grain growth changes from abnormal to normal growth. When sintered at 1400°C, the grains are polyhedral with sharp edges (and corners) and grow abnormally because their singular surfaces move by nucleation of surface steps. When sintered at 1600°C, the edges become round, indicating the surface roughening transition. The grains still grow abnormally, but their number density is larger than that at 1400°C because of the smaller surface step free energy. When sintered at 1820°C, the grains are nearly spherical, but the flat‐surface segments still remain. The grain growth at this temperature is nearly normal because of very small surface step free energy. The surface roughening transition is reversed when a specimen initially sintered at 1820°C is heat‐treated again at 1400°C, but some grains show transition shapes with nearly flat edges and slope discontinuities (shocks).

Constitutional supercooling surface roughening

This paper consists of my recollections about the studies of crystal growth which I have been involved with over the past decades. I am deeply indebted to many outstanding co-workers who have collaborated with me. The descriptions are drawn from my memory of events, and subject to all of the failing which that might entail. I only regret that I will not be able to mention all of my collaborators over the years, due to a limitation of space and a faulty memory. This paper includes sections on interface instabilities, on the surface roughening transition, on the Jackson alpha factor, on transparent analogues of metals, on analytical models for crystallization, and finally, a few remarks about on-going and future of studies of crystal growth processes.

Interplay between phase ordering and roughening on growing films

We study the interplay between surface roughening and phase separation during the growth of binary films. Renormalization group calculations are performed on a pair of equations coupling the interface height and order parameter fluctuations. We find a larger roughness exponent at the critical point of the order parameter compared to the disordered phase, and an increase in the upper critical dimension for the surface roughening transition from two to four. Numerical simulations performed on a solid-on-solid model with two types of deposited particles corroborate some of these findings. However, for a range of parameters not accessible to perturbative analysis, we find non-universal behavior with a continuously varying dynamic exponent.

Kinetic Monte Carlo simulations of the surface roughening of binary systems

Although the surface roughening transition in single component systems has been studied extensively, the surface roughening transition in binary alloys has not. We have conducted a kinetic Monte Carlo computer study to simulate alloy crystallization using the spin-one Ising Model with diffusion. The surface roughness, which is defined as the density of lateral unsatisfied bonds at the interface, has been determined for both the pure components and for alloys of various compositions. The surface roughness for a pure material depends on the change in bond energy associated with the transformation, divided by the melting point. The solid phase contains essentially one component, the surface roughness of the alloys was found to depend on the change in bond energy of the pure component, divided by the liquidus temperature of the alloy.

Using Fluctuations To Locate The Surface Roughening Transition

AbstractAn analysis of the fluctuations of a {100} FCC interface has been used to locate the surface roughening transition in kinetic Monte Carlo simulations. These simulations were conducted using a spin 1/2 Ising model treatment of the solid-liquid interface. A temperature gradient was imposed on the simulation cell providing a restoring force to the interface. Fluctuations in the number of solid atoms in the system were measured using time correlations. The fluctuation dissipation theorem was employed to relate the characteristic times retrieved from the time correlation to the kinetic coeffcients of the system. A dramatic change was found in these characteristic times, which locates the surface roughening transition.

Using Fluctuations To Locate The Surface Roughening Transition

AbstractAn analysis of the fluctuations of a {100} FCC interface has been used to locate the surface roughening transition in kinetic Monte Carlo simulations. These simulations were conducted using a spin 1/2 Ising model treatment of the solid-liquid interface. A temperature gradient was imposed on the simulation cell providing a restoring force to the interface. Fluctuations in the number of solid atoms in the system were measured using time correlations. The fluctuation dissipation theorem was employed to relate the characteristic times retrieved from the time correlation to the kinetic coefficients of the system. A dramatic change was found in these characteristic times, which locates the surface roughening transition.

Extended phase diagrams for guiding plasma-enhanced chemical vapor deposition of silicon thin films for photovoltaics applications

Real time spectroscopic ellipsometry has been applied to develop extended phase diagrams that can guide the deposition of hydrogenated silicon (Si:H) thin films for highest performance solar cells. Previous such studies have shown that optimization of amorphous Si:H intrinsic layers by rf plasma-enhanced chemical vapor deposition (PECVD) is achieved using the maximum possible H2 dilution of SiH4 while avoiding a transition to the mixed-phase (amorphous+microcrystalline) growth regime. In this study, we propose that optimization of amorphous Si:H in higher rate rf PECVD processes further requires the largest possible thickness onset for a surface roughening transition detected in the amorphous film growth regime.

Two-dimensional equilibrium surface roughness for dissociative dimer dynamics.

Equilibrium crystal surfaces, constrained to equilibrate by means of dissociative dimer deposition and evaporation, have anomalous global surface roughness. We generalize earlier results for one-dimensional interfaces to two dimensions. The global surface width scales with surface size L as W2 approximately equals ln[L/(ln L)(1/4)] instead of the conventional form W2 approximately equal to ln L. The surface roughening transition does not change in nature, but its location is subject to a large and slowly varying logarithmic finite-size-scaling shift.

The Interface Kinetics of Crystal Growth Processes

A brief review of the present state of our understanding of the kinetic processes which take place on the atomic scale at the interface during crystal growth is presented in this paper. Computer simulations have played a central role in the development of this understanding. Three aspects will be discussed: (1) There are two classes of materials based on their different modes of crystallization. Molecular dynamics modeling has demonstrated that the growth rate for many simple materials is not thermally activated, but instead depends on the thermal velocity of the atoms. (2) The cooperative processes which give rise to the surface roughening transition. Kinetic Monte Carlo studies played a central role in the development of our understanding of how interface roughness dominates growth morphologies. (3) Solute trapping in alloys. Kinetic Monte Carlo simulations of alloys have led to an understanding of these kinetic effects during alloy crystallization.

Phase Diagrams for the Optimization of rf Plasma Enhanced Chemical Vapor Deposition of a-Si:H: Variations in Plasma Power and Substrate Temperature

ABSTRACTReal time spectroscopic ellipsometry (RTSE) has been applied to characterize surface roughening transitions during the growth of undoped hydrogenated silicon (Si:H) thin films by rf plasma-enhanced chemical vapor deposition (PECVD). In particular, the amorphous–to–amorphous surface roughening transition [→] and the amorphous–to–(mixed-phase-microcrystalline) roughening transition [a→(a+µc)] observed during Si:H growth have been studied under different PECVD conditions of hydrogen dilution ratio R=[H2]/[SiH4], rf plasma power P, and substrate temperature T. For Si:H growth on crystalline Si substrates under the different conditions, phase diagrams have been constructed by plotting the bulk film thicknesses at which these transitions occur as a function of R. The effects of the substrate (c-Si wafers versus amorphous Si:H films) on the a→(a+µc) transition of the phase diagram are also explored. The results provide deeper insights into recent attempts to improve the material properties and solar cell performance for a-Si:H i-layers prepared by rf PECVD at higher rates.

Thermal roughening of a thin film: A new type of roughening transition

The equilibrium thermal roughening of thin Ge layers (one and two monolayers) deposited on Si(001) has been investigated with low-energy electron microscopy. A Ge-coverage-dependent roughening is observed. For two monolayers, the temperature at which imaging contrast is lost due to surface roughness is 900+/-25 degrees C, between the roughening temperatures of Ge(001) and Si(001). Lower Ge coverages move this temperature closer to that of Si(001). The roughening is confined to the Ge overlayers. It is believed that this phenomenon represents a new type of surface roughening transition that should be generally applicable for heteroepitaxial films.

Lattice Monte Carlo models of thin film deposition

Monte Carlo models of crystal growth have contributed to the theoretical understanding of thin film deposition, and are now becoming available as tools to assist in device fabrication. Because they combine efficient computation and atomic-level detail, these models can be applied to a large number of crystallization phenomena. They have played a central role in the understanding of the surface roughening transition and its effect on crystal growth kinetics. In addition, columnar growth, vacancy and impurity trapping, and other growth phenomena that are closely related to atomic-level structure have been investigated by these simulations. In this chapter we review some of these applications and discuss MC modeling of sputter deposition based on materials parameters derived from first principles and molecular dynamics methods. We discuss models of deposition which include the atomic scale, but can also simulate film structure evolution on time scales of the order of hours. By the use of advanced computers and algorithms, we can now simulate systems large enough to exhibit clustered, columnar, and polycrystalline film structures. The event distribution is determined from molecular dynamics simulations, which can give diffusion rates, defect production, sputtering yields, and other information needed to match real materials. We discuss simulations of deposition into small vias and trenches, and their extension to the length scale of real devices through scaling relations.

Phase diagram of the restricted solid-on-solid model coupled to the Ising model

We study the phase transitions of a restricted solid-on-solid model coupled to an Ising model, which can be derived from the coupled XY-Ising model. There are two kinds of phase transition lines. One is a Ising transition line and the other is surface roughening transition line. The latter is a KT transition line from the viewpoint of the XY model. Using a microcanonical Monte Carlo technique, we obtain a very accurate two dimensional phase diagram. The two transition lines are separate in all the parameter space we study. This result is strong evidence that the fully frustrated XY model orders by two separate transitions and that roughening and reconstruction transitions of crystal surfaces occur separately.