Grain Growth In Thin Films Research Articles

Effect of boundary grooving on grain growth by Potts model simulations

Grain growth in thin films is a technologically important and scientifically fascinating topic: As the grain size influences many mechanical and optical properties of a material, any change in the grain structure and therewith in the grain size changes also the materials properties. There are many factors influencing coarsening of the microstructure such as grain boundary properties or the presence of particles; additionally the film thickness becomes rather important. It has been observed that when the grain size reaches the magnitude of the layer thickness, coarsening slows down and may even stagnate. For thin films, surface effects such as grain boundary grooving become important. In general, grooves are totally left out of implementations in the standard Potts model. In the current investigation, we introduce grain boundary grooves in the Potts model as physical entities. In the analysis, we show how the mere presence of such grooves reduces the driving force of grain boundary migration at the surface tremendously at any stage of coarsening.

Magnetically Enhanced Thin‐Film Coarsening by a Magnetic XPFC Model Allowing to Decouple Magnetic Anisotropy and Magnetostriction

External magnetic fields provide a macroscopic control mechanism to influence the microstructure of polycrystalline materials. Herein, the influence of strong magnetic fields on grain growth in thin films is modeled with a magnetic extended phase field crystal model. The magneto–structural effects are incorporated into the correlation function in reciprocal space. With this approach, magnetic anisotropy, magnetostriction, and mobility of grain boundary can be controlled and a variety of geometrical and topological properties consistent with experimental results can be determined.

Grain growth in thin film under strong temperature gradients

Grain growth in thin films has attracted lots of attention due to the numerous applications of polycrystalline films, on one hand, and modified properties due to changes in the microstructure caused by thermal effects, on the other hand. While the phenomenon of grain growth is well understood in general, effects of e.g. temperature gradients leave questions open still today. In the present study, we investigate the influence of strong temperature gradients on grain growth in thin films. To that aim, a modified three-dimensional Potts model algorithm is employed, where the annealing temperature depends on the position within the sample leading to spatial heterogeneities in the grain boundary network. As a consequence of different mobilities, a drag effect occurs on the boundary network evolution that has serious consequences for the microstructures evolving during grain growth.

Impact of a strong temperature gradient on grain growth in films

The migration of grain boundaries and, therewith, the phenomenon of grain growth depend strongly on the annealing temperature. Generally, higher temperatures are associated with higher mobilities of the boundaries and therewith faster microstructural coarsening. In the present study, the influence of a strong temperature gradient on grain growth in thin films is investigated. To that aim, a modified three-dimensional Potts model algorithm is employed, where the annealing temperature changes with the thickness of the sample taking grain boundary mobility and energy into account. The resulting drag effect has serious consequences for the temporal and spatial evolution of the grain microstructure.

Topological evolution of thin films during grain growth

Grain growth in thin films is a complex three-dimensional problem. It is shown by Potts model simulation how a bulk-like microstructure transforms into a columnar structure with a focus on the topology of the microstructure. Classical coarsening laws such as the Aboav-Weaire-law and the von Neumann-Mullins-law are fulfilled for the different stages of coarsening. Nevertheless, two types of grains are found in the film—namely columnar grains and non-columnar, spherical grains—that show clearly different behaviors. The results confirm the importance of modeling thin films in three dimensions.

Triple junction controlled grain growth in thin films

In the present study triple junction controlled grain growth in thin films is compared to the case of normal (grain boundary controlled) grain growth. To that aim, a modified three-dimensional Potts model algorithm is employed and the results are compared using previously developed mean-field theories, for which the increased importance of the triple junctions at small grain sizes is taken into account modeling grain growth in nanocrystalline materials. In particular, the influence of the transition from bulk-like growth to columnar microstructures on the two different kinetics is analyzed in detail for the first time.

Grain growth stagnation in thin films due to shear-coupled grain boundary migration

Normal grain growth in thin films attached to a substrate has been considered. It has been shown that shear-coupled grain boundary migration results in the build-up of elastic stresses in the film. A semi-quantitative model of grain growth combining the elements of disclinations theory with the Burke-Turnbull model of normal grain growth has been proposed. It has been shown that shear-coupled grain boundary migration may lead to stagnation of normal grain growth, whereas for high coupling factor values, the grains in the film do not grow at all. Finally, the mechanisms of stress relaxation enabling some grain growth are discussed.

Grain Growth Stagnation in Thin Films Due to Shear-Coupled Grain Boundary Migration

We have considered normal grain growth in thin films attached to a substrate and shown that shear-coupled grain boundary migration results in the build-up of elastic stresses in the film. We proposed a semi-quantitative model of grain growth, combining the elements of disclinations theory with the Burke-Turnbull model of normal grain growth. We have shown that shear-coupled grain boundary migration may lead to stagnation of normal grain growth, whereas for high coupling factor values, the grains in the film do not grow at all. Finally, we discussed the possible mechanisms of stress relaxation and resulting activation energies of grain growth.

Texture Controlled Grain Growth in Thin Films Studied by 3D Potts Model

AbstractThin films have numerous applications in optics, mechanics, and electricity, where the grain microstructure and the corresponding crystallography are key factors influencing materials properties as well as coarsening kinetics. However, even today this problem is often tackled by 2D means of treating thin films as quasi‐2D objects. In the present work, it is shown by modified 3D Potts model simulations that not only is the third dimension non‐negligible, but also how texture changes the growth kinetics, when the percentage of low angle grain boundaries varies systematically.

Critical assessment 30: Grain growth – Unresolved issues

ABSTRACTGrain growth is a broad field of study that covers several scientifically and technically interesting characteristics. Normal grain growth kinetics and its associated parabolic growth law and self-similar size distributions, grain boundary pinning, abnormal grain growth, the effect of triple junctions and quadruple points as well as grain growth in thin films are the most important current topics of grain growth research. In this critical assessment of grain growth in metals, we present and discuss some of the most important unresolved issues that have been a subject of research for many years.

Treating grain growth in thin films in three dimensions: A simulation study

Historically, metallography has been the two-dimensional characterization of materials microstructures by optical microscopy. Consequential problems have long been known: A two-dimensional section through a three-dimensional object gives us only a very poor idea about size and form of the object. Therefore, many attempts have been made to gain three-dimensional information experimentally. Nevertheless, in simulations and also analytical theories thin films are commonly still treated as two-dimensional objects making comparisons with 3D experimental data rather hard. In the present paper, grain growth in thin films is investigated for the first time by three-dimensional Potts model simulations focusing particularly on the transition from bulk-like growth to columnar microstructures.

C-Axis oriented SrMoO4 thin films by chemical solution deposition: Self-assembled orientation, grain growth and photoluminescence properties

In this study, SrMoO4 thin films have been prepared by chemical solution deposition using rapid thermal annealing. The results show that the thus-derived thin films are c-axis oriented, irrespective of the annealing temperature or the substrate orientation, indicating the self-assembled nature of the c-axis orientation, which is attributed to the combined effects of rapid thermal annealing, the lowest surface energy of the (001) plane, and the large anisotropic thermal expansion coefficient between the ab-plane and along the c-axis orientation in SrMoO4. Abnormal grain growth in thin films annealed under high temperatures or on substrates with large lattice/thermal expansion coefficient mismatch is observed and can be attributed to “strain-induced secondary recrystallization” or “strain annealing”. Bimodal-like rocking curves that are observed in some of the present thin films are attributed to the different interface structures, as confirmed by transmission electron microscopy observations. The photoluminescence properties show different characteristics due to the variations in the degree of orientation, the microstructure and the strain. The results will provide an instructive route to optimizing the properties of scheelite-structured molybdate thin films.

Phase field modeling of grain growth in thin films on rigid substrates

AbstractA phase field model of grain growth in thin films on rigid substrates is presented, in which the motion of grain boundaries is driven by curvatures as well as anisotropy in surface, interface, and strain energies. The match between the proposed model and the corresponding sharp interface model is demonstrated by asymptotic analysis. The effect of diffuse interface on grain boundary mobility is minimized. The parameters in the model are connected to the measurable properties of materials, and therefore can be obtained from experiments or atomic scale simulations. In numerical simulations, the influence of the texture dependent surface, interface, and strain energies on grain growth is illustrated. The effects of film thickness, strain level, and grain boundary grooving on grain size distribution and texture evolution are also investigated. The effectiveness as well as the limits of the present model are discussed, and the ways to extend it are proposed.

Abnormal grain growth in thin films not caused by decreased energy of their free surface

We modeled the grain growth process in thin films on substrate. The only driving force for grain growth was a decrease in the total grain boundary energy; possible difference in the energies of the free surface of neighboring grains was not taken into account. The main features of capillarity-driven microstructure evolution qualitatively agree with those observed experimentally. This leads to the conclusion that abnormal grain growth in thin films can develop without assistance of decreased energy of the free film surface.

Neutrons and Photons in Materials Research for Thin Film Solar Cells

AbstractThe understanding of the interplay between structural and electronic properties of photovoltaic materials as well as a deeper insight into growth pathways and phase formation kinetics of the absorber layer in a thin film solar cell give a crucial contribution to the continuous improvement of the solar cell efficiency. Among the various experimental methods used for the investigation of the structure and microstructure of photovoltaic materials, neutron, and X‐ray (photon) scattering are key techniques of choice. Both techniques are complementary, which is demonstrated in the present paper. Neutron powder diffraction is used to detect different kinds of intrinsic point defects in chalcopyrite type and kesterite type semiconductors. The calculated defect concentrations may lead to the expectation of a clustering of anti‐site defects and vacancy to the electrical inactive defect pairs (2 VCu + InCu) and (InCu + CuIn). By the means of energy dispersive X‐ray diffraction (EDXRD) phase formations and grain growth in thin films are studied in real time. The potential of EDXRD for in situ studies of reactions during the formation of chalcopyrite thin films is demonstrated.

A Phase Field Model of Surface-Energy-Driven Abnormal Grain Growth in Thin Films

A phase field model is established to investigate the surface-energy-driven abnormal grain growth in thin films. It is consistent with sharp interface model and its parameters are connected to material properties. Numerical simulations show that surface energy anisotropy and drag effect are required to motivate the abnormal grain growth. The size of a single abnormal grain increases linearly as a function of time, and it exhibits power-law scaling with film thickness and Arrhenius relationship with temperature. For multiple abnormal grains, their area fraction can be characterized by the Avrami equation with exponent around 2 at large times. These features agree well with the theoretical and experimental results. [doi:10.2320/matertrans.M2011227]

Grain growth in thin films with a fibre texture studied by phase-field simulations and mean field modelling

The evolution of fibre textured structures is simulated in two dimensions using a generalised phase field model assuming two forms for the misorientation dependence of the grain boundary energy. In each case, a steady-state regime is reached after a finite amount of grain growth, where the number and length weighted misorientation distribution functions (MDF) are constant in time, and the mean grain area A as a function of time t follows a power growth law A − A 0 = kt n with n close to 1 and A 0 the initial mean grain area. The final shape of the MDF and value of the prefactor k in the power growth law clearly correlate with the misorientation dependence of the grain boundary energy. Furthermore, a mean field approach is worked out to predict the growth exponent for systems with non-uniform grain boundary energy. The conclusions from the mean field approach are consistent with the simulation results. In previous studies on grain growth in anisotropic fibre textured systems, this steady-state regime was often not reached, which resulted in wrong conclusions on the growth exponent n and evolution of the MDF.

Microstructure and texture developments in multiple pulses excimer laser crystallization of GaAs thin films

In this paper, we observed and characterized changes in the microstructure and texture during recrystallization and grain growth in polycrystalline GaAs thin films using multiple pulses crystallization by a KrF excimer laser. Films of various thicknesses were studied to assess film thickness and laser energy density effects. In the low temperature domain corresponding to the partial melting regime, normal grain growth was observed. In the superlateral grain growth regime the increase in grain size was notable with grain sizes much greater than the film thickness. A bimodal grain size distribution emerged implying the onset of secondary grain growth. The change in grain size distribution, texture, and grain boundary texture were analyzed using scanning electron microscopy and electron backscatter diffraction. It was found that grain growth is accompanied by a strengthening in {001} texture, indicating that the grain growth phenomenon is strain energy driven. The experimental results are explained with theory of secondary grain growth in thin films.

A high-throughput approach for cross-sectional transmission electron microscopy sample preparation of thin films

Cross-sectional transmission electron microscopy (XTEM) is a very useful technique to study the interfacial diffusion and reactions and the grain growth of thin films. However, the preparation of XTEM samples of thin films is tedious and challenging. Difficulties may include the delamination of films from the substrate, fracture of brittle substrates and differential milling rates of the substrate and the film. This paper describes an improved technique using a combination of tripod polishing and focused ion beam milling to prepare XTEM samples of thin films. The technique can be widely used for high-throughput production of samples having varying film and substrate properties. Two different geometries are introduced. The first one is suitable for XTEM sample preparation of most films at a high yield rate, but with a limited view area. The other geometry is able to give a larger view area and is more suitable for thicker films. The technique is illustrated by an example of the sample preparation of Fe/Pt multilayer films on SiO2/Si substrates.

Effect of a Ta-Si-N diffusion barrier on the texture formation in thin Cu films

Analyzing pole figures obtained by x-ray diffraction experiments, quantitative texture analyses are carried out on 50 nm thick Cu films sputter-deposited on oxidized Si substrates and on Ta-Si-N diffusion barrier films of various compositions. To explain the observed Cu texture formation during layer deposition, a model of two-dimensional grain growth in thin films is applied. In the case of silicon oxide substrates, a ⟨111⟩ Cu fiber texture component resulting from the minimization of the surface and interface energy is observed, in addition to a ⟨100⟩ component resulting from the minimization of the strain energy. For Cu films deposited onto Ta-Si-N diffusion barriers, preferred growth occurs only for ⟨111⟩-oriented Cu grains and their twins. The volume fractions of both components decrease with increasing N content of the diffusion barrier. Annealing of the Cu films at Tan=600 °C results in a strengthening and a sharpening of the ⟨111⟩ component. Hence, both the chemical composition of the underlayer as well as a postdeposition anneal are observed to have a significant impact on the texture of thin Cu films.