Heteroepitaxial Growth Of Thin Films Research Articles

Mechanism of stacking fault formation in metal(100) heteroepitaxial growth

Strain in metal(100) heteroepitaxial thin-film growth can lead to a variety of effects such as the formation of stacking faults (SFs). To gain a better understanding we have carried out temperature-accelerated dynamics (TAD) simulations of the submonolayer growth of Cu islands on a biaxially strained Cu substrate at 200 K. In the case of 4% compressive strain we find that SF formation occurs with a morphology very similar to the structures found experimentally in Cu/Ni(100) growth. We also find that islands play a key role by lowering the barrier for vacancy formation. In particular, once two substrate vacancies are formed and diffuse to form the appropriate configuration this leads to the formation of a SF in both the substrate and island. While the activation barrier for SF formation is very high, due to the presence of a large number of low-frequency vibrational modes, the saddle-point entropy is large while the corresponding Vineyard prefactor is more than 10 orders of magnitude larger than is typical of atomic processes in fcc metals. Similarly, an analysis of the entropy of the SF state indicates that the reverse prefactor is much smaller, while the free energy of this state is lower than that of the initial state.

High-Quality Heteroepitaxial Growth of Thin Films of the Perovskite Oxynitride CaTaO2N: Importance of Interfacial Symmetry Matching between Films and Substrates.

Perovskite oxynitrides have been studied with regard to their visible light-driven photocatalytic activity and novel electronic functionalities. The assessment of the intrinsic physical and/or electrochemical properties of oxynitrides requires the epitaxial growth of single-crystalline films. However, the heteroepitaxy of perovskite oxynitrides has not yet matured compared to the progress realized in work with perovskite oxides. Herein, we report the heteroepitaxial growth of CaTaO2N thin films with (100)pc, (110)pc, and (111)pc crystallographic surface orientations (where the subscript pc denotes a pseudocubic cell) on SrTiO3 substrates using reactive radio frequency magnetron sputtering, along with investigations of crystallinity and surface morphology. Irrespective of surface orientation, stoichiometric CaTaO2N epitaxial thin films were grown coherently on SrTiO3 substrates and showed clear step and terrace surfaces in the case of low values of film thickness of approximately 20 nm. A (110)pc-oriented film was also more highly crystalline than (100)pc- and (111)pc-oriented specimens. This relationship between crystallinity and surface orientation is ascribed to the number of inequivalent in-plane rotational domains, which stems from the symmetry mismatch between the orthorhombic CaTaO2N and cubic SrTiO3. A CaTaO2N thin film grown on a lattice- and symmetry-matched orthorhombic DyScO3 substrate exhibited a significant crystallinity and a clear step and terrace surface even though the film was thick (∼190 nm). These results are expected to assist in developing the heteroepitaxial growth of high-quality perovskite oxynitride thin films.

Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene.

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

Stress generation and evolution in oxide heteroepitaxy

Many physical properties of oxides can be changed by inducing lattice distortions in the crystal through heteroepitaxial growth of thin films. The average lattice strain can often be tuned by changing the film thickness or using suitable buffer layers between film and substrate. The exploitation of the full potential of strain engineering for sample or device fabrication rests on the understanding of the fundamental mechanisms of stress generation and evolution. For this study an optical measurement of the substrate curvature is used to monitor in situ how the stress builds up and relaxes during the growth of oxide thin films by pulsed laser deposition. The relaxation behavior is correlated with the growth mode, which is monitored simultaneously with reflection high-energy electron diffraction. The stress relaxation data is fitted and compared with theoretical models for stress evolution which were established for semiconductor epitaxy. The initial stage of the growth appears to be governed by surface stress and surface energy effects, while the subsequent stress relaxation is found to be fundamentally different between films grown on single-crystal substrates and on buffer layers. The first case can be rationalized with established theoretical models, but these models fail in the attempt to describe the growth on buffer layers. This is most probably due to the larger average density of crystalline defects in the buffer layers, which leads to a two-step stress relaxation mechanism, driven first by the nucleation and later by the migration of dislocation lines.

Uniformity of strained islands in heteroepitaxial thin film growth with patterned substrates: A fast kinetic Monte Carlo study

We study heteroepitaxial thin films grown on patterned substrates using a two-dimensional ball and spring model. Island size and its uniformity as a function of patterned substrate parameters and growth conditions are investigated in an early stage of growth using fast kinetic Monte Carlo simulations. In contrast to the flat substrate, patterned substrates provide preferred sites for island formation at the beginning of growth and promote island size uniformity. A patterned substrate parameter enables us to modify island size in a considerable range. Island size uniformity is further enhanced by stress suppression at the island edges when lattice mismatch is increased. Island size, however, decreases as the growth temperature is increased due to the constraint of periodicity of patterned substrates imposed. An upper bound of a patterned substrate parameter before island size becomes less uniform seems to closely connect to island period and island edge spacing of the flat substrate.

Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain.

Study of layered complex oxides emerge as one of leading topics in fundamental materials science because of the strong interplay among intrinsic charge, spin, orbital, and lattice. As a fundamental basis of heteroepitaxial thin film growth, interfacial strain can be used to design materials that exhibit new phenomena beyond their conventional forms. Here, we report a strain-driven self-assembly of bismuth-based supercell (SC) with a two-dimensional (2D) layered structure. With combined experimental analysis and first-principles calculations, we investigated the full SC structure and elucidated the fundamental growth mechanism achieved by the strain-enabled self-assembled atomic layer stacking. The unique SC structure exhibits room-temperature ferroelectricity, enhanced magnetic responses, and a distinct optical bandgap from the conventional double perovskite structure. This study reveals the important role of interfacial strain modulation and atomic rearrangement in self-assembling a layered singe-phase multiferroic thin film, which opens up a promising avenue in the search for and design of novel 2D layered complex oxides with enormous promise.

Enhancement of Surface Diffusivity for Waviness Evolution on Heteroepitaxial Thin Films

The present study deals with a numerical analysis on the island growth of heteroepitaxial thin-films through local surface diffusivity enhancement. A non-linear governing equation for the surface waviness evolution in lattice-mismatched material systems is developed for the case of spatially-varying surface diffusivity. Results show that a flat film that is stable under constant diffusivity conditions evolves to form nanostructures upon externally-induced spatial diffusivity modulation. The periodicity of waviness can be controlled by changing the modulation parameters, which allows for generation of pattern arrays. The present study therefore points towards a post-deposition treatment technique that achieves controllability and order in the structure formation process for applications in nanoelectronics and thin-film devices.

A DFT study on the effect of surface termination in CdTe (111)/α-Al2O3 (0001) heteroepitaxy

The heteroepitaxial growth of thin films on complex oxide substrates is highly dependent on chemical interactions in the interfacial material layers. We use first-principle density functional theory calculations to investigate the effect of alterations in the substrate termination in CdTe heteroepitaxy on c-plane sapphire, i. e., α-Al2O3 (0001). Comparing to previous experimental results, we rationalize the observations that an artificial alteration of the surface termination, through the addition of aluminum on the substrate surface, will affect rotational domain alignment. Our findings show that Al adatoms will occupy sites on the sapphire surface that otherwise would be beneficial for the binding of CdTe dimers, effectively altering the orientation of dimers during the initial growth process.

Submonolayer growth study using a solid-on-solid model for 2 × 1 reconstructed surfaces of diamond-like lattices

We have developed a simple cubic solid-on-solid (SOS) model of growth that captures many of the features of 2×1 reconstructed surfaces. Energetics in the model is based on bond-counting and the tendency to form dimer rows is controlled by a reconstruction energy parameter δ. We examine the dependence of the shapes and sizes of islands formed at submonolayer coverage on the parameter δ. The aspect ratio distribution of islands indicates a transition from two-dimensional to one-dimensional growth as δ is increased. At coverages below 0.2ML, the islands are mostly made of dimer chains with aspect ratios as large as 14. On annealing the system, the islands coalesce into larger islands with a much smaller aspect ratio of approximately 3. Through extensive simulations, we have found a set of parameters that reproduces the known experimental results on submonolayer growth on Si(001) in Molecular Beam Epitaxy(MBE). These results include island size and shape distribution and their dependence on temperature. This model can be used to study heteroepitaxial thin film growth on reconstructed surfaces.

Kinetic Monte Carlo simulations of heteroepitaxial growth with an atomistic model of elasticity

We have implemented Kinetic Monte Carlo (KMC) simulations of growth of heteroepitaxial thin films. A simple cubic Solid-on-Solid (SOS) model is used to describe the atomic configurations and nearest neighbor bonds are used to describe the energetics. Elastic effects are modeled using harmonic springs between atoms displaced from their lattice positions. The misfit strain is a consequence of different equilibrium spring lengths for the substrate and film. The consistency of this elastic model with continuum theories for strained surfaces has been shown by performing elastic energy calculations for various morphologies. KMC simulations for submonolayer deposition show scaling behavior in the island size distribution. The resulting island shapes are predominantly square and do not show any shape transitions in the physically relevant range of conditions. This method gives a detailed understanding of elastic interactions and their interplay with surface diffusion in heteroepitaxial systems.

Computer simulation of quantum dot formation during heteroepitaxial growth of thin films

Influence of mismatch on quantum dot formation during heteroepitaxial growth of thin films with zinc blende structure on GaAs substrate is investigated. A kinetic Monte Carlo model is used for simulation of thin films with different values of lattice mismatch in the range of 4% to 14%. Simulation is performed at a substrate temperature of 700 K and deposition rate of 0.3 ML/s. Also, ‘reflection high-energy electron diffraction’ (RHEED) intensity of simulated thin films is evaluated during growth. Results of simulation show that at constant temperature and deposition rate, quantum dot size decreases with increasing lattice mismatch, and their distribution function becomes sharp. Simulated RHEED oscillations have a good agreement with morphology of simulated thin films, and results show that growth mode changed from Stranski-Krastanov to Volmer Webber by increasing lattice mismatch.

Directed assembly in epitaxial zinc oxide films on focused ion beam modified sapphire substrates

A new method for directed self-assembly using focused ion beam (FIB) and physical vapor deposition is presented. The high resolution and site-specific patterning capabilities of FIB are coupled with the self-assembly process in heteroepitaxial thin film growth. An efficient FIB-induced damage mechanism is exploited to pattern amorphous regions in sapphire substrates which direct the subsequent assembly of a sputter-deposited zinc oxide film. This novel approach allows for the fabrication of in-plane nano- to microscale heterostructures comprising epitaxial regions with high strain and defect density that are separated by regions of randomly oriented (in-plane) grains with much lower strain and defect density.

Localized dry-etch substrate thinning for dislocation reduction in heteroepitaxial CdTe/Si(211)

Threading dislocations are a significant problem for heteroepitaxial growth of thin films on large lattice-mismatched substrates. In the case of HgCdTe thin films on Si, Ge, or GaAs, the molecular beam epitaxy (MBE) of 10–15-μm-thick CdTe buffer layers has historically played a crucial role in reducing threading dislocation densities to current state-of-the-art levels. In this work, the authors investigate a localized substrate thinning approach and its overall effect on further reducing dislocation densities in the CdTe/Si heteroepitaxial system. In using substrates with regions thinned to thicknesses on the order of the CdTe buffer, the attempt is to reduce the dislocation image force acting from the interface toward the epilayer surface. The authors employ both wet- and dry-etching techniques to create locally back-thinned regions of Si(211) wafers. Localized rather than whole wafer thinning was necessary to maintain sufficient substrate thickness for handling. The opposite sides of the wafers were cleaned using standard techniques prior to CdTe MBE. Scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure epilayer and substrate thicknesses. Using CdTe defect-decoration techniques, a decrease in threading dislocation density by up to 60% has been observed in regions for which the underlying Si substrate was thinned to 2 μm. Results obtained for wet-etch and dry-etch back-thinning approaches suggest that the dislocation-reduction mechanism is not solely based on substrate-thickness induced image forces.

GdN (1 1 1) heteroepitaxy on GaN (0 0 0 1) by N 2 plasma and NH 3 molecular beam epitaxy

We report on the heteroepitaxial growth of thin films of rocksalt GdN on c-plane (0 0 0 1) wurtzite GaN by molecular beam epitaxy (MBE) using either an N 2 plasma or NH 3 as the nitrogen source. In both cases, epitaxial films with fully oriented GdN (1 1 1)∥GaN (0 0 0 1) were deposited as demonstrated by θ–2 θ X-ray diffraction. φ scans of GdN peaks demonstrate 6-fold symmetry along the growth axis implying the presence of two 3-fold-symmetric GdN (1 1 1) crystal variants in-plane. Electrical transport and magnetometry measurements on films grown using N 2 plasma show that these GdN films are ferromagnetic below T C=70 K and degenerately doped or metallic from 10 to 300 K with magnetotransport signatures associated with T C.

Spatial ordering and anisotropy in surface stress domains and nanostructural evolution

The role of elastic interaction has been well recognized for various growth modes and nanostructural self-organization on solid surfaces. Examples include spontaneous formation of stress domains in reconstructed surfaces or in-phase-separating monolayers, meandering and bunching of surface steps, and quantum dot formation in heteroepitaxial thin film growth. The long-range spatial ordering in these strain-mediated nanostructural evolution phenomena on solid surfaces is critically dependent on the anisotropy. A universal formulation has been developed to examine the energetically favored orientation with respect to parameters that represent the anisotropy in elasticity and applied stress.

207 Kinetic Monte Carlo法によるヘテロエピタキシャル薄膜成長のモデリング(OS15.電子・原子・マルチシミュレーションに基づく材料特性評価(2),オーガナイズドセッション)

207 Kinetic Monte Carlo法によるヘテロエピタキシャル薄膜成長のモデリング(OS15.電子・原子・マルチシミュレーションに基づく材料特性評価(2),オーガナイズドセッション)

Formation of surface structures during heteroepitaxial thin film growth on prepatterned substrates

Guided quantum dot self-assembly grown heteroepitaxially on a substrate surface prepatterned with cosinelike humps is studied by computer simulation. The $\mathrm{Si}\mathrm{Ge}∕\mathrm{Si}$ material system is used as a model system, in which the strain energy, surface energy, wetting effect, surface anisotropy, and elastic anisotropy are taken into account. The simulation results reveal the formation of surface structures which may potentially be of practical applications.

General Guidelines for the Heteroepitaxial Thin Film Growth Established by Combinatorial X-ray Diffraction

Heteroepitaxial thin films are widely used in a broad range of modern electronic devices, for example as the high-dielectric-constant (high-k ) gates in field effect transistors. In order to improve the crystal quality of epitaxial layers, researchers develop intricate and costly deposition processes. An alternative paradigm should involve deposition of the complex compounds that could exhibit unusual behavior and yield the high quality films by simple and industry-friendly processes. Successful search for the new complex compounds must employ combinatorial methods. In this publication, we demonstrate fabrication of composition spread materials libraries by Combi-PLD and their high-throughput characterization by two different XRD techniques. Generally, the composition spreads have polycrystalline, textured, and epitaxial areas of varied quality. We use statistical analysis to establish general guidelines for the epitaxial growth. Using the NiO-Co3O4-Mn2O3 system on SrTiO3 as an example, we demonstrate that for the chemically inert interfaces lattice mismatch is the most important factor of the commensurate growth. For example, the rocksalt-perovskite interface requires the optimal lattice mismatch of ∼6.5%. Examination of the Y2O3-HfO2-Al2O3 system on Si(100) revealed a narrow epitaxial area comprised of a new metastable crystal phase. Thus, stable epitaxial interface is attainable even for the chemically reactive boundaries. In addition to lattice mismatch, the high-quality heteroepitaxial film requires balancing the reaction with the outdiffusion of the reaction products away from the interface.

Surface evolution of elastically stressed films under deposition by a diffuse interface model

We consider the heteroepitaxial growth of thin films by numerical simulations within a diffuse interface model. The model is applicable to describe the self-organization of nanostructures. The influence of strain, surface energies and kinetics on the surface evolution is considered. A matched asymptotic analysis shows the formal convergence of an anisotropic viscous Cahn–Hilliard model to a general surface evolution equation. The system is solved by adaptive finite elements in three dimensions and in special cases compared with sharp interface models.

Bending of nanoscale ultrathin substrates by growth of strained thin films and islands

Mechanical bending is ubiquitous in heteroepitaxial growth of thin films where the strained growing film applies effectively an ``external'' stress to bend the substrate. Conventionally, when the deposited film is much thinner than the substrate, the bending increases linearly with increasing film thickness following the classical Stoney formula. Here we analyze the bending of ultrathin (nanometer range) substrates induced by growth of coherently strained thin films. The behavior deviates dramatically from the classical linear dependence: when the film thickness becomes comparable to the substrate thickness the bending decreases with increasing film thickness. This complex bending behavior can be understood by considering evolution of strain sharing between the film and substrate. We demonstrate experimentally such counterintuitive bending of a nanoscale thin Si substrate induced by a coherently strained Ge film, in the form of islands, grown on silicon-on-insulator substrate. Larger dome islands, representing a thicker film, induce much less bending of the substrate than smaller hut islands, representing a thinner film, in direct contrast to their behavior on thick Si. We explain these observations by properly considering the island shape and strain relaxation within the island.