Dislocations interfaciales et cohérence dans le système épitaxique fer-or

Iron was deposited in ultrahigh vacuum onto thin single-crystal films of gold oriented with (111) parallel to the surface plane. Study by electron diffraction and electron microscopy has shown that the initial growth of iron occurs by successive monolayers. Three stages in the deposit growth have been found: first the deposited film of iron is strained to match the gold substrate exactly; then interfacial dislocations appear at the gold-iron interface; and finally iron nuclei of b.c.c. structure are formed. The interfacial dislocations are in mixed orientation with their Burgers vectors $\text{1}\text{2}$a 〈110〉 inclined to the gold-iron interface; it is found that they have a poor efficiency for accommodating the misfit between the substrate and the deposit, so that for deposits of average thickness greater than 8 Å the interface is mostly incoherent.     

Equilibrium surface roughness of a strained epitaxial film due to surface diffusion induced by interface misfit dislocations

In recent years, developments in the microelectronics industry have led to extensive studies of the growth and characterization of thin solid films and their implementation in electronic and opto-electronic devices. A goal is to produce thin films with minimal bulk and surface defects. For those systems produced by epitaxial growth of a film on a substrate that has a slightly different lattice parameter, the stress associated with the elastic mismatch strain needed to satisfy the constraint of epitaxy provides a driving force for nucleation and growth of undesirable defects in the film material or on its surface. Among the most common defects are interface misfit dislocations, arranged more or less periodically on the film-substrate interface, which partially relax the elastic mismatch strain in the film. It has been observed that, for some material systems, surface roughness or waviness arises which correlates spatially with the positions of interface misfit dislocations. It is suggested here that the waviness along the surface may be a result of surface diffusion which is driven by a gradient in the chemical potential of the material along the surface. The chemical potential gradient arises from the nonuniform strain field of the interface misfit dislocations, as well as from the unrelaxed elastic mismatch strain. The focus here is on the development of a relatively simple model of this system which leads to an estimate of the magnitude and profile of surface waviness under conditions of thermodynamic equilibrium, i.e., after the material responds to the chemical potential gradient by seeking out a new configuration for which stresses are redistributed and the chemical potential is again uniform. The condition of uniform chemical potential for the final shape leads to an integro-differential equation for the equilibrium surface shape which is solved numerically. For representative values of system parameters, estimates of equilibrium surface roughness are obtained which can vary from less than one percent of film thickness to a significant fraction of film thickness. Although transient aspects of the process are not studied here, the characteristic time for achieving an equilibrium configuration is estimated.     

Use of misfit strain to remove dislocations from epitaxial thin films

Misfit strain can be used to drive threading dislocations out of epitaxial films and thus to improve their perfection. This process is influenced by film thickness, the orientation of the interface, the dimensions of the interface parallel to its plane, and the misfit between film and substrate. A simple theoretical model, and the experimental observations made on deposits of Ga(As, P) on GaAs, suggest that it is desirable for the film thickness to be small. This in turn implies that the misfit should be large. It should not, however, be large enough to cause dislocation nucleation. If the film is face-centered cubic, and the threading dislocations are uniformly distributed over the 〈110〉 {111} slip systems, then the most desirable interface orientations lie near {012} or {013}. If the Burgers vectors of the threading dislocations are not uniformly distributed then other interfaces may become desirable. Multilayers are able to remove threading dislocations more effectively than single films.     

Microstructure of Epitaxial La_(0.7)Ca_(0.3)MnO_3 Thin Films Deposited by Direct Current Magnetron Sputtering on LaAlO_3 Substrate

Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been used to study the microstructural properties of La0.7Ca0.3MnO3 films on (001) LaAlO3 substrates prepared by direct current magnetron sputtering technique.The as-grown thin films with different thickness are perfectly coherent with the substrates.The film suffers a tetragonal deformation in the area near the interface between the film and the substrate.With increasing thickness, the film is partially relaxed.It was found that La0.7Ca0.3MnO3 films consist of two types of oriented domains described as: (1) (110)f[001]f||(001)s[100]sand (110)f[001]f||(001)s[100]s and (2) (110)f[001]f||(001)s[010]s and (110)f[001]f//(001)s[010]s.Upon annealing, the film is relaxed by the formation of misfit dislocations.Other than misfit dislocations, two types of threading dislocations with Burgers vector of <100> and <110> were also identified.     

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100)_B3, (110)_B3, and (111)_B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100)_B3 substrates, Fe(100)_bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110)_bcc and Fe(111)_bcc single-crystal films are respectively obtained on GaAs(110)_B3 and GaAs(111)_B3 substrates at RT-300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0-9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc-Fe crystal.     

Generation of grown-in dislocations at NiCu misfit boundaries during three-dimensional nucleation and growth

The origin of a high density of grown-in dislocations in electrolytic nickel films grown epitaxially on bulk single-crystal Cu{001} and their relationship to misfit dislocations were investigated using transmission electron microscopy. Consistent with previous observations, two types of misfit dislocation were found to be present at the NiCu interface; the first type had the mixed Burgers vector of $\text{1}\text{2}\text{〈011〉}$ inclined at 45° to the interface and it probably originated from the deposit top surface and slipped into the interface. The second type, being most effective in accomodating an interfacial lattice misfit (about 2.5%), had the Burgers vector of $\text{1}\text{2}\text{〈110〉}$ which is parallel to the interface. Weak-beam imaging revealed that these two types of dislocation were segmented, actually forming a half-loop, suggesting that they must have been generated locally during three-dimensional nucleation and growth stages. Furthermore, these half-loop segments are replicated into the deposit side approximately perpendicular to the interface and eventually become a part of grown-in dislocations. The second type, the origin of which was not well understood previously, is shown to be generated during coalescence of three-dimensional epitaxial islands. The segmentation of misfit dislocations is found to be responsible for the production of the high density of grown-in dislocations observed in epitaxially grown nickel deposits on copper.     

Some features of the behavior of misfit dislocations during diffusion

The misfit between one film and another is often accommodated by misfit dislocations. If the crystals are miscible and are allowed to interdiffuse the misfit dislocations become distributed in the alloyed volume. Frequently, some parts of one dislocation move away from the interface into one crystal and other parts move into the other crystal. The parts in one crystal are connected to those in the other by dislocations that thread the diffusion zone. The density of these threading dislocations depends on the misfit between the two crystals, and may be influenced by the Kirkendall effect and by the misfit accommodated by elastic strain. Interaction between misfit dislocations in the diffusion zone often leads to the creation of new grains. These grains are unusual in that their lattices are curved to accommodate misfit between the upper and lower film surfaces. The boundaries to the grains are approximately perpendicular to the original interface and are made up of misfit dislocations that once occupied the material inside the curved grain. As dislocations and grain boundaries enhance diffusion the threading dislocations, and the boundaries to the curved grains, are expected to contribute to mixing of the films.     

Transition from a disordered to a crystalline state in II–VI and III–V films

The initial stages of HgCdTe growth on Al₂O₃, GaAs, CdTe, and KCl substrates have been studied by electron diffraction. HgCdTe films were produced by pulsed laser deposition and isothermal vapor phase epitaxy. InGaAs films were grown by isothermal chloride epitaxy on GaAs substrates. In the initial stages of the growth process, we observed a transition from an amorphous to a textured polycrystalline phase and then to a mosaic single-crystal structure. We have calculated the critical size of crystalline grains below which amorphization occurs in II-VI and III-V compounds. The critical grain size agrees with the grain size of the disordered (amorphous) phase that forms in the initial stage of epitaxy. We have determined some characteristics of the heterostructures: critical film thickness below which pseudomorphic growth is possible without misfit dislocation generation, elastic stress in the epitaxial system, surface density of dangling bonds at dislocations, and the critical island radius above which no interfacial misfit dislocations are generated.     

Boundary Tension at the Interface of Nanoscopic Helium Films on an Heterogeneous Substrate

This paper reports the first calculation of the two-dimensional interfacial profile and energetics of nanoscopically thin films of helium, on an heterogeneous planar substrate consisting of two adjoining metals. The calculations are performed in the frame of density functional theory at zero temperature, with the purpose of identifying the formation process of the interface at the boundary between the two substrates when few atomic layers are involved, to elucidate the possible relationship of the magnitude of the boundary tension with the displacement of layers between the half films, and to extract keys to organize future calculations of film coexistence at finite temperatures.     

Identification of atomic steps at AlSb/GaAs hetero-epitaxial interface using geometric phase method by high-resolution electron microscopy

Geometric phase analysis (GPA) is applied to determining the strain fields in AlSb/GaAs hetero-epitaxial film from high-resolution electron microscopy (HREM) images. The misfit dislocations along the hetero-interface are shown to be predominantly 90° Lomer dislocations. The epitaxial film is almost fully relaxed by a high density of misfit dislocations. The 90° Lomer dislocations are assumed to be formed by either recombination of two 60° mixed misfit dislocations through a glide and climb process or direct nucleation at the interface. The atomic steps (ASs) are visualized in the strain map, providing a new method for identifying the ASs at the interface.     

Initial epitaxial growth of (111) Au/(111) Cu and (111) Cu/(111) Au

The room temperature modes of growth of Au/(111) Cu and Cu/(111) Au are described. For the former growth mode initial deposits (2.4 Å) of gold on copper form smooth flat islands delineated by coincidence lattice misfit dislocations. For 6.0 Å of gold deposit, both thick and thin gold areas were observed with almost complete substrate coverage. For a 10 Å deposit, surface coverage was complete. Strain measurements and dislocation densities obtained on the (111) Au/(111) Cu films suggest the presence of two separate misfit dislocation networks at the interface. The coincidence lattice networks were large enough for transmission electron microscopy observation but contributed little to total overlayer strain. The (van der Merwe) natural lattice misfit dislocations were too closely spaced for direct observation but their presence was inferred because of the strain measurements. The initial epitaxy of Cu/(111) Au was similar to the Stranski-Krastanov model: the initial monolayer of copper (also delineated by coincidence misfit dislocations) grew smoothly on the gold; additional copper formed essentially stress-free “nuclei” on top of the initial copper layer.     

Dislocations in GaAs p-i-n diodes grown by hydride vapour phase epitaxy

Hydride vapour phase epitaxy grown all-epitaxial p-i-n structures were studied by synchrotron X-ray topography. Three types of process induced dislocations were found: short threading dislocations, long straight interfacial dislocations and circular arc dislocations. The majority of the dislocations observed are short straight threading dislocations, the density of which is typically about 5000 cm⁻². The dislocations at the p-i interface are long straight lines parallel to [110]. They are screw dislocations having their Burgers vector parallel to [110], calculated from the contrast analysis of the well resolved dislocation images. One sample also showed a dense misfit dislocation network at the n-side. However, no misfit dislocations were seen in the back-reflection topographs of the n-side of the other samples, which shows that it is possible to grow a misfit-dislocation free n-type GaAs layer onto the substrate side of a hydride vapour phase epitaxy grown GaAs surface after proper substrate removal.     

Anisotropy effect on strain-induced instability during growth of heteroepitaxial films

The use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations.     

Line defects, planar defects and voids in SrTiO3 films grown on MgO by pulsed laser and pulsed laser interval deposition

Two and three dimensional growth of SrTiO₃ films on (001) MgO substrate was achieved by pulsed laser interval and pulsed laser deposition respectively. The growth mode was monitored by in-situ reflection high energy electron diffraction. Interval deposition forces layer-by-layer growth of materials even with such a large lattice misfit (~ 7.9%). A titanium dioxide buffer monolayer was deposited to allow the film to wet the substrate to encourage two dimensional growth of the strontium titanate. A variety of defects was investigated using transmission electron microscopy and high resolution scanning transmission electron microscopy. Misfit dislocations, steps at the interface, Ti-rich defects and regularly shaped nano-holes connected by anti-phase boundaries were found to be the dominant defects in these films grown layer by layer. The edges of the nano-holes were mainly along [010] and [100] for a [001] growth direction. The large strain between the two crystal systems with large lattice mismatch leads to in-plane tensile stress during the layer-by-layer growth. The stress is relieved in part by the holes. The films with a three dimensional growth mode possess a uniform surface with dislocations as the dominant defects. The individual densities of the various defects, including a Ti-rich phase and misfit and threading dislocations, are determined by the kinetics of the deposition method.     

Dislocation wall formation during interdiffusion in thin bimetallic films

Transmission electron microscopy observations are described of the diffusion-induced behaviour of misfit dislocations originally present in the interface of thin bimetallic films. Experiments were carried out with specimens consisting of a layer of approximately 500 Å Cu vapour deposited onto an electropolished Ni substrate approximately 1000 Å thick. Diffusion anneals were performed in situ in the electron microscope at annealing temperatures in the range 450–600°C. The dislocation behaviour in Cu/Ni bicrystals with originally a (100) interface was photographed and video-recorded. A cross-grid of misfit dislocations parallel to 〈110〉 directions was present in the original interface. The Burgers vectors were of type $\text{1}\text{2}$a〈110〉 lying in the interface. During diffusion the misfit dislocations became distributed in the diffusion zone. When $\text{2Dt}$ (where D is the diffusion coefficient and t is the annealing time at a given temperature) exceeded a value of 40–50 Å, the dislocations started to align vertically forming dislocation walls along 〈110〉 directions parallel to the original interface. This resulted in a dislocation cell structure. Lengthwise the dislocation walls grew with shocks. The elastic strain energy of a finite edge dislocation array was estimated. Using this result an energy criterion for the formation of dislocation walls was derived. From this criterion it followed that dislocation wall formation may start to occur when $\text{2Dt}$≈ 45 Å, in good correspondence with the experimental results. Some additional observations of recrystallization phenomena during interdiffusion are reported.     

Misfit dislocations and stress in In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs heterostructures

We have estimated the elastic properties of In_{1 − x} Ga _x As/GaAs heterostructures and the characteristics of misfit dislocations in such heterostructures: misfit dislocation spacing, Burgers vector length in various interfaces, surface density of dangling bonds, film/substrate interface energy, critical film thickness below which pseudomorphic growth is possible without misfit dislocations, elastic strain energy of the film-substrate system, average elastic strain of a thin-film island as a function of its radius, thermal stresses induced by the thermal-expansion and lattice mismatches between the layers in contact, and crack length in the film.     

Molecular Dynamics Study on Interfacial Energy and Atomic Structure of Ag/Ni and Cu/Ni Heterophase System

The results of molecular dynamics calculations on the interfacial energies and atomic structures of Ag/Ni and Cu/Ni interfaces are presented. Calculation on Ag/Ni interfaces with low-index planes shows that those containing the (111) plane have the lowest energies, which is in agreement with the experiments. Comparing surface energy with interfacial energy, it is found the order of the interfacial energies of Ag/Ni and Cu/Ni containing the planes fall in the same order as solid-vapor surface energies of Ag, Cu and Ni. In this MD simulation, the relaxed atomic structure and dislocation network of (110)Ag||(110)Ni interface are coincident to HREM observations.