Effects Of Coherency Strain Research Articles

On the stability of thin epitaxial films

The deposition of thin epitaxial layers from the vapour phase is a subject of broad and current interest. The authors have recently become aware of a particularly simple epitaxial system, (copper deposited on monocrystalline silver), for which the misfit is large, and where the two f.c.c. metals are essentially immiscible at the temperature of deposition. It is the purpose of this contribution to estimate the strain energy densities involved in the coherent deposition of copper on silver, and to evaluate the effect of coherency strains on the vapour pressure of the deposited material. The structure of copper overlays evaporated at room temperature and a pressure of about 10/sup -7/ Pa on the (111) plane of silver was investigated recently by Tyliszczak, De Crescenzi and Hitchcock. Their results indicate that layer-by-layer epitaxial growth occurs during the deposition of the first few monolayers. The copper atomic spacing is commensurate with the silver substrate. During later deposition the silver surface is covered with copper islands. At later states, the copper spacing corresponds to that of bulk copper. The lattice parameter of copper is about 0.361 nm while that of silver is 0.408 nm. Clearly, the first few layers of epitaxial copper are severelymore » strained and Hitchcock, in private communication, notes that these initial layers tend to re-evaporate readily. The authors examine a simple thermodynamic model to predict the increase in equilibrium vapour pressure arising from the epitaxial strain. The extra energy associated with this strain is calculated using linear elastic theory, with its accompanying simplifications. The calculation indicates that epitaxial growth on (100) planes would minimize this strain energy, in contrast to deposition on (111), which maximizes it.« less

Effects of coherency strain on the band gap of pseudomorphic InxGa1−xAs on (001) InP

The effects of lattice mismatch induced coherency strain on the direct (Γ) band gap of InxGa1−xAs alloys for commensurate growth on (001) InP substrates are presented. It is found that the low-temperature (T≊2 K) band gap of the strained ternary varies between 1.0 and 0.48 eV for 0≤x≤1.0. The band-gap difference ΔEg between the strained ternary and cubic InP varies therefore between 0.42 and 0.94 eV. The partitioning of ΔEg between conduction and valence bands has yet to be determined for nonlattice-matched compositions (i.e., x≠0.53). In view of the large calculated variation in ΔEg it is anticipated that coherency strain may provide a means for tailoring band discontinuities in this technologically important materials system. Potential applications are briefly reviewed.

Physics and applications of GexSi1-x/Si strained-layer heterostructures

This paper reviews recent advances in our current level of understanding of the physics underlying transport and optical properties of Ge x Si 1-x /Si strained-layer heterostructures. Included are discussions of critical (maximum) layer thicknesses, effects of coherency strain on the bandgaps of both Si and Ge x Si 1-x and the influence of layer strains on the band alignments of Ge x Si 1-x /Si strained-layer heterostructures. Transport studies will center on the modulation doping results of both n and p type heterostructures. Indeed, these earlier transport studies provided essential information which led to an understanding of the band-alignment in these strained layer heterostructures. Recent measurements of the indirect bandgap of Ge x Si 1-x strained layers on

Effects of capillarity and coherency on δ' (Al 3Li) precipitation in dilute AlLi alloys at low undercoolings

The influence of capillarity and coherency on the δ' (Al 3Li) solvus line in dilute AlLi alloys are described. Capillarity effects are small but detectable in very dilute alloys (~ 8 at.% Li) at temperatures near the solvus line. The effect of coherency strain on the position of the solvus line is calculated to be extremely small and was not measurable experimentally. However, evidence for coherency effects on δ' stability are observed when particle sizes are very small. The heterogeneous precipitate distributions observed on aging a 7.9 at.% Li alloy at small undercoolings are interpreted in terms of a combination of excess vacancy effects on particle growth at low temperature and capillarity effects on particle stability at elevated temperatures. This interpretation contrasts with earlier reports of a coherent δ' solvus in dilute AlLi alloys.

Thermodynamics of the massive transformation

The thermodynamic nature of the massive transformation may be revealed by finding the limiting conditions for the massive mode of growth. A review of the experimental information available indicates strongly that the solvus line rather than theT0 line is the natural limit. Some observations of massive transformation inside the two-phase field may be explained by the effect of coherency strains in the composition spike formed in front of the interface. This spike has a significant thickness in many cases and can explain the role of the solvus. It is exceedingly thin in iron-base alloys, and the role of the solvus may there be explained by higher diffusivity inside the interface. A comparison is made with the transformation of γ in Fe-M-C alloys where the situation is very similar.

The influence of coherency strain on the elevated temperature tensile behavior of Ni-15Cr-AI-Ti-Mo alloys

The effect of coherency strain on elevated temperature tensile strength was examined in a model, two-phase y’-strengthened Ni-15Cr-Al-Ti-Mo alloy series. The temperature dependence of coherency strain as represented by the γ-γ’ mismatch was determined over the temperature range 25 to 800 °C. The flow stress incrementAσγ, due to precipitation of γ’, was found to correlate well to the magnitude of the γ-γ’ mismatch over the same temperature interval. The correlation was strongest for high misfit alloys regardless of the Antiphase Boundary Energy (APBE). The predominance of by-pass type dislocation-particle interactions in high coherency alloys confirms that strengthening is primarily due to coherency strains. Conversely, alloys with low misfit exhibit two distinct particle shear mechanisms believed to be dependent upon the relative APBE and matrix stacking fault energy of the alloy.

Effect of coherency strain and misfit dislocations on the mode of growth of thin films

The mode of growth of epitaxial deposits can usually be predicted from Young's equation. However, there are some observations of three-dimensional islands in systems where layer by layer growth is predicted. Examples of these systems are copper on nickel and gold on palladium. This paper contains a simple calculation which slows that one can account for the formation of three-dimensional islands of copper on nickel or gold on palladium if the energies associated with misfit strain and misfit dislocations are considered.

Effect of coherency strain on the climb of misfit dislocation during difuss

Effect of coherency strain on the climb of misfit dislocation during difuss

Effect of Coherency Strains on Diffusion in Copper-Palladium Alloys

Interdiffusion coefficients have been measured in Cu–Pd alloys containing 70–90 at.% Pd at temperatures from 355° to 440°C using films containing short wavelength composition modulations produced by evaporation. From the relative intensities of the high- and low-angle satellites about the 111 Bragg peak, it was established that the composition modulations were coherent for λ<28 Å (where λ is the wavelength of the modulation) and were incoherent for λ>38 Å. Over the range 28 to 38 Å, there was a progressive loss in coherency and this was accompanied by a twofold decrease in the diffusivity. This decrease confirms quantitatively the effect of coherency strains on diffusion predicted by Cahn. The dependence of the effective diffusion coefficient on wavelength in the coherent modulations yielded K=−10.3×10−6 erg·cm−1 for the gradient-energy coefficient.