Dislocation misfit

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

When a mismatch is inevitable, as in the combination Gej-Sii j. — Si, it is found that up to a value of jc = 0.4, there is a small mismatch which leads to a strained silicide lattice (known as commensurate epitaxy) and at higher values of jc there are misfit dislocations (incommensurate epitaxy) at the interface (see p. 35). From tlrese and other results, it can be concluded that up to about 10% difference in the lattice parameters can be accommodated by commensurately strained thin films. 

While the c/a ratio deviates only by about 2% from one, it is not ideal and this has significant consequences for the pseudotwin and 120° rotational fault. It results in a misfit at these interface which is compensated by a network of misfit dislocations (Kad and H2izzledine 1992). In contrast, the non-ideal c/a ratio does not invoke any misfit at ordered twins. However, the misfit dislocations present at interfaces are about fifty lattice spacings apart and thus there are large areas between them where the matching of the lamellae is coherent. The structures and... 

Misfit dislocations are favored for films thicker than five layers. This can be seen from the two data points near the minima shown in Fig. 8 for layers 2—8. In each case the data point at the smaller value of N... 

Each new layer is populated by adding the particles in rows that are uniformly spaced along the y axis that is, by changing the density of misfit dislocations. The grouping of atoms into 2D clusters is an important effect that is excluded by this approach. However, the effects of this type of clustering can be inferred from these results. Since the chemical potential of the film material is fx=dE/dN, the tangent to this curve is ... 

Figure 9.1 Double-crystal X-ray topograph of a heavily relaxed (OOl)-oriented GaAsSb layer on GaAs showing the characteristic tweed contrast from two sets of misfit dislocations running parallel to the (110) directions...

Figure 9.5 Simulated rocking topograph of misfit dislocations parallel to the surface of a (OOl)-oriented GaAs wafer. Dislocation line runs up the page...

Figure 9.10 Magnified section of the topograph of Figure 9.9(b) showing threading and misfit dislocations. (Courtesy Dr R.Kohler)...

Figure 10.16 Double-crystal topographs of misfit dislocations in InGaAs on GaAs (a) below, (b) at, and (c) above the critical thickness for relaxation...

In general, twinning of the modulated structures is present. Figure 11 shows a [001] 90° rotation twin in the 2212 sample. On the left part (A), the modulation is in the plane of view and on the right (B), it is along the viewing direction. No misfit dislocations are observed at the twin plane. 

Stress builds up at a coherent interface between two phases, a and / , which have a slight lattice mismatch. For a sufficiently large misfit (or a large enough interfacial area), misfit dislocations (= localized stresses) become energetically more favorable than the coherency stress whereby a semicoherent interface will form. The lattice plane matching will be almost perfect except in the immediate neighborhood of the misfit dislocation. Usually, misfits exist in more than one dimension. Sets (/) of nonparallel misfit dislocations occur at distances... 

Here, G denotes the shear modulus, and f(c/r) is a function of the ratio c/r in which c and r are the spheroidal semiaxes of the precipitate. For spheres, f(c/r= 1) = 1 = /max. For discs as well as for rods, /< 1. In principle, shear stress energies and energies arising from misfit dislocation networks also have to be added. They influence AG by additional energy terms. 

Boundaries between solids transmit shear stress, particularly if they are coherent or semicoherent. Therefore, the strain energy density near boundaries changes over the course of solid state reactions. Misfit dislocation networks connected with moving boundaries also change with time. They alter the transport properties at and near the interface. Even if we neglect all this, boundaries between heterogeneous phases are sites of a discontinuous structural change, which may occur cooperatively or by individual thermally activated steps. 

A thin recombination layer is employed between a substrate and a photodetective layer in the imager of JP-A-61067958. The lattice constant of the recombination layer is different from the lattice constant of the substrate and the photodetective layer. To prevent misfit dislocation, the thickness of the recombination layer is chosen to be thinner than a critical value. 

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