Mismatch relaxed

If the dislocations are generated by the lattice mismatch, an extra-half plane should occur in the Si substrate, which has a small lattice constant [29]. Because both the room temperature lattice constant and the thermal expansion coefficient of GaP are larger than those of Si, GaP should have the larger lattice constant than Si at the growth temperature. Hence, the lattice mismatch relaxation at the growth temperature is responsible for the generation of the A-type dislocation. The B-type dislocations cannot be explained merely on the grounds of lattice mismatch. 

In this chapter we discuss the measurement and analysis of simple epitaxial stractures. After showing how to select the experimental conditions we show how to derive the basic layer parameters the composition of ternaries, mismatch of quaternaries, misorientation, layer thickness, tilt, relaxation, indications of strain, curvature and stress, and area homogeneity. We then discuss the hmitations of the simple interpretation. 

Figure 3.1 A schematic representation of the defects common in epilayer structures, (a) mismatch, (b) misorientation, (c) dislocation content, (d) mosaic spread, (e) curvature, (f) relaxation, (g) inhomogeneity...

The true or relaxed mismatch m is defined with respect to the relaxed (i.e. the normally tabulated) lattice parameters a i and a of the epilayer and substrate materials as... 

Dislocations are commonly present in two regions. A layer with high mismatch may relax so that interface dislocations are created to accommodate the strain. A network at the interface is thns observed. Shp dislocations may be generated by local plastic deformation due to thermal or mechanical strain and propagate elsewhere in the layer. Dislocations in the layer itself may also be generated during the growth process, dne, for example, to the presence of inclusions. 

Equation (3.4) contains the assumption that the interface is fully coherent. If it is only partially coherent, i.e. it contains interface dislocations, it is said to be relaxed, and equation (3.4) is not valid for the determination of the relaxed mismatch. Note the two different usages of the word relaxed in the last sentence It is necessary to measure the misfit parallel to the interface as well as... 

Subject to the caveat that there can be a significant shift in peak position when the total layer thickness is sub-micrometre we can determine the average composition of the MQW using the zeroth order, or average mismatch, peak. Asymmetric reflections are often used, both to determine any relaxation and to enhance the diffraction from thin layers. Let the period of the superlattice in real space be A, and the thickness of layers of A B i of composition x i and x 2 be D 1 and D 2 respectively. Then... 

Another class of toroidal NMR probes is formed by toroid cavity detectors (TCD) [31]. These TCD have been mounted into cylindrical metallic autoclaves to study spin relaxation effects of gases under pressure [31]. If the toroid cavity detector is the metallic pressure vessel itself it is called a toroid cavity autoclave (TCA) probe [32, 33]. These probes can be tuned to higher resonance frequencies than TCDs and also show better spectral resolution [29]. Figure 2.12 shows the design of a TCA where the autoclave body is built from phosphorus bronze [33] keeping the field distortions of the magnetic field Bq, induced by susceptibility mismatches... 

GaP has a much larger mismatch with CdS (-7%) compared with InP. Yet a fair degree of epitaxy was obtained for CD CdS on the (111) GaP surface [58]. In this case, a mixture of cubic and hexagonal CdS with a large density of stacking faults, presumed due to strain relaxation arising from the large mismatch, was obtained. 

Whatever composition and structure is chosen, a perfect match between the lattice spacings of the different layers is virtually impossible to achieve. Thus all perovskites show some evidence of lattice-induced strain. Section 13.3 describes some of the ways in which these strains can be relaxed, and the more effective the method of relaxation, the greater the amount of mismatch that can be accommodated. Since many relaxation mechanisms have already... 

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