Alloys isovalent

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

Of particular interest is the fundamental science and technology of the solid solutions between II-VI binary compounds. These isovalent alloys may be classified according to the scheme introduced previously (see Sect. 1.2.3) - a convenient matrix diagram comprising their observed structures can be found in a publication of Wei and Zunger [102]. 

Isovalent (alloys where the valence of the atoms being exchanged is the same... 

Conversely, one can have non-isostmctural or non-isovalent alloys. Note, however, that because compounds with different structures or valences do not readily dissolve in each other, these latter categories are rare and must be produced by nonequilibrium processes such as sputter deposition (see Chapter 11). All commonly used alloys are both isostructural and isovalent, therefore this is not a very useful description. Henceforth, we will stick to the pseudobinary,. .. identification. Examples of non-isovalent and non-isostructural alloys are considered in Section 6.5. 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

The choices of compounds from which to construct semiconductor alloys are limited. For example, the common elemental semiconductors are Si, and Ge. Other more rare examples include diamond, cubic Sn, and amorphous Se. The simple binary alloys therefore consist only of mixtures of these elements (excluding Se, which is not isovalent with the others) either in crystalline or amorphous form. All except Si-Ge alloys are extremely limited in usable compositions. Amorphous semiconductors have their own chapter (Chapter 8) and so will be ignored here. Carbon forms SiC rather than a Si-C alloy when mixed with Si. Sn has little or no solid solubihty with the other materials. By contrast. Si and Ge are completely miscible. Si-Ge alloys are of sufficient importance that they are discussed in detail in Section 6.4. Until then, we will leave the elemental alloys in favor of the compound semiconductor alloys. These provide much more flexibility in the resulting properties but are also much more complex and difficult to work with. 

Alloys include binary (two elements), pseudobinary/temary (two compounds), or multinary (more than two elements or compounds). Most alloys are isostructural and isovalent. 

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