Choosing alloy constituents

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. 

Individual lines on these diagrams represent the energy gap dependence in individual pseudobinary alloys. Regions contained by four curves are for quartemary alloys, and define the region of phase space that should, if the alloy were completely miscible, be accessible with a given quaternary alloy. 

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In designing an alloy, polymer chemists choose candidate resins according to the properties, cost, and/or processing characteristics required in the end product. Next, compatibility of the constituents is studied, tested, and either optimised or accommodated. 

In what follows we shall take A to be the minority constituent (x < 0.5), and choose 5 > 0. Except when explicitly noted, we restrict Vb to have a constant value V, independent of x, and connect nearest nei bors only. Although in this section we consider electrons in an alloy, a very similar Hamiltonian is used to describe lattice vibrations in the presence of mass disorder. The results of this section may be extended by a simple transformation to describe such phonon systems (Economou, 1971b). 

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