Semiconductors ternary

The resulting stability curves are ellipses or squared ellipses in the quaternary phase space. Each ellipse becomes smaller at higher temperature as the spinodal decomposition region shrinks (as is the case in Figure 4.18c), driven by the stronger entropy contribution. Onabe [8] carried out calculations for all of the common 111-V semiconductor ternary and quaternary alloys. Selected results are shown in Figure 6.7. 

There are hundreds of semiconductor materials, but silicon alone accounts for tire overwhelming majority of tire applications world-wide today. The families of semiconductor materials include tetraliedrally coordinated and mostly covalent solids such as group IV elemental semiconductors and III-V, II-VI and I-VII compounds, and tlieir ternary and quaternary alloys, as well as more exotic materials such as tire adamantine, non-adamantine and organic semiconductors. Only tire key features of some of tliese materials will be mentioned here. For a more complete description, tire reader is referred to specialized publications [6, 7, 8 and 9]. 

III-V compound semiconductors with precisely controlled compositions and gaps can be prepared from several material systems. Representative III-V compounds are shown in tire gap-lattice constant plots of figure C2.16.3. The points representing binary semiconductors such as GaAs or InP are joined by lines indicating ternary and quaternary alloys. The special nature of tire binary compounds arises from tlieir availability as tire substrate material needed for epitaxial growtli of device stmctures. 

Figure C2.16.3. A plot of tire energy gap and lattice constant for tire most common III-V compound semiconductors. All tire materials shown have cubic (zincblende) stmcture. Elemental semiconductors. Si and Ge, are included for comparison. The lines connecting binary semiconductors indicate possible ternary compounds witli direct gaps. Dashed lines near GaP represent indirect gap regions. The line from InP to a point marked represents tire quaternary compound InGaAsP, lattice matched to InP.

Temary and quaternary semiconductors are theoretically described by the virtual crystal approximation (VGA) [7], Within the VGA, ternary alloys with the composition AB are considered to contain two sublattices. One of them is occupied only by atoms A, the other is occupied by atoms B or G. The second sublattice consists of virtual atoms, represented by a weighted average of atoms B and G. Many physical properties of ternary alloys are then expressed as weighted linear combinations of the corresponding properties of the two binary compounds. For example, the lattice constant d dependence on composition is written as ... 

The first semiconductor lasers, fabricated from gallium arsenide material, were formed from a simple junction (called a homojunction because the composition of the material was the same on each side of the junction) between the type and n-ty e materials. Those devices required high electrical current density, which produced damage ia the region of the junction so that the lasers were short-Hved. To reduce this problem, a heterojunction stmcture was developed. This junction is formed by growing a number of layers of different composition epitaxially. This is shown ia Figure 12. There are a number of layers of material having different composition is this ternary alloy system, which may be denoted Al Ga his notation, x is a composition... 

Heterostructures and Superlattices. Although useful devices can be made from binary compound semiconductors, such as GaAs, InP, or InSb, the explosive interest in techniques such as MOCVD and MBE came about from their growth of ternary or quaternary alloy heterostmctures and supedattices. Eor the successful growth of alloys and heterostmctures the composition and interfaces must be accurately controlled. The composition of alloys can be predicted from thermodynamics if the flow in the reactor is optimised. Otherwise, composition and growth rate variations are observed... 

These materials are useful semiconductors and have a wide range of industrial applications, particularly in opto-electronics. One of their attractive features is the possibility of tailoring the band gap and the lattice constant in the ternary alloys by varying the composition. CVD is now a major production process of these materials. 

Examples of known ternary (and quaternary) chalcogenide compounds, classified according to a formal valence combination scheme, are given in Table 1.4. These compounds were collected from a compilation of Madelung [40] regarding semiconductor materials. To be sure, numerous other systems exist. Some important ternary compounds or classes will be considered in the relevant sections of the present chapter. 

Table 1.4 Some ternary (and quaternary) semiconductor chalcogenides...

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

The ECALE deposition of ternary II-VI compound semiconductors such as CdxZni xS, CdxZni xSe, and CdSjcSei c, on Ag(lll), has been reported [51-53]. The compounds were prepared by sequential deposition of the corresponding binaries in submonolayer amounts for instance, alternate deposition of CdS and ZnS was carried out to form Cd cZni cS. The stoichiometry of the ternaries was seen to depend on the deposition sequence in a well-defined and reproducible way, with the limit that only certain discrete x values were attainable, depending on the adopted sequence profile. Photoelectrochemical measurements were consistent with a linear variation of the band gap vs. the composition parameter x of the mixed compounds. 

The majority of important semiconducting materials are isoelectronic with elemental silicon. Important semiconductor materials include the III-V (13-15) materials such as GaAs or InP, and II-VI (12-16) materials such as CdS or ZnSe (Table 1). These compound semiconductors are most often formed by combining elements displaced on either side of silicon by one place (i.e., Ill = Ga or In and V = N or As for a III-V material) or two places (i.e., II = Zn or Cd and VI = S or Se for a II-VI material) in the periodic table. Other materials are of specialist importance, especially ternary materials such as CuInE2 (E = S and Se), which find applications in solar cell technologies, as do materials of III-VI composition such as InxS, although their properties are often complicated by the potential for the formation of a wide range of similar phases. 

As a final comment on terminology, we note that elemental semiconductors are formed from a single element, e.g., Si or Ge, whereas compound semiconductors are formed from two binary), three ternary), four quaternary), or, rarely, more elements. Semiconductor alloys refer to solid solutions where either one anion or one cation can substitute for another, or possibly two or more such substitutions can occur for a binary semiconductor AB a simple alloy with C would be represented as Ai CjcB. Semiconductors are often classified by the group numbers in the periodic table. Thus, for example, I-VII semiconductors include Cul and AgBr, II-VI semiconductors include ZnS, CdTe, and HgTe, III-V semiconductors include GaAs, GaN, InP, and InSb, and IVx-VIv semiconductors include PbSe and Sn02. Fundamental physical properties are compiled in a recent handbook [22]. 

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