Ternary compound semiconductors

Ga(III) leads first to Ga(I), then upon further reduction the elemental Ga forms from Ga(I). On glassy carbon the electrodeposition involves instantaneous three-dimensional nucleation with diffusion-controlled growth of the nuclei. No alloying with A1 was reported if deposition of Ga was performed in the Ga(I) diffusion regime. Reproducible electrodeposition of Ga is a promising route to binary and ternary compound semiconductors. A controlled electrodeposition of GaX quantum dots (X = P, As, Sb) would be very attractive for nanotechnology. 

Schimmel MI, Bottechia OL, Wendt H (1998) Anodic formation of binary and ternary compound semiconductor films for photovoltaic cells. J Appl Electrochem 28 299... 

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.

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... 

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. 

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]. 

Heteroepitaxy. Heteroepitaxy (e.g., deposition of Al Ga As on GaAs) is somewhat different, because the solid and liquid cannot initially be in equilibrium, that is, a chemical potential difference exists across the solid-liquid interface. In compound semiconductors, the chemical potential of each element is constrained by compound stoichiometry. For example, for a ternary solid (A B C) in equilibrium with a ternary liquid, the conditions of equilibrium are given by equations 8 and 9 ... 

Liquid-Solution Models. The simple-solution model has been used most extensively to describe the dependence of the excess integral molar Gibbs energy, Gxs, on temperature and composition in binary (142-144, 149-155), quasi binary (156-160), ternary (156, 160-174), and quaternary (175-181) compound-semiconductor phase diagram calculations. For a simple multicomponent system, the excess integral molar Gibbs energy of solution is expressed by... 

The analysis of the surface reaction zone has been applied to laboratory-scale PVD of binary and ternary group II-VI compound semiconductors, such as CdS, ZnS, (Cd ZnJS, CdTe, HgTe, and (Cd HgjTe, and the ternary group I-III-VI chalcopyrite CuInSe2 (17). For example, Figure 14 shows the comparison between the predicted and measured compositions of ternary (Zn Cd jS alloys. The predicted composition is within 3 atom % of the measured composition across the range of composition from 10 to 90 atom %. 

In general, all of the properties of ternary compounds can be treated just as we will treat them for binary compounds. In each case there will be uncertainties, as there are in the treatment of susceptibilities, about the parameters used and whether the dominant effects have been included. For a complete and current review of ternary semiconductors, see Kaufmann and Schneider (1974, p. 229). 

Ternary compounds (not alloys) such as CdCr2Sc4 can be grown from a PbCl2-CdCl2 flux. These are magnetic semiconductors, but they have been eclipsed by the manganese- and iron-based alloys with zinc and cadmium chalcogenides, such as Cdi -,cMn,tTe. The pure compounds, usually made by direct reaction, are often used as... 

When the compound formation step is performed in the gas phase, as in ILGAR, the end product is less mobile and therefore more homogeneously distributed over the surface. Very thin continuous layers can therefore be prepared in this way, as shown by Muffler et al. (2002). In their work one or more metallic components are deposited from solution on the substrate and then converted to the semiconductor compound by exposure to a reactant gas. The method is able to produce extremely thin coatings of chalcopyrite, chalcogenide and oxide semiconductors on nearly arbitrarily shaped substrates, including very deep nanoporous structures. A number of binary and ternary compounds, such as CdS, ZnS, CulnSa, In2Sc3, ZnO, ZrOa, 263 and others, have been prepared. 

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