III—V Compounds

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.

Figure C2.16.2 shows tire gap-lattice constant plots for tire III-V nitrides. These compounds can have eitlier tire WTirtzite or zincblende stmctures, witli tire wurtzite polytype having tire most interesting device applications. The large gaps of tliese materials make tliem particularly useful in tire preparation of LEDs and diode lasers emitting in tire blue part of tire visible spectmm. Unlike tire smaller-gap III-V compounds illustrated in figure C2.16.3 single crystals of tire nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growtli of device stmctures. Substrate materials such as sapphire and SiC are used instead.

Some of tliese problems are avoided in heterojunction bipolar transistors (HBTs) [jU, 38], tlie majority of which are based on III-V compounds such as GaAs/AlGaAs. In an HBT, tlie gap of tlie emitter is larger tlian tliat of tlie base. The conduction and valence band offsets tliat result from tlie matching up of tlie two different materials at tlie heterojunction prevent or reduce tlie injection of tlie base majority carriers into tlie emitter. This peniiits tlie use of... 

The supplanting of germanium-based semiconductor devices by shicon devices has almost eliminated the use of indium in the related ahoy junction (see Semiconductors). Indium, however, is finding increased use in III—V compound semiconductors such as indium phosphide [22398-80-7] for laser diodes used in fiber optic communication systems (see Electronic materials Fiber optics Light generation). Other important indium-containing semiconductors include indium arsenide [1303-11-3] indium antimonide [1312-41 -0] and copper—indium—diselenide [12018-95-0]. 

Arsenic from the decomposition of high purity arsine gas may be used to produce epitaxial layers of III—V compounds, such as Tn As, GaAs, AlAs, etc, and as an n-ty e dopant in the production of germanium and silicon semiconductor devices. A group of low melting glasses based on the use of high purity arsenic (24—27) were developed for semiconductor and infrared appHcations. 

The CVD of Ceramic Materials Borides, SUicides, III—V Compounds and II—VI Compounds (Chalcogenides)... 

The CVD of the III-V compounds is usually obtained by reacting an alkyl of a Group-IIIb element with a hydride of a Group-Vb element. These reactions have largely replaced the co-reduction of the halides, The general reaction is as follows ... 

Other gallium-based III-V compounds are also produced by CVD, either experimentally or in production. The major ones are listed in Table 13.4. 

Commonly used II-VI compounds include zinc sulfide, zinc selenide, zinc telluride, cadmium sulfide, cadmium telluride, and mercury cadmium telluride. These materials are not as widely used as the III-V compounds, one reason being that it is difficult to achieve p-type doping. Mercury cadmium telluride is used extensively in military night sights, which detect in the 8-13 im spectral band (a similar material, platinum silicide, is being developed for that purpose). The major applications ofCVD II-VI compounds are found in photovoltaic and electroluminescent displays. 

Since indentation hardness is determined by plastic deformation which is determined in turn by dislocation kink mobility, hardness is expected to be proportional to the bond modulus. Figure 5.2 shows that indeed it is for the Group IV elements, and the associated isoelectronic III-V compounds. 

Figure 5.4 Effect of ionicity on hardnesses of cubic III-V compounds.

A good indication of the importance of chemical bond strengths in determining hardness is the correlation between the heats of formation of compounds and their hardnesses. An example for III-V compounds is shown in Figure 5.13.The heat of formation density is equivalent to the bond modulus. This provides further evidence of the importance of chemical bond strength in determining hardness. 

Figure 5.13 Dependence of hardness on the heat of formation density for the isoelec-tronic III-V compounds.

Other semiconductor crystals for which photoplasticity has been observed during hardness measurements include III-V compounds (such as GaAs— Koubaiti et al., 1997), and II-VI compounds (such as ZnS and ZnO—Klopfs-tein et al., 2003). Flowever, since the effect declined in these studies with the depth of indentation, it is likely that the observations are artifacts associated with changes of the indenter/specimen friction coefficients. An extensive review of photoplastic effects in II-VI compounds is given by Osip yan et al. (1986). 

Since chemical hardness is related to the gaps in the bonding energy spectra of covalent molecules and solids, the band gap density (Eg/Vm) may be substituted for it. When the shear moduli of the III-V compound crystals (isoelec-tronic with the Group IV elements) are plotted versus the gap density there is again a simple linear correlation. 

It is shown that the stabilities of solids can be related to Parr s physical hardness parameter for solids, and that this is proportional to Pearson s chemical hardness parameter for molecules. For sp-bonded metals, the bulk moduli correlate with the chemical hardness density (CffD), and for covalently bonded crystals, the octahedral shear moduli correlate with CHD. By analogy with molecules, the chemical hardness is related to the gap in the spectrum of bonding energies. This is verified for the Group IV elements and the isoelec-tronic III-V compounds. Since polarization requires excitation of the valence electrons, polarizability is related to band-gaps, and thence to chemical hardness and elastic moduli. Another measure of stability is indentation hardness, and it is shown that this correlates linearly with reciprocal polarizability. Finally, it is shown that theoretical values of critical transformation pressures correlate linearly with indentation hardness numbers, so the latter are a good measure of phase stability. 

Commonly used material classes are the III-V compounds (especially when dynamic or active functions are needed), LiNbCh (because of its electro-optical properties), the indiffused glasses, the SiON-materials, the polymers and materials obtained from sol-gel technology. Last three will be treated in other chapters of this book. As an example we show the cross section of a simple channel structure based on SiON technology in Figure 6. 

At present, the elements used in the formation of compounds by EC-ALE include the chalcogenides S, Se, and Te the pnictides As and Sb the group three metals Ga and In the group II metals Zn, Cd, and Hg as well as Cu, and Co. The range of compounds accessible by EC-ALE is not clear. The majority of work has been performed on II VI compounds (Table 1). The III-V compounds InAs and InSb have recently been formed, and the first deposits of a III-VI compound, InSe, have been made [151], In addition, Shannon et al. have begun studies of CoSb [152] with the intent of forming thermoelectric materials. 

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