Binary semiconductors

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.

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]. 

Four simple crystal structural types encompass the majority of elemental or binary semiconductors. The high symmetry of the structures has important consequences for the NMR spectra in several respects ... 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Fig. 4.5.. sp-orbital model of AB-binary semiconductor of 2N atoms interacting with adatom a. Reprinted from Davison and Huang (1974) with permission from Elsevier. 

Lanthanum arsenide (La + As " —> LaAs) is used as a binary semiconductor. It is very toxic. 

The next five chapters deal with deposition of specific groups of semiconductors. In Chapter 4, II-VI Semiconductors, all the sulphides, selenides, and (what little there is on) tellurides of cadmium (most of the chapter), zinc (a substantial part), and mercury (a small part). (Oxides are left to a later chapter.) This chapter is, understandably, a large one, due mainly to the large amount of work carried out on CdS and to a lesser extent on CdSe. Chapter 5, PbS and PbSe, provides a separate forum for PbS and PbSe, which provided much of the focus for CD in earlier years. The remaining sulphides and selenides are covered in Chapter 6, Other Sulphides and Selenides. There are many of these compounds, thus, this is a correspondingly large chapter. Chapter 7, Oxides and Other Semiconductors, is devoted mainly to oxides and some hydroxides, as well as to miscellaneous semiconductors that have only been scantily studied (elemental selenium and silver halides). These previous chapters have been limited to binary semiconductors, made up of two elements (with the exception of elemental Se). Chapter 8, Ternary Semiconductors, extends this list to semiconductors composed of three elements, whether two different metals (most of the studies) or two different chalcogens. 

In the general case, the reaction of decomposition of a semiconductor material can be both anodic [cf. Eq. (42a)] and cathodic. For instance, for a binary semiconductor MX (where M, X are the electropositive and electronegative components of the compound, respectively) the reaction of anodic... 

A = (Ec - E )bulk or (Ev - EF)huik for an n-type or p-type semiconductor, respectively, M is the metal work function and K and S are constants that depend on the particular interface being considered. The ideal Schottky barrier, such as that discussed above, has S = 1, whereas perfect pinning would have S = 0. Experimentally [53], it is found that S remains small if Ax (the electronegativity difference A — xB ) for the binary semiconductor AB is less than 0.7 and changes rapidly towards unity for Ax > 0.7. Thus, for GaAs where Ax = 0.4, S = 0.04 (p-GaAs), 0.07 (n-GaAs). 

Most classical electronic semiconductors, such as Ge, Si, GaAs, InP, GaP, CdSe or CdS, are characterized by an electronic structure, in which the upper state of the valence band is occupied by electrons involved in the chemical bonding between the atoms of the crystal. Holes occurring at the surface are equivalent to missing bonds, which leads to a weakening of the structural stability and finally, in contact with a liquid, to an anodic decomposition. Taking a binary semiconductor as an example, the decomposition reaction is given for instance by... 

Use The last three are used as high-purity binary semiconductors. 

The quasi-thermodynamic approach enables one to obtain a criterion for finding out whether a semiconductor is liable to (photo)corrosion and suggest a way of preventing it. For example, in the case of a binary semiconductor MX (where M denotes the electropositive and X the electronegative components) the equation for a partial electrochemical reaction of cathodic decomposition with conduction electrons involved can be written in the form... 

Table 2.5. Binary semiconductor/metal systems observed to exhibit solid-state amorphization by interdifiiision reactions. Type of experiment, typical reaction temperature (rR), maximum thickness of the amorphous layer (2f and references are listed. (S interdiffusion of polycrystalline metal with semiconductor single crystal, and A interdiffusion of amorphous semiconductor layers with polycrystalline metal)...

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