Zincblende

Yet a third simple cubic structure, not found among the alkali halides, occurs nevertheless in a limited number of ionic compounds of composition AX. This is the zincblende arrangement, shown in fig. 3.04, in which the co-ordination is fourfold or tetrahedral, each ion being 

Tig 3 °5 The ionic radii of the elements. The values are those appropriate to 6-co-ordination. 

It is clear that from the observed interionic distances we can deduce only the sum of two ionic radii, but that if any one radius is known then other radii may be found. Various independent methods are available for estimating the radii of certain ions, and the values so determined, taken in conjunction with data from the crystal structures not only of the alkali halides but also of many other compounds, lead to the semi-empirical ionic crystal radii shown in table 3.02 and in fig. 3.05. The interpretation of the radii given in this table is subject to a number of qualifications which will be discussed below. For the present, however, it is sufficient to treat the radii as constant and characteristic of the ions concerned. For the alkali halides with the sodium chloride structure it will be seen that the interatomic distances quoted in table 3.01 are given with fair accuracy as the sum of the corresponding radii from table 3.02. 

Certain general points which emerge from the data in this table call for explicit mention  

Values are in Angstrom units and correspond to 6-co-ordination The representative and transition elements 

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Figure Bl.8.4. Two of the crystal structures first solved by W L Bragg. On the left is the stnicture of zincblende, ZnS. Each sulphur atom (large grey spheres) is surrounded by four zinc atoms (small black spheres) at the vertices of a regular tetrahedron, and each zinc atom is surrounded by four sulphur atoms. On the right is tire stnicture of sodium chloride. Each chlorine atom (grey spheres) is sunounded by six sodium atoms (black spheres) at the vertices of a regular octahedron, and each sodium atom is sunounded by six chlorine atoms.

Potassium chloride actually has the same stnicture as sodium chloride, but, because the atomic scattering factors of potassium and chlorine are almost equal, the reflections with the indices all odd are extremely weak, and could easily have been missed in the early experiments. The zincblende fonn of zinc sulphide, by contrast, has the same pattern of all odd and all even indices, but the pattern of intensities is different. This pattern is consistent with a model that again has zinc atoms at the comers and tlie face centres, but the sulphur positions are displaced by a quarter of tlie body diagonal from the zinc positions. 

The first crystal structure to be detennined that had an adjustable position parameter was that of pyrite, FeS2 In this structure the iron atoms are at the comers and the face centres, but the sulphur atoms are further away than in zincblende along a different tln-eefold synnnetry axis for each of the four iron atoms, which makes the unit cell primitive. 

There is a great number of mostly covalent and tetraliedral binary IV-IV, III-V, II-VI and I-VII semiconductors. Most crystallize in tire zincblende stmcture, but some prefer tire wairtzite stmcture, notably GaN [H, 12]. Wlrile tire bonding in all of tliese compounds (and tlieir alloys) is mostly covalent, some ionic character is always present because of tire difference in electron affinity of tire constituent atoms. 

Figure C2.16.2. The sequence of atoms in tlie two poly types of SiC, zincblende and wurtzite, along tlie c-direction. The zincblende lattice has perfect tetraliedral angles.

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.

Table 1. Physical Properties of the Cubic Zincblende Structure III—V and II—VI Semiconductors...

Figure 2 Calculated Hg L3-edge specu a (a) at 1.5 GPa with hypothetical zincblende (full line) and cinnabar (dashed line) structures (b) at 8GPa with hypothetical cinnabar (dashed line) and rocksalt (full line) structures.

Predicted by Dmitri Ivanovich Mendeleev (1834-1907) as "eka-aluminum" was first identified spectroscopically in Spanish zincblende. 

Since covalent bonding is localized, and forms open crystal structures (diamond, zincblende, wurtzite, and the like) dislocation mobility is very different than in pure metals. In these crystals, discrete electron-pair bonds must be disrupted in order for dislocations to move. 

The most common—and perhaps most important—hybrid orbitals are the tetrahdral ones formed by adding one s-, and three p- type orbitals. These can be arranged to form various crystal structures diamond, zincblende, and wurtzite. Combinations of the s-, p-, and d- orbitals allow 48 possible symmetries (Kimball, 1940). 

The glide planes on which dislocations move in the diamond and zincblende crystals are the octahedral (111) planes. The covalent bonds lie perpendicular to these planes. Therefore, the elastic shear stiffnesses of the covalent bonds... 

Crystals whose structures are not centrosymmetric are polar because their centers of positive charge are displaced slightly from their centers of negative charge. Examples are crystals with the wurtzite structure which have polar axes along their (0001) directions. Also, crystals with the zincblende structure are polar in their (111) directions. 

Similar differences have been observed for crystals with the zincblende structure, such as GaAs, on the (111) vs. the (-1-1-1) surfaces (Le Bourhis et al., 2004). However, in this case the effect is quite small a difference of only 1.5 percent. 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

Fig. 2 Differences between wurtzite (WZ) and zincblende (ZB) forms, (a, b) Handedness of the fourth interatomic bond along the right (R) for WZ and left (L) for ZB. (c, d) Eclipsed (WZ) and staggered (ZB) conformation of atoms. Reprinted with permission from [23], Copyright 1992 by the American Physical Society...

The remark just made suggests that a natural place to begin our discussion of equilibrium equations is with the occupation of different charge states. Let a hydrogen in charge state i(i = +, 0, or - ) have possible minimum-energy positions in each unit cell, of volume O0, of the silicon lattice. (O0 contains two Si atoms, so our equations below will be applicable also to zincblende-type semiconductors.) To account for spin degener-ancies, vibrational excitations, etc., let us define the partition function... 

Zinc antimonide, 3 44, 53—54 Zinc atomizing process, 26 598 Zinc baths, 9 828-829, 830t Zincblende semiconductors, 22 141 band structure of, 22 142-144 transport properties of, 22 148, 149t Zinc borates, 4 282-283 Zinc brass... 

John Champion patented in 1758 the process of producing brass and zinc from the common ore of zincblende (ZnS) or black jack (patent number 726). This ore had previously been considered worthless, but even with this advance it is obvious that metallic zinc was still far too expensive to use in brass production by direct mixing, a process which was patented by James Emerson in 1781 (no. 1297). Watson (1786), however, describes the use of zinc in the production of high-quality gilding brasses such as pinchbeck, tomback and Mannheim gold (see below). 

It is not found in its pure metallic form in nature but is refined from the mineral (compound) zinc sulfide (ZnSO ) known as the ores sphalerite and zincblende. It is also recovered from minerals and ores known as willemite, hydrozincite, smithsonite, wurtzite, zincite, and Franklinite. Zinc ores are found in Canada, Mexico, Australia, and Belgium, as well as in the United States. Valuable grades of zinc ores are mined in Colorado and New Jersey. 

Black cubic crystal zincblende structure density 5.775 g/cm melts at 525°C density of melt 6.48 g/mL dielectric constant 15.9 insoluble in water. 

All the synthesized particles are zincblende and are nearly spherical, with their average radii being 1.5 0.3 nm, irrespective of A A concentration. Thus, the particles are in a single-nanometer regime and their radii are considerably smaller than the Bohr radius of ZnS, i.e., 2.5 nm. 

Detailed analysis of x-ray diffractometry reveals the local structure of semiconductor clusters. Bawendi et al. (21) applied the method to CdSe nanoclusters of 3.5-4.0 nm. They concluded that these clusters have a mixture of crystalline structures intermediate between zincblende and wurtzite, on the basis of detailed simulation studies taking the thermal fluctuation into account. Conventional diffractometry could overlook the coexistence of wurtzite component. 

This article focuses primarily on the properties of the most extensively studied III—V and II—VI compound semiconductors and is presented in five sections (/) a brief summary of the physical (mechanical and electrical) properties of the zincblende cubic semiconductors (2) a description of the metal organic chemical vapor deposition (MOCVD) process. MOCVD is the preferred technology for the commercial growth of most heteroepitaxial semiconductor material (J) the physics and (4) applications of electronic and photonic devices and (3) the fabrication process technology in use to create both electronic and photonic devices and circuits. 

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