Compound wurtzite materials

Most surfaces of compound semiconductors are polar, that is, the number of anions and cations per surface unit cell is not balanced. While for the zinc blende materials there is only one nonpolar exception, the (110) face, for the wurtzite structures, there are two nonpolar surfaces, the m-plane (1100) and a-plane (1120) [98]. In wurtzite materials, a (110) surface does not exist because of the different crystal structure. 

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.

In the wurtzite form of ZnS the sulfur atoms are arranged in hexagonal close packing, with the metal atoms in one-half of the tetrahedral positions. There are two layers of tetrahedra in the repeat distance, c, and these point in the same direction. This gives the materials a unique axis, the c axis, and these compounds show piezoelectricity. 

Nevertheless, some conclusions may be drawn from the set of results presented here. First, with the notable exception of InN, the group III nitrides form a family of hard and incompressible materials. Their elastic moduli and bulk modulus are of the same order of magnitude as those of diamond. In diamond, the elastic constants are [49] Cu = 1076 GPa, Cn = 125 GPa and Cm = 577 GPa, and therefore, B = (Cn + 2Ci2)/3 = 442 GPa. In order to make the comparison with the wurtzite structured compounds, we will use the average compressional modulus as Cp = (Cu + C33)/2 and the average shear modulus as Cs = (Cu + Ci3)/2. The result of this comparison is shown in TABLE 8. 

CdTe, HgS, HgSe and HgTe. Each binary compound (including zinc blende) is an intrinsic semiconductor. The wurtzite lattice is also important in semiconducting materials ZnO, CdSe and InN are examples of compounds adopting this structure. 

The wurtzite crystal structure is prominent in the class of II-VI compounds (ZnO, CdS, etc.) and, concerning the III-V materials, the III nitrides, tliat is, GaN, AlN, InN, and their multinary compounds. In the following, we will discuss surface structures of wurtzite crystals, using III nitrides as examples. The fundamental rules of surface formation are the same as for the zinc blende structures, treated in the last chapters 12.3.2 in detail. 

For noncubic compounds, the bulk crystal may be optically anisotropic. This is the case not only for wurtzite or chalcopyrite crystal structures but also for ternary and quaternary III-V and II-VI compounds showing ordering effects on the sublattices. In aU these cases, the compound semiconductor is an optically anisotropic material. The RAS signal is then a superposition of two contributions, one originating from the bulk and the other from the surface. In order to analyze the surface, the two contributions have to be separated, a task for optical modeling. 

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