Wurtzite Materials

By combining an anion and a cation layer to a double-atomic layer, one obtains an ABAB stacking in the c-direction of alternating close-packed layers. Depending 

Croup Element Symbol Covalent radii (nm) Electro negativity Electron configuration 

The two independent lattice parameters for wurtzite are named a and c. The lattice parameter c lies parallel to the direction of the stacking of the close-packed planes and parallel to the crystallographic direction identified by the Miller indices 

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An enormous range of properties is found in oxides. The most successful and most widely used substrate for GaN to date is sapphire, AI2O3. Except for wurtzite materials, few of the unit cells in these materials match with GaN, but it is usually useful to think of these systems as having a close-packed nitrogen lattice in GaN matching to a near close-packed oxygen lattice in the oxide. 

To measure the spin coherence time and to estimate the spin-polarized carrier injection efficiency from the electroluminescence data, the selection rules and the valence band structure in ZnO must be understood. The valence band in wurtzite materials is split into three bands (A, B, and C) due to crystal field and spin-orbit coupling as discussed before in Chapter 3. The spin degeneracy of these three bands and the conduction band is lifted in magnetic field resulting in small symmetric Zeeman splittings as shown in Figure 5.6 near the F point [48]. The allowed transitions following the selection rules AI = 1 (for 0 polarization) are indicated... 

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.

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm. ... 

Oluwafemi, O. S. Revaprasadu N and Adeyemi O. O. (2010). A new synthesis of hexadecylamine-capped Mn-doped wurtzite CdSe nanoparticles. Material Letter, 64, 1513-1516. 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

Hampden-Smith and co-workers have prepared [Zn(SEt)Et]10 (Figure 30) by the insertion of sulfur into the Zn—C bond of diethylzinc. 7 Although this decameric thiolate possesses an arrangement of zinc and sulfur atoms similar to that found in wurtzite, pyrolysis of the material at 250 °C led to predominantly cubic ZnS. Cubic ZnS was also formed when the precursor is used in a spray CVD process. 

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). 

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. 

STRUCTURE. CdSe forms the same three crystal stractures as described earlier for CdS. The main difference between the CD films of the two materials is that, while CdS can be commonly found in both the wurtzite and sphalerite forms, CdSe is more commonly deposited in the cubic zincblende form. Mixtures of the two forms have been reported in some cases, particularly when a visible Cd(OH)2 precipitate is present in the initial deposition solution. 

H2GaNH2)3 at 500 °C produces nanocrystalline GaN with a significant impurity of gallium metal.The nanocrystalline material slowly converts to the wurtzite phase at 900 °C. Since dimethylhydrazine H2NNMe2 decomposes at much lower temperatures than ammonia, the dimeric four-membered ring (H2GaNHNMe2)2 has also been considered as a precursor to GaN. ° ... 

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