Wurtzite, limiting structure

Most wurtzite GaN films have been grown on either 6H-SiC(0001) (see Datareview A7.8) or sapphire (A1203) substrates. The orientation of sapphire most frequently used is C-plane (0001) although there have been some structural characterisation studies made for growth on A-plane (1120) [1-4] and R-plane (0112) [1,2,5-7] substrates. Other defects found in the a-phase include inversion domain boundaries, prismatic faults, nanopipes, pits, voids and cracks. The limited structural information available on bulk single crystals of a-GaN shows that they contain a low density of line dislocations and stacking faults near inclusions [12] (see Datareview A7.5). 

Zinc oxide (ZnO) has useful piezoelectric properties. It has an hexagonal structure (wurtzite type) with a density of 5.66 g/cm. It is relatively unstable and decomposes above 1700°C, which is below its melting point (1975°C). It is readily attacked by all common acids and bases. It has limited CVD applications at this time. 

From these numbers we can readily see why the structures of BeO (Wurtzite, coord, no. 4) and MgO (Rocksalt, coord, no. 6) differ. The Mg 0 radius ratio is 0.47, well above the limit of 0.44 for six-coordination but the Be 0 ratio is 0.23 which is just above the limiting value for tetrahedral coordination of four. 

Fig. B.4. Ball and sticks model of the wurtzite structure. Along the c direction, the period of the crystal is equal to the height c of the cell and it contains two stacks of atoms of one type (the AB stacking period of wurtzite). To better appreciate the symmetry, the limits of one hexagonal subunit have been outlined. The minimum distance between superposed non-bonded atoms along the c-axis is 0.625c.

Among the covalent AX compounds which crystallize with either the zincblende or wurtzite structure are ZnS itself, ZnO, AIN, A1P, HgS, CuCl and many more. At first sight it would seem that such an arrangement is inconsistent with the known covalencies of the atoms involved. In ZnS, for example, the zinc and sulphur atoms, with two and six electrons, respectively, in the outermost shell, would be expected to be limited to a covalency of 2 and therefore to be capable of covalent... 

This method was used to calculate the nearest distance (uq/c) between the A1 and the N ions along the c axis from the measured values of the square of the structure amplitude. We found (Fig. 2) that Uj/c = 0.386 0.001 and that It was independent of temperature (within the limits of the experimental error). Jeffrey et al. [1] reported 0.385 for this parameter. If all the shortest atomic spacings were equal and c/a = 1.600, we should obtain Uq/c = 0.380, whereas in the case of a perfect wurtzite structure (c/a = 1.633) we should have 0.375. Hence, we concluded that the ions located within tetrahedra in tiie aluminum nitride lattice were displaced somewhat toward the base of the unit cell. The shortest distance between the Al and the... 

The results of studies conducted to date have indicated that O-substitution in wurtzite-structured GaN, and N-substitution in Ga203 phases, can be achieved effectively by using a variety of chemical and physical approaches, and that this has resulted in an important tuning of the optical and electronic properties. As yet, the oxygen and nitrogen substitution limits have not been determined, and the effects of... 

So far our tests of the Gaussian Approximation Potentials were limited to singlespecies systems, but the framework can be extended to multispecies systems. Here we report our first attempt to model such a system, the cubic phase of gallium nitride. Gallium nitride (GaN) is a two-component semiconductor with a wurtzite or zinc-blende structure. There is a charge transfer between the two species. 

Fig. 17.7 Size-induced PL blueshift of a InAs [10], lnP-01 [12, 76], b Zn-blende-I and wurtzite structure-11 of CdS and CdSe [7], CdS-I [13], CdSe [71-75], and c InP [11, 76] and CdSe [80] nanostructures. The and curves diverge at the lower end of the size limit (reprinted with...

Figure6.2 Optical bandgapsofCdyZni yOand Mg> Zni, Palloys as a function of Cd and Mg composition in the ZnO lattice. Data are limited for CdyZni yO to small compositional fractions. The data shown for Mg,Zni are for compositions corresponding to wurtzite structure. (Data provided by A. Ohtomo.)...

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