Thread dislocations

Figure 9.2, Left Structure of a CeOj(l 11) surface as relaxed on a cubic ZrOj(l 11) substrate, generated by dynamic simulation. Zr (light blue), Ce(magenta), oxygen in 2hO,(red), oxygen in CeO,(green). Right Stick representation of a screw-edge dislocation threading through the CeOj layer and the first ZtO sub-layer. From ref. 62, reproduced by permission of the Royal Society of Chemistry.

Freund, L. B. (1987), The stability of a dislocation threading a strained layer on a substrate. Journal of Applied Mechanics 54, 553-557. 

N-type nature of ZnO is due to the sensitiveness of ZnO lattice constants to the presence of extended defects (planar dislocations/threading) and structural point defects (interstitials and vacancies) that are commonly found in ZnO resulting in a non-stoichiometric compound. The excess zinc atoms in Zni+dO have the tendency to act as donor interstitials that give its natural N-type conductivity. In ionic form, the excess zinc tends to occupy special Zn interstitial sites with Miller index (1/3, 2/3, 0.875) as shown in fig. 12. These special sites offer passage routes for zinc interstitials to easily migrate within the wurtzite structure [109]. 

Dislocation density is measured as the total length of dislocation lines in a unit volume of crystal, meters per meter cubed. However, experimentally it is often simpler to determine the number of dislocations that intersect a surface, so that a common measure of dislocation density is the number of dislocation lines threading a surface, that is, the number per meter squared. In a fairly typical material there will be on the order of 108 dislocation lines crossing every square centimeter of solid. However, it is known that if a solid is deformed, the dislocation density rises, perhaps by a factor of 103 or 104. Clearly, dislocations must be able to multiply under the conditions that lead to deformation. 

If a specimen is heavily dislocated it may be impossible to distinguish peaks in double-crystal rocking curves. Figure 7.5 shows a five-layer Si-Ge specimen grown with 0.5 //m thick layers, with the Ge content in each layer respectively 10, 20, 30, 40 and 50% the aim being to produce a moderate amount of relaxation at each layer so that the top layer is fully relaxed 50% Ge but without excessive threading dislocations. This was achieved—the dislocation density was... 

Figure 9.10 Magnified section of the topograph of Figure 9.9(b) showing threading and misfit dislocations. (Courtesy Dr R.Kohler)...

Fig. 7.151. Screw-thread spiral analogy to a screw dislocation.

FIGURE 1 Cross-sectional TEM image of GaN on c-plane sapphire (grown by MOCVD) taken near the [1100] zone with diffraction vector g-2g (g = 1120). Threading dislocations extend from a highly defective low temperature GaN buffer layer to the film surface. The density of threading dislocations is 1010 cm 2. The majority of dislocations are edge defects with b = <1120>. 

FIGURE 2 (a) Schematic illustration of a hexahedron shaped pit in GaN with a threading dislocation at the apex, The vicinal sides of the pit are 1011 surfaces, (b) TEM micrograph of a pit in a ten layer Ino.30Gao.70N/GaN MQW structure. The GaN barrier, b, is thinner inside the V-defect, where growth occurs on the 1011 surfaces, than outside the V-defect, where growth occurs on the (0001) surface (L.T. Romano, MQW film courtesy of Hewlett Packard Optoelectronics Division). 

Bulk plate shaped GaN crystals do not have threading dislocations along the c-axis which would end at the (0001) surfaces. This is very different in comparison with GaN layer crystals grown on any substrate. It is also important with respect to application of these plates as substrates for homoepitaxial growth, since threading dislocations in a substrate propagate into the epitaxial layers. 

It should be noted that a reduction in the threading dislocation density in GaN has been achieved through the application of selective area epitaxy and lateral epitaxial overgrowth ((LEO), also known as epitaxial lateral overgrowth (ELO), or epitaxial laterally grown GaN (ELOG)). This epitaxial technique is discussed in Datareview B2.10 in this book. 

The FWHMs of 00.2 reflection for GaN layers on sapphire 00.1 vary between 30 arc sec [17] and more than 1000 arc sec [18], Such a large scatter of the results is caused by different types and concentrations of threading dislocations generated by a large (about 16%) lattice mismatch between GaN layers and sapphire. 

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