Sapphire dislocation types

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. 

Figures 3.89 and 3.90 show twist boundaries in SiAlON and in sapphire ceramics, respectively. In Fig. 3.89, both tilt and twist boundaries are indicated. Rows of parallel and more complex dislocations are observed. These dislocation structures are periodic. The Burgers vector determined for the dislocations are of type b = a/3 (110). The experimental results show that these twist boundaries are stable without an amorphous grain-boundary phase. It appears, according to the experimental results, that boundaries with low L misorientation possess relatively low energies and, therefore, are formed favorably during a sintering process.

Figure 4.51 indicates that the yielding in sapphire undergoing basal slip is a consequence of dislocation multiplication and is not due to the unpinning of a Cottrell-type atmosphere, where dislocation pinning results from impurities. The study of two types of sapphires, with different initial dislocation densities, was meant to point out the difference in their surface dislocation densities and the consequent differences in their yield phenomena. 

In this section, as examples of nonpolar GaN on lattice-mismatched substrates, the surface morphology and microstructure of a-plane GaN on an r-plane sapphire substrate and m-plane GaN on m-plane 4H-SiC are presented. Next, the SELO method of reducing threading-dislocation and stacking-fault densities is described in detail. This is followed by a description of the properties of the conductivity control of n-type andp-type nonpolar GaN, and the growth of the heterostructure/quantum well structure. Finally, the performances of the violet and green LEDs on nonpolar GaN are discussed with respect to the threading-dislocation density dependence of the output power. 

There exists a strong difference in the predominant types of dislocations found in c- and a-plane GaN grown on sapphire. In general, the c-plane GaN... 

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