In this paper, different methods for lowering the dislocation density such as incorporation of the buffer layers, substrate patterning and nitridation, silan-ammonia treatment are reviewed and compared. Advantages and limitations of these methods as well as a specific mechanism to reduce the dislocation amount are discussed. Usually, high densities of threading dislocations within the range 10<^>10... 10<^>11 cm<^>-2 are present in typical thin nitride films that are directly grown on sapphire substrate. Using these methods for substrate preparation, the density of dislocations can be reduced to the value <$E1~cdot~10 sup 7 ~roman cm sup -2>. An important process that enables to obtain the high-quality GaN layers with the low dislocation density is patterning the sapphire substrate. The dislocation density of these substrates depends on the pattern shape and orientation of patterned strips on c-plane sapphire. Layers of GaN grown on a cone-shaped pattern have the lowest dislocation density. In addition, patterning the sapphire substrate increases the external quantum efficiency of radiative structures and reduces the mechanical stresses in the nitride layers.

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