One of the main problems of nitride-based technology is the lack of commercially available large native substrates. Commonly used substrates, such as sapphire or 6H-SiC, are poorly matched in terms of lattice parameters and thermal expansion coefficients. This leads to high densities of threading dislocations (typically in the range of 10¹⁰-10¹¹ cm^-2), which seriously limits the performance of nitride-based devices. Dislocations are believed to reduce the mobility and lifetime of electrical carriers, provide paths for diffusion of impurities and serve as leakage current pathways. They also seem to be responsible for the degradation of nitride laser structures. To address these problems several different methods of dislocation density reduction have been proposed in recent years. Early methods utilized growth of buffer layers and were aimed at controlling the number of nucleation sites and lowering the strain at the interface area. They brought down the dislocation density to the range of 10⁸-10⁹ cm^-2. However, further reduction was possible due to the development of other methods. First group of such methods, known as epitaxial lateral overgrowth methods, are designed to change the dislocation propagation from vertical to horizontal redirecting them from the active layer of the nitride structure. In other methods an insertion of intermediate layers or growth of thick nitride layers increases the probability of interactions between dislocations resulting in their combination and/or annihilation. There are also methods that attempt to combine these approaches. They include growth on porous substrates or growth interruption combined with in situ surface treatment with anti-surfactants. In this paper all different methods will be reviewed and compared. Their advantages and limitations, as well as, specific mechanisms of dislocation reduction will be discussed.

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