In this work the mechanism which helps to reduce the dislocation density by deposition of a SiN_x interlayer is discussed. It is shown that the dislocation reduction by SiN_x interlayer deposition is influenced by dislocation density in the underlying GaN layers. The SiN_x interlayer is very effective when the original dislocation density is high, while in the case of lower dislocation density the deposition of SiN_x is not effective for crystal quality improvement. Although it is widely accepted that SiN_x serves as a barrier for dislocation propagation, similarly to the enhanced lateral overgrowth method, it is shown that after masking the SiN_x deposition cannot be the dominant dislocation reduction mechanism. The most probable mechanism is the annihilation of bended neighbouring dislocations during the coalescence of 3D islands. The SiN_x layer cannot serve as a barrier for dislocations, since it is probably dissolved during the following GaN growth and dissolved Si atoms are incorporated into the above-grown GaN layer which stimulates the 3D island formation. Then the use of the SiN_x interlayer for dislocation reduction is recommended only for the improvement of layers with a high dislocation density. On the other hand, the PL signal was strongly enhanced for both low and high dislocation density structures with the SiN_x interlayer, suggesting that the interlayer might help to suppress the nonradiative recombination in subsequent GaN that is not related to the dislocation density, which remained the same. But its origin has to be studied further.