TEM dark field cross-section of an MOCVD GaN structure on c-sapphire with low temperature LT-GaN buffer layer at the sapphire/GaN interface and two low temperature GaN interlayers in between the high-temperature HT-GaN layers. The number of dislocations clearly decreases in growth direction from bottom to top. The inset at the top left shows a dislocation half loop formed in the HT-GaN layer in the vicinity of an LT-interlayer. Obviously, the interlayers act as barriers for the propagation of dislocations. Adapted from Benamara et al. [50].