This chapter investigates the structural properties of non-polar interfaces as well as on the diffusion mechanisms of the most favourable point defect along polar interfaces. The family of nitrides semiconductors is one of the most motivating classes of optoelectronic materials. One of the most interesting areas where there is a lack of extensive research till now is the influence of the structural characteristics and defects on the thermal properties of the III-N compounds. Stacking faults are common defects in III-nitrides and a high density of stacking faults is usually observed in III-nitride epitaxial layers near the interface with the substrate. In group-III nitride semiconductor technology nonpolar growth directions are promising for increasing the output efficiency of optoelectronic devices. III-Nitrides are well established components of the opto- and micro-electronics industry being implemented in light-emitting diodes (LEDs), laser diodes (LDs) as well as high electron mobility transistors for chemical and biological sensors.