GaN exhibits a decomposition tendency for temperatures far below its melting point and common growth temperatures used in metal-organic vapour phase epitaxy (MOVPE).This characteristic is known to be a major obstacle for realising GaN bulk substrate. Therefore, a foreign substrate is always required to achieve GaN epiwafers for device applications. Nevertheless, InGaN quantum well (QW)-based blue light-emitting diodes (LEDs) can still reach an excellent efficiency even when deposited on a sapphire substrate. To achieve high efficiency InGaN QWs, an indium-containing layer must be introduced prior to the QW deposition. Only recently, the mechanism responsible for this efficiency enhancement has been pinpointed to be related to trapping of defects lying at the GaN surface. Unveiling of the critical impact of the (sub)surface defects (SDs) on the InGaN QW efficiency has raised questions regarding their origin, nature and migration during the (In)GaN growth. This thesis attempts to answer these questions. We start with the study of the GaN (0001) surface stability under MOVPE growth conditions. A defect creation process, shown to be primarily dependent on the temperature, is revealed. These defects created at the (sub)surface of GaN will stay at the surface during low temperature (LT) growth but incorporate into the InGaN QW by forming efficient non-radiative recombination centres (NRCs). These results strongly suggest that the well-established high temperature (HT) thick GaN buffer layer, commonly believed to be beneficial for the surface morphology and crystal quality, may be quite detrimental to the efficiency of InGaN QWs. The migration dynamics of these SDs during (In)GaN growth has also been explored. We found they are created during HT growth and have a tendency to segregate towards the surface. The maximum internal quantum efficiency (IQE) of an InGaN QW with the introduction of an underlayer (UL) is predominantly determined by the SD equilibrium concentration of this UL at the surface. This equilibrium concentration of SDs is fixed by two factors: the growth temperature and the indium composition. The superior capability of the InGaN UL has been tentatively attributed to a divancancy complex formation mechanism. Additionally, we found that the SD migration has shown to be a kinetic-driven process, which occurs at the surface. A faster growth rate at LT can be exploited to rapidly incorporate these defects into the bulk, at the expense of smooth surface morphology. In our final study, we demonstrate the applicability of using selective area epitaxy (SAE) to fabricate micro light-emitting diodes (Ό-LEDs). Importantly, by exploiting the presence of semipolar planes on the SAE grown micro pyramids, we demonstrate that the InGaN UL remains beneficial to the InGaN QW grown on the c-plane of the HT formed GaN pyramids. In addition, the QW grown on the semipolar facets appears to be insensitive to the presence of an InGaN UL, suggesting the SD creation/segregation/incorporation mechanisms may be a manifestation of the intricate properties of the GaN polar [0001] plane. Overall, we show the profound impact of the (In)GaN growth on the InGaN QW optical properties independent of the presence of dislocations. Our findings demonstrate that III-nitride growth is yet to be fully understood, even after more than three decades of intensive research.