In this work we focus on the impact of different buffer and nucleation layers on the corresponding crystalline quality of gallium nitride (GaN), grown by metal-organic chemical vapour deposition (MOCVD). In situ curvature measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used for advanced characterization. The influence of various growth modes on meltback etching and cracking is analyzed. Also the effect of a silicon nitride (SiN) mask on the growth of GaN and its coalescence is investigated. Furthermore, associated potential consequences for the growth of aluminium nitride (AlN) interlayers are examined to obtain a homogeneous surface without cracks and with good crystalline quality. Our studies indicate that an incomplete coalesced GaN surface located underneath the AlN interlayer leads to an increased defect density. Additionally we studied the influence of growth temperature at nucleation on the material quality and the process stability. Finally we demonstrate interlayer induced compressive strain during GaN growth with an XRD rocking-curve full width half maximum (FWHM) as low as 450 arc seconds for the reflection.