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Abstract: We present MOVPE-grown, high-quality Al x Ga1− x N layers with Al content up to x=0.65 on Si (111) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al x Ga1− x N was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0002) reflection of 540 and 1400arcsec for the (101¯0) reflection were achieved for a ∼1-μm-thick Al0.1Ga0.9N layer and 1010 and 1560arcsec for the (0002) and (101¯0) reflection of a ∼500-nm-thick Al0.65Ga0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al x Ga1− x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations. [Copyright &y& Elsevier]