Si(111) is an opaque substrate while sapphire and SiC are transparent.
The major challenges in heteroeptaxial growth of GaN epi-layer on Si (111) is the defects especially dislocation density due to the lattice and thermal mismatched between the Si substrate and the above GaN epi-layer. But these challenges can be overcome by using buffer layer, stacks of intermediate multi-layer and superlattices layer before the active region.
Advantages of using Si substrate including low cost, available in large wafer sizes, good electrical and thermal conductivity and easy integration with Si based optoelectronic devices.
You may refer to journal as attached:
https://www.researchgate.net/publication/260957379_InGaN-based_multi-quantum_well_light-emitting_diode_structure_with_the_insertion_of_superlattices_under-layer
Article InGaN-based multi-quantum well light-emitting diode structur...
The main issue is integration of device with substrate, for Si it is easy but for sapphire it is too tough to electrical contact. Sic can but very expensive that`s why Si is preferable to integrate because Si technology is already matured enough in this field.
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