A method of producing a substrate for epitaxial growth of a gallium-based III-N alloy layer comprises the following consecutive steps: —providing a donor substrate of semi-insulating monocrystalline silicon carbide, —implanting ionic species in the donor substrate so as to form a zone of weakness defining a thin layer of semi-insulating monocrystalline SiC to be transferred, —bonding the donor substrate to a first receiving substrate by means of a bonding layer, —detaching the donor substrate along the zone of weakness so as to transfer the thin layer of semi-insulating monocrystalline SiC on to the first receiving substrate, —forming an additional layer of semi-insulating SiC on the transferred thin layer, —bonding the additional layer to a second receiving substrate having a high electrical resistivity, —removing at least a portion of the bonding layer so as to detach the first receiving substrate and expose the layer of transferred semi-insulating monocrystalline SiC.