The invention concerns a method of fabricating a composite substrate comprising at least one thin insulating layer interposed between a support substrate and an active layer of semiconductor material. The method comprises: providing a source substrate that comprises a semiconductor material and a support substrate; forming or depositing an insulating layer on the source substrate; providing recovery heat treatment of the insulating layer; providing plasma activation of a front face of the recovery heat treated insulating layer or a front face of the support substrate; molecular bonding, after the plasma activation, the front face of the insulating layer with the front face of the support substrate to form a bonded substrate; and lifting off a back portion of the source substrate from the bonded substrate to retain an active layer that comprises a remaining portion of the source substrate.