A method of fabricating a semiconductor wafer, the method comprising: applying a stop layer onto a Group III nitride on a substrate; wherein the stop layer comprises amorphous hydrogenated carbon, applying a first sacrificial layer onto the stop layer; applying a structured mask to the sacrificial layer having openings; removing portions of the sacrificial layer exposed in the openings and the stop layer to expose discrete areas of the at least one Group III nitride; removing the first structured mask; using the remaining portions of the sacrificial layer as a mask and removing the at least one Group III nitride in regions uncovered by the sacrificial layer to form at least one mesa on the upper surface of the substrate, removing the sacrificial layer, depositing an insulating layer onto the substrate comprising the mesa, the mesa comprising the stop layer arranged on at least one Group III nitride such that an upper surface of the substrate and the mesa are covered with the insulating layer; forming a second structured mask on the insulating layer, the second structured mask having an opening above the mesa, the opening having a lateral area smaller than a lateral area of the mesa; removing a portion of the insulating layer within the opening and reducing a thickness of the portion of the insulating layer arranged above the mesa; and removing the second structured mask and progressively removing portions of the insulating layer by chemical mechanical polishing to produce a planarised surface comprising a surface of the stop layer and a surface of the insulating layer and afterwards removing the stop layer such that the planarized surface comprises an outer surface of the mesa and the outer surface of the insulating layer. applying a stop layer onto a Group III nitride on a substrate; wherein the stop layer comprises amorphous hydrogenated carbon, applying a first sacrificial layer onto the stop layer; applying a structured mask to the sacrificial layer having openings; removing portions of the sacrificial layer exposed in the openings and the stop layer to expose discrete areas of the at least one Group III nitride; removing the first structured mask; using the remaining portions of the sacrificial layer as a mask and removing the at least one Group III nitride in regions uncovered by the sacrificial layer to form at least one mesa on the upper surface of the substrate, removing the sacrificial layer, depositing an insulating layer onto the substrate comprising the mesa, the mesa comprising the stop layer arranged on at least one Group III nitride such that an upper surface of the substrate and the mesa are covered with the insulating layer; forming a second structured mask on the insulating layer, the second structured mask having an opening above the mesa, the opening having a lateral area smaller than a lateral area of the mesa; removing a portion of the insulating layer within the opening and reducing a thickness of the portion of the insulating layer arranged above the mesa; and removing the second structured mask and progressively removing portions of the insulating layer by chemical mechanical polishing to produce a planarised surface comprising a surface of the stop layer and a surface of the insulating layer and afterwards removing the stop layer such that the planarized surface comprises an outer surface of the mesa and the outer surface of the insulating layer.