An example embodiment includes method for forming a layer of a Group III-Nitride material. The method includes providing a substrate having a main surface comprising a layer of a first Group III-nitride material. The substrate further includes, on the main surface, a dielectric layer comprising an opening exposing the first Group III-nitride material. A thermal treatment process is performed while subjecting the substrate to a gas mixture comprising a nitrogen containing gas, thereby increasing temperature of the substrate up to a temperature for growing a layer of a second Group III-nitride material. At least one Group III-metal organic precursor gas is subsequently introduced into the gas mixture at the growth temperature, thereby forming, at least in the opening on the exposed Group III-nitride material, a layer of the second Group III-nitride material by selective epitaxial growth, characterized in that the gas mixture is free of hydrogen gas.