Method for selective epitaxial growth of a group III-nitride layer

A method for forming a layer of a Group III-Nitride material comprising:

• providing a substrate having a main surface comprising a layer of a first Group III-nitride material, the substrate further comprising, on the main surface, a dielectric layer comprising an opening exposing the first Group III-nitride material;
• performing a thermal treatment process 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;
• introducing, subsequently, at least one Group III-metal organic precursor gas into the gas mixture at the temperature for growing, thereby forming, at least in the opening on the exposed first Group III-nitride material, a layer of the second Group III-nitride material by selective epitaxial growth, wherein the gas mixture is free of hydrogen gas;
• and
• upon formation of the layer in the opening at a moment the layer and the dielectric layer abut a common planar surface, introducing H2 gas to the gas mixture.