NOVELTY - The process comprises providing a MmXn-substrate (1) with first and second surfaces, where M is aluminum and/or gallium, X is oxygen and/or nitrogen and m and n are numbers of 1-3, treating the first surface of the substrate with an etching medium, so that a part of the first surface of the substrate is removed, applying a leveling material on the first surface of the substrate, where the leveling material possesses chemical or mechanical ablation rate that corresponds to the substrate, and then chemically or mechanically treating the modified first surface.    USE - Process for machining the surfaces of the MmXn-substrates, which are useful for producing a semiconductor device of a radiation-emitting device such as LED (all claimed) and for manufacturing of solar cells.    ADVANTAGE - The process is capable of simply and efficiently machining the surface of the substrate with improved quality, homogeneous structure, improved mechanical stability and low roughness without crystal defects.    DETAILED DESCRIPTION - The process comprises providing a MmXn-substrate (1) with first and second surfaces, where M is aluminum and/or gallium, X is oxygen and/or nitrogen and m and n are numbers of 1-3, treating the first surface of the substrate with an etching medium, so that a part of the first surface of the substrate is removed, applying a leveling material on the first surface of the substrate, where the leveling material possesses chemical or mechanical ablation rate that corresponds to the substrate, and then chemically or mechanically treating the modified first surface, so that the leveling material is completely removed and simultaneously the part of the first surface of the substrate is removed. The surface areas of the first surface are removed and have a defective crystal structure. The etching medium acts on the substrate at 200 degrees C. The effect of the etching medium is ended such that the etching medium is separated from the substrate and/or is diluted by a solvent and/or a mixture of the etching medium and substrate is cooled. The second surface of the substrate is partially provided with a protective layer and/or a part of the first surface is provided with the protective layer. The protective layer lies in the form of parallel strips. The first surface is provided with a sacrificial layer, which is removed in the treated step, where the sacrificial layer is obtained by changing the crystal structure of the surface of the substrate. INDEPENDENT CLAIMS are included for:    (1) MmXn-substrate for defect-free semiconductor device;    (2) a semiconductor device;    (3) a method for producing a semiconductor device; and    (4) a radiation-emitting device such as LED.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of a structure of a radiation-emitting device.    Substrate (1)    Functional layer (2)    Radiation conversion material (3)    Primary radiation (4)    Electrode layers. (21)