The formation of structurally and optically well-defined Si3N4/GaAs(100) interfaces is presented. A high reproducibility both with highly doped and semi-insulating GaAs crystals was achieved. The thickness of the insulator prepared by rf sputtering under high-vacuum conditions was 100 nm. The general applicability of the plasma treatment used is documented on a plasma CVD grown a-SiGe:H/(Si(100) interface, which exhibits similar structural properties. The thickness of the amorphous layer was 1 mu m. The surface pretreatment was done by hydrogen and/or argon capacitively coupled plasma. The most significant result was obtained by X-ray diffraction at grazing incidence (XRDGI) on the plasma pretreated samples on both types of interfaces or semiconductors. The diffraction pattern at grazing angle of 1.5 degrees is dominated by a single extremely sharp 311 reflection corresponding to the coherence length of similar to 300 nm, The intensity of this reflection is by two orders of magnitude lower on the reference sample distributed by commercial producers.