Semiconductor heterostructures based on III-Nitride materials are widely used in opto- and microwave electronics. Application of composit substrates or templates of GaN/sapphire, AlN/sapphire is perspective for the quality improvement of nitride heterostructures and cutting of production costs. Earlier we have shown the possibility to produce crystalline aluminum nitride layers on sapphire (AlN/sapphire templates) by thermochemical nitriding (TChN) of sapphire [1]. The essence of this method is the following. While annealing sapphire at temperatures of 1300-1450^оС in nitrogen-containing atmosphere under reductive conditions (СО, Н₂ - reagents), there occurs nitriding of the surface layer followed by the formation of the crystalline phase of aluminum nitride of a varying composition AlN(O) with the wurtzite crystal structure.
     The aim of the present work was to establish the conditions for chemical-mechanical polishing of the AlN/sapphire templates obtained by the TChN method , and to study the morphology and element composition of the surface at layer-by-layer removal of the nitridated layer.
     The study was performed on (0001)AlN//(0001)Al₂O₃ templates. The thickness of AlN layers was determined by means of X-ray diffractometry from the interference lines intensity in the symmetrical Bragg geometry and run into 0.3-0.5μm. According to atomic forse microscopy the AlN surface with an initial roughness R_a=7.7nm represent the hills with a diameter up to 0.1 μm and a height up to 20nm, the depth of individual pores was 20nm.
    Series of polishing suspensions based on colloidal silica, surfactants, alkaline agents were applied for polishing AlN/sapphire templates. The scratches appeared on the surface of nitrided layer or the etching of surface defects with formation of etch patterns occurred during the polishing in certain cases. The surface of the best quality has been produced at use of polishing suspension consisted aerosil and KOH at рН 10.3. Roughness R_a of 0.95 nm was achieved. Polishing with this suspension has allowed to produce a surface without scratches on whole thickness of nitrided layer.
     The study of the element composition of template surface with X-ray photoelectron spectroscopy has shown gradual reduction of nitrogen atoms concentration at layer-by-layer removal of the nitrided layer. It is proving the diffusive mechanism of process of sapphire nitriding. However, even at achievement of a sapphire substrate (with the nitrogen concentration 1 at.%) the funnel-shaped pores of depth up to 10 nm was remained on the surface. The possible origin of these pores can be dissociative decomposing of sapphire with generation the gaseous products which form pores.

     1. S.V. Nizhankovskiy, A.A. Kruhmaljov, N.S. Sidelnikova et al., Fiz.Tverd.Tela, 54, 1777 (2012) [in Russian].

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