Sapphire is the material most widely used in the capacity of substrates for technical devices based on the nitrides GaN, InN, AlN and  their solid solutions. However, the method of heteroepitaxy of the nitrides on sapphire substrates results in high dislocation density and other structure defects, due to discrepancy in the crystal lattice parameters and thermal expansion coefficients of these materials. This essentially complicates the obtaining of heterostructures with high functional characteristics. Since the commercial production of the substrates for homoepitaxial growth is expensive, much attention is being paid nowadays to the use of modified sapphire substrates with buffer nitride layers (GaN/sapphire or AlN/sapphire templates) which play the role of inherent nitride quasi-substrates. We have developed a new method for the obtaining of AlN/sapphire templates based on nitridation of sapphire in a gaseous mixture containing N₂, СО, СО₂, Н₂, Н₂О with low concentration of СО₂ and Н₂О [1]. This method allows to obtain polar ((0001)AlN//(0001)Al₂O₃, (0001)AlN//(11-20) Al₂O₃), semipolar ((10-13) AlN//(10-10) Al₂O₃) and nonpolar ((11-20) AlN//(10-12) Al₂O₃) crystalline AlN layers on the surface of sapphire. Their thickness ranges between 20-30nm and several microns, the rocking curve half-width is less than 1º [2, 3]. In contrast to well-known deposition methods in which epitaxial films are obtained using external source, in the given method AlN layer is formed due to dissolution of nitrogen in anion-deficient corundum followed by crystal-chemical transformations in the surface-adjacent layer of sapphire substrate. Therefore, the nitride layer is to be formed after the stage of corundum reduction followed by etching of the substrate surface [4]. The goal of the present work was to establish regularities of the influence of reducing annealing of sapphire substrate on the morphology of the surface of AlN/sapphire templates.

The study was performed on sapphire substrates with a diameter of 50.8mm with the crystallographic orientation (0001) and a deviation angle of 3–9′. The substrate surface underwent chemico-mechanical polishing. The substrates were annealed at 1450°С. For reducing annealing there was used a mixture of Ar, СО, H₂ gases (0.1–0.12MPa, Ar and N₂ concentrations were ~ 99vol.%, ~ 0.1–0.4vol.%, respectively). Nitridization was realized in a mixture of N₂, СО, Н₂ gases (0.1–0.12MPa, ~ 99vol.% concentration of N₂.

        The surface morphology was investigated on an atomic-force microscope Solver 47 Pro (NT-MDT) in contact mode. After chemico-mechanical polishing the initial surface of the substrates had a surface with terraces and steps, the step width ranged between 30 and 140nm. Such a nanostructured lay may be formed on extremely smooth sapphire surface at low angles of inclination to the crystallographic plane. The roughness R_a was 0.2–0.4nm.

      Shown in Fig.1 are the AFM images of the substrate inclination to the crystallographic plane. The roughness R_a was 0.2–0.4nm surface after reducing annealing in gaseous medium with low (Fig. 1 а,c) and high (Fig. 1 b,d) reduction potential which value is defined by the ratios  СО₂/СО and Н₂О/Н₂ in the annealing medium.

      In the process of reducing annealing thermochemical etching leads to transformation of the initial surface structure accompanied with splitting and junction of the steps, increase of their width and height, deformation of the edges. Within 15 min. of the annealing at low reduction potential (Fig. 1. а) the width of the steps is 0.3 μm, the roughness R_a is 0.49 nm. When the duration of isothermal holding increases to 5 hours, there is observed splitting of the surface structure followed by the formation of islets located in the direction which coincides with that of the terraces (Fig.1. с). In the process of annealing at high reduction potential  the steps undergo essential deformation, their boundaries are sharper, the shape becomes toothed representing parts of etch patterns for the crystallographic orientation (0001) of sapphire (Fig. 1 b), the step width increases to 0.8–1.05μm, R_a = 1.23nm. At long-term holding (Fig. 1 d) the surface deformation is even more essential: the step width increases up to 1–2μm, the roughness R_a reaches 6.1nm. The crystallographic orientation of the steps is well defined from  the  etch patterns at their edges.

      Fig. 2 presents the AFM images of the surface of AlN layer obtained on the substrate surface after preliminary reducing annealing (Fig. 2 а), and of the initial surface after chemico-mechanical polishing (Fig. 2 b,с). The nitride layer replicates the structure of the surface obtained after thermochemical etching of the substrate. On the surface of the sample which has not been subjected to preliminary reducing annealing the structure with steps is weakly pronounced, it is seen only at large magnification of the AFM images (Fig. 2 с).

     Thus, the preformed investigation shows that the morphology of the surface of  AlN obtained by the method of thermochemical nitridation of sapphire is essentially defined by thermochemical etching of the initial substrate realized prior to the formation of the nitride layer.

Referenses

1. Pat. UA90239C2, C30B 25/00, C30B 29/38, C01B 21/072, C01F 7/00, Ukraine. Method obtainment of crystalline film of aluminum nitride / Kh. Sho. Kaltaev, N.S. Sidelnikova, S.V. Nizhankovskiy, A.Y. Dan’ko, M.A. Rom; Declar. 29.09.2009; Pablish. 12.04.2010, Byul. N7, 2010.

2. Kh. Sho. Kaltaev, N.S. Sidel’nikova, S.V. Nizhankovskiy, A.Y. Dan’ko, M.A. Rom, P.V. Mateychenko, M.V. Dobrotvorskaya, and A.T. Budnikov. Obtainment of Textured Films of Aluminum Nitride by Thermochemical Nitridation of Sapphire // Semiconductors, 2009, Vol. 43, N.12, P. 1606–1609.

3. S.V. Nizhankovskiy, A.A. Krukhmalev, H. Sh.-ogly Kaltaev, N.S. Sidelnikova, A.T. Budnikov, V.F. Tkachenko, M.V. Dobrotvorskaya, E.A. Vovk, S.I. Krivonogov, and G.T. Adonkin. Thermochemical Nitridation of Sapphire Substrates of Different Crystallographic Orientations // Physics of the Solid State, 2012, Vol. 54, N.9, P. 1896–1902.

4. Kh. Sh.-ogly Kaltaev, N.S. Sidelnikova, A.Ya. Dan'ko, A.T. Budnikov, S.V. Nizhankovskiy. Thermochemical etching of sapphire in CO+H₂ gas atmosphere // Functional Materials. 2010. Vol.17, N.3. – P.395–400.

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