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The optical absorption in large Ti: sapphire crystals (up to 175 × 175 × 40 mm in size) grown by horizontally directed crystallization with the use of zone leveling of the activator distribution in different reducing atmospheres has been investigated. It has been shown that there is a high uniformity of the optical characteristics and the distribution of color centers based on Ti3+, Ti4+, and activator-vacancy complexes in grown crystals, which is largely determined by the stability of the reduction potential of the growth medium. It has been established that the activator concentration in the Ti4+ charged state in crystals grown in the CO + H2 low-pressure atmosphere does not exceed 1.5% and accounts for 0.2–0.5% of the total activator amount in an Ar atmosphere. The Ti4+ concentration decreases to ∼0.01% after the additional reducing annealing.

It is shown that the formation of longitudinal aggregation of inclusions in bulk sapphire and yttrium-aluminum garnet (YAG) grown by the method of HDC is caused by local accumulation of impurities, disturbance of morphological stability of the crystallization front and capture of inclusions and impurities in the nodal region of the melt two-vortex convection. Studied is the influence of thermal and geometrical parameters of the melt and the shape of the crystallization front on the conditions of the formation of the capture of inclusions.

The influence of the reducing annealing of sapphire substrate during the thermochemical nitrida� tion of sapphire on the structure of the nitride layer in the AlN/sapphire template has been investigated. Iso� thermal annealings of (0001) and sapphire substrates have been performed at the nitridation temper� ature (1450°C) and different reducing potentials of the gas medium. It is found that the initial terracestep structure of substrates is transformed during annealing as a result of the diffusion and thermochemical etching of sapphire substrates; this process is accompanied by the fragmentation and merging of steps and by an increase in their width and height. The degree of stepstr ucture distortion increases with an increase in the reducing potential of the gas medium and the annealing time. Hexagonal etching figures are observed for (0001) sapphire substrates. It is established that the AlN/sapphire template surface relief is formed mainly in the initial stage during thermochemical etching of the stating substrate, which precedes the formation of a nitride layer. The sapphire-nitride transformation does not lead to a further increase in the surface roughness.

The coefficient of dislocation gliding over the planes of easy glide depending on the shape of the crystallization front, is determined. The regions of non-uniform distribution of thermal field in the crystal are established. Optimized are the temperature conditions for the growth of large-size (350×500×45 mm³) sapphire crystals by the method of horizontally directed crystallization. The change of the density of dislocations and of their distribution in the crystal bulk after correction of the technological growth conditions, is considered. It is shown that optimization of the axial, horizontal and vertical components of the temperature gradient makes it possible to improve uniformity of the thermal field distribution at the crystallization front in the process of the stationary crystal growth.

The optical and luminescence characteristics of sapphire crystals grown in nitrogen-containing reducing media based on CO have been investigated. It is shown that the nitrogen dissolution in the oxygen sublattice of sapphire under reducing conditions is accompanied by the formation of F + centers and aggregate F 2 centers (defects that significantly influence the optical absorption and X-ray-, photo-, and thermoluminescence in sapphire). It is established that the formation of Al2O3:N occurs at fairly low nitrogen concentrations in the growth medium (10–20 vol % when a crystal is grown at P ∼ 0.2 Torr and 1 vol % when growth is carried out at P = 800–950 Torr).

This paper presents the results of the study of the influence of physicochemical parameters of thermochemical nitridation of sapphire substrates of different crystallographic orientations in an N2 + (CO,H2) reducing atmosphere on structural characteristics, composition, and kinetics of AlN layer formation. The effective diffusion coefficients of nitrogen in sapphire at T = 1473−1773 K have been determined. The temperature dependences of the structural quality, thickness, and roughness of the AlN layer surface have been found; their strong dependence on the composition and reduction potential of the nitridation atmosphere has been shown. The structural models of matching the AlN and sapphire lattices during thermochemical nitridation have been proposed.

The possibility of obtaining textured aluminum nitride films by thermochemical nitridation of sapphire in the temperature range of 1200–1500°C in nitrogen atmosphere with the CO content from 0.1 to 5 vol % is shown. The diffusion mechanism of formation of the aluminum nitride film on the sapphire surface under the effect of the reducing medium is established.

This paper reports on the results obtained during the development of the technological process of growth of sapphire crystals for optoelectronics through horizontal directional crystallization in a gaseous argon medium at a pressure of 800 mmHg. The sapphire crystals intended for the use in optoelectronics have been grown from purified molten alumina according to the authors’ technology. It has been demonstrated that, under conditions of a high temperature gradient across the crystallization front and at a low content of reducing components (H2, CO) in the growth medium, it is possible to grow sapphire crystals satisfying the requirements of optoelectronics.

This paper reports on the results of investigations into the transformation of the corundum structure upon reducing annealing at high temperatures in the range from 1700 to 2050°C. It is established that the reduction results in the transformation of corundum into new phases with a lower oxygen content, including the phase with a spinel structure. These structures are assumed to be stabilized by anion vacancies. A model of the crystal structure of the spinel phase is proposed. This model provides an adequate description of the compound obtained in the experiment.

The mechanism of the formation of light-scattering centers in sapphire crystals grown by horizontal directed crystallization in gas atmospheres reducing with respect to the Al2O3 melt was studied. The experimental regularities here significantly differ from those observed upon formation of other conventional defects in sapphire crystals (vacancy pores, gas bubbles, and so on). It is shown that the known formation mechanisms of macroscopic ≥1 mm) inclusions in crystals are not acceptable in this case. Using the model of bulk crystallization is proposed to describe the obtained regularities.

The results of investigation of thermally stimulated and photoluminescence as well as absorption spectra of a-sapphire grown by a technique of horizontally directed crystallization in a protective gas medium of varying composition and pressure are presented. This technique has an advantage of minimization of W and Mo erosion, thus providing a considerably reduced prime cost. At the same time, there are some problems connected with emergence of considerable number of anion vacancies, whose in-axis concentration gradient is directed oppositely to that of dopants (mainly Ti). The correlations are found between the conditions of -sapphire growing and the data of absorption and luminescent spectroscopy. An interaction between the anion vacancies and Ti dopants, which induces a transition from electric-charge state Ti⁴+ to Ti³+, is discussed.