At present physical vapor transport (PVT) growth method has been recognized as the only reasonable approach to produce true bulk crystals of aluminum nitride.

(i) Sublimation conditions and growth rate. In the course of dissociative evaporation AlN is completely transformed into gas. Vapor pressure over solid AlN at temperatures ensuring sufficient surface diffusion is rather high in the order of 10² mbar. AlN demonstrates technologically favorable evaporation behavior, as completely reversible transformation of AlN virtually allows effective growth process with continuous supply of raw material. The growth rate of AlN in sublimation process remains the subject of wide disagreement. In some publications the growth rate of AlN is treated as completely dependent on vapor transport, i.e. the temperature difference between the crystal and the powder source is considered as the only parameter relevant for growth rate. In some other publications, contrary, the surface kinetics (and also very little investigated surface chemistry of condensation process) are predicted accountable for growth rate limitations. Additional complications are arising from the wide range of processing parameters used in the growth process of AlN: for example the crystal temperature reported in the available literature varies for bulk AlN from 1800 to 2250ºC. In this presentation we compare our experimental data on PVT growth of single crystals of AlN grown at different temperatures with the maximum condensation rate calculated according Hertz-Knudsen equation. It is concluded that the growth rate is determined mainly by the surface kinetics as it is very sensitive to crystal temperature and little dependent on temperature difference in sublimation system. (ii) Self-nucleation and formation of freestanding crystals. Self-nucleation and subsequent growth of cm-sized freestanding crystals known as Lely procedure for SiC has been established by our group for AlN also. The presentation will explain the habit and zonar structures in freestanding AlN. (iii) Directional crystallization and seeded growth. Unidirectional crystallization of AlN is possible with growth rates in the range of 0.1-1.0 mm/h. Seeded growth of AlN still remains a challenge, but successful seeding has been already demonstrated. In the presentation our recent results on growth of large nominally pure single crystals of AlN up to 1.5 inch in diameter will be presented (iv) Specific defects. Open core screw dislocations (micropipes), of particular concern in SiC, are not observed in AlN. Dislocation density in freestanding and seeded crystals of AlN revealed by wet chemical etching in our recent studies was in the range of 10⁴-10⁵ cm^-2. The lowest defect density and best optical properties are observed in crystal zones grown on basal Al-terminated (0001) and adjacent pyramidal faces.

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1. Processes of dissociative evaporation involved into sublimation growth of bulk AlN crystals
2. Influence of growth orientation on microstructure of AlN grown by solid-source solution growth (3SG) method