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In this Rapid Communication, we report positron annihilation results on in-grown and proton irradiationinduced vacancies and their decoration in aluminium nitride (AlN) single crystals. By combining positron lifetime and coincidence Doppler measurements with ab initio calculations, we identify in-grown V Al −O N complexes in the concentration range 10 18 cm −3 as the dominant form of V Al in the AlN single crystals, while isolated V Al were introduced by irradiation. Further, we identify the UV absorption feature at around 360 nm that involves V Al .

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The technology of sublimation growth of 15 mm diameter bulk single AlN crystals is scaled to grow similar 2‐inch diameter crystals. The best results are currently achieved with the two‐stage technique including 1) seeding and initial growth of 2‐3 mm thick single‐crystal AlN layers on 2‐inch diameter 6H‐SiC wafers in pre‐carbonized Ta crucibles in graphite equipment and 2) growth of bulk AlN crystals on the above AlN layers in tungsten crucibles and equipment. The initial AlN layers prove of good crystallographic quality but may contain up to 6 at.% of Si impurities. The eventual bulk AlN crystals consist of about 40 mm diameter round single‐crystal core and polycrystalline rim. No impurities in concentration higher than 0.01 at.% are found in the bulk crystals. The bulk AlN crystal may be separated from the initial AlN layer to use the latter repeatedly for growth of a new crystal. Such “secondary” crystals were found to be semi‐insulating. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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