Single-crystalline AlN is the best substrate material for high Al content AlGaN-based optoelectronic devices performing in deep UV spectral range. High-purity AlN is optically transparent in UV and closely lattice and thermally expansion matched to the layers needed for the device structure, allowing for low dislocation density in the active region of the device. During last years the feasibility of bulk AlN growth using sublimation growth method at temperatures above 2200 C has been demonstrated by few research teams including our CrystAl-N GmbH (former research group at University of Erlangen; Germany). However, an understanding of sublimation and growth mechanisms of AlN during high-temperature PVT is still quite limited (an example here could be the well-known fact that nobody until now was able to explain very different growth habit of the crystals obtained in different crucibles/hot zone materials).

In his pioneering work [1] G. A. Slack assumed the growing (as well as evaporating) surface of AlN to be covered with a layer of liquid Al metal. This layer captures monatomic nitrogen atoms with unit probability, but it is unable to utilize N₂ molecules, as their dissociation energy is too large. Slack proposed crystal growth rate to be limited by supply of thermally dissociated (active) nitrogen. An interesting model for thermal decomposition of nitrides has been proposed by B. V. Lvov [2]. He succeeded to fit main kinetic characteristics of AlN evaporation considering partial evolution of nitrogen in the form of free atoms according to the equation AlN(s)->Al(g)+0.6N+0.2N2.

In this presentation we will discuss different aspects of dissociative evaporation involved into sublimation growth of bulk AlN crystals and show the influence of liquid Al condensation on the interface of the growing crystal on its morphological stability and crystalline perfection:

 (i) Evaporation process inside the charge powder. Evaporation rate in the beginning of the process is strongly different from that of the later stages. Complex character of the initial evaporation stage is the result of superposition of two contrary effects: (1) enhanced evaporation of small particles/sharp corners consumed within the initial process stage and (2) slowed evaporation during the incubation period of accumulation of Al-melt having strong catalytic influence on evaporation process. Charge powder will be shown to be an important source of active (atomic) nitrogen additionally to thermal activation.

 (ii) Growth process. Correct prediction of crystal growth rate requires consideration of transport mechanisms relevant for both components: aluminium gas and atomic nitrogen (including its recombination into N2). Al-melt accumulation on the growth surface depends on component supply rate and also on growth temperature. Surface diffusion driven by radial temperature gradient appears as an important redistribution mechanism of Al-melt adsorbed on the growing surface.

 (iii) Specific crystal defects related to dissociation are bubble-like Al inclusions ranging from sub-micron up to 10 µm in diameter and locally disturbed crystal areas grown under strong excess of bulk Al-melt.

 Finally the latest results of successful growth of large (> 2 inch) AlN crystals nearly free of Al inclusions achieved by CrystAl-N GmbH will be presented. The crystals grown in tungsten crucibles are of excellent chemical high purity ensuring their transparency to UV.

 [1] G. A. Slack: Aluminum nitride crystal growth, Final Report, Contract No. F49620-78-C-0021 (1979)

[2] B. V. Lvov. Thermochimica Acta 373 (2001) 97-124

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