THE ESTIMATION OF MAXIMUM GROWTH-RATE FOR ALUMINUM NITRIDE CRYSTALS GROWN BY DIRECT SUBLIMATION

The largest crystals if AlN reported have been grown by direct sublimation and this paper examines each step in the sublimation process. It is shown that the large energy (9.8 eV) required for homogeneous dissociation of N2 precludes any contribution from the transport of atomic nitrogen. The steps considered in the evaporation direction are: (1) AlN(s) --> Al(ads) + N(ads), (2) Al(ads) --> Al(s), (3) N(ads) --> 1/2N2ads --> 1/2N2(g) or N(ads) --> N(g) --> 1/2N2(g). The likely rate-limiting step is identified and simple kinetic theory used to estimate the maximum possible rate of material transport in sublimation. The sticking coefficient of N2 has been obtained from the data of Hoch and Ramakrishnan on the nitrogen exchange reaction on AlN. Choice of useful growth temperature is shown to be restricted to the range 2000-2400-degrees-C, where maximum possible growth rates range from 0.1-4 mum/s.