AlN is the most promising material for wide bandgap opto-electronics and high power electronic devices. An almost equal thermal expansion coefficient and a small lattice mismatch with GaN (2.4 % along the a-axis) favour AlN substrates over sapphire or even over SiC. Its use results in a reduced defect density in devices which drastically improves their performance and lifetime.

Due to the lack of high quality AlN wafers for homoepitaxially- seeded sublimation growth of AlN bulk crystals, other lattice-matched materials have to be employed as seeds. SiC covered by an epitaxial AlN layer is the only foreign substrate reported so far. In the present work, we introduce {111} TaC as an alternative substrate material. The misfit between the {111} TaC and the {0001} AlN plane is only 1.2%. This value is comparable with the lattice mismatch of 1.0% between SiC and AlN. As advantage over SiC, TaC brings a nearly complete thermal and chemical stability at the normal growth conditions (T > 2000 °C).

Unfortunately, there are no TaC single crystals exceeding 4 mm commercially available. Therefore, we fabricated our {111} TaC substrates by carburization of {111} Ta wafers (12 mm in diameter, 1 mm thickness) at 1900 °C in pure graphite powder and in Ar atmosphere. A main challenge is to prevent small-angle grain boundaries that occur under non-optimized process conditions. These grain boundaries result from stress induced by the little lattice expansion during the carburization.

AlN growth on TaC seeds was carried out in a rf heated reactor. A TaC crucible containing purified AlN powder was placed in a graphite assembly and was enclosed in a porous graphite foam insulation.

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