  A major issue in the development of the technology of nitride based materials
is the choice of substrate. The structural and optical properties of the layers
are intimately connected to the substrate material used in the epitaxial
growth. Key issues include lattice matching of substrate and epitaxial layer
and the difference in the thermal expansion between the substrate and epitaxial
layer. Due to the extremely high vapor pressure of nitrogen over Gallium
Nitride (GaN), pure GaN substrate are very difficult to produce and currently
almost all nitride-based device are fabricated by hetero-epitaxy on either
Sapphire or Silicon Carbide (SiC). Alternatively, AlN has a significantly lower
vapor pressure of nitrogen and can be grown in bulk form. In this paper we
report on our results in the growth of AlN by physical vapor transport. A
theoretical model was developed to investigate the growth process by to
determine optimal growth parameters. Several different seed crystals were
investigated, singular 6H-SiC, 3.50 off-axis 6H-SiC and 80 off-axis 4H-SiC, and
AlN grown by hydride vapor deposition on silicon substrates (the silicon
substrates were subsequently removed by etching). Auger electron microscopy,
transmission electron microscopy, and various X-ray diffraction techniques were
used to investigate the samples grown.
