  Homoepitaxial chemical vapour deposition (CVD) on high pressure high
temperature (HPHT) synthetic diamond substrates allows production of diamond
material with controlled point defect content. In order to minimize the
extended defect content, however, it is necessary to minimize the number of
substrate extended defects that reach the initial growth surface and the
nucleation of dislocations at the interface between the CVD layer and its
substrate. X-ray topography has indicated that when type IIa HPHT synthetic
substrates are used the density of dislocations nucleating at the interface can
be less than 400 cm-2. X-ray topography, photoluminescence imaging and
birefringence microscopy of HPHT grown synthetic type IIa diamond clearly show
that the extended defect content is growth sector dependent. <111> sectors
contain the highest concentration of both stacking faults and dislocations but
<100> sectors are relatively free of both. It has been shown that HPHT
treatment of such material can significantly reduce the area of stacking faults
and cause dislocations to move. This knowledge, coupled with an understanding
of how growth sectors develop during HPHT synthesis, has been used to guide
selection and processing of substrates suitable for CVD synthesis of material
with high crystalline perfection and controlled point defect content.
