  In order to improve the performance of existing technologies based on single
crystal diamond grown by chemical vapour deposition (CVD), and to open up new
technologies in fields such as quantum computing or solid state and
semiconductor disc lasers, control over surface and bulk crystalline quality is
of great importance. Inductively coupled plasma (ICP) etching using an Ar/Cl
gas mixture is demonstrated to remove sub-surface damage of mechanically
processed surfaces, whilst maintaining macroscopic planarity and low roughness
on a microscopic scale. Dislocations in high quality single crystal CVD diamond
are shown to be reduced by using substrates with a combination of low surface
damage and low densities of extended defects. Substrates engineered such that
only a minority of defects intersect the epitaxial surface are also shown to
lead to a reduction in dislocation density. Anisotropy in the birefringence of
single crystal CVD diamond due to the preferential direction of dislocation
propagation is reported. Ultra low birefringence plates (< 10-5) are now
available for intra-cavity heat spreaders in solid state disc lasers, and the
application is no longer limited by depolarisation losses. Birefringence of
less than 5*10-7 along a direction perpendicular to the CVD growth direction
has been demonstrated in exceptionally high quality samples.
