  We report on the effects of thermal treatment and ultraviolet irradiation on
the point defect concentrations and optical absorption profiles of single
crystal CVD synthetic diamond. All thermal treatments were below 850 K, which
is lower than the growth temperature and unlikely to result in any structural
change. UV-visible absorption spectroscopy measurements showed that upon
thermal treatment (823 K), various broad absorption features diminished: an
absorption band at 270 nm (used to deduce neutral single substitutional
nitrogen (NS 0) concentrations), and also two broad features centred at
approximately 360 and 520 nm. Point defect centre concentrations as a function
of temperature were also deduced using electron paramagnetic resonance (EPR)
spectroscopy. Above ~500 K, we observed a decrease in the concentration of NS 0
centres and a concomitant increase in the negatively charged
nitrogen-vacancy-hydrogen complex (NVH \bar) concentration. Both transitions
exhibited an activation energy between 0.6 and 1.2 eV, which is lower than that
for the NS 0 donor (~1.7 eV). Finally, it was found that illuminating samples
with intense short-wave ultraviolet light recovered the NS 0 concentration and
also the 270, 360 and 520 nm absorption features. From these results, we
postulate a valence-band mediated charge-transfer process between NVH and
single nitrogen centres with an acceptor trap depth for NVH of 0.6- 1.2 eV.
Because the loss of NS 0 concentration is greater than the increase in NVH \bar
concentration we also suggest the presence of another unknown acceptor existing
at a similar energy as NVH. The extent to which the colour in CVD synthetic
diamond is dependent on prior history is discussed.
