Charge transfer effects, thermo and photochromism in single crystal CVD synthetic diamond

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 (NS0) 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 NS0 centres and a concomitant increase in the negatively charged nitrogen-vacancy-hydrogen (NVH) complex (NVH-) concentration. Both transitions exhibited an activation energy between 0.6 and 1.2 eV, which is lower than that for the NS0 donor (∼1.7 eV). Finally, it was found that illuminating samples with intense short-wave ultraviolet light recovered the NS0 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 NS0 concentration is greater than the increase in NVH- concentration we also suggest the presence of another unknown acceptor existing at a similar energy to NVH. The extent to which the colour in CVD synthetic diamond is dependent on prior history is discussed.