Acceptor level of nitrogen in diamond and the 270-nm absorption band

The 270-nm optical-absorption band is seen in a wide variety of diamonds but there are conflicting opinions about its relationship with the common substitutional nitrogen center. Here we use density-functional theory to show that in addition to a deep donor level, substitutional nitrogen has an acceptor level lying in the gap, which is involved in the 270 nm transition. Specifically we show that the calculated level and its stress response are consistent with those of the 270 nm defect. This indicates that substitutional nitrogen in diamond has three charge states and not two as has been commonly assumed. We also show that N+
s has an absorption band with a peak around 270 nm and hence can account for the lack of correlation between the 270-nm band and N0
s in those diamonds containing centers which compensate nitrogen. Finally, we discuss the origin of the 271-nm absorption feature and suggest that the nitrogen-hydrogen center is a strong candidate.