Sub-bandgap spectroscopy of chemical vapor deposition diamond

Sub-bandgap absorption of chemical vapor deposition (CVD) diamond grown on silicon was investigated by the constant photocurrent method (CPM), transmission/reflection measurements, electron spin resonance (ESR) and light-induced electron spin resonance (LESR). The CPM spectra of nominally undoped and nitrogen doped samples show distinct nitrogen features around 2.3 and 3.3 eV usually found in type Ib synthetic diamond and broad absorption bands most likely due to amorphous carbon. UV-illumination reduces the absorption coefficient in the range 4.2-5.2 eV and gives rise to occupation of a defect level at 1.2 eV. The density of carbon dangling bond defects deduced by ESR (g = 2.0029) increases linearly with the nitrogen content. Paramagnetic nitrogen (P1 center, g = 2.0024) is detected only under UV-illumination in samples with N > 35 ppm. In films containing less than 35 ppm nitrogen, only 1-5% of the incorporated nitrogen is paramagnetic and therefore located at an isolated substitutional site.