The sub-bandgap absorption of chemical-vapor deposited films, with N contents ranging from 10 to 132ppm, was investigated by using constant photo-conductivity, photo-thermal deflection spectroscopic and electron spin resonance methods. The spectra which were measured by means of photo-thermal deflection spectroscopy increased monotonically and became structure-less with increasing photon energy; thus indicating an absorption which was due to amorphous C and graphite. The constant photo-conductivity data exhibited distinct features, with absorption bands at 1.6, 4.0 and 4.7eV in nominally undoped films, and 2.4 and 4.7eV in N-rich layers. It was noted that the constant photo-conductivity spectra of the doped films were comparable to photo-conductivity data for synthetic Ib diamond. The defect densities increased with increasing N content. By using electron spin resonance methods, a vacancy-related defect density (g = 2.0028) was deduced.

E.Rohrer, C.F.O.Graeff, R.Janssen, C.E.Nebel, M.Stutzmann, H.Güttler, R.Zachai: Physical Review B, 1996, 54[11], 7874-80