The origin of the broadband luminescence and the effect of nitrogen doping on the optical properties of diamond films

Raman and various photoluminescence (PL) techniques were employed to investigate the role of nitrogen doping on the optical spectra of chemical-vapor-deposited (CVD) diamond films and to determine the origin of the characteristic broadband luminescence which is observed from approximately 1.5 to 2.5 eV and centered at ∼2 eV. The PL transitions attributed to the zero-phonon lines (ZPL) of nitrogen centers are observed at 1.945 and 2.154 eV. A new possible nitrogen center at 1.967 eV is also observed as well as the band A luminescence centered at ∼2.46 eV. The experimental results preclude the possibility of the broadband PL being due to electron-lattice interaction of the nitrogen ZPL centers. We establish the presence of an in-gap state distribution in CVD diamond films attributed to the sp² disordered phase and show that its optical transitions are the likely cause of the broadband luminescence. A model of the in-gap state distribution is presented which is similar to models previously developed for amorphous materials.