1.681-EV LUMINESCENCE CENTER IN CHEMICAL-VAPOR-DEPOSITED HOMOEPITAXIAL DIAMOND FILMS

The 1.681-eV luminescence center characteristically observed in chemic al-vapor-deposited diamond films is studied in a homoepitaxially grown diamond film. Homoepitaxial growth relaxes the strain typical for fil ms grown on heterosubstrates with lattice mismatch, thus reducing dram atically the optical linewidths down to 0.2 meV. The no-phonon lumines cence transition that we observe exhibits fine structure consisting of a fully resolved doublet with line components at 1.6820 and 1.6828 eV . The doublet thermalizes with an activation energy of (0.80 +/- 0.04) meV equal to the spectroscopic spacing of 0.8 meV. In addition, eithe r doublet component has itself an associated close satellite in a mirr orlike arrangement. Three other partly resolved lines enhance the tota l number of components in the no-phonon transition to at least seven. Photoluminescence and photoluminescence excitation measurements under uniaxial stress along the (001) crystal direction reveal a splitting o f the no-phonon structure into four main components. These are studied at varying temperatures and stress values for their thermalization be havior. We deduce an electronic level scheme of two excited states fro m which electrons radiatively relax to two lower states. The data are not consistent with excitonic recombination or electron-to-hole recomb ination. They indicate that the optical center is under uniaxial inter nal overpressure of approximately 0.06 GPa, probably due to its large size. The luminescence decay time of the optical center was measured t o be 4 ns (5 K) through 2.7 ns (300 K) in the homoepitaxial film and a pproximate to 1 ns nearly dependent of temperature in a polycrystallin e diamond film.