1.681-eV luminescence center in chemical-vapor-deposited homoepitaxial diamond films

Abstract

The 1.681-eV luminescence center characteristically observed in chemical-vapor-deposited diamond films is studied in a homoepitaxially grown diamond film. Homoepitaxial growth relaxes the strain typical for films grown on heterosubstrates with lattice mismatch, thus reducing dramatically the optical linewidths down to 0.2 meV. The no-phonon luminescence 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\ifmmode\pm\else\textpm\fi{}0.04) meV equal to the spectroscopic spacing of 0.8 meV. In addition, either doublet component has itself an associated close satellite in a mirrorlike arrangement. Three other partly resolved lines enhance the total 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 of the no-phonon structure into four main components. These are studied at varying temperatures and stress values for their thermalization behavior. We deduce an electronic level scheme of two excited states from which electrons radiatively relax to two lower states. The data are not consistent with excitonic recombination or electron-to-hole recombination. They indicate that the optical center is under uniaxial internal overpressure of approximately 0.06 GPa, probably due to its large size. The luminescence decay time of the optical center was measured to be 4 ns (5 K) through 2.7 ns (300 K) in the homoepitaxial film and \ensuremath{\approxeq}1 ns nearly independent of temperature in a polycrystalline diamond film.