Recombination Radiation from Diamond

Abstract

The spectrum of the recombination radiation from diamond has been measured over the photon energy range 4.9-5.5 eV at 90, 160, 207, and 320\ifmmode^\circ\else\textdegree\fi{}K. At 90\ifmmode^\circ\else\textdegree\fi{}K, recombination emission has been detected from ten samples out of a batch of fifteen single crystals, the majority of which were known to be relatively defect free. The persistent and usually dominant spectral features are interpretable in terms of exciton annihilation with the emission into the lattice of one or more phonons. The energies of the single phonons and the proposed classifications are 0.085 eV (transverse acoustical), 0.134 eV (longitudinal optical and acoustical), and 0.142 eV (transverse optical). The details of these intrinsic features of the emission are in good agreement with predictions obtained from a recent analysis of the edge absorption spectrum of diamond. Other recombination bands can be interpreted in terms of exciton annihilation with the emission of the transverse optical phonon, together with the simultaneous intervalley scattering of the electron by a longitudinal acoustical phonon which is produced during the recombination process. The assignment of the intervalley phonon energies is supported by an analysis of the band-broadening contribution to the temperature dependence of the indirect energy gap, in which intravalley scattering is also apparent. The magnitudes of the experimental intervalley phonon energies show that the conduction band minima of diamond are situated at about 95% of the distance from the center of the Brillouin zone to the zone boundary in the $〈100〉$ directions. This result defines the wave vector of the three phonons quoted above. Recombination emission bands are also observed which appear respectively to involve at the low-energy extremities the simultaneous emission of two or three phonons of energy comparable to the Raman energy (0.165 eV). Two sets of sharp two-component extrinsic recombination bands are present. Each component of a given set is displaced by approximately the same energy below the low-energy thresholds of the intrinsic longitudinal acoustical (LA) + longitudinal optical (LO) and transverse optical (TO) bands (displacement energies 0.056 and 0.009 eV). These bands are thought to arise from the recombination of excitons bound to certain impurity or defect sites. One system (0.056-eV binding energy) is tentatively associated with the acceptor center of semiconducting (type IIb) diamond, which is thought to be substitutional aluminium. The anticipated corresponding recombination bands occurring without the emission of a phonon have not been identified, however.