Ž . Diamond and Related Materials 10 2001 168173 Evidence for excitonic behavior of photoluminescence in polymer-like a-C:H films C. Godet a,  , M.N. Berberan-Santos b a ( ) Laboratoire PICM CNRS-UMR 7647 Ecole Polytechnique, 91128 Palaiseau-Cedex, France b Centro de Quımica-Fısica Molecular, Instituto Superior Tecnico, A . Ro isco Pais, Lisboa 1049-001, Portugal ´ ´ ´ Abstract To understand the dynamics of energy transfer and randomization of photoluminescence polarization in hydrogen-rich polymer-like amorphous carbon a-C:H films, time-resolved investigations of intensity and anisotropy decays have been performed recently. The intensity decay rates increase exponentially as a function of emission energy with a behavior very similar to that observed in wide band-gap C-rich a-Si C :H. In addition, in polymer-like carbon, the observation of a plateau of PL anisotropy 1  x x in the 100  1000 ps range, is taken as strong evidence for the existence of a finite density of excitonic species in radiative recombination phenomena; it does not fit the phonon-assisted depolarization models proposed earlier. Polarization anisotropy decays and steady-state values are consistently interpreted using a dipole dipole non-radiative energy transfer mechanism Ž . Forster mechanism with a characteristic depolarization time of 50 ps rather independent of the emission energy. The latter ¨ value is likely to be related to the density of radiative centers distribution estimated independently in the constant exciton radius approximation, rather than the result of hopping in an exponential distribution of tail states.  2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous carbon; Luminescence; Exciton; Polarization spectroscopy 1. Introduction As a consequence of the allotropic properties of carbon, both nanostructure and optoelectronic proper- ties of carbon-based films are strongly dependent on the deposition conditions. Hydrogen incorporation in- troduces additional degrees of freedom into the atomic structure, leading to low density carbon films. The -bonding favors a segregation between sp 2 and sp 3 sites, so that sp 2 sites can pair up to form chain or ring configurations. This medium range organization related to conjugation of -bonds confers a high versatility to optical and transport properties of carbon thin-films  1.  Corresponding author. Tel.: 33-1-69-33-32-10; fax: 33-1-69- 33-30-06. Ž . E-mail address: godet@poly.polytechnique.fr C. Godet . Ž . Hydrogenated amorphous carbon a-C:H films are also potentially interesting for the deposition of large area electronic devices at low temperatures, compatible with polymer substrates. However, so far, the develop-  ment of light emitting diodes 2 , heterojunction photo-   voltaic devices 3 , thin-film transistors 4 and field  emission devices 5 remains far from expectations. The achievement of a good quality electronic material can- not be separated from a detailed understanding of the relationship between growth mechanisms, film nanostructure and the resulting electronic density of Ž . states DOS . In amorphous carbon films containing both sp 2 - and sp 3 -hybridized C atoms, the weaker -bonding and the orientation dependence of the  interaction cause the localization of the entire - and   -bands within the     -gap 6 . Strong site-to-site potential fluctuations result from the large difference between - and -bond 0925-963501$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 9 2 5 - 9 6 3 5 00 00464-7