Photoluminescence properties and local structure of polymer-like a-C : H films

In order to understand better the electronic properties of x-bonded materia ls, some optical and photoluminescence properties of amorphous carbon films have been investigated as a function of film density (0.9 to 1.7 g.cm(-3)) . This study gives an overview of the radiative recombination properties in relation with local structure characterizations (in situ infrared ellipsom etry and Raman spectroscopies) for a series of dual-plasma deposited polyme r-like a-C:H films. Medium range topology has consequences in the hyperconj ugation effects seen on infrared bands, as well as in optical and resonant Raman characteristics. Photoluminescence (PL) excitation spectroscopy revea ls resonance features which are attributed to exciton-like electron-hole pa irs in close Coulomb interaction. The PL efficiency shows a sharp quenching for densities above 1.3 g.cm(-3) where a clear transition also occurs in t he Raman "fingerprint". In addition, quantitative analysis of IR ellipsomet ry and Elastic Recoil Detection give evidence of a strong decrease of both the effective dynamical charge e*(C-H) and the bandwidth of sp(3) C-H vibra tions; this is interpreted as being a result of the increase of local strai ns in the carbon skeleton, meaning that matrix distortions already appear a t H content values as high as 46 H at.% due to film densification. An expec ted consequence is the mixing between pi and sigma molecular orbitals and t he enhancement of the dissociation rate of confined electron-hole pairs. PL quenching would thus result from both a decrease of exciton confinement an d an increase of the density of accessible nonradiative centers.