Photoluminescence properties and local structure of polymer-like a-C : H films (vol 14, pg 274, 2000)

In order to understand better the electronic properties of pi-bonded materi als, some optical and photoluminescence properties of amorphous carbon film s 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 i n relation with local structure characterizations (in situ infrared ellipso metry and Raman spectroscopies) for a series of dual-plasma deposited polym er-like a-C:H films. Medium range topology has consequences in the hypercon jugation effects seen on infrared bands, as well as in optical and resonant Raman characteristics. Photoluminescence (PL) excitation spectroscopy reve als resonance features which are attributed to exciton-like electron-hole p airs in close Coulomb interaction. The PL efficiency shows a sharp quenchin g for densities above 1.3 g.cm(-3) where a clear transition also occurs in the Raman "fingerprint". In addition, quantitative analysis of IR ellipsome try 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 vibr ations; this is interpreted as being a result of the increase of local stra ins in the carbon skeleton, meaning that matrix distortions already appear at H content values as high as 46 H at.% due to film densification. An expe cted consequence is the mixing between pi and sigma molecular orbitals and the enhancement of the dissociation rate of confined electron-hole pairs. P L quenching would thus result from both a decrease of exciton confinement a nd an increase of the density of accessible nonradiative centers.