Nanostructure of thin amorphous hydrogenated carbon films studied by positron annihilation and photoluminescence

We deposited polymer-like a-C:H films using the plasma-enhanced chemical va por deposition technique and characterized film microstructure by variable- energy positron lifetime spectroscopy, photoluminescence (PL), and UV-visib le absorption spectroscopy. It was confirmed that PL occurs from a chromoph ore in a sp(2) cluster as a result of fast recombination of a photoexcited electron-hole pair. Positron annihilation lifetime spectroscopy showed that positronium (Ps) formation takes place via electron-positron recombination in the sp(3) matrix. The lifetime of ortho-positronium (o-Ps) in our a-C:H films was similar to that in polyethylene, indicating their polymer-like n ature. The relative PL efficiency increased by about an order of magnitude with increasing film band gap from 1.3 to 3.4 eV, which can be related to t he decreasing concentration of nonradiative centers. On the other hand, Ps formation was much less influenced by the band gap and nonradiative centers . Comparison of this result with that for polyethylene mixed with carbon-bl ack nanoparticles, where a considerable reduction in Ps formation was obser ved, showed that nonradiative centers were of a different nature from the d efects on the carbon nanoparticle surface. This work demonstrated the usefu lness of positron lifetime spectroscopy combined with optical measurements to study the nanostructure of a-C:H. (C) 2001 American Institute of Physics .