Ultraviolet photoluminescence and its relation to atomic bonding properties of hydrogenated amorphous carbon

Abstract

We have observed photoluminescence in hydrogenated amorphous carbon (a-C:H) samples at substantially higher energy than the visible band. The emitted light is in the ultraviolet (UV) region, the spectrum consists of three bands with peak positions of ∼4.46 eV, ∼4.01 eV and ∼3.63 eV, depending slightly on sample properties. The UV photoluminescence (PL) has been observed in samples of different band gaps which show either high or low PL efficiency in the visible region. The excitation spectrum of UV luminescence exhibits high efficiency in the photon energy range of 5.6–6.2 eV and a strong decrease at excitations below this energy range. The experimental fact, that, the peak energies of UV bands exceed the optical gap energy of the studied samples, supports the light emission via radiative recombination of localised geminate electron–hole pairs. Strong localisation is expected for the excitation of π–π* transitions in conjugated double bonded fragments of small sizes. An infrared study of the UV light emitting a-C:H films confirms the presence of conjugated double bonds with aromatic and olefinic local configurations as well, however, the unambiguous relation between UV luminescence and small aromatic structures cannot be established. It is more probable that the olefinic fragments with chain lengths of 2–4 give π electronic levels through which UV light emission takes place. The role of twofold co-ordinated carbon sites cannot be excluded yet.