Photoluminescence behavior of hydrogenated amorphous carbon

The photoluminescence (PL) of amorphous hydrogenated carbon (a-C:H) has been investigated as a function of optical gap and excitation energy. The PL spectra exhibit a red shift when the excitation energy falls below the optical gap. The bandwidth of the PL spectra increases with the excitation energy but depends very weakly on the gap. The results are discussed in terms of the electronic structure of a-C:H and the configuration of sp2 sites. It is proposed that PL arises by the geminate radiative recombination of the electron–hole pairs confined to the π states of the sp2 bonded clusters. The quantum efficiency is found not to decrease for excitation above the gap, indicating that the electron–hole pair remain bound, and suggesting that any mobility edges lie beyond the optical band edges. The PL efficiency is found to increase roughly exponentially with optical gap, displaying two regimes. At higher gap the efficiency depends just on the gap, while at lower gap the dependence may depend on the capture radius of nonradiative recombination centers.