Deep trap states in rubrene single crystals induced by ion radiation

Rubrene single crystals have been irradiated with 1-MeV protons and helium ions at various fluence levels ($0.1\ifmmode\times\else\texttimes\fi{}{10}^{12}\ensuremath{-}5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ particles/cm${}^{2}$), and the resulting increase of the density of bulk trap states (DOS) has been studied by temperature-dependent space-charge-limited current measurements. Irradiation creates a peak in the trap DOS (about ${10}^{16}$ traps/cm${}^{3}$ for a radiation dose of $\ensuremath{\approx}$35 J/g), centered at $\ensuremath{\approx}$0.35 eV above the valence band edge. With incrementally increasing radiation dose, the induced trap density rises sublinearly and saturates at high dose. Three to five times more traps are created if the crystal surface is not covered during irradiation. We attribute this trap creation primarily to C--H bond breaking accompanied by hydrogen loss.