EXCITATION MECHANISM OF TM3+ CENTERS IN ZNS ELECTROLUMINESCENT THIN-FILMS

The electroluminescence (EL) and photoluminescence (PL) characteristics of blue emitting ZnS:Tm3+ phosphors have been studied. The difference of the EL excitation mechanism from that of PL is also discussed. For EL excitation, the emission is concentrated more in the infra-red regions, whereas for PL excitation, blue emission dominates the spectrum. It is found that the PL excitation mechanism of Tm3+ centers is attributable to the non-radiative energy transfer from the ZnS host to the Tm3+ centers. In contrast with PL, the dominant EL excitation mechanism results from the direct impact excitation of the Tm3+ centers by hot electrons rather than energy transfer. The cause for the relative inefficiency of blue emission in EL is that the excitation of the lower lying 3F4 state is dominant compared with that of the higher lying 1G4 state.