The optical excitation mechanism in ZnS: Sm3+ grown by molecular-beam epitaxy

We fabricated a thin film of Sm3+ doped ZnS by molecular-beam epitaxy (MBE) and investigated its photoluminescence properties. The excitation spectrum of the Sm3+ luminescence at 10 K is dominated by broad bands peaked at the free exciton energies, in contrast to many earlier reports on rare-earth-doped semiconductors. This result shows that the free exciton contributes significantly to the excitation of Sm3+ in our MBE sample, probably because the concentration of the trap centers which capture free excitons but do not transfer energy to Sm3+ is sufficiently low in our sample. The thermal quenching characteristic of the Sm3+ luminescence under interband excitation of the ZnS host is fitted well to a formula expressed by taking into account two deactivation channels. The origins of these deactivation channels are argued on the basis of previously proposed excitation mechanisms of the rare-earth ions in semiconductors.