Hot-Electron Impact Excitation of Tb3+ Luminescence in ZnS: Tb3+ Thin Films

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Abstract
It is experimentally demonstrated in a new and direct manner that electroluminescence in thin films of ZnS: Tb3+ is excited by the direct impact of hot electrons on the Tb3+ ions. The scheme consists of monitoring as a function of applied voltage across the structure studied‐a ZnS: Tb3+, Ta2O5 sandwich‐the ratio of fluorescence intensities I(5D3)/I(5D4) originating from the 5D3 and 5D4 levels of Tb3+ at 3.25 and 2.54 eV, respectively. As the electron energy distribution is shifted to higher energies by increasing the voltage across the structure, I(5D3)/I(5D4) grows. The Tb3+ ion thus acts as a probe for the electron distribution in these films. By comparing the results obtained to simple theory, it is concluded that in considering low‐energy losses of the electrons in the active layer, inelastic scattering by low‐lying 7F levels of Tb3+ can not be the sole energy‐loss mechanism at the Tb concentration of interest. The optical phonons of ZnS must also be taken into account.