Thermofield Cr→Cr²⁺ recharging resulting in anomalous intensification of Cr²⁺ emission in ZnS:Cr thin-film electroluminescent structures

Abstract

For the first time, an anomalous strong increase of the Cr²⁺ emission intensity (I) with increasing the applied voltage (V) has been discovered in ZnS:Cr thin-film electroluminescent structures (TFELS) instead of the I(V) dependence saturation typical of TFELS of the MISIM type, where M is an electrode, I is an insulator layer and S is an EL film. The dependence of I on the transferred charge (Q) is very superlinear, whereas the luminance of the emission of hot electrons, which takes place simultaneously with the Cr²⁺ emission, increases proportionally to Q as it happens usually in TFELS. The increase of I and Q is accompanied by rising the sample temperature up to 30 – 50 °C. However, the emission spectrum that is inherent to the ⁵E → ⁵T₂ transition in the 3d shell of a Cr²⁺ ion is not changed in this case. The above effects are explained by Cr⁺ → Cr²⁺ thermofield recharging, which results in an increase of the number not only of free electrons, but also of Cr²⁺ radiation centers. The most probable mechanism of such a recharging is the Frenkel-Pool field-stimulated thermal ionization of Cr⁺ ions, whose ionization energy is 0.65…0.82 eV.