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

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 degrees C. However, the emission spectrum that is inherent to the E-5 -> T-5(2) 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.