AGING CHARACTERISTICS OF ZNSMN ELECTROLUMINESCENT FILMS GROWN BY A CHEMICAL VAPOR-DEPOSITION TECHNIQUE

The stability of luminance-voltage characteristics (L-V) has been investigated on ZnS:Mn electroluminescent films grown by chemical vapor deposition. It is found that the aging pattern varies between the positive shift (P shift) and the negative shift (N shift) with the growth temperature. In both cases the L-V curve, the current-voltage relationship, and the symmetry of the light emission with respect to the voltage polarity are strictly cooperative during the aging process. The N shift appears to be dominant when the current is limited by electron injection from the interface, whereas the P shift becomes dominant under the bulk limited conduction, accompanying an excellent symmetry of the emission. A high-field-conduction characteristic in the ZnS layer, which is a decisive factor to determine the aging pattern, is significantly affected by its stoichiometry, grain size, and impurity content. It has been further clarified that the N shift consists of the combination of a softening and a shifting in the L-V relation, which are defined as the effect due to the asymmetric polarization and the band bending in the ZnS layer, respectively. This study demonstrates a stable L-V characteristic without shifting the threshold voltage. The difference between the P shift and the N shift is discussed on the basis of the crystalline property and the carrier transport mechanism.