Kousuke Terada

Aging characteristics of ZnS:Mn 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 ...

Chemical vapor deposition of ZnS : Tb electroluminescent films in a bypass-HCl flow system

Green-emitting ZnS : Tb electroluminescent films have been successfully grown by low-pressure chemical vapor deposition, in which a Tb source is transported as gaseous halide by using a HCl carrier gas. A secondary HCl flow, bypassing the source material, is additionally supplied to the growth ambience. It is shown that the ZnS : Tb film changes from the cubic structure to the hexagonal structure with increasing the bypass-HCl flow rate, whereas the Tb concentration is basically determined by the mass transfer of the Tb-halide. The film growth rate decreases significantly in the presence of the HCl flow, and simultaneously the crystal quality is much improved by the incorporation of the Tb-dopant. Such growth behaviors are explained by supposing two kinds of competetive relations; one is a surface migration process competing between ZnCl2 and an isolated Zn atom, the other is a hexagonal nucleation process competing between TbCl3 and ZnCl2. The critical HCl flow rates to control the crystal structure are completely determined. The growth kinetics and the roles of Tb3+ and Cl- ions are discussed on the basis of the competitive adsorption model.