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.