The microstructure and electroluminescence (EL) characteristics of ZnS:Mn doped with KCl for alternating current thin film electroluminescent devices deposited by a RF sputter system from ZnS and Mn targets are investigated. Transmission electron microscopy (TEM), X-ray diffractometry, energy dispersive spectrometry (EDS), secondary ion mass spectrometry, EL brightness and luminous efficiency (eta) measurements were carried out. The as-deposited films have a 100 nm layer of equiaxed grains at the insulator/phosphor interface and columnar grains in the rest of the film. K is uniformly distributed through the 1 mum ZnS film after 5 min anneals at Tgreater than or equal to600 degreesC, while Cl requires an anneal at Tgreater than or equal to700 degreesC before diffusion results in a uniform concentration. Upon annealing at 700 degreesC for 5 min, the equiaxed grains were consumed by growth of the columnar grains, and growth of the columnar grain diameter is enhanced by the presence of KCl. The mixed cubic and hexagonal atomic stacking observed in as-deposited films became more cubic upon annealing with KCl co-dopants. EDS data from plan-view TEM samples indicate that K did not segregate to the grain boundaries. Considering the large ionic radius of K+ relative to Zn2+ (1.51 vs. 0.74 Angstrom), and the small temperature dependence of K diffusion into ZnS, it was postulated that K+ diffuses by an interstitial mechanism. In addition, it occupies an interstitial site in the cubic structure where it acts as a donor, similar to substitutional Cl-. ZnS:Mn co-doped with KCl shows significant improvement in EL brightness and luminous efficiency. It was concluded that the improved EL properties result from both improved crystallinity and modification of point defects and space charge in the ZnS:Mn, K, Cl films.