Microstructure and light emission of ac thin-film electroluminescent devices

Abstract

The microstructure of ac-thin film electroluminescent devices was studied by transmission electron microscopy (TEM) of cross-sectional specimens, and was correlated to electro-optical characteristics of the devices. The cross sections reveal the microstructure of the BaTiO3/ZnS:Mn/BaTiO3 layer structure as a function of film depth. The rf-sputtered BaTiO3 films are amorphous; the electron-beam evaporated ZnS films are polycrystalline. The first-to-grow region of the ZnS layers always exhibits a very small grain size. With increasing film thickness the growth of larger columnar grains is observed. At higher substrate temperatures the grains have a more conical shape. The Mn-concentration, measured by x-ray microanalysis on the cross-sectional specimens in a scanning TEM, shows an overall increase with growing ZnS film thickness, because the ZnS was evaporated from a single ZnS:Mn source. Annealing at 550 °C improves the electro-optic characteristics of the samples considerably, without changes in microstructure being observable. Annealing at 850 °C induces strong recrystallization. The initial fine-grained region disappears and the grains extend from the bottom to the top of the ZnS film.