Interface states between an insulating layer and a transparent electrode were investigated, being concerned with the stability and the luminescent properties of CaS thin film electroluminescent (EL) devices. It was found that Y2O3 and CaS thin films deposited on an ITO transparent electrode at 500°C were peeled off when these films were heat-treated at 500°C for one hour. The peeling-off is thoroughly prevented by inserting a SiO2 thin film as a buffer layer between the Y2O3 and ITO films. It was shown that the luminescent properties of the EL device was also improved by using the SiO2 buffer layer. The roles of the buffer layer are discussed from the results of the elemental depth profile measured by Auger electron spectroscopy.
