DC electroluminescent ZnS (Cu, Mn) thin-film devices are prepared by successive vacuum deposition on SnO2 substrate, followed by heat treatment and deposition of an Al or an Au electrode. The devices are completed by forming at elevated temperatures and also completed by forming without heating for comparison. It is clarified that the forming at elevated temperatures can easily create a high-resistance layer in the phosphor films and improve luminous efficiency. Efficiencies of 1.54 lm/W and 0.5 lm/W are obtained in the devices with Al and Au electrode, respectively. The analysis of Auger electron spectroscopy shows that there is an Al oxide layer between the phosphor film and Al electrode. The oxide layer is considered to contribute towards making the higher resistance layer. The results of the measured photovoltaic spectral responses suggest that the interface between the unformed high-conductance region and the formed high-resistance region seems to play an important role as a Schottky barrier.