A systematic study of the effect of microstructure on the EL properties of ZnS:Er thin film devices fabricated by thermal evaporatio n was carried out using XRD, XPS and EL techniques. The experimental results i ndicate that the high brightness of the devices is attributed to the deposition growth of cr ystallites oriented in the (311), (400) directions. This can be explained by ass uming a higher population of erbium being in the trivalent charge state in the ( 311), (400) planes of the films grown under certain deposition condition. It is concluded that only the part of rare earth dopant being in trivalent charge stat e in the film contributes to luminescence. The mechanism of the formation of lum inescence center and the excitation of Er 3+ ion by the electric field are discussed.
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