Electroluminescence of Schottky diodes rare-earth-doped zinc sulphide

Trivalent impurities substituting for divalent host atoms in insulators are normally charge-compensated by other impurities or defects. In semiconductors they behave as donors with charge compensation by electrons. The rare earths in ZnS have an intermediate behaviour. Absorption and luminescence show the trivalent configuration even in n-type material, except for Eu 2+ with its half-filled shell. This is in contrast to 3d n impurities, where the d-shell occupancy can, other than for Mn, be changed. The authors show that the luminescence fine structure of ZnS:Sm is different after zinc and sulphur annealing, indicating local charge compensation. However, it is easy to make conducting n-type ZnS with rare-earth doping and zinc treatment, which suggests that at least some rare-earth atoms act as donors. Such material shows, in addition to 4f n lines, broad green and red bands in photoluminescence. In electroluminescence at a Schottky barrier for conducting ZnS:Re a broad band with fast decay is observed. This is attributed to transitions between different parts of the conduction band. In addition there is a green band similar to that seen in photoluminescence, but any 4f n lines are below the noise background. The results show that the presence of luminescent rare-earth centres and hot electrons is not sufficient for rare-earth emission in electroluminescence.