character exhibited by a free-ion. These 4f levels are subsequently unaffected by the host

lattice.

 Chase et al [47] investigated the EL emission of the device with rare earth fluorides

in the phosphor layer. For ZnS:ErF3, the device has a faint green emission which results

from the transitions 2H11/2 -* 15/2 and 4S3/2 4115/2. Fig 2.12 shows the complete

spectrum. About 10 states are observed to fluoresce.

 For ZnS:TmF3, weak emission in the blue and the red is observed (Fig 2.13). The

emissions are believed to originate from the 1G4 state.

2.6.3 Charge Compensation

 Earlier works [46] have noted that samples doped with Re metals show only spotty

luminescence, whereas halide coactivation (F-, C1-) gives uniform emission over the

entire surface. Moreover, it's difficult to introduce rare earth ions substitutionally into

ZnS with relatively high concentration because of the mismatch in ion size and charge

valence between Zn2+ and RE3 ions. Chase et al. [47] proposed a new type of emission

center which introduced rare earth fluoride in ZnS. Studies by Jayaraj et al. [48] on the

effect of halides (F-, C1-, Br-) and oxide (02-) on the EL emission spectra of ZnS: Pr

TFEL devices show that the fluoride dopant produces the maximum brightness. In this

study, the ZnS film is doped with fluorine for charge compensation.