Recently much attention has been paid to the study in blue LEDs of ZnS. In order to make efficient LEDs of ZnS

a basic problem

i.e.the reproducible preparation of low-resistivity bulk materials must be solved.In this paper

we present a method to produce low-resistivity ZnS:I crystals end discuss the influence of iodine concentration in ZnS:I crystals on annealing condition.ZnS single crystals studied in this work were grown by I-chemical vapor transport.The crystals were cut into slices 1.5mm thick.After chemical cleaning and etching

they were heated in a molten mixture of Zr-Al (1.5-2%) in a sealed quartz ampoule which was coated by careen film for 40-100 hours at 950℃ and then were quenched immediately in cold water.Both faces of samples were lightly polished and toiled in a solution of 25% NaOH-in-H2O for 30-60sec and then washed in hot deionized water.The chmic contacts were made by heating In-Ga alloy in N2 flow at 400℃ for 5-10 min. The resistivity of samples were mesured by Van der Pauw method of from I-V characteristics.ZnS:I crystals usually shew different bulk colour. According to difference of the bulk colour

ZnS:I crystals can be divided into three kincis:A--Colourless and transparent;B--Yellowish and C--Yellow.Chemical analysis shows that their I concentration is 45

100 and 130 ppm respectively.It is identified that the bulk colour of crystals is correlated with iodine concentration in the crystals. The higher the iodine concentration

the darker the bulk colour.It is found that different annealing conditions are needed for ZnS:I crystals with different bulk colour in order to obtain the same resistivity.Having taken suitable annealing condition

Al concentration in ZrS:I crystals can be controled at the order of 10

-3

g/g and the resistivity of ZnS:I slices can be reduced down to 10-102.cm.The MS(metal-semiconductor) junction made by the low-resistivity ZnS crystals can produce blue EL. The emission peak is located at about 4500A at room temperature. Finally

the influence of iodine on Al diffusion was discussed by means of volume compensation principle.In general

iodine ions enter into ZnS crystals at the substitution site and yet the ion radius of iodine is bigger than sulphur ion. On the other hand

aluminum ion is smaller than zinc ion. So Al ions occupy more easily those vacancies which are the nearest neighbors of the iodine ions during annealing.That is

doping of iodine ion may influence on Al ion diffusion to a certain degree. Therefore under the same annealing conditions

for a higher iodine concentration in ZnS crystals

a large amount of Al can be incorporated in the lattice.It is concluded that the diffusion of Al impurity have not only a connection with annealing condition and Zn-Al alloy composition but also with the iodine concentration in ZnS crystals. Different annealing conditions are needed for ZnS:I crystals with different iodine concentration(i.e. different bulk collour) o obtain same low resistivities.