The cylindrical surfaces of ZnS and ZnS:Cu single crystals grown from the vapour phase frequently show some parallel striations perpendicular to the c-axis. They are sparse in the smooth zone of crystal surface and dense in the step-like rough surface zone. We call these striations "the growth-layer lines" (GLL) in the present paper. When the ZnS:Cu crystal is put in an a.c. electric field electroluminescent lines (ELL) lying on the GLL and situated frequently on the

<

1120

>

irection are observed with the shape of comet and appearing in pairs. The degree of the dense of the ELL and GLL along c-axis are corresponding to each other. But these lines are not in one to one corresponding

i.e.

there may be no ELL in some regions where there is GLL.The crystals were grown from vapour phase at 1050℃. The structure of three crystal regions has been analysed by means of the X-ray rotation potograph. The results indicate that the structure of the zone without GLL is ordered 2H-ZnS; in the zone where GLL’s are sparse the structure is 2H seperated by a lot of stacking faults with random distributions in the zone with dense GLL

most part has the 2H structure while the rest is of the form of 3C-ZnS and 4H-ZnS (or other polytypes)

all of them being also separated by stacking faults with random distribution. It should be pointed out that in the c-axis rotation photogaph for the zone with dense GLL

an unsymmetrical arrangement

with respect to the zero-layer line

of the spots of 3C and 4H on the h-k=3r (

r

=0

±1

±2

...) row has been observed. Such kind of asymmetry results from a small displacement of the corresponding reciprocal lattice point along e* direction. The small displacement may be due to a displacement of the atom on the close-packed plane.It is shown that the structure of GLL zone is formed during crystal growth and during the cooling down period. The structure transformation of 2H--3C-ZnS occurs through a disordering process of 2H.The random distribution of the stacking faults (disorder) always appears in the zone of GLL. The experimental results in the present paper and in a later investigation (to be published) show that the random distribution of the stacking faults is a necessary condition for the appearance of EL in the ZnS:Cu crystal grown from vapour phase.