Crystallographic defects in epitaxial layers of cadmium sulphide

Thin epitaxial films of CdS deposited in high vacuum on ionic single crystal substrates have been studied by transmission electron microscopy. The (100), (110) and (111) faces of NaCl and the (111) face of BaF₂ were used as substrate surfaces. Both cubic sphalerite and hexagonal wurtzite structure films have been produced. The orientations of the sphalerite structure films were (100) and (110) and were produced on substrate faces having the same two orientations. The wurtzite structure films were in (0001) orientation and grew on (111) oriented substrate faces. For a fixed rate of deposition both the number and type of defects found in the films appear to be dependent upon the growth temperature and the crystal structure. Annealing the films at a high temperature has been tested as a means for reducing their defect content and the effect is very different for the two crystal structures. A reduction in the defect content of the wurtzite structure films is induced but no change in the crystal structure occurs. In contrast to this, the sphalerite structure films undergo a progressive phase transformation to the wurtzite stucture while at the same time losing a high proportion of their defects.