A combined optical, SEM and STM study of growth spirals on the polytypic cadmium iodide crystals

Abstract

Some novel results of a combined sequential study of growth spirals on the basal surface of the richly polytypic CdI₂ crystals by optical microscopy, scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) are presented and discussed. In confirmation of the known structural data, the STM pictures clearly reveal the value of unit cell a(=b) dimension to be equal to 0.428 nm a and the value of the interaxial angle γ to be equal to 120°. Under the high resolution and magnification achieved in the scanning electron microscope, the growth steps of large heights seen in the optical micrographs are found to have a large number of additional steps of smaller heights existing between any two adjacent large height growth steps. When further seen by a scanning tunneling microscope, which provides still higher resolution, several unit substeps are seen to exist between two consecutive additional steps. The height of each sub step is found to be equal to the unit cell height of the underlying polytype. The height of the larger additional steps are found to be integral multiple of the unit cell height. The horizontal distance between two consecutive growth steps is found to be an integral multiple of the lattice parameter a. The interspacing of the growth steps of same step height is found to vary at different positions on the same crystal face.