Dislocation Structure and Growth Mechanism of Cadmium Sulfide Crystals

Abstract

Crystals, needles, ribbons, and platelets, grown from pure and impurity-added CdS vapor, have been studied morphologically. Needles grow parallel to the [0001], [101̄0], and [21̄1̄0] axes of the hexagonal system. Ribbons grow parallel to the [101̄0] and [21̄1̄0] axes and their surfaces are parallel to the (12̄10) and (01̄10) planes, respectively. The surfaces of the platelets are parallel to the (12̄10) planes. The dislocation structures of grown crystals have been observed by x-ray diffraction microscopy and chemical and thermal etch techniques. In order to hinder the dislocations from slipping out of the crystals, some impurity atoms have been put in the crystals. From the observations of growth manners and dislocation structures of the platelets, ribbons, and needles, it is concluded that whiskers grow in the initial growth stages of every crystal, and when the whiskers contain two parallel dislocations along the axis, the platelets are formed by the thickness increase of the whiskers in a direction perpendicular to the axis. One of the dislocations in the whisker has a Burgers vector 1/3[21̄1̄0] and the other is an edge dislocation with a Burgers vector [0001]. Behaviors of these two dislocations during the growth process of the crystal are inferred as follows: The dislocation with a Burgers vector 1/3[21̄1̄0] is responsible for the axial growth of the whisker and is easy to slip out of the whisker, leaving some dislocation half-loops on one of its side surfaces. The edge dislocation with the Burgers vector [0001] is also easy to slip out of the whisker, leaving many dislocation half-loops on another surface. The ends of these dislocation half-loops act as new sites of crystal growth in the direction perpendicular to its axis and a platelet is formed. Some observations which justify the above inference are illustrated. It is discussed theoretically that the two parallel axial dislocations are much more stable than any single axial dislocation in a whisker.