Orientation Inversions in Polycrystalline CdS Bulk Crystals and Thin Films

Abstract

A study of the crystallographic orientation of the single‐crystal grains of large (8.5‐cm‐diam) polycrystals of CdS and of thin films of CdS has been made. Chemical and sputter‐etching observations, x‐ray crystallographic techniques, and piezoelectric polarity measurements have been used to classify the growth habits of the thin films as a whole and of the individual grains of the bulk crystals. Three types of grains have been identified in the bulk CdS polycrystals. These are type I: c axis normal to a Cd‐rich growth face; type II: c axis greater than 15° from the normal to an S‐rich growth face; and inverted type II: c axis greater than 15° from the normal to a Cd‐rich growth face. Type I and inverted type II grains exhibit negative compression piezovoltages, while type II grains are positive. Type I grains are identical to the type I single crystals previously discussed by Reynolds, and type II grains are similar to the 14° misoriented type II single crystals of Reynolds. Similar results were obtained from the evaporated CdS thin films which could be classified as type I and type II, with the normal to the type II films about 20° off the c axis, indicating a close parallelism between the processes occurring in evaporated thin films and in bulk crystal growth. Sputtered CdS films were always type I. It is suggested that the variable efficiencies that have been observed in piezoelectric thin‐film transducers with similar crystallographic orientations are related to orientation inversions in the films.