Epitaxial growth of films of ZnS evaporated on to NaCl

Abstract

Films of ZnS were deposited on to NaCl by vacuum evaporation. The influence of the growth conditions on the structure of the films was studied by transmission electron microscopy. The most important single factor in producing the widest range of epitaxial substrate temperatures and the most perfect films was the use of focused electron-beam evaporation. Contamination raised the minimum substrate temperature for epitaxial growth. It also resulted in the incorporation of doubly positioned wurtzite-structure grains in the sphalerite-structure films. Co-evaporation of an excess of either Zn or S with ZnS produced sphalerite-structure films containing singly positioned wurtzite grains. The use of conventional hot tantalum evaporators resulted in the incorporation of 0·2 wt.% of tantalum in the films and in poor structural quality. Reproducible current-voltage characteristics were obtained in the best structural quality films. These current-voltage characteristics could be understood in terms of the effect of Schottky barriers but not of space-charge-limited currents.

The structure of epitaxial films of ZnS evaporated onto NaCl in a vacuum

“…The complete absence of wurtzite-structure included grains and of microtwins in the best films produced by focused electron beam evaporation [1,2] constituted a considerable advance in structural perfection compared with previous work in the literature (reviewed in [8 ]). The presence of only one type of included grain in the films prepared at non-optimum substrate temperatures, namely double-(or single-) positioned wurtzite is significant.…”

“…The films grown under the cleanest conditions were free of included grains and no satellite spots appeared in their diffraction patterns [1,2]. The diffraction patterns from films of ZnS free of included grains however were prominently streaked in the (111) directions.…”

“…ZnS films were epitaxially grown on vacuumcleaved faces of NaC1 by focused electron beam evaporation in high and ultra-high vacua as previously described [1,2]. The films were floated off the substrates in water and examined by transmission electron microscopy.…”

“…1 [2]. These grains all occur with the (0001) basal plane parallel to the (100) plane of the cubic matrix of the film.…”

“…Co-evaporation of either Zn or S with ZnS produced cubic films, large areas of which contained wurtzite grains in only one of the two positions. It was also shown that the single-phase single crystal ZnS films gave reproducible current-voltage characteristics which were interpreted as due to thermionic emission of carriers over field-lowered contact barriers (the Schottky effect) [2].…”