Nanoscopic trigonal pyramidal crystallites in WS 2− x sputtered thin films: a scanning tunnelling microscopy study of initial growth

Abstract

We present original scanning tunnelling microscope (STM) images of the first growth stages of crystalline, textured WS 2− x thin films ( x=0.2−0.4) prepared by reactive RF magnetron sputtering. In the first 10–20 nm of thickness, the films grow in the shape of trigonal pyramids with typical lateral size in the 20 nm range. Step heights of 0.6 nm show that the films grow by molecular layers rather than by complete 2H or 3R polytypic unit cells of c parameters 1.2 or 1.8 nm. The triangular shapes reflect the symmetry of the trigonal prismatic unit and the pyramidal arrangement indicates a rhombohedral stacking. The growth process is three-dimensional, including some spiral growth.