Protrusions, Depressions, and Indentation Hardness of Pressure-Gradient-Sputtered (Bi2Te3)x(Sb2Te3)1−x Thin Films

Bi2Te3-based thin films for micro-thermoelectric devices must carry metal contacts and withstand device processing. A roughness amplitude alone, the usual surface metric, distinguishes neither protrusions from depressions nor film from substrate features. We examined pressure-gradient-sputtered (Bi2Te3)x(Sb2Te3)1−x films at five compositions on polished alumina by stylus profilometry, white-light interferometry, electron microscopy, and instrumented indentation. Among the thinnest films, protrusions dominate only at x = 0.48 and give a positive skewness (+2.03) and the largest areal roughness (442 nm). At approximately 2 µm thickness, the film at this composition has faceted grains (0.2–0.5 µm), coarser than those at x = 0.00 and 0.24. In the other four thinnest films, sparse depressions 4–11 µm wide give a negative skewness (−3.10 to −10.42). At x = 1.00, two depressions reach below the level of the adjacent bare substrate and thus replicate its defects; elsewhere, they stay within the film thickness. Indentation separates hard Sb-rich films (x = 0.00 and 0.24; hardness 1.76–1.89 GPa; creep 5.1%–8.2%) from soft Bi-rich films (x = 0.73 and 1.00; hardness 0.80–0.99 GPa; creep 20.0%–23.4%). The film at x = 0.24 is also the smoothest (areal roughness 4.2 nm). A stylus profile across the film edge identifies depressions below the bare-substrate level as substrate-derived.