Influence of the substrate thermal expansion coefficient on the morphology and elastic stress of CoSb₃ thin films

Abstract

During the post-annealing and cooling process of CoSb 3 thin films deposited on thermally oxidized Si(100) substrates, cracks occur at the surface of the films, which can be caused by the difference in thermal expansion coefficient of the substrate and the film. To investigate the crack formation, 40-nm-thick CoSb 3 films were deposited at room temperature under ultrahigh vacuum (UHV) conditions onto various substrates, exhibiting different thermal expansion coefficients (2 Â 10 À6 to 12 Â 10 À6 K À1 ). All samples were post-annealed in UHV at 500 8C for 1 h. The composition of the films was verified by Rutherford backscattering spectrometry. The phase formation and elastic stress of the films were analyzed by X-ray diffraction, confirming the formation of the desired skutterudite phase, while the individual grains were studied by electron backscatter diffraction. In addition, the surface morphology and the roughness of the films were investigated by atomic force microscopy. For substrates with a thermal expansion coefficient between 9 Â 10 À6 and 11 Â 10 À6 K À1 , crack formation can be prevented and a minimum in roughness was found, resulting also in a minimal value of the electrical resistivity.

Structural and thermoelectric properties ofFeSb3skutterudite thin films

“…3 (b)). The cracks occur at the grain boundaries due to the mismatch in lattice expansion coefficient of substrate and film 34 , which can be seen also in the EBSD band contrast map shown in fig. 3 (c).…”

“…Band contrast maps reveal a bright contrast for positions, where a crystalline skutterudite structure is clearly observed, and a dark contrast for points, where no crystalline structure is detected (like amorphous regions or grain boundaries). Each separated area corresponds to a grain with different orientation 34 and the grain boundaries exhibit a thickness smaller than 50 nm.…”

Structural properties of thermoelectric CoSb3 skutterudite thin films prepared by molecular beam deposition

“…Above 77 at.% Sb, a fine-grained surface structure distorted by several cracks is observed for the skutterudite films. In a previous work [31], it was shown by EBSD that the cracks occur at the grain boundaries. Each large area enclosed by the cracks exhibits a specific crystal orientation and corresponds therefore to a single grain [31].…”

“…In a previous work [31], it was shown by EBSD that the cracks occur at the grain boundaries. Each large area enclosed by the cracks exhibits a specific crystal orientation and corresponds therefore to a single grain [31]. Thus the determined grain size of the annealed single-phase skutterudite films lies between 3 lm and 10 lm depending on the film composition.…”

Thermal stability of thermoelectric CoSb3 skutterudite thin films

“…The strongest increase is caused by cracks that form during the annealing process at temperatures larger than 300°C. The films are deposited on SiO 2 (100 nm)/Si(100) substrates and the cracks occur due to the mismatch of the thermal expansion coefficient of the film and the substrate [22]. The cracks become more pronounced with further increase of the annealing temperature.…”