Nanoindentation and Bending Fracture Behavior of Flexible Sulfide Thin Films Grown at Near Room Temperature With in Situ Tensile/Compressive Stress

Residual stress is known to affect significantly the mechanical fracture behavior of flexible thin films. Here, an in situ chemical deposition process that allows for control of extra compressive or tensile stress in PbS thin films is suggested. This process uses the fixed convex or concave surface of a flexible substrate to impart compressive or tensile stress, respectively, after the releasing step to the flat mode. Using two technical evaluations of mechanical fracture behavior, the existence of extra stress is verified with expected enhancements of the fracture resistance only in the case of compressive stress. For example, a ≈36% improvement of the elastic modulus is confirmed by nanoindentation testing for compressively stressed PbS thin films. The bending fracture resistance is also enhanced for the compressively stressed films, as evidenced by a ≈18.5% improvement in the crack-initiating critical strain and a ≈121% increment in the fracture energy under the bending motion.