Nanoindentation Behaviour of As-Deposited and Annealed CuO/GaAs Thin Films

CuO thin films with thicknesses of 200 nm and 300 nm, respectively, are deposited on GaAs substrates. The mechanical properties of the CuO/GaAs thin-film systems are evaluated under room temperature conditions by nanoidentation tests performed to depths ranging from 150 to 350 nm. The tests are performed using both as-deposited samples and samples annealed at 500℃ for 30 minutes. The results show that for the as-deposited specimens, a pop-in effect occurs in the loading curve for all film thicknesses and nanoindentation depths (150 nm, 250 nm and 350 nm) due to a delamination of the thin film from the substrate. However, for the annealed samples, no pop-in events are observed, irrespective of the film thickness or nanoindentation depth. For both samples (as-deposited and annealed), the maximum indentation load increases with an increasing indentation depth and film thickness. However, the shape of the hardness-depth curve for the annealed specimens is different from that of the as-deposited specimens. Moreover, the hardness of the annealed specimens is less than that of the as-deposited specimens at the maximum indentation depth. Similar tendencies are observed for the variation of the Young's modulus with the indentation depth. Scanning electron microscopy (SEM) observations show that the indentation area increases with an increasing indentation depth and thin film thickness, but decreases in the annealed condition. Furthermore, the transmission electron microscopy (TEM) observations reveal that no delamination occurs for the annealed specimen with the maximum thickness of 300 nm under indentation depths of 150 nm and 350 nm. In addition, dislocations within the GaAs substrate are apparent only in the annealed specimen with a thickness of 300 nm and an indentation depth of 350 nm. The selected area (electron) diffraction (SAED) patterns confirm that the as-deposited and annealed CuO film has a polycrystalline structure, while the GaAs substrate has a single crystal structure. Finally, the high-resolution TEM (HRTEM) micrographs show that the as-deposited and annealed CuO film and GaAs substrate have lattice spacings of 0.25 nm and 0.283 nm, respectively.