We studied the grain growth in thin nanocrystalline Au films deposited on sapphire substrate with and without ultrathin underlayer of Ti (adhesion promoter). The ultrathin film of Ti was first deposited onc-oriented sapphire by electron beam, followed by the deposition of 80 nm-thick Au film. The samples were annealed at 200°C for 2 h, under the conditions at which the bulk diffusion of Ti in Au is negligible.
The thickness and the grain size of the films before and after annealing were determined by transmission electron microscopy (TEM). The surface morphology of the films was analyzed using an Atomic Force Microscope (AFM). Both methods indicated that the initial grain size in the films with and without Ti underlayer was about 50 nm. While significant grain growth occurred in thin Au films on sapphire after annealing, the microstructure remained stable in the Au films with Ti underlayer. The depth dependence of the chemical composition of the annealed Au/Ti films was measured using Time of Flight - Secondary Ion Mass Spectrometry (TOF-SIMS), while the chemical composition of the grain boundaries (GBs) was determined by energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscope (STEM) and by energy-filtered TEM. The concentration profiles measured with the aid of TOF-SIMS indicated that Ti diffused across the Au film and revealed some Ti oxide at the surface of Au. The HAADF STEM images of the Au/Ti film cross-section indicated the presence of Ti oxide at the surface of Au film, with typical thermal groove morphology in the vicinity of GBs. The dihedral angle at the bottom of the groove was very sharp (about 60deg), indicating that the energy of Au/titania interface is low in comparison with the surface energy of Au. Also, the elements mapping in the STEM EDS mode revealed that Ti is presented in some of the GBs. This was correlated with the results of AFM study, showing the characteristic “ridges” at some, but not all GBs. These topography ridges were associated with titania phase observed in TEM.
We attributed the stability of the microstructure of Au/Ti films to the stabilizing effect of titania phase formed at the sites where the GBs of Au emerge to the free surface. These sharp wedges of titania pin the GBs in Au and prevent the process of grain growth. We formulated a diffusion model describing the kinetics of growth of the titania-filled GB grooves at the surface of Au film. The calculated morphologies of titania phase were in good agreement with the experimental data.