| AVS 55th International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuP |
| Session: | Surface Science Poster Session |
| Presenter: | J. Sun, University of New Hampshire |
| Authors: | J. Sun, University of New Hampshire J.B. Hannon, IBM T. J. Watson Research Center G.L. Kellogg, Sandia National Laboratories K. Pohl, University of New Hampshire |
| Correspondent: | Click to Email |
Controlling compositional heterogeneity is important in ultrathin films growth, but determining exactly how and why heterogeneity develops is extremely challenging. The reason is that the three-dimensional compositional and structural profile of the film is difficult to measure because of the lack of surface techniques that combine high spatial resolution, subsurface sensitivity, chemical identification capability and high temporal resolution. For example, STM is not sensitive to the subsurface region and LEED averages over a large surface area. To overcome these limitations, we have developed a novel analysis approach1 that allows us to measure the evolution of the 3D compositional and structural profile of a heterogeneous alloy surface in real time. We do this by quantitatively analyzing the pixelated intensity in the low-energy electron microscopy (LEEM) images. In the dynamical IV (intensity-vs.-voltage) analysis, a proper model for the inner potential, representing the atomic muffin-tin constant and the inelastic optical scattering, was adapted to overcome the challenges in very low-energy electron scattering. The structural and non-structural parameters are optimized simultaneously in search of the real surface structure that gives a best fit between the calculated and experimental IV curves. We have measured the composition of a CuPd surface alloy in the three topmost atomic layers, during growth, with 8.5 nm lateral resolution and monolayer depth resolution. From the 3D compositional and structural profiles, we have identified a generic step-overgrowth mechanism that leads to inherent alloy heterogeneity at steps. The heterogeneity can be traced to the difference between bulk and surface diffusion of Pd. Furthermore, Monte Carlo simulations are described to reproduce the time evolution of the compositional heterogeneity and give support to the step-overgrowth model. By the LEEM-IV analysis technique, the surface structural and compositional information measured in situ can be correlated with other surface properties, such as surface strain, diffusion mechanisms, and growth and decay processes. This work is supported by the National Science Foundation, the Department of Energy, Office of Basic Energy Sciences, and the Petroleum Research Fund.
1Sun, J., Hannon, J. B., Kellogg, G. L. & Pohl, K., Phys. Rev. B 76, 205414 (2007); Hannon, J. B., Sun, J., Pohl, K. & Kellogg, G. L., Phys. Rev. Lett. 96, 246103 (2006).