Surface microtopography in equilibrium at elevated temperatures: step fluctuation spectroscopy with low-energy electron microscopy.

M. Ondrejcek^1,2, W. Swiech^2,* and C. P. Flynn^1,2

¹ Department of Physics, ² Frederick-Seitz Materials Research Laboratory and ^* Center for Microanalysis of Materials, University of Illinois at Urbana-Champaign, Urbana, IL 61820

We explored the surface microtopography evolution and equilibrium fluctuations of steps on close packed surfaces of model metals. The work employed low-energy electron microscopy (LEEM) in the temperature range 1100-1680K for Mo samples and 1200-1500K for Pt, respectively. We describe investigations of step fluctuations on Mo (011) thin single crystal films with various azimuthal orientations of miscut as well on Pt (111) bulk single crystal in order to determine the step stiffnesses. Using capillary wave analysis we obtain fluctuation amplitudes and step relaxation times as function of wave number q by calculating the Fourier components of the step edge displacement. Step fluctuations on metals under investigation are generally smaller and faster than results for elemental semiconductors. The analysis of Mo data yields an anisotropic stiffness of about 0.36 eV/nm along [01] and about 0.15 eV/nm along [100]. The step free energies derived from the stiffnesses are less anisotropic by about a factor 3. From the temperature dependence of the relaxation rates, activation energy of 0.8 ± 0.2 eV is determined for the mass diffusion of the mobile defects responsible for the fluctuations. Pt exhibits a more complex behavior. Below 1400K the relaxation times vary with wavevector q as q3, and above 1400K as q2.  The data show that surface diffusion is responsible for the rates at low temperatures, and that bulk vacancy diffusion becomes dominant above 1400K where the observed activation energy changes from 1.25 eV to 2.76 eV. We infer that a similar crossover from surface to bulk diffusion must occur for steps on most metal surfaces.

This research was supported in part by the DOE under grant DEFG02-91ER45439.