Mahesh Rajappan

Low energy electron microscopy investigations of kinetics and energetics on clean close-packed metal surfaces

Experiments are described in which low energy electron microscopy is employed to investigate the kinetics and energetics of clean close-packed metal surfaces, using step fluctuation spectroscopy. The data contain systematic trends of surface mass diffusion and step energetics that are identified and discussed. Further experiments and equipment development have been undertaken to explore the influence of accelerated mixing on surface evolution caused by ion beams, through the resulting kinetic and energetic perturbations of surface processes.

Surface mass diffusion over an extended temperature range on Pt(111)

Surface mass diffusion is investigated on Pt(111) at temperatures in the range 710–1220 K. This greatly extends the range over which diffusion is known from step fluctuation spectroscopy (SFS). In the present research, a beam of Pt− self-ions is employed to create a suitable structure on step edges. The surface mass diffusion coefficients then follow from the decay of Fourier components observed by low-energy electron microscopy (LEEM) at selected annealing temperatures. The results agree with SFS values where they overlap, and continue smoothly to low temperature. This makes it unlikely that diffusion along step edges plays a major role in step edge relaxation through the temperature range studied. The surface mass diffusion coefficient for the range 710–1520 K deduced from the present work, together with previous SFS data, is Ds = 4 × 10−3 exp(−1.47 eV/kBT) cm2 s−1.

LEEM investigations of surfaces using a beam of energetic self-ions

This article reviews recent research using a low‐energy electron microscope, built by Tromp at IBM, and equipped with an accelerator that permits in situ irradiation with a beam of self‐ions. The available ion energies of 20 eV to 5 keV span the range from epitaxial growth by a hyperthermal beam to sputtering at the level of ∼10 atoms per incident ion. The design criteria and instrument calibration are described. The research described is surface science that requires a vacuum maintained below 10−10 Torr, with all components contained in the same vacuum. Two general categories of applications are sketched. Experiments that accurately measure important physical quantities include surface mass diffusion over an extended temperature range; determining the critical chemical potential at which island nucleation occurs; observation and explanation of the universal evolution by which adatom and advacancy islands both grow and shrink by beam‐driven processes; and the study of sublimation (regarded as negative ion beam intensity). Experiments described here with other goals include beam‐assisted synthesis first of large pans and mesas for isolating surface experiments (e.g., nucleation) from the surrounding crystal, and second of Fourier waves on steps, for studies of diffusive relaxation. Operation of exotic structures including Bardeen‐Herring sources and Frank growth spirals deformed by crystal anisotropy are also described. Microsc. Res. Tech., 2009.

Surface sink action during irradiation for Pt- on Pt(111) by LEEM

A beam of negative Pt − self-ions was employed on Pt(111) at 1050–1150 K, for various impact energies in the range 60–2500 eV, to observe the way lattice sites are added to the crystal by the ion beam. The measurements were made in situ, in a low-energy electron microscope, by direct observation of surface sink (i.e. step edge) motion. The results lie in the regime in which the surface responds linearly to the ion beam intensity. An absolute calibration of the surface sputter yield is made by comparison with recent molecular dynamics (MD) simulations. Within this comparison, the results show that current MD simulations afford a good description of ion beam impacts on crystalline surfaces, and reproduce the observed neutral energy of 245 eV for Pt(111), at which sputtering balances the self-ion input. The net flow of surface steps exhibits the energy dependence expected of combined epitaxial growth and ion beam erosion processes, with bulk and surface lattice sites otherwise conserved.

Surface mass diffusion and step stiffness on V(011)

By step fluctuation experiments on V(011) thin films grown on (112¯0) α-Al2O3, using low energy electron microscopy, we determine the coefficient of surface mass diffusion Ds in the temperature range of 1170K<T<1560K, centered near 0.6Tm, with the melting temperature Tm=2183K for V. As is common to annealed V, Nb, and Ta in UHV, submonolayer coverages of O were present on the otherwise clean and well-defined surface. We obtain Ds=0.8exp (−1.43eV∕kBT)cm2∕s for this temperature interval. Compared to Nb(011), the step stiffness obtained from the measurements is relatively small and isotropic at ∼60meV∕nm. Sublimation is made visible by uphill step flow above 1460K, with a temperature dependence consistent with the known cohesive energy.

Ripples formed in the sputter erosion of Pd(111)

During the sputter erosion of Pd(111) by 1 keV Ar+ ions in the temperature range we observe the development of clearly defined ridges that deepen as sputtering continues up to removed. Step edges observed by low energy electron microscopy appear as contour lines that define the detailed topography of the rippled surfaces. Different characteristic step profiles, and hence surface geometries, are observed after short and long sputtering times. The sequence resembles the interfacial structures reported for the Mullins–Sekerka instability of driven solidification interfaces. At long sputtering times the ridges adopt crystallographic orientations, as reported in recent studies of the ion erosion of crystal surfaces.