Joshua M. Pomeroy

Hyperthermal ion beam system optimized for studying the effects of kinetic energy on thin-film growth

A hyperthermal and low-energy ion beam (10–1000 eV) optimized for studying morphological trends in epitaxial metal thin films as a function of atomic kinetic energy has been built and characterized. The ion beam line produces metal and inert gas ions and is specially designed to produce up to 2.9 μA of highly collimated ions with single amu mass resolution while precisely controlling the ion’s energy, achieving a ΔE/E∼0.1. Energy resolution can be enhanced further at the expense of flux. Varying the focal length of the final electrostatic lens allows the flux density to be adjusted from 10 to 500 nA/mm2. The beam line has been coupled to an ultra-high-vacuum deposition chamber with a versatile sample manipulator, an electron beam deposition source, residual gas analysis, and real-time reflection high-energy electron diffraction (RHEED). Once prepared, the sample can be moved in situ to perform Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The high fluxes with narrow energy di...

STM investigation of energetic insertion during direct ion deposition

Thin copper films have been deposited on single crystal copper substrates and characterized using a UHV Scanning Tunneling Microscope to probe the effect of atomic insertions during hyperthermal ion deposition. At low temperatures, atomic insertions are predicted to provide a net downhill current that offsets the roughening effect due to uphill “Schwoebel” currents leading to a net smoothing of the surface. Films have been grown at several different energies targeted to observe a crossover from insertion driven smoothing to adatom-vacancy dominated roughening. Copper thin films are deposited near 20 eV using a mass selected ion deposition system that allows precise control (+/− 2 eV) over the energy of constituent atoms. Experimental observations are compared with a sophisticated Kinetic Monte Carlo and Molecular Dynamics hybrid (KMC-MD) simulation.

STM characterization of Cu thin films grown by direct ion deposition

In certain cases, the incidence energy of constituent atoms activates an atomistic insertion mechanism, which decreases the surface roughness of metal thin films. In an effort to probe this effect, homoepitaxial copper films were grown using a mass/energy selected direct ion deposition technique that allows precise control of the incidence energy. Surface roughness is measured using a Scanning Tunneling Microscope (STM) within the same UHV surface analysis system. The activation of the insertion mechanism near 20 eV triggers smoother crystal growth. The beneficial effects begin to be obscured by adatom/vacancy creation near 30 eV. A sophisticated Kinetic Monte Carlo/Molecular Dynamics (KMC-MD) model supports this interpretation.

STM studies of island nucleation during hyperthermal atom deposition

We report fundamental changes in island nucleation dynamics as the kinetic energy of the constituent particles used for film grown is increased. A hyperthermal energy ion beam-line with precise control over ion kinetic energy was used to grow copper islands on a Cu(100) substrate. Dramatic increases in island densities were observed with increasing kinetic energy from thermal energies to 150 eV. We find that sputter erosion and the formation of adatom-vacancy pairs contribute to this increase. In addition, variations in flux and temperature suggest that the mean-field scaling exponent is sensitive to atomistic mechanisms activated by the ion beam.

STM analysis of copper thin films using hyperthermal copper ions

STM analysis of thin copper films grown on Cu(111) and Cu(100) at room temperature using hyperthermal ions reveals several morphological features not present in thermally grown films. Hyperthermal deposition of thin films has become a popular industrial technique due to observed decreases in film roughness and stress with increased grain sizes, but the link between incidence energy and these properties is poorly understood. Using a sophisticated molecular dynamics (MD) simulation, morphologies observed in experimentally grown films are correlated with the activation of hyperthermal atomistic mechanisms in the MD. Previous work has provided strong evidence for activation of adatom-vacancy pair production near 20 eV on Cu(111). Evidence will be presented here for the activation of sputter erosion near 40 eV. A proposal for quantifying the effect of energy on nucleation density by modifying the flux term in mean field nucleation theory will be presented. Other processes which directly contribute to smooth growth, such as atomic insertion near 10eV, have been proposed but are more difficult to correlate with the data due to their subtle morphological signatures.

Time-Resolved X-Ray Scattering Study of Co Surface Evolution during Low-Energy Ion Irradiation

The sputter-erosion of hcp Co (0001) single crystal with Ar + ions in the 100 to 700 eV energy range was investigated using in-situ time-resolved x-ray scattering. At temperatures above 300°C the surface remains relatively smooth, erosion evolving through a layer-by-layer or step flow mechanism. In this regime the ions have a smoothening effect on the surface and the resulting roughness decreases with increasing ion energy. Below 300°C the surface develops a pattern of mounds or pits with a characteristic wavelength. The time, ion energy and temperature dependence of this wavelength were studied in detail. Epitaxial Co thin films thermally evaporated on sapphire were also sputtered through in order to synthesize self-assembled arrays of Co nanoclusters with a narrow size distribution. The degree of local order within the Co dot arrays was examined using atomic force microscopy.