B. H. Cooper

Simulations of energetic beam deposition: From picoseconds to seconds

We present a method for simulating crystal growth by energetic beam deposition. The method combines a kinetic Monte Carlo simulation for the thermal surface diffusion with a small scale molecular-dynamics simulation of every single deposition event. We have implemented the method using the effective medium theory as a model potential for the atomic interactions, and present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to 35 eV. The method is capable of following the growth of several monolayers at realistic growth rates of 1 ML per second, correctly accounting for both energy-induced atomic mobility and thermal surface diffusion. We find that the energy influences island and step densities and can induce layer-by-layer growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag), which correlates with where the net impact-induced downward interlayer transport is at a maximum. A high step density is needed for energy-induced layer-by-layer growth, hence the effect dies away at increased temperatures, where thermal surface diffusion reduces the step density. As part of the development of the method, we present molecular-dynamics simulations of single atom-surface collisions on flat parts of the surface and near straight steps, we identify microscopic mechanisms by which the energy influences the growth, and we discuss the nature of the energy-induced atomic mobility.

Low energy ion-surface interactions

In this paper we present measured energy and angular distributions for 10–100 eV Na+ scattering from Cu(001). Excellent agreement with the measured spectra is achieved with a simulation that uses a model potential for the ion-surface interaction. From these spectra we can extract information about the scattering dynamics, such as the scattering trajectories, energy transfer to the surface, and particle trapping. For energies below 50 eV we find that these quantities are particularly sensitive to the attractive well in the potential. We also present measurements that probe the charge transfer process (the exchange of electrons between the atom and surface) for 5–1600 eV alkali ion scattering from Cu. We briefly discuss the role that charge transfer plays in influencing the scattering dynamics.

Charge transfer dynamics of low energy collisions of Li+ with alkali-covered Cu(001)

Abstract We present measurements of branching ratios to different final states of the scattered particles for 400 eV Li + ions incident on alkali-covered Cu(001). In particular, we have measured the charge state fractions and the relative yield of excited states in the scattered flux as a function of the adsorbate-induced work function shift. The results of a resonant charge transfer theory which includes excited states, negative ions, and level crossings are compared to the measurements. Qualitative agreement is achieved with the observed work function dependence of both the charge state fractions and the relative yield of Li(2p). In particular, the theory predicts that the peak observed in the Li(2p) yield is due to a competition between the Li(2p) state and the Li − (2s 2 ) state.

Persistent layer-by-layer sputtering of Au(111)

Persistent layer-by-layer removal of Au(111) during Ar+ ion irradiation was observed in a real-time x-ray scattering study. Over 100 specular beam intensity oscillations were measured. For a given ion energy, a smoother surface morphology is obtained when the ion flux is reduced. For a fixed erosion rate, ion energy in the range of 70–500 eV does not have a strong influence on the evolution of surface morphology. Diffuse scattering measurements show the development of features with a characteristic lateral length scale on the surface during ion irradiation.

Influence of step edge diffusion on surface morphology during epitaxy

The role of step edge diffusion in epitaxy is discussed using kinetic Monte Carlo simulations. Three models with different rates of edge diffusion are investigated. Increasing the rate of edge diffusion results in an early onset of second layer nucleation, pattern formation in thick films, and increased surface roughness. An expression for the uphill current due to edge diffusion is derived in the limit of fast edge diffusion.

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.

Surface Diffusion: Atomistics and Surface Morphology (Summary of MRS Symposium B Panel Discussion)

The paper gives some of the highlights of a panel discussion on surface diffusion held Monday, November 30, 1992 at the Fall MRS Meeting in Boston, Massachusetts. Four invited speakers discussed computer modeling techniques and scanning tunneling microscopy experiments that have been used to provide new understanding of the atomistic processes that occur at surfaces. We present a summary of each of the invited talks, indicate other presentations on surface diffusion in this proceedings, and provide a transcript of the two discussion sessions.

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.

Real-Time X-Ray Scattering Study of Surface Dynamics on Au(111) During Ar+ Ion Irradiation

X-ray scattering is used to investigate the surface dynamics on Au(111) during Ar + ion irradiation. During 500 eV Ar + ion irradiation, we observe the three regimes of step retraction, quasi-layer-by-layer removal and three dimensional rough erosion, analagous to molecular beam epitaxy. The quasi-layer-by-layer sputtering regime has been studied to identify similarities and differences in surface evolution during ion irradiation and molecular beam epitaxy. X-ray measurements suggest that 500 eV Ar + ion irradiation does not lead to stable adatom island formation. Also, in contrast to molecular beam epitaxy, adatom detachment and diffusion seems important in describing the surface kinetics during ion irradiation.