| AVS 56th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS1-ThA |
| Session: | Nucleation and Growth - Metals |
| Presenter: | R. Sathiyanarayanan, University of Maryland, College Park |
| Authors: | R. Sathiyanarayanan, University of Maryland, College Park A. Bhadj Hamouda, University of Monastir, Tunisia A. Pimpinelli, Univ. B–Pascal Clermont-2, France & Science Attaché, French Embassy, Houston T. Einstein, University of Maryland, College Park |
| Correspondent: | Click to Email |
Spontaneous pattern formation through kinetically controlled epitaxial growth provides a viable route for nanostructuring of surfaces. Ernst and co-workers found that during growth, Cu(100) undergoes a mounding instability, and its vicinal surfaces develop a meandering instability.2 More specifically, (i) the meandering wavelength (λm) scales with deposition rate (F) as λm ~ F-γ, with γ ≈ 0.19, (ii) both close-packed <110> and open <100> steps undergo meandering instability and (iii) above 10ML deposition, small pyramids appear near the steps. No previously-known instability mechanism, (esp. Bales-Zangwill, kink Ehrlich-Schwoebel effect, or unrestricted step-edge diffusion) could account for all of the experimental observations.
Using kinetic Monte Carlo simulations, A. Ben-Hammouda et al. showed that impurities codeposited on the surface could reproduce the λm–F scaling behavior and the formation of small pyramids.3 Further, they found that only those impurity atoms (i) whose bond to Cu adatom is about 1.6 times the strength of the Cu-Cu bond and (ii) whose diffusion barrier is about 1.6 times the barrier of Cu adatom could cause the observed instabilities. Due to their stronger bonds with Cu adatoms, impurity atoms hinder Cu adatom diffusion, thereby shortening the diffusion length and making λm less sensitive to deposition rate (F). Also, impurity atoms act as nucleation centers for the formation of small pyramids.
By computing the binding energies and diffusion barriers for certain candidate impurity atoms, we could eliminate several species as possible sources of the observed instabilities. Using density-functional theory (DFT)-based Vienna Ab-initio Simulation Package (VASP), we computed the adsorption energies and diffusion barriers for many candidate impurity atoms. Our calculations show that common impurities such as oxygen, sulfur, and carbon actually repel Cu adatoms. The bonds formed by elements that alloy with Cu, e.g. Zn, Ag and Sn, are too weak to cause the observed instabilities; also, these atoms have smaller diffusion barriers than Cu. Our results indicate that either Fe or Mn atoms are causing the observed instabilities.4 We discuss the results of our calculations and the possible role of impurities in nanostructuring of surfaces.
1Supported by NSF MRSEC Grant DMR 05-20471; NSF supported computer usage at NCSA, UIUC.
2N. Néel et al., J. Phys.: Condensed Matter 15 (2003) S3227.
3A. Ben-Hammouda et al., Phys. Rev. B 77 (2008) 245430.
4R. Sathiyanarayanan et al., in preparation.