Paper SS2-MoA3
Stress Balance in Nanopatterned N/Cu(001) Surfaces

Monday, October 18, 2010, 2:40 pm, Room Santa Ana

Helium atom scattering (HAS) and density functional theory (DFT) within pseudopotential methods have been used to investigate stress balance in nano-patterned N/Cu(001) surfaces. HAS shows that the stress-relief-driven lateral expansion of the averaged lattice parameter within finite-sized N containing patches reduces, with increasing N coverage (and decreasing stripe widths), from 3.5% to 1.8% and then, beyond a critical exposure, the patches’ lateral expansion increases again slightly to 2.4%. This implies that, in this higher coverage range, the compressive stress is partially relieved with another mechanism; namely Cu vacancy trenches are nucleated. The trenches serve to enable further surface stress reduction and expansion in the N lattice parameters. In full agreement with above and previous experimental observations, DFT calculations show that an optimized N-induced c(2×2) structure has a net surface stress level ~ 4 N/m and such stress is effectively relieved when stripes of clean Cu(001) form along the <100> direction or when trench-like steps of Cu atoms form along the <110> direction. On the other hand, the calculations demonstrate that (contrary to the suggestions of Driver et al.[1]) rumpling displacements within the outermost Cu layer do not act to relieve the compressive surface stress levels while clock-like displacements could relieve stress levels, although such displacements are energetically unstable.

[1] S. M. Driver, J-T Hoeft, M. Polcik, M. Kittel, R. Terborg, R. L. Toomes, J-H. Kang, and D. P. Woodruff, J. Phys. Cond. Mat. 13, L601 (2001).

Work supported in part by NSF Grant CHE-0741423