Chemical reactivity and Misfit Dislocation Behavior

1
Chemical Reactivity and Dislocation Behavior in
Strained Thin Films
Emerging points of dislocations are presumed to
be extremely reactive, but no there is a lack of
studies of this reactivity at an atomic
scale. Strained thin films often present well
defined misfit dislocation networks. We have
looked into the interplay of dislocation changes
and film reactivity in a model system 2ML
Cu/Ru(0001)
Juan de la Figuera, Karsten Pohl, Andreas K.
Schmid, Norm C. Bartelt Jan Hrbek and Robert
Q. Hwang
Sandia National Laboratories Brookhaven National
Laboratory
Funding DOE-AC04-94AL85000 Fulbright- MEC/Spain
2
Misfit Dislocations

• Misfit dislocation networks arise in thin films
  to relieve the lattice mismatch between film and
  substrate. If the lattice mismatch is small
  enough, or the film is thin enough, the full film
  can be elastically strained preventing the
  formation of misfit dislocations. This is not the
  case for many metal/metal systems even for one
  layer films.

Elastically strained thin film
Film with misfit dislocations

• In hexagonal substrates, either fcc(111) or
  hcp(0001), there are two adsorption sites for the
  films, and the perfect edge misfit dislocations
  dissociate into partial dislocations which bound
  areas with different stacking sequence.

3
Misfit Dislocations in hexagonal substrates

• The networks of misfit dislocations on hexagonal
  substrates are quite general, and the same kind
  of networks have been observed in
  Ag,Au,Cu/Ru(0001), Ag,Cu,Co,Pt/Pt(111),
  Au(111)...
• These networks have been proposed at template
  layers for the growth of nanostructures, as they
  supply ordered arrays of defects. But it is
  important to realize that the periodicity of this
  arrays is the result of a delicate balance of the
  elastic strain in the film and the interaction
  with the substrate.

Au(111)
50nm
3 ML Cu/Ru(0001)
4
Building blocks of the dislocation networks
2ML Cu/Ru(0001)

• The building blocks of the networks of misfit
  dislocations on hexagonal substrates are Partial
  Shockley dislocations run at the film-substrate
  interface, with threading edge dislocations
  (perpendicular to the film) were different
  partials meet.
• The atomic arrangement is quite distorted only at
  the emerging points of the threading
  dislocations.
• The partial dislocations appear as bright lines
  in Scanning Tunneling Microscopy images.

150nm x100nm
Threading edge dislocation
Shockley Partial Dislocations
5
Effects of Oxygen Exposure

• Exposure to molecular oxygen produces a
  mesoscopic arrangement of the dislocation network
  of the film.
• Oxygen is observed as a black decoration around
  the threading dislocations. It has not been
  resolved at an atomic scale.
• After the formation of triangular structures,
  oxygen starts to etch the second copper layer.

0L
0.6L
0.9L
2.4L
6
Nucleation of new threading dislocations
Trigon

• Once the preexisting threading dislocations have
  been decorated with oxygen, new threading
  dislocations are created in the film through the
  extraction of atoms at the next-most reactive
  site the partial dislocations (the creation of a
  vacancy is equivalent to the formation of two
  threading edge dislocations). The two new
  dislocations can glide only in the observed
  direction, getting decorated in turn by oxygen.

Image of a cut, exposures above 0.4L
Image of a developed trigon, gt0.9L
The creation of new threading dislocations
appears as cuts with a fixed orientation that
break the ribbons of partial dislocations. The
mesoscopic dislocation network of the film
changes in response of the reactive element
exposure. The result of this process is the
transformation of the initial array of parallel
partial dislocations into trigons dislocation
loops bounded by three partial dislocations and
three threading dislocations.
7
Effect of sulfur exposure on the dislocation
network
Under sulfur exposure, the film undergoes a
similar transformation, although S does not etch
the film (as oxygen eventually does). And the
trigon phase is far more ordered than that with
oxygen. Sulfur is imaged either as protrusions or
as depressions, depending on the tip
condition. The mechanisms for the dislocation
multiplication are, as with oxygen, the creation
of cuts on top of the partial dislocations, and
the dissociation of preexisting dislocations.
cut
50nm
8
Initial Stages of exposure to Sulfur
The sulfur atoms are atomically resolved, and at
the first stages of the exposure to sulfur, they
decorate the preexisting threading dislocations
of the film. The distance between sulfur atoms
at the core of the threading dislocations is
6Å. We propose that the reactivity toward
sulfur is due to the pseudo-fourfold hollow sites
that the cores of the dislocations present. A
series of different reconstructions of S on
Cu(100) and Cu(111) have in common such a
tetramer of Cu bonded to a S atom.
(2x2)S/Cu(100)
structures of S/Cu(111)
9
Dissociation of threading dislocations

• The other dislocation multiplication mechanism
  observed in 2ML Cu/Ru (under either sulfur or
  oxygen, although show here for sulfur), is the
  dissociation of threading edge dislocations. This
  mechanism in impossible in the bulk, due to the
  increase in the energy of the final configuration
  following Franks criterion.

10
From stripes to trigons

• Both oxygen and sulfur react with the dislocation
  network on Cu/Ru(0001) breaking the initial
  arrangement of domains of parallel partial
  dislocations giving rise to small trigon
  structures. Oxygen and sulfur decorate the
  threading dislocations of the film. We can
  understand how the trigon network is created by
  starting with the stripe phase, cutting the
  ribbons in alternate positions and decorating all
  the threading dislocations with sulfur or oxygen.

11
Summary

• Exposure of the dislocations networks present on
  2ML Cu/Ru(0001) to oxygen1 or sulfur2 produces
  major changes in the ordering of those networks.
• Both elements produce the same sequence of
  changes
• At first, only the preexisting threading
  dislocations are decorated. We can resolve the
  individual features due to sulfur.
• Later, new threading dislocations are generated
  in the film in several ways dissociation of
  preexisting threading dislocations, and
  nucleation of pairs of threading dislocations on
  top of the Shockley partials. All the new
  threading dislocations are observed decorated in
  the same way as the preexisting ones.
• The introduction of threading dislocations
  eventually modifies the mesoscopic ordering of
  the film, changing from domains of parallel
  stripes of partial dislocations to a triangular
  arrangement of loops of dislocations (trigons).
• The observed changes remark the role of the
  dislocation network in the reactivity of the film.

1 J. de la Figuera, K. Pohl, A.K. Schmid, N.C.
Bartelt and R.Q. Hwang, Surf. Sci. 415 (1998)
L993 2 J. de la Figuera, K. Pohl, A.K. Schmid,
N.C. Bartelt, J. Hrbek and R.Q. Hwang, Surf.
Sci., in press.