Nanowires and Nanorings

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Nanowires and Nanorings
at the Atomic Level
Midori Kawamura, Neelima Paul, Vasily Cherepanov,
and Bert Voigtländer
Institut für Schichten und Grenzflächen ISG 3,
Forschungszentrum Jülich, 52425 Jülich, Germany
Presentation Kim Jae-Hee
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Introduction (I-1)

• Because most electronics devices are fabricated
  on Si
• substrates, nanostructures grown epitaxially on
  Si substrates
• are most desirable

• For the case of the important Si/Ge
  nanostructure system
• the observation of such a growth behavior may
  not be
• observed

• In the case of the system Si/Ge it has been
  difficult to
• differentiate between Si and Ge due to their
  similar electronic
• structure.

Homogeneous mixed composition Displacive
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Introduction (I-2)

• In a recent approach to distinguish Si and Ge
  atoms,
• a termination of the surface with Cl was used.

• this termination of the surface was performed
  after growth it could not prevent the displacive
  adsorption of Ge
• ? Si Ge atoms are located at random
  locations at the surface in this case.

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Introduction (II)

• The step-growth mode is used to fabricate Si
  and Ge nanowires
• with a width of 3.5nm and a thickness of one
  atomic layer(0.3nm) by
• self-assembly.

• Alternating deposition of Ge and Si results in
  the formation of a
• nanowire superlattice covering the whole
  surface..

• One atomic layer of Bi terminating the surface
  is used to distinguish between
• the elements Si and Ge.

• Also, different kinds of two dimensional Si/Ge
• nanostructures like alternating Si and Ge
  nanorings
• having a width of 5-10nm were grown

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Introduction (III-1)

• Property of Bi surfactant ..

The property of Bi surfactant
Si
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Introduction (III-2)
Step-flow growth mode
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Experiment I
750K
740K
720K
Bi, Ge, and Si were deposited on a clean Si(111)
substrate by solid source molecular beam epitaxy
(MBE).
Bi 1.000 ML / min Si 0.010 ML /
min Ge 0.015 ML / min
STM home-built beetle-type scanning tunneling
microscope sample bias
voltage between 2.22.6V
tunneling current 0.1 nA
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Result I
Ge
Si
(b) The cross section along the white line shows
the dimensions of the Si and Ge nanowires
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Result (I-2)
Different width of the wires can be easily
achieved by different amounts of Ge and Si
deposited.
(DC current heating during cleaning of the Si
substrate was used)

• Si/Ge nanowires on a larger scale
• growing at four step edges.
• The homogeneity of the nanowires is visible
  in this STM image.

(b) Alternating deposition of nine wires per
step edge results in the formation of a
superlattice of Si/Ge nanowires covering the
whole surface. (3-5nm wide)
1
3
5
7
2
4
6
8
9
(Ge 0.1 ML, Si 0.15 ML)
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Experiment - nanorings
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Result II - nanorings
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Result II
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
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Analysis
Several possible reasons for the observed height
difference for Si and Ge areas.
(height difference 25)
I.

• Difference of lattice constant ? vertical
  relaxation reliving strain

II. Difference surface reconstruction of the
terminating Bi layer on Si and Ge
Third explanation is confirmed by the fact that
the measured height difference shows a
pronounced dependence on the applied bias voltage
III. Different electron density of states for Bi
bond to Si and Ge, respectively
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Result III
Ge
Ge
Si
Because of Bi trimers
Submonolayer Ge deposition on a Sb terminated
Si(111)
Ge 0.2 ML Sb 0.5 ML / min
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Summary
1. Control formation of different kind of
two-dimensional Si/Ge nanostructure.
2. The property of the nanostructures grown
? width 3.5 nm ? thickness 0.3 nm
? cross section consisting of only
21atoms ? strong lateral covalent bond
3. A simple and general method has been presented
to distinguish between Si and Ge in
two-dimensional nanostructures using the height
difference in STM images after termination of
the surface with Bi