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Nanowires and Nanorings at the Atomic Level. Midori Kawamura, Neelima Paul, Vasily Cherepanov, and Bert Voigtl ä nder. Institut f ü r Schichten und Grenzfl ä chen ISG 3, Forschungszentrum J ü lich, 52425 J ü lich, Germany. Presentation : Kim Jae-Hee. : Ge atom. - PowerPoint PPT Presentation
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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
Introduction (I-1)
In the case of the system Si/Ge it has been difficult to
differentiate between Si and Ge due to their similar electronic
structure.
For the case of the important Si/Ge nanostructure system
the observation of such a growth behavior may not be
observed
Because most electronics devices are fabricated on Si
substrates, nanostructures grown epitaxially on Si substrates
are most desirable
: Ge atom: Si atom
Homogeneous mixed composition [Displacive]
In a recent approach to distinguish Si and Ge atoms,
a termination of the surface with Cl was used.
Introduction (I-2)
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.
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
The property of Bi : surfactant
Si
Property of Bi : surfactant &..
Introduction (III-1)
Bi
GeBiGe
J.H.G. Owen and H.W.Yeom et al, PRL.88, 226104
Step-flow growth mode
Introduction (III-2)
Preexisting step
Experiment I
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 +2.2~+2.6V
tunneling current : ~ 0.1 nA
Si
BiGe
740K
750K 720
K
Result I
(a) STM image of two-dimensional
Ge/Si nanowires grown by step
flow at a preexisting step edge
on a Bi terminated Si(111)
substrate.
An STM image after repeated alternating deposition of 0.15 atomic layers of Ge and Si
Both elements can be easily distinguished by the apparent heights in the STM image
Initial step position
Final step position
Si
Ge
~0.07nm
~3.5nm(b) The cross section along the white line shows the dimensions of the Si and Ge nanowires
(c) Atomic structure of a 3.3 nm wide Ge wire on Si substrate capped by Bi
Top view
Side view
Result (I-2)
(a) Si/Ge nanowires on a larger scale
growing at four step edges.
The homogeneity of the nanowires is visible in this STM image.
Different width of the wires can be easily achieved by different amounts of Ge and Si deposited.
(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)
12
34
57
86
9
(Ge : 0.1 ML, Si : 0.15 ML)
(DC current heating during cleaning of the Si substrate was used)
Experiment - nanorings
Template - islands
Substrate – clean Si(111)-7x7
At 740K, clean surface was initially covered by a one atomic layer
thick Bi layer
Ge and Si deposition of 0.15 ML was performed at T=720 K
and 750 K
Result II - nanorings0.09 nm
~5-10nm
Bi
Ge
Ge
Ge
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Result II
Analysis
Several possible reasons for the observed height Several possible reasons for the observed height difference for Si and Ge areas.difference for Si and Ge areas.
(height difference : ~25%)(height difference : ~25%)I.
I. Difference of lattice constant vertical relaxation reliving strain
II. Difference surface reconstruction of the terminating Bi layer on Si
and Ge
III. Different electron density of states for Bi bond to Si III. Different electron density of states for Bi bond to Si and Ge, and Ge,
respectivelyrespectively
Third explanation is confirmed by the fact that the measured height difference shows
a pronounced dependence on the applied bias voltage
Result IIIGe GeSi
Because of Bi trimers
Submonolayer Ge deposition on a Sb terminated Si(111)
Ge: 0.2 ML
Sb : 0.5 ML / min
Summary1. 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