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