1. 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

2. 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]

3. 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.

4. 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

5. The property of Bi : surfactant Si Property of Bi : surfactant &.. Introduction (III-1) BiGeBiGe J.H.G. Owen and H.W.Yeom et al, PRL.88, 226104

6. Step-flow growth mode Introduction (III-2) Preexisting step

7. 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 Bi Ge 740K 750K 720K

8. 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

9. 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) 1 2 3 4 5 7 8 6 9 (Ge : 0.1 ML, Si : 0.15 ML) (DC current heating during cleaning of the Si substrate was used)

10. 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

11. Result II - nanorings 0.09 nm ~5-10nm

12. Bi Ge Bi Result II

13. Analysis Several possible reasons for the observed height 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 and Ge, respectively respectively Third explanation is confirmed by the fact that the measured height difference shows a pronounced dependence on the applied bias voltage

14. Result III Ge Si Because of Bi trimers Submonolayer Ge deposition on a Sb terminated Si(111) Ge: 0.2 ML Sb : 0.5 ML / min

15. 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