Formation and Dynamics of Si Clusters on Pb/Si (111) Surface Rakesh Kumar a,b,c , Yi-Hsien Leea and Ing-Shouh Hwang a * (a) Surface and Nano Science Laboratory, Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan (b) Nanoscience and Technology Program (TIGP), Academia Sinica (c) Department of Engineering and System Science, NTHU, Hsinchu, Taiwan
Abstract :-We have studied the very early stages of Si deposition on Pb/Si (111) surface with use of VT-STM. We have observed the formation of Si clusters on Pb/Si(111). The clusters formation depends on the sample preparation conditions. Different types of cluster have been found on Si-Pb/Si(111) surface- C5, and C7. C5 cluster found at low temperature and stable up to 245K 5K. C7 cluster found when the sample temperature is > 260K. C7 cluster is stable at room temperature and shows rotational property. Some clusters found near to defect sites and are very stable. Both C7 and C5 clusters shows different dynamic behavior on Pb/Si(111) surface. Si clusters played a role of nucleation site for coming Si adatoms during deposition and mass transportation.
Si Clusters at LT:-
Motivation:-
We have been reported about the nucleation and growth behaviors of Si deposition on the Pb/Si (111) surface. In addition of nucleation of 2-D Si islands, also find the self assembled 1-D Si atomic wires, when the coverage is not high enough. Formation of Si-atomic wires suggests that single Si adatoms are mobile at Pb/Si(111) surface. So, we believed that there should be another nanostructures before formation of Si-atomic wires at the surface. T. C. Chang, et al., Nucleation and growth of Si on Pb monolayer covered Si (111) surfaces, Surf. Sci. (2011), 605, 1249-1256. a
b
Method 2: Si deposit on Si(111)-(1*1)-Pb at 225 K and sample cool down. 158 K
200 K
Si5
158 K
Si5
Si5
c
3.5 nm
5 nm
2V
5 nm
2V
2V
STM images of Si5 resides near to the (√7*√3) domain boundary.
Behavior of Si Clusters with Sample Temperature 158 K STM images of the surface morphology after deposition of Si atoms of 0.018 BL on the Pb-(1*1) phase at RT (a and b). (c) High resolution image of Si-atomic wire with estimated unit cell.
Sample Preparation:-
Si5 Si5 5 nm
(a) Submonolayer of Pb on Si (111)-(7*7) surface. (b) Anneal sample to 400oC briefly. (√7*√3)
(1X1) (√7*√3)
(1X1)
255 K
242 K
3 nm
2V
5 nm
1.5 V
1.5 V
STM images of clusters, C5 cluster disappear from the (√7*√3) domain and the surface structure of the domain changed. The cluster may dissociate in to individual adatom or either covalent with other cluster or island. 170 K
140 K
263 K
Sid
(1X1)
Sid
STM image of the (1*1) phase and in inset the atomic model with Pb atoms on the T1 site
STM image of the (√7*√3) phase at Ts< 260 K and in inset the atomic model.
STM images of Pb-(1*1) surrounded by Si (111)-(7*7). Sample voltage is -2 V.
Sid
5 nm
2 nm
5 nm
1.75 V
1.5V
1.25 V
STM images of a cluster at different temperature. We have found that cluster showed static behavior with respect to temperature.
Si Clusters at RT:-
Cluster behave as Nucleation Site
Method 1: Si (0.036 ML) deposit on Si(111)-(1*1)-Pb surface at Ts > 265 K a
c
b
0.036 ML
a
0.048 ML
b
Si cluster Si7
A orchid flower
2V 2V 2V STM images of clusters (a) Si7 cluster found on defect free area, (b) Other type of cluster, and (c) a cluster like a orchid flower. These cluster formed by self-organization mechanism..
Dynamics of Cluster at RT a
0 min
c
180 min
b
2V
2V
STM images at RT, Si 0.048 ML deposit on the existing cluster, the cluster starts to transform to another structure at RT.
Epitaxial Growth mechanism of Covalent Material
210 min
EB
EB Monomer
Si7
Dimer
Trimer
n-mers
Stable cluster
EB Atomic wire
Stable structure
Si7
Si7
Unstable Species unless trap inside defect sites Increase of Si coverage
2 nm
5 nm
5 nm
2V 2V 2V STM images of Si cluster at RT, (a) initial state of C7 cluster, (b) the cluster shows rotating property, but the center of axis is fixed and (c) after 210 minutes. C7 cluster shows rotational property.
Possible mechanism for the epitaxial growth of covalent material involving clusters. These transitions are one directional because their reverse transitions occurs rarely. In this growth system found different types of clusters and nanostructures. Clearly covalent material during epitaxial growth required to cross energy barriers before become the stable nano-structure.
Summary
Model for Cluster Rotation
1.
2 2 1 3
1
7
5
7
5
6
3
2.
4
4
6
30O rotation with in 3 hours
T4 T1
T1
T4
H3
H3 3
2
2
6
2 3
3
1
5
5
1
7
6
5
4
1
3.
6 4
4
T1
T4
7
7
H3
4. 5.
The formation of clusters depends on the sample preparation conditions. We have found different types of Si clusters, those are formed by selforganization mechanism. Si7 cluster found on the surface at Ts > 265K. This cluster is stable at RT. The dangling bonds of this cluster fully saturated and the diffusion barrier high. Si7 clusters shows rotational property (which direction not sure due to its isotropic shape) but the center of axis is fixed. Si adatoms in a cluster very tightly bound to each other, so; their interaction with the substrate is weakened. When Ts > 245K, the diffusion barrier is low for Si5 cluster because this cluster became disappear from its original position. The surface structure underneath of the cluster changed as sample temperature (Ts) increased. The cluster which found on/near to the surface point defect (Sid), showed static behavior with respect to time and sample temperature (Ts). Si7 cluster played a role of nucleation site for coming Si adatoms. These clusters can be consider the building units for homogenous epitaxial