1. Size dependent surface potential studies on Si and GeSi quantum dots
Fengfeng Ye, Yi Lv, Shihua Zhao, Zuimin Jiang* and Xinju Yang*
Phys. Dept., Fudan Univ., Shanghai, People’s Republic of China
I. Introduction
Self-assembled Si-based quantum structures have received great interests due to their potential applications in optoelectronic devices and quantum computers. However as compared with the intensive studies on the growth and electronic properties, the local electrical property studies of the individual quantum structures are relatively lacked.
Now we investigate the surface potential distributions on Si and GeSi quantum dots (QDs) by Kelvin Probe Force Microscopy (KPFM) , while their conductance distributions are measured by Conductive atomic force microscopy (CAFM). The dependences of the conductance and surface potential on the QD’s sizes are obtained for both Si and GeSi QDs. The surface potential results provide experimental evidences to explain the QDs’ conductance distributions.
III. CAFM and KPFM result of GeSi QDs
II. CAFM and KPFM Result of Si QDs
10nm
10nm
(a)
(d)
500pA
-500pA
200nm
10nm
-10nm
5mV
-5mV
200nm
10nm
-10nm
(a)
(d)
GeSi QDs have dome-like current distribution
GeSi QDs have much lower CPD compared with Si QDs because of their composition and stress difference
Higher GeSi QDs are more conductive
100nm
100nm
-10nm
-10nm
20mV
200pA
Four step to fabricate Si QDs
(b)
(e)
(e)
(b)
Si QDs have ring-like current distribution
Si QDs have lower CPD
-20mV
-200pA
(c)
(f)
(a), (d) Topography of GeSi QDs;
(b) Current measured by CAFM;
(e) Contact potential difference (CPD) measured by KPFM
(c) (f) Profile of an individual GeSi QD along the black line.
(c)
(f)
(a), (d) Topography of Si QDs;
(b) Current measured by CAFM;
(e) Contact potential difference (CPD) measured by KPFM
(c) (f) Profile of an individual Si QD along the black line.
(a)
(b)
(c)
IV. In situ KPFM and CAFM measurement
15nm
15nm
(a), (b) CPD difference of GeSi QDs compared to wetting layer increase with size;
And (c) show that CPD is mainly dominant by Height
Lower CPD
More conductive
V. Explanation of QDs’ conductance
1μm
Wetting layer
Tip QD
Tip
-15nm
-15nm
Current
Surface potential
100nA
40mV
The Schottky barrier height at the interface is lower for larger QDs
-100nA
-40mV
VI. Conclusion
GeSi QDs have much lower CPD compared with Si QDs because of their composition and stress difference;
Higher QDs have lower CPD, which mainly dominated by QDs’ height;
The surface potential results show the reason why the QDs are more conductive than the wetting layer, and larger QDs are more conductive than smaller QDs is that larger QDs have a lower barrier for electrons to flow from sample to tip because of their lower CPD.