1. Investigation on Ge surface diffusion via growing Ge quantum dots on top of Si pillars
Yuwen Jiang, Delin Mo, Xiaofeng Hu, Zuimin Jiang*
Department of Physics, Fudan University
Introduction:
In order to understand the mechanism and have a better control of heteroepitaxial growth of Ge on Si substrates, knowledge about the surface diffusion length L and activation energy Ea of Ge on Si at atomistic levels is very useful and desirable.
The surface diffusion length and activation energy of Ge play an important role in the growth of ordered and addressable GeSi quantum structures which could further improve the optical and electronical properties of GeSi materials in device application
The Ea values which have been reported both experimentally and theoretically so far scatter in a large range.
Further investigations on the Ea are still necessary, in particularly, a simple and straightforward experimental method is anticipated.
Surface chemical potential
Expermental procedure:
(a) AFM image of Si pillar after Si buffer growth, (b) are the corresponding 2D surface chemical potential. The two black dotted rings indicate the region of local surface chemical potential minima.
Si pillar edge acts as a sink of Ge adatoms:
L : surface diffusion length; r: radius of Si pillar.
L> r : no Ge QDs appear at the central region of the pillar;
L≤ r : some QDs appear at the central region of the pillar.
Results and discussion:
AFM images of the surface morphologies of the pillars with different radii after Ge growth at a temperature of 620℃, and with a rate of 0.1 Å/s. The marked r values are designed radii of pillars for lithography.
AFM images of 1/4 Si pillar morphologies after Ge growth at a growth temperature of 580 °C but different growth rates with the designed radii ranging from 2.0 to 3.0 μm.(a)-(d) v=0.1 Å/s; (e)-(h) v=0.05 Å/s. The insets show the corresponding overall views. eV
v=0.1 Å/s ：
v=0.05 Å/s ：
Surface diffusion length as a function of growth rate for two fixed growth temperatures.
Arrhenius plots of the diffusion lengths as a function of growth temperature for two growth rates.
Surface diffusion lengths are directly proportional to ,
further demonstrating the reliability of the method.
Two Ea values agree well with each other, implying that the method is reliable and self-consistent.
Conclusion:
A simple and intuitionistic experimental method to quantitatively measure surface diffusion lengths of Ge adatoms on Si(001) and its activation energy is proposed and demonstrated, which is realized by depositing 10 ML Ge on top surfaces of Si pillars with different radii and taking an advantage of preferential nucleation and growth of Ge QDs at the top surface edges of the Si pillars.The obtained results demonstrate that this method is feasible and reliable..