1. Large-area ordered Ge-Si compound quantum dot molecules on dot-patterned Si (001) substrates Hui Lei, Tong Zhou, Shuguang Wang, Yongliang Fan, Zhenyang Zhong (State Key Laboratory of Surface Physics, Fudan University, Shanghai , PRC)
1 Introduction
We report on self-assembled Ge-rich quantum dots (QDs) on dot-patterned Si (001) substrates. Truncated-pyramid-like Si dots with {11n} facets are readily formed via nanosphere lithography, which are ordered in large areas with controlled period and size. Each Si dot induces four self-assembled Ge-rich dots at its base edges that can be four-fold symmetric along <110> directions. A model based on the surface chemical potential well accounts for these results. Such a configuration of one Si QD surrounded by four-fold symmetric Ge-rich QDs can be referred as a compound QD molecule with unique features.
Methods and Results
2.1 Fabrication of dot-patterned substrates
Figure 1. Schematic illustration of the fabrication of the dot-patterned Si (001) substrate, (a) a close-packed monolayer of PS nanospheres on a clean Si substrate, (b) after RIE of O2, (c) after RIE of CF4, (d) after removing the PS spheres.
2.2 Ge-Si compound quantum dot molecules
Figure 3. (a) AFM image (1×1μm2) of the Ge-Si CQDMs after deposition of 5ML Ge on a Si dot-patterned substrate, (b) an enlarged single Ge-Si CQDM, (c) the height profiles along the dashed lines in (a) and in Fig. 2(a) and (c), (d) AFM image (1×1μm2) of the Ge QDs after deposition of 5ML Ge on a flat substrate.
2.3 Correlation calculation
Figure 4. (a) 3D surface CP around a single Si dot (195 x 195 nm), (b) the surface CP along the line AB (across the center of the Si dot) and CD (along the base edge of the Si dot) in (a) and the height profile across the center of the Si dot. The CP along the line CD is shifted upwards by 8 meV to clearly show the features of all curves in (b).
Figure 2. AFM images of (a) the Si dot-patterned substrate (1×1μm2), (b) an enlarged single Si dot before growth, (c) the Si dot-patterned substrate (1×1μm2), (d) an enlarged single Si dot after Si buffer layer growth. The height profiles along the dashed lines in (a) and (c) are shown in Fig. 3 (c).
3 Conclusion
In summary, large-area ordered Si dots with the controlled size and period are realized on Si (001) substrates via nanosphere lithography. Four-fold symmetric Ge-rich dots along <110> directions can be readily induced around the centers of the base edges of the truncated pyramid-like Si dots. This promising feature of self-assembled Ge-rich dots on dot-patterned Si (001) substrates is well explained by a model based on of the surface chemical potential. Our results disclose the critical effect of the surface curvature on the diffusion and the aggregation of Ge adatoms at the beginning of Ge deposition, which further clarify the unique features and the inherent mechanism of self-assembled QDs on patterned substrates. The obtained Ge-Si CQDMs will have much more promising physical properties in comparison with other configurations of QDs due to the unique confinement and the strong interaction of electrons and holes. Accordingly, such Ge-Si CQDMs will have great potential in device applications.