Strain-Induced Formation of Fourfold Symmetric SiGe Quantum Dot Molecules

The strain field distribution at the surface of a multilayer structure with disklike SiGe nanomounds formed by heteroepitaxy is exploited to arrange the symmetric quantum dot molecules typically consisting of four elongated quantum dots ordered along the [010] and [100] directions. The morphological transition from fourfold quantum dot molecules to continuous fortresslike quantum rings with an increasing amount of deposited Ge is revealed. We examine key mechanisms underlying the formation of lateral quantum dot molecules by using scanning tunneling microscopy and numerical calculations of the strain energy distribution on the top of disklike SiGe nanomounds. Experimental data are well described by a simple thermodynamic model based on the accurate evaluation of the strain dependent part of the surface chemical potential. The spatial arrangement of quantum dots inside molecules is attributed to the effect of elastic property anisotropy.

Figure 1

(a) Cross-sectional TEM micrograph of threefold stack of buried SiGe QDs, which was grown at $700{}^{°}\mathrm{C}$. Each QD layer was formed by deposition of seven MLs of Ge. The top QD layer was completed by the deposition of a 5 nm Si overlayer. (b) AFM image of the surface ($1.5\times 1.5\mu m^2$) with disklike SiGe nanomounds, which were used as formation sites to originate QDMs. In panel (c) we display a [110] line profile of the typical nanomound [indicated by a solid line in Fig. 1(b)].

Figure 2

STM images of surface ($1\times 1\mu {\mathrm{m}}^2$) with the groups of closely spaced SiGe islands grown at $600{}^{°}\mathrm{C}$ on a three-layered QD superlattice template with different amounts of deposited Ge: (a) 3.5 MLs, (c) four MLs, (e) five MLs. Image sides are oriented along the $⟨011⟩$ directions. Figures 2(b), 2(d), and 2(f) correspond to three-dimensional representations of the fourfold symmetric QDMs [marked by dash circles in Figs. 2(a), 2(c), and 2(e), respectively].

Figure 3

Three-dimensional representation of the surface ($1\times 1\mu {\mathrm{m}}^2$) with fourfold symmetric QDMs formed after deposition of five MLs of Ge at $600{}^{°}\mathrm{C}$ on a three-layered QD superlattice template. The top QD layer was completed by the deposition of a 10 nm Si overlayer at $700{}^{°}\mathrm{C}$.

Figure 4

(a) Surface distribution of strain-dependent chemical potential on the top of a SiGe disklike nanomound covered by a thin Ge wetting layer, which was calculated by FEM. The disklike nanomound was represented by a truncated cone with a height to base aspect ratio of 0.05 and a sidewall inclination angle of 14°. (b) STM image of spatial configuration of small 3D islands originated on the disklike nanomound after deposition of 3.5 MLs of Ge at $600{}^{°}\mathrm{C}$.

Figure 5

STM images of surface ($1\times 1\mu {\mathrm{m}}^2$) with the close-packed groups of SiGe islands formed after deposition of five MLs of Ge at $600{}^{°}\mathrm{C}$ on the top of vertically stacked SiGe QDs. The top QD layer was completed by the deposition of a 35 nm Si capping layer at $700{}^{°}\mathrm{C}$.