Self-organization of the InGaAs/GaAs quantum dots superlattice
Introduction
The fabrication of quantum dots (QDs) by self-organization processes has attracted increasing attention in the last few years [1]. Although Stranski–Krastanow growth is regarded as a promising road towards zero-dimensional quantum structures, their random nucleation in a single layer results in a broad distribution of QDs size and positions. Because arrays of QDs of the same size and shape are required for practical applications, the identification of growth mechanisms that lead to a narrowing of the size distribution is of great importance.
Lateral and vertical ordering of QDs in multilayers has been studied in both the SiGe/Si [2], [3], [4], and the InGaAs/GaAs [5], [6] systems. Tersoff et al. [7] have presented a model not only accounted for the vertical alignment, but which also predicted a reduction of the size fluctuation if dot layers are stacked. Xie et al. [5] have also studied the QDs vertical alignment in the InAs/GaAs multilayers based upon mechanochemical diffusion caused by the QDs-induced strain. These studies indicate that QDs superlattice provides a possible route to obtain the size uniformity needed for electronic applications of QD arrays.
Here, we provide a direct observation by cross-sectional transmission electron microscopy (XTEM) of the vertical alignment of QDs in InGaAs/GaAs multilayers. Moreover, we observe an interesting double-crystal X-ray diffraction (DCXD) spectrum, which clearly shows two sets of satellite peaks, corresponding to the QDs superlattice, and multi-quantum wells (QW) formed by wetting layers.
Section snippets
Experimental procedure
The sample was grown on semi-insulating GaAs (1 0 0) substrates in a Riber-32p molecular beam epitaxy chamber. It consists of a 1.5 μm GaAs contact layer, a 30 period InGaAs/GaAs superlattice and 1.2 μm GaAs contact cap layer. The growth rate for GaAs was 2.82 Å/s. The indium component of InGaAs was about 0.3, and the corresponding growth rate was 2.22 Å/s. The thickness of InGaAs and GaAs was 4.0 and 20 nm. Substrate temperature for the growth of contact layers and the InGaAs/GaAs QDs superlattice
Results and discussions
Fig. 1 shows a cross-sectional bright-field micrograph of a 30 period InGaAs/GaAs QDs superlattice. The formation of QDs is evident in the XTEM image through the strain contrast. One striking observation is that the QDs in subsequent layers are almost perfectly vertically aligned along the growth direction. Most importantly, this vertical alignment is dependent on the QDs positions in the first layer. In contrast to regime A, where the QDs are perfectly one-to-one vertically aligned, in regime
Conclusions
In summary, we have shown experimental evidence for the nearly perfect vertically aligned InGaAs/GaAs QDs superlattice. It is found that the main character of controlling vertical ordering is the QDs spacing in the first layer. Merging of QDs is the main mechanism leading to vertical alignment. PL measurements demonstrate a narrowing line width of 50 meV. We also observe two distinct sets of satellite peaks, which correspond to the QDs superlattice, and multi-quantum wells formed by the wetting
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