Enhancing the in-plane spatial ordering of quantum dots

We report on the use of $(\text{In},\text{Ga})\text{As}∕\text{GaAs}$ multilayer stacking at elevated growth temperatures to produce enhanced in-plane spatial ordering. Cross-sectional transmission electron microscopy images reveal that the $(\text{In},\text{Ga})\text{As}$ islands are vertically correlated while atomic force microscopy images demonstrate lateral ordering of quantum dots that are closely aligned along the $\left[0\overline{1}1\right]$ direction as chains which are themselves positioned periodically along the $\left[011\right]$ direction. The in-plane spatial ordering along the $\left[0\overline{1}1\right]$ and $\left[011\right]$ directions is directly seen by asymmetric (311) glancing exit x-ray diffraction with the x-ray beam along the respective direction. Growth studies as a function of temperature indicated that the observed lateral ordering results from enhanced surface diffusion and the vertical transfer of corresponding anisotropic strain pattern due to the anisotropy of surface diffusion.

Figure 1

AFM top views of the samples grown at $540°\mathrm{C}$ (a) and $480°\mathrm{C}$ (b). The bottom of (a) shows the line scan corresponding to the position marked as a white line on the top of (a).

Figure 2

XRD rocking curves around the asymmetric GaAs (311) glancing exit reflection of the sample grown at $540°\mathrm{C}$. (a) and (b) are measured when the scattering plane is along the $\left[011\right]$ and $\left[0\overline{1}1\right]$ directions, respectively. The inset shows the schematic diffraction geometry of (a) and (b). $d_{-1}$ and $d_1$ denote the $-1$st and 1st satellites, respectively.

Figure 3

The cross-sectional TEM images of sample $a$ taken along the $\left[0\overline{1}1\right]$ projection showing the whole structure (a) and the initial seven periods (b). With increasing the stacking layers, the wetting layer gets thinner and thinner and the QD’s get taller and taller as can be seen, for example, from the column marked as the white box.