Ordering mechanism of stackedCdSe∕ZnSxSe1−xquantum dots: A combined reciprocal-space and real-space approach

Abstract

The vertical and lateral ordering of stacked CdSe quantum dot layers embedded in $\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ has been investigated by means of grazing incidence small angle x-ray scattering and transmission electron microscopy. Different growth parameters have been varied in order to elucidate the mechanisms leading to quantum dot correlation. From the results obtained for different numbers of quantum dot layers, we conclude on a self-organized process which leads to increasing ordering for progressive stacking. The dependence on the spacer layer thickness indicates that strain induced by lattice mismatch drives the ordering process, which starts to break down for too thick spacer layers in a thickness range from $45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}80\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Typical quantum dot distances in a range from about $110\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}160\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ have been found. A pronounced anisotropy of the quantum dot correlation has been observed, with the strongest ordering along the $[1\overline{1}0]$ direction. Since an increased ordering is found with increasing growth temperature, the formation of stacking faults as an additional mechanism for quantum dot alignment can be ruled out.