X-ray atomic mapping of quantum dots

Abstract

We report a model-independent atomic-mapping technique for quantum dots (QDs) by combining Bragg reflection x-ray standing wave (XSW) and grazing incidence diffraction (GID) measurements. In this study, we choose GaAs capped InGaAs QDs/GaAs(001) as a model system to show the locations and arrangements of indium atoms within the QDs along various $[hkl]$ directions. This technique directly reveals the actual amount of positional anisotropy and ordering fraction of indium atoms within the QDs by probing the ($\overline{1}11$), (111), (311), ($\overline{1}31$), (113), and ($\overline{1}13$) crystallographic planes. We find that indium atoms are outwardly shifted along the [001] direction by small fractions of the lattice constant, $0.04{a}_{\mathrm{GaAs}}$ and $0.06{a}_{\mathrm{GaAs}}$ from Ga sites for 50- and 150-\AA{} GaAs capped InGaAs QDs, respectively. We observe that an improved coherency factor of the indium atoms within the QDs by 45--60% along the [001] and [011] directions reduces the photoluminescence linewidth by 22%, thus making the QDs efficient for QD-laser and optoelectronic device applications. We also find that the position and ordering of In atoms along the (113) and ($\overline{1}13$) planes are most sensitive to the thickness of the GaAs cap layer. Our XSW-based results are supported by numerical calculations using a QD-macroscopic structural model based on our GID study. We thus show that this atomic-mapping technique will be useful for studying various quantum structures and tuning their properties.