Structural Characterisation of Quantum Dots by X-Ray Diffraction and TEM

Abstract

X-ray diffraction and transmission electron microscopy (TEM) provide complementary structural data on semiconductor quantum dots. While TEM characterizes single structures with atomic resolution X-ray diffraction yields information on statistical averages of large ensembles. For the work reported here, established methods were refined and some new methods were developed. Materials systems investigated were (In,Ga)As/GaAs, Ga(Sb,As)/GaAs and (Si,Ge)/Si. The composition of wetting layer and quantum dots could be quantitatively determined, showing a good agreement between quite a number of different X-ray and TEM methods. For all systems the depletion of the wetting layer and the enrichment of the strain providing species in the upper part of the quantum dot could be quantitatively analysed. For the model system (Si,Ge)/Si it was found that a ring of deep wetting layer depletion forms around the (Si,Ge)-islands. It was shown that this strain driven depletion prevents further lateral nucleation and thus eventually limits the size of the islands. A specific three-dimensional ordering of multilayered (In,Ga)As quantum dots grown on GaAs high-index surfaces was demonstrated and could be explained in the framework of elasticity theory and surface kinetics.