Glancing angle EXAFS of encapsulated self-assembled InAs/InP quantum wires and InAs/GaAs quantum dots

Abstract

We have used X-ray absorption spectroscopy to study the structural properties of encapsulated semiconductor quantum wires (QWrs) and quantum dots (QDs), obtained by self-organized molecular beam epitaxy (MBE) growth. Extended X-ray absorption fine structure (EXAFS) spectroscopy gives information on the local microscopic structure of the sample, and allows to determine the strain and composition of the nanostructures. We measured EXAFS at the As K-edge, of the InAs QWrs, and at the In K-edge of the InAs QDs, in glancing-angle geometry (GIXAFS) at the ERSF, keeping the X-ray incidence angle close to the substrate critical angle to enhance the contribution of the thin nanostructured epilayer. The XAFS spectra of QWrs and QDs show a good signal-to-noise ratio that allows quantitative analysis. We have obtained bond distances up to the third coordination shell, and composition. The results show, in agreement with previous anomalous scattering measurements, that contrary to what expected due to diffusion and intermixing of the Group V species, the wires are made essentially of InAs. In the case instead, of the QDs samples a strong In diffusion is observed, in agreement with the well known In tendency to diffuse and segregate towards the surface, in strained InGaAs systems. We have also determined the elastic strain content of the samples.