A grazing incidence small angle X-ray scattering study of the effect of growth interrupt on the structure of InAs quantum dots

There is considerable interest in InAs quantum dots (QD) grown in a GaAs matrix by molecular beam epitaxy. Electrons and holes may be confined in bound states within the dots due to the smaller band gap of InAs. Optical transitions in the infrared energy range offer the potential of opto-electronic devices such as lasers for optical communications. One of the crucial factors that determines the electrical properties of quantum dots is their structure, including strain within the dot and the matrix and changes in composition due to alloying. The pyramid shape of the InAs islands prior to burial by the GaAs cap is well known from standard surface science studies. There is, however, evidence that the dot shape and composition changes dramatically upon burial. Further changes occur if the sample is subjected to post-growth annealing. The details of such processes are poorly understood, but vital for greater understanding and hence control of QD properties. We present grazing incidence small angle X-ray scattering (GISAXS) studies of QDs measured at the ESRF synchrotron. A series of InAs dots were grown on GaAs(001) capped with 25 nm of GaAs. The samples were prepared with different growth interruptions during the production process. The X-ray scattering reveals differences in the shape of the dots that can be correlated with their photoluminescence spectra. The GISAXS signal also reveals a distinct asymmetry in the shape of the dots. Detailed structural information on buried QDs will allow more realistic computer modelling.