The study of misfit dislocations in In_xGa₁-_xAs/GaAs strained quantum well structures

Abstract

In_xGa₁-_xAs/GaAs strained-layer structures containing four single quantum wells (SSQWs) were grown by molecular beam epitaxy on a (001) semi-insulating GaAs substrate with In contents of x = 0.09 and x = 0.16. The structures contained four In_xGa₁-_xAs quantum wells with well width and growth sequence given by 20, 40, 80 and 160 Å and were separated by 500 Å GaAs layers. The optical and structural properties of the SSQW samples have been analysed by low-temperature cathodoluminescence (CL) and transmission electron microscopy. Misfit dislocations were observed in the sample with higher In content. Large-angle convergentbeam electron diffraction has been newly applied to this system as a reliable method for determining dislocation character. All misfit dislocations studied in plan-view specimens revealed dark line contrast in appropriately chosen monochromatic CL images irrespective of the dislocation character or its α or β form. A simple one-to-one correlation between dislocations and dark lines was obtained. The very different well widths for In_xGa₁-_xAs wells in the structures resulted in very different emission energies in CL experiments. It was therefore possible to deduce in which quantum well a particular dislocation segment lay by the energy of the monochromatic image which showed dark-line contrast. As a result, dislocations were traced stepping progressively from one well to another. It was also proved that most dislocations lay in the first-grown well although it was the narrowest. The implication of these results and potential applications of the experimental technique are discussed.