Formation and role of defects in stacked large binary InAs/GaAs quantum dot structures

A detailed study of defect formation, as a result of large dots synthesis, in stacked InAs/GaAs quantum dots (QDs) was performed using double crystal x-ray diffraction, transmission electron microscopy, atomic force microscopy and photoluminescence. Three stacked samples with varying InAs nominal thicknesses but fixed GaAs 'spacer' of 25 nm were grown by molecular beam epitaxy (MBE). For these large dots emitting near 1.3 mu m, the formation of coherent QDs was accompanied by 'volcano-like' defects and extended defects such as threading dislocations. Power dependence photoluminescence revealed a bimodal distribution of small dots coexisting with the large dots. Surface depressions due to lattice bending within the sample were also observed. The 'volcano-like' defects which extended all the way to the surface had a surface density of the order of 10(9) cm(-2). Remarkably, significant improvements in photoluminescence intensity were observed when the material was etched to selectively remove the topmost stacks thus demonstrating that the 'volcano-like' defects were responsible for the severely suppressed photoluminescence efficiencies. For nominal InAs thickness of 2.73 monolayers, an optimal stack of three periods yielded the highest photoluminescence intensity before the onset of defects.