Study of electrically active defects in structures with InAs quantum Dots in GaAs matrix

In this work, InAs(QD)/GaAs(001) structures are analyzed in order to identify the conditions for plastic strain relaxation via dislocations. To this aim samples with a continuously varying coverage, going from 1.5 to 2.9 ML, have been studied mainly by means of electrical characterization and with the fundamental support of structural TEM analysis, PL measurements and AFM. In the whole coverage range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2E11 cm?2, respectively. At the same time, for coverage above 2.4 ML, large-sized QDs with diameters of 25 nm and densities ranging from 2E8 to 1.5E9 cm?2 coexist with small-sized QDs. We explain the formation of large-sized QDs as the unavoidable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with coverage, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. The relaxation of those incoherent islands substantially affects the properties of QD structures by (i) reducing the free carrier concentration near the QD plane, (ii) formation of deep levels with typical features related to extended structural defects, and (iii) considerably quenching the QD photoluminescence intensity. Moreover a preliminary work on Scanning Capacitance Microscopy measurements on these structures have been carried out. The first tests demonstrates a sample ageing due to many hours of ambient atmosphere exposition.