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We report on quantum dot ͑QD͒ lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (d l у4ϫ10 10 cm Ϫ2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence ͑PL͒ intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In 0.3 Ga 0.7 As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm 2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.

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Annealing at higher temperature (700 °C) of structures with two-dimensional and three-dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan-view transmission electron microscopy (TEM) images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan-view TEM images taken under [001] zone axis illumination as well as by the cross-section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time (10–60 min) in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the intensity of the QD luminescence decreases, and the intensity of the wetting layer and the GaAs matrix luminescence increase with the increase in the annealing time.

In 0.5 Ga 0.5 As/GaAs ͑001͒ quantum dots ͑QDs͒ were grown by metalorganic chemical vapor deposition ͑MOCVD͒ on exactly ͑001͒ oriented substrates using the Stranski-Krastanow growth mode. The dot density and their relative geometrical arrangement are found to be strongly dependent on the substrate temperature. The dots have identical square shaped bases oriented along ͗100͘. For high densities a preferential relative alignment of the dots along the ͗110͘ directions is found. These dots tend to be arranged in a chainlike pattern with decreasing dot size towards the ends of the chains. From these observations the dot formation process for In 0.5 Ga 0.5 As quantum dots is suggested to be driven by energetics whereas the relative orientation is governed by kinetic effects. © 1996 American Institute of Physics.͓S0003-6951͑96͒00923-0͔Heteroepitaxial growth of highly strained structures like In x Ga 1Ϫx As/GaAs has gained increasing interest as it offers the possibility to fabricate self-organized structures like quantum dots ͑QDs͒ and quantum wires without any substrate patterning process. These low dimensional structures show unique physical properties, particularly interesting for novel optoelectronic devices like QD lasers with low threshold current density and high T 0 . 1 A fundamental question for future applications is whether dot formation is kinetically driven or due to minimization of total energy. The latter leads to sharper dot size and shape distributions. Thermodynamical equilibrium calculations predict a hierarchical series of self-organization processes. 2 According to these calculations the equilibrium state for a dilute quantum dot array is an ensemble of identically shaped pyramids of square base parallel ͗100͘. Their equilibrium size is defined by the ratio of facet energy and strain relation at facet edges. For a dense array the interaction between the individual dots via strain fields leads to an alignment of the dots on a two-dimensional square array oriented along the ͗100͘ directions thus representing an even higher level of self-organization.For QD structures grown by molecular beam epitaxy ͑MBE͒ there exists a wealth of information 3-7 including the first observation of self-organization of InAs quantum dots. 7 For MBE grown In x Ga 1Ϫx As dots ͑0.3ϽxϽ0.6͒ on singular ͑001͒ surfaces the formation of wirelike structure oriented along ͗110͘ has been observed. 8,9 In contrast there exist only very few reports on metalorganic chemical vapor deposition ͑MOCVD͒ grown InGaAs/GaAs QDs. 10,11 Preferential dot formation at surface steps has been observed for grown on misoriented substrates. 10 Self-ordering of dots grown by MOCVD on exactly oriented substrates has not yet been reported. In this letter, we present the first observation of selforganization of InGaAs/GaAs quantum dot structures grown by MOCVD on exactly oriented substrates. We find the dots to be aligned along the ͗110͘ directions in contrast to the alignment found for MBE grown InAs dots, though their base shape is identical to MBE grown ...