Tomographic nanometer-scale images of self-assembled InAs͞GaAs quantum dots have been obtained from surface-sensitive x-ray diffraction. Based on the three-dimensional intensity mapping of selected regions in reciprocal space, the method yields the shape of the dots along with the lattice parameter distribution and the vertical interdiffusion profile on a subnanometer scale. The material composition is found to vary continuously from GaAs at the base of the dot to InAs at the top.

Room temperature photoluminescence at 1.6μm is demonstrated from InGaAs quantum dots capped with an 8nm GaAsSb quantum well. Results obtained from various sample structures are compared, including samples capped with GaAs. The observed redshift in GaAsSb capped samples is attributed to a type II band alignment and to a beneficial modification of growth kinetics during capping due to the presence of Sb. The sample structure is discussed on the basis of transmission electron microscopy results.

Stacked layers of self-assembled In͑Ga͒As quantum rings on GaAs grown by solid source molecular beam epitaxy are studied by ex situ atomic force microscopy ͑AFM͒, low temperature photoluminescence ͑PL͒ and cross-sectional transmission electron microscopy ͑XTEM͒. The influence of the strain field and InAs segregation on the surface morphology, optical properties and vertical ordering of three quantum ring layers is analyzed for GaAs spacers between layers from 1.5 to 14 nm. AFM and PL results show that samples with spacers Ͼ6 nm have surface morphology and optical properties similar to single layers samples. XTEM results on samples with 3 and 6 nm GaAs spacers show that the rings are preserved after capping with GaAs, and evidence the existence of vertically ordered quantum rings. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1866228͔ Molecular beam epitaxy ͑MBE͒ is a powerful technique for the fabrication of lattice mismatched semiconductor nanostructures. In particular, InAs on GaAs ͑001͒ selfassembled quantum dots ͑QDs͒ are one of the most studied systems. QDs are particularly interesting for their potential use in detectors, memories, quantum computing and photonic devices applications. [1][2][3][4] The development and design of QDs based devices requires a precise control of the morphology and composition of the QDs. As an example of the achieved capabilities to control QDs size and shape it has been proved that it is possible to self-assemble In͑Ga͒As quantum rings ͑QRs͒. 5 QRs are obtained by covering a layer of QDs with a thin cap ͑ϳ20% of the dot height͒ followed by a growth pause. In this way, the original islands reshape into ring-like structures. 6 Several authors have reported interesting differences in the optical and confining properties between QDs and QRs. Pettersson et al. 7 found an oscillator strength for the fundamental transition three times higher for QRs than for QDs. Warburton et al. 8 showed that the permanent dipole moment of exciton in QRs is three times higher and with opposite sign than those found for lens shaped QDs. Lorke et al. 9 proved that the ground state of QRs with zero angular momentum transits into a chiral state under the influence of an external magnetic field, concluding that ring shaped morphology translates into a ring-like electronic structure. Their nontrivial geometry, together with the possibility of controlling the energy levels, oscillator strength, polarizability and magnetic properties, make QRs good candidates for the development of new devices. The use of stacked layers increases quantum efficiency and avoids saturation gain effects in laser devices. 10 In the case of QRs, stacking the nanostructures is even more important, as the large in-plane mean dimensions of the rings ͑100 nmϫ 90 nm by AFM͒ require low density ring ensembles ͑1-9 ϫ 10 9 cm −2 ͒ to avoid overlap problems.In spite of all the work focused on optical characterization, little is known about the structural properties of embedded rings. This work mainly presents structural charact...

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We have studied the emission and absorption properties of type II GaSb/ GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ϳ500 meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2827582͔In recent years, GaSb/ GaAs structures have aroused great interest due to their type II band alignment and intrinsically different behavior compared to the well known InAs/ GaAs system. Fundamental issues regarding their growth process, energy level structure, and optical properties in addition to their technological applications in photodetection and photovoltaics have been already investigated in different configurations such as quantum dots ͑QDs͒, 1-5 quantum wells 6 ͑QWs͒ or ternary compounds. 7 In this work, we present various results regarding the GaSb/ GaAs QDs system, which extend and complete previous works.The QDs studied here were grown by solid source molecular beam epitaxy on a n-type GaAs͑001͒ substrate after deposition of a n-type GaAs buffer layer ͑Si: 1 ϫ 10 18 cm −2 ͒. The QDs were nucleated at 480°C, using a growth rate of 0.1 ML/ s. The formation of the GaSb QDs was detected by the change of the reflection high energy electron diffraction pattern after the deposition of 1.3 ML of GaSb. The GaSb layer, with a nominal thickness of 2 ML, was then exposed to Sb flux for 20 s and then annealed for 20 s without an Sb flux to limit the amount of Sb segregated during capping. The GaAs capping was done at 0.4 ML/ s in two steps. In the first step, a 10 nm thick GaAs layer was grown at the temperature of QD nucleation to avoid their destabilization. In the second one, a 40 nm thick GaAs layer was deposited at 570°C. During growth, the As and Sb beam equivalent pressures were 1.0ϫ 10 −5 and 1.9ϫ 10 −6 mbar, respectively. This scheme was repeated six times, with a 3 min growth interruption under an As 4 flux to lower the substrate temperature before the nucleation of the next QD layer. On top, a p-type 300 nm thick GaAs layer ͑Be: 1 ϫ 10 18 cm −2 ͒ was grown at 580°C. Finally, standard optical lithography and wet etching techniques were used to define mesas and metal Ohmic contacts. Figure 1͑a͒ shows the photoluminescence ͑PL͒ spectra recorded at 20 K as a function of excitation power at 532 nm. We can clearly identify two bands centered at 1.32 and 1.05 eV. The narrow high energy band corresponds to the wetting layer ͑WL͒ recombination and dominates the spectrum at low temperatures. Its peak energy position is compatible with a GaSb WL thickness of 0.7 nm, 2 which is larger than the total amount of GaSb deposited ͑0.57 nm͒...