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Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.

By using a marker technique based on nanowire (NW) heterostructure, we have identified the Ga-limited and N-limited GaN NW growth regimes, which are shifted in comparison to those in two-dimensional GaN layers. The results show that the Ga atoms diffusing along NW sidewalls have a significant contribution to the NW vertical growth. By reducing the substrate temperature, Ga-rich conditions locally activate the lateral growth. In contrast to Ga atoms, the contribution of Al and N adatom diffusion to the NW vertical growth is negligible. Finally, the control of GaN/AlN heterostructures in NWs is demonstrated.

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...

ZnO microrods were grown by laser assisted flow deposition technique in order to study their luminescence behaviour in the near band edge spectral region. Transmission electron microscopy analysis put in evidence the high crystallinity degree and microrod’s compositional homogeneity. Photoluminescence revealed a dominant 3.31 eV emission. The correlation between this emission and the presence of surface states was investigated by performing plasma treatments with hydrogen and nitrogen. The significant modifications in photoluminescence spectra after the plasma treatments suggest a connexion between the 3.31 eV luminescence and the surface related electronic levels.