T. Ben

Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell

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.

Vertical order in stacked layers of self-assembled In(Ga)As quantum rings on GaAs (001)

Stacked layers of self-assembled In(Ga)As quantum rings on GaAs grown by solid source molecular beam epitaxy are studied by ex situ atomic force microscopy (AFM), low temperature photoluminescence (PL) and cross-sectional transmission electron microscopy (XTEM). 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 >6nm 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.

Identification of III?N nanowire growth kinetics via a marker technique

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.

Size control of InAs∕InP(001) quantum wires by tailoring P∕As exchange

The size and emission wavelength of self-assembled InAs∕InP(001) quantum wires (QWrs) is affected by the P∕As exchange process. In this work, we demonstrate by in situ stress measurements that P∕As exchange at the InAs∕InP interface depends on the surface reconstruction of the InAs starting surface and its immediate evolution when the arsenic cell is closed. Accordingly, the amount of InP grown on InAs by P∕As exchange increases with substrate temperature in a steplike way. These results allow us to engineer the size of the QWr for emission at 1.3 and 1.55μm at room temperature by selecting the range of substrate temperatures in which the InP cap layer is grown.

Stacking of InAs/InP(001) quantum wires studied by in situ stress measurements: Role of inhomogeneous stress fields

Size and spatial distribution homogeneity of nanostructures is greatly improved by making stacks of nanostructures separated by thin spacers. In this work, we present in situ and in real time stress measurements and reflection high-energy electron diffraction observations and ex situ transmission electron microscopy (TEM) characterization of stacked layers of InAs quantum wires (QWRs) separated by InP spacer layers, d(InP), of thickness between 3 and 20 nm. For d(InP)<20 nm, the amount of InAs involved in the created QWR from the second stack layer on, exceeds that provided by the In cell. Our results suggest that in those cases InAs three dimensional islands formation starts at the P/As switching and lasts during further InAs deposition. We propose an explanation for this process that is strongly supported on TEM observations. The results obtained in this work imply that concepts like the existence of a critical thickness for two- to three-dimensional growth mode transition should be revised in correlate...

Strong suppression of internal electric field in GaN/AlGaN multi-layer quantum dots in nanowires

Photoluminescence (PL) studies of GaN/Al(x)Ga(1-x)N quantum dots (QDs) in nanowires demonstrate an efficient carrier confinement, resulting in thermally stable decay times up to 300 K. The evolution of the PL transition energy as a function of both the QD height and the Al mole fraction in the barriers, as well as the evolution of the decay time as a function of the QD height, point out that a built-in electric field is significantly smaller than the value expected from the spontaneous polarization discontinuity. This is explained by the uniaxial compressive strain resulting from the spontaneously formed Al-rich shell that envelops the QD stack

Incorporation of Sb in InAs∕GaAs quantum dots

The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs∕GaAs quantum dots emitting at 1.3μm. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmission electron microscopy. The intermixing process giving rise to the formation of this quaternary alloy takes place despite the large miscibility gap between InAs and GaSb binary compounds, and is probably driven by the existence of strain in the quantum dots.

Direct imaging of quantum wires nucleated at diatomic steps

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place preferentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step.

InAs/AlGaAs quantum dot intermediate band solar cells with enlarged sub-bandgaps

In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon sub-bandgap (SBG) photocurrent, and output voltage preservation, and both have been experimentally demonstrated at low temperature. At room temperature (RT), however, thermal escape/relaxation between the conduction band (CB) and the IB prevents voltage preservation. To improve this situation, we have produced and characterized the first reported InAs/AlGaAs QD-based IBSCs. For an Al content of 25% in the host material, we have measured an activation energy of 361 meV for the thermal carrier escape. This energy is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a specifically designed QD capping layer. This high value is responsible for the suppression of the SBG quantum efficiency under monochromatic illumination at around 220 K. We suggest that, if the energy split between the CB and the IB is large enough, activation energies as high as to suppress thermal carrier escape at room temperature (RT) can be achieved. In this respect, the InAs/AlGaAs system offers new possibilities to overcome some of the problems encountered in InAs/GaAs and opens the path for QD-IBSC devices capable of achieving high efficiency at RT.

Effect of N2 and H2 plasma treatments on band edge emission of ZnO microrods.

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.