Microstructural and morphological properties of homoepitaxial (001)ZnTe layers investigated by x-ray diffuse scattering

The microstructural and morphological properties of homoepitaxial (001)ZnTe layers are investigated by x-ray diffuse scattering. High resolution reciprocal space maps recorded close to the ZnTe (004) Bragg peak show different diffuse scattering features. One kind of cross-shaped diffuse scattering streaks along directions can be attributed to stacking faults within the epilayers. Another kind of cross-shaped streaks inclined at an angle of about 80deg with respect to the in-plane direction arises from the morphology of the epilayers. (abridged version

Nanometer-scale Tomographic Reconstruction of 3D Electrostatic Potentials in GaAs/AlGaAs Core-Shell Nanowires

We report on the development of Electron Holographic Tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as X-ray tomography

GaAs hetero-epitaxial layers grown by MOVPE on exactly-oriented and off-cut (1 1 1)Si: Lattice tilt, mosaicity and defects content

Integration of III-V devices with Si-photonics and fabrication of monolithic III-V/Si tandem solar cells require the heteroepitaxy of III-V compounds on Si. We report on the lattice tilt, mosaicity and defects content of relaxed GaAs grown by MOVPE on exactly-oriented and 4°-offcut (1 1 1)Si. Thin GaAs single-layers grown at 400 °C and annealed at 700 °C show ∼ 3×10^8 cm−2 density of surface pinholes. Double-layer samples were obtained by GaAs overgrowth at 700 °C. GaAs epilayers are tilted by (0.05–0.14)° with respect to Si. Rotational twins were observed in X-ray diffraction (XRD) pole figures: the most abundant ones originate from 60°-rotation of GaAs around the [1 ̄1 ̄1 ̄] growth direction and are identified as micro-twins along the GaAs/Si hetero-interface. Twins obtained by rotations around the [1 ̄1 ̄1], [11 ̄1 ̄], and [1 ̄11 ̄] directions or by combined rotations around the growth direction and one of the former, were also observed. The GaAs mosaicity and block size were studied through high-resolution XRD intensity mapping: for single-layer samples crystal blocks are ascribed to 3–5 nm thin micro-twins, whose size does not change upon annealing. In double-layer samples thicker (32–35 nm) micro-twins occur. GaAs samples grown on offcut (1 1 1)Si show less rotational twins but a reduced mosaic block size with respect to exactly-oriented Si

Translating land cover/land use classifications to habitat taxonomies for landscape monitoring: A Mediterranean assessment

Periodic monitoring of biodiversity changes at a landscape scale constitutes a key issue for conservation managers. Earth observation (EO) data offer a potential solution, through direct or indirect mapping of species or habitats. Most national and international programs rely on the use of land cover (LC) and/or land use (LU) classification systems. Yet, these are not as clearly relatable to biodiversity in comparison to habitat classifications, and provide less scope for monitoring. While a conversion from LC/LU classification to habitat classification can be of great utility, differences in definitions and criteria have so far limited the establishment of a unified approach for such translation between these two classification systems. Focusing on five Mediterranean NATURA 2000 sites, this paper considers the scope for three of the most commonly used global LC/LU taxonomies—CORINE Land Cover, the Food and Agricultural Organisation (FAO) land cover classification system (LCCS) and the International Geosphere-Biosphere Programme to be translated to habitat taxonomies. Through both quantitative and expert knowledge based qualitative analysis of selected taxonomies, FAO-LCCS turns out to be the best candidate to cope with the complexity of habitat description and provides a framework for EO and in situ data integration for habitat mapping, reducing uncertainties and class overlaps and bridging the gap between LC/LU and habitats domains for landscape monitoring—a major issue for conservation. This study also highlights the need to modify the FAO-LCCS hierarchical class description process to permit the addition of attributes based on class-specific expert knowledge to select multi-temporal (seasonal) EO data and improve classification. An application of LC/LU to habitat mapping is provided for a coastal Natura 2000 site with high classification accuracy as a result

MOCVD of II-VI compounds

The metalorganic chemical vapor deposition (MOCVD) of II-VI semiconducting compounds (II-IVs) has been studied extensively since 1980, due to the relevance of these materials in many ®elds of optoelectronics. The MOCVD growth of II-VIs today includes wide-band-gap binary compounds, ternary and quaternary pseudobinary alloys for applications in blue-green optoelectronics, as well as narrow-gap compounds for infrared detectors. The growth of multilayer heterostructures, including multiple quantum wells (MQWs), and superlattices (SLs), has been demonstrated for many of these materials. In this article, the MOCVD process of II-VIs is illustrated along with the associated growth mechanisms. To grow device-quality II-VIs, several requirements must be met by the MOCVD process: reduced growth temperatures, high purity, efficient n- and p-type doping, and high crystalline and optical properties. Established solutions and current limitations of the technology are pointed out

III-V nanowires by self-assembly MOVPE technology for novel and efficient opto-electronic and photovoltaic devices

We report on the self-assembly by Au-catalyzed metalorganic vapor phase epitaxy (MOVPE) of GaAs-based nanowires (NWs) and their applications to novel and efficient nano-devices. The growth of GaAs and GaAs-AlGaAs core-shell NWs is presented as case study, focusing on the dependence of their structural, optical and electrical properties on MOVPE conditions. MSM diodes fabricated using as-grown core-shell NWs are reported, along with their photoelectric performances. These devices show potentials for applications as fast photo-detectors and efficient solar cells

Principles of VPE and MOVPE and applications

The fundamental laws and mechanisms behind vapour phase epitaxy (VPE) are illustrated and discussed, along with different methods and reactor architectures currently used for the growth of both elemental (Si) and compound (III-V, III-N and II-VI) semiconductors for micro- and optoelectronic devices; applications of cloride-, hydride- and metalorganic-VPE are also discussed

Dilute nitride III-V nanowires for high-efficiency intermediate-band photovoltaic cells: Materials requirements, self-assembly methods and properties

This paper deals with dilute nitride III-V (III-N-V) semiconductor nanowires and their synthesis by bottom-up (so-called self-assembly) methods for application to novel and high efficiency intermediate-band solar cells (IBSCs). Nanowire-IBSCs based on III-N-V compounds promise to overcome many of the limitations encountered so far in quantum-dots or planar-heterostructure IBSCs; indeed, thanks to the combination of IBSC functionality with the unique physical properties associated with nanowires-based devices, photovoltaic cells with unprecedentedly high power conversion efficiency, simpler junction geometry, reduced structural constraints, low materials usage and fabrication costs could be conceived. The fabrication of III-N-V nanowire-IBSCs requires however, careful engineering of the inner nanowire-device structures to comply with both IBSC stringent operational requirements and the peculiar physical properties of III-N-V semiconductor alloys. Herewith, we propose for the first time perspective III-N-V core-multishell nanowire heterostructures as potential candidates to IBSC applications, their fabrication requiring however, precisely controlled self-assembly technologies. The present status of research on the topic is reviewed, focusing in particular on the bottom-up growth of III-N-V nanowires by molecular beam and metalorganic vapor phase epitaxy methods and properties of as-grown nanostructures. Major results achieved in the current literature and open problems are presented and discussed, along with advantages and limitations of employed self-assembly methods for the fabrication of dilute nitride III-V based nanowire-IBSCs