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

Recent developments in the MOVPE growth of ZnSe-based compounds and heterostructures

This work briefly reviews the state of the art of p-type doping metalorganic vapor phase epitaxy (MOVPE) grown ZnSe-based compounds and heterostructures. In particular, the origin of unintentional hydrogen (H) incorporation during the MOVPE of these wide band-gap II-VI compounds is discussed. H in ZnSe effectively passivates intentional nitrogen (N) acceptors, during the MOVPE fabrication of blue-light emitting diodes and laser diodes. It is shown that H enters in MOVPE-grown ZnSe as result of specific surface reactions between alkyls, whose efficiency depends on the chemical nature of the VI-group precursor. The existence of a marked trade-off between the proclivity of most common Se alkyls to incorporate H and their thermal stability is pointed out. The use of a novel class of VI-group alkyl precursors of the form R2X2 [where X=Se, S and R is an ethyl (Et) or methyl (Me) radical] will be then proposed as a convenient solution of the above problem. These alkyls allow a strong reduction of H incorporation in ZnSe-based materials, whilst retaining the low temperatures required by the MOVPE growth of device quality wide band-gap II-VI compounds. Dimethyldiselenide (Me2Se2) and diethyldisulphide (Et2S2) allow the pyrolytic MOVPE growth of Zn(S)Se-based heterostructures below 400 °C, i.e. ca. 150°C lower than what necessary for the usual dimethylselenide and diethylsulphide alkyls. Mass spectrometry fragmentation experiments performed on the alkyl molecular ions allowed us to investigate the alkyl relative bond strengths and suggest likely decomposition paths. The reduced thermal stability of these alkyls is then attributed to a weakening of the X-C bonds in the R2X2 molecule induced by the stronger X-X bond. Secondary ion mass spectrometry (SIMS) analysis shows that as-grown ZnSe epilayers have [H] values around (1-3)x10^17 cm-3, i.e. amongst the lowest ever reported for MOVPE-grown layers. SIMS elemental depth profiling measurements performed on the ZnSe samples show unintentional halogens (Cl and I) which contaminate the epilayers at the 10^14-10^15 cm^-3 level; these impurities originate from residual halides in the Me2Se2 batch. The functional validation of the new S and Se alkyls is completed by the structural, morphological and optical characterization of Zn(S)Se-based heterostructures. The results of double-crystal X-ray diffraction (XRD) measurements and scanning electron microscopy observations performed on ZnSe, ZnS and ZnSSe epilayers grown on (100)GaAs will be presented. The epilayer structural properties, determined by a statistical XRD theory under the kinematical approximation, compare well with what reported for molecular beam epitaxy grown ZnSe. 10K cathodoluminescence (CL) measurements performed on ZnSe and ZnS samples are also presented. CL spectra of as-grown ZnSe epilayers appear of good quality, with pronounced band-edge emissions and reduced deep level contributions