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The luminescence of InxGa1−xN nanowires (NWs) is frequently reported with large red-shifts as compared to the theoretical value expected from the average In content. Both compositional fluctuations and radial built-in fields were considered accountable for this effect, depending on the size, structure, composition, and surrounding medium of the NWs. In the present work, the emission properties of InGaN/GaN NWs grown by plasma-assisted molecular beam epitaxy are investigated in a comprehensive study combining ultraviolet-Raman and photoluminescence spectroscopy (PL) on vertical arrays, polarization-dependent PL on bundles of a few NWs, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and calculations of the band profiles. The roles of inhomogeneous In distribution and radial fields in the context of optical emission properties are addressed. The radial built-in fields are found to be modest, with a maximum surface band bending below 350 meV. On the other hand, variations in the local In content have been observed that give rise to potential fluctuations whose impact on the emission properties is shown to prevail over band-bending effects. Two luminescence bands with large positive and moderate negative polarization ratios of ≈+80% and ≤−60%, respectively, were observed. The red-shift in the luminescence is associated with In-rich inclusions in the NWs due to thermodynamic decomposition during growth. The negative polarization anisotropy is suggested to result from spontaneously formed superlattices in the In-rich regions of the NWs. The NWs show a preferred orthogonal absorption due to the dielectric boundary conditions and highlight the extreme sensitivity of these structures towards light polarization.

The influence of self-assembled short-period superlattices (SPSLs) on the structural and optical properties of InGaN/GaN nanowires (NWs) grown by PAMBE on Si (111) was investigated by STEM, EDXS, µ-PL analysis and k·p simulations. STEM analysis on single NWs indicates that in most of the studied nanostructures, SPSLs self-assemble during growth. The SPSLs display short-range ordering of In-rich and In-poor InxGa1-xN regions with a period of 2–3 nm that are covered by a GaN shell and that transition to a more homogenous InxGa1-xN core. Polarization- and temperature-resolved PL analysis performed on the same NWs shows that they exhibit a strong parallel polarized red-yellow emission and a predominantly perpendicular polarized blue emission, which are ascribed to different In-rich regions in the nanostructures. The correlation between STEM, µ-PL and k·p simulations provides better understanding of the rich optical emission of complex III-N nanostructures and how they are impacted by structural properties, yielding the significant impact of strain on self-assembly and spectral emission. Graphical abstract

In this work, the effect of atomic defects created by gallium ion irradiation on the optical properties of single-layer molybdenum disulfide is studied by means of micro-photoluminescence measurements. The induced defects give rise to an additional emission band located at about 170 meV below the free exciton. The micro-photoluminescence intensity of this defect-related emission band is found to be proportional to the defect density. The large spectral width suggests the presence of binding sites with different binding energies available for excitons that remain optically active up to 230 K.

Laser-thinning of 2D materials such as MoS2 is a promising approach for a local reduction of the number of multilayers down to a monolayer. For a precise control of the thinning process real-time m...

The ternary mixed metal oxides with high specific surface area are industrially important for sensors, catalysis, energy storage, and optoelectronics. However, synthesis of such metal oxides is alw...

International audience; We report on the microscopic behavior of residual hydrogen on nanometric field emitters. By using homogeneous or heterostructured semiconductor specimens analyzed in a laser-assisted atom probe, it is possible to study how the relative abundances of the ionic species H+, H2+, and H3+ depend on the microscopic electric field, estimated through post-ionization statistics. In the case of Ga-containing semiconductors, the relative abundances of H+, H2+, and H3+ follow a common trend, independent of the nonmetallic component of the matrix. The dependence of the total H flux on the electric field exhibits a more complex behavior, which depends also on the spatial direction of the variation of the field (in-depth or on-surface). The analysis of multiple detection events provides further insights into surface chemistry. Noticeably, H+–H3+ ion pairs are both correlated in number and separated by very small distances on the detector space, suggesting a possible reaction 2H2 → H3++H++2e– taking place on the field emitter surface.

The measurement of the composition of ε-Ga2O3 and the quantification of Sn doping in ε-Ga2O3:Sn by laser-assisted atom probe tomography (APT) may be inaccurate depending on the experimental conditions. Both the role of the laser energy and surface electric field were investigated, and the results clearly indicate that deviations from stoichiometry are observed changing the electric field conditions during APT. The measured atomic fraction of Ga can change from 0.45 at low field to 0.38 at high field, to be compared with the expected 0.4. This was interpreted in terms of preferential evaporation of Ga at high field and deficit of O at low field, which was caused by the formation of neutrals. The quantification of Sn-doping is accurate at low-field conditions, with an overestimation of the detected Sn-metallic fraction at high field. This suggests that Sn has a higher evaporation field compared to Ga. Finally, multiple detection events were in-depth studied, revealing that three dissociation reactions occur during APT: GaO2+ → Ga+ + O+; Ga2O22+ → Ga+ + GaO2+; Ga3O22+ → Ga+ + Ga2O2+. Nevertheless, only 2% of the detected events are related to such dissociation reactions, too small a fraction to fully explain the observed deviation from the stoichiometric composition in ε-Ga2O3.

The measurement of the composition of ε-Ga2O3 and the quantification of Sn doping in ε-Ga2O3:Sn by laser-assisted atom probe tomography (APT) may be inaccurate depending on the experimental conditions. Both the role of the laser energy and surface electric field were investigated, and the results clearly indicate that deviations from stoichiometry are observed changing the electric field conditions during APT. The measured atomic fraction of Ga can change from 0.45 at low field to 0.38 at high field, to be compared with the expected 0.4. This was interpreted in terms of preferential evaporation of Ga at high field and deficit of O at low field, which was caused by the formation of neutrals. The quantification of Sn-doping is accurate at low-field conditions, with an overestimation of the detected Sn-metallic fraction at high field. This suggests that Sn has a higher evaporation field compared to Ga. Finally, multiple detection events were in-depth studied, revealing that three dissociation reactions occur during APT: GaO2+ → Ga+ + O+; Ga2O22+ → Ga+ + GaO2+; Ga3O22+ → Ga+ + Ga2O2+. Nevertheless, only 2% of the detected events are related to such dissociation reactions, too small a fraction to fully explain the observed deviation from the stoichiometric composition in ε-Ga2O3.

Ga2O3 micro- and nanowires-based optical microcavities have been obtained by patterning pairs of distributed Bragg reflectors (DBRs) with a focused ion beam (FIB) microscope. DBRs result in widely tunable high reflectivity bands. The microcavities have been designed and optimized with the aid of simulations and optically characterized by micro-photoluminescence. Tunable strong modulations are confirmed in the NUV-blue as well as in the red-NIR ranges for unintentionally doped and chromium doped wires, respectively. Experimental, analytical and simulations results will be compared and some possible applications of these cavities will be assessed.

In this Letter, we report optical confinement in the near-ultraviolet (near-UV) range in G a 2 O 3 nanowires (NWs) by distributed Bragg reflector (DBR) nanopatterned cavities. High-contrast DBRs, which act as the end mirrors of the cavities of the desired length, are designed and fabricated by focused ion beam etching. The resonant modes of the cavities are analyzed by micro-photoluminescence measurements, analytical models, and simulations, which show very good agreement between each other. Experimental reflectivities up to 50% are obtained over the 350–410 nm region for the resonances in this wavelength range. Therefore, G a 2 O 3 NW optical cavities are shown as good candidates for single-material-based near-UV light emitters.

In this Letter, we report optical confinement in the near-ultraviolet (near-UV) range in

The present review defines the state of the art of parabolic trough collectors and identifies the concepts in the framework of innovation’s requirements. These are the increase of operating temperatures and of the power plant’s overall efficiency, as well as the cost reductions of the solar field by eliminating components and the reduction of parasitic power consumption (Pitz-Paal et al., 2007). The study investigates 34 collector concepts and classifies them according to their main structural features like mechanisms or materials. The categories include conventional collectors in Category A, collectors with alternative structure and sheet reflectors in Category B, with non-metallic materials like sandwich composite structures and concrete in Category C, with enclosed aperture in Category D and with fixed focus in Category E. For the technical-economic comparison, two conventional collectors vs. 14 innovations are analysed, following a design engineering method. The definition of a metric table defines the technical comparison criteria for those collectors suitable for thermo-oils or molten salts as heat transfer fluid. Results show that collectors operating with molten salts, instead of thermo-oil, have a significant techno-economic potential for utility scale application, for instance, the Molten Salt Trough, the UltimateTrough and the SkyFuelDSP (dispatchable solar power) as forerunners of their respective categories. Results show also that the inflatable Heliovis collector operating with thermo-oil has a relevant economic potential.

We show that one-dimensional (1d) GaN quantum-wires (QWRs) exhibit intense and spectrally sharp emission lines. These QWRs are realized in an entirely self-assembled growth process by molecular beam epitaxy (MBE) on the side facets of GaN/AlN nanowire (NW) heterostructures. Time-integrated and time-resolved photoluminescence (PL) data in combination with numerical calculations allow the identification and assignment of the manifold emission features to three different spatial recombination centers within the NWs. The recombination processes in the QWRs are driven by efficient charge carrier transfer effects between the different optically active regions, providing high intense QWR luminescence despite their small volume. This is deduced by a fast rise time of the QWR PL, which is similar to the fast decay-time of adjacent carrier reservoirs. Such processes, feeding the ultra-narrow QWRs with carriers from the relatively large NWs, can be the key feature towards the realization of future QWR-based devices. While processing of single quantum structures with diameters in the nm range presents a serious obstacle with respect to their integration into electronic or photonic devices, the QWRs presented here can be analyzed and processed using existing techniques developed for single NWs., This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 787). SMS acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”. SMS and JA acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish Ministerio de Economía y Competitividad project ValPEC (ENE2017-85087-C3). ICN2 acknowledges support from the Severo Ochoa Program (Ministerio de Economía y Competitividad, Grant SEV-2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of the Universitat Autónoma de Barcelona Materials Science PhD program. Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative - I3). This work has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement No. 654360 NFFA-Europe.

4D-scanning transmission electron microscopy (4D-STEM) can be used to measure electric fields such as atomic fields or polarization-induced electric fields in crystal heterostructures. The paper focuses on effects occurring in 4D-STEM at interfaces, where two model systems are used: an AlN/GaN nanowire superlattice as well as a GaN/vacuum interface. Two different methods are applied: First, we employ the centre-of mass (COM) technique which uses the average momentum transfer evaluated from the intensity distribution in the diffraction pattern. Second, we measure the shift of the undiffracted disc (disc-detection method) in nano-beam electron diffraction (NBED). Both methods are applied to experimental and simulated 4D-STEM data sets. We find for both techniques distinct variations in the momentum transfer at interfaces between materials: In both model systems, peaks occur at the interfaces and we investigate possible sources and routes of interpretation. In case of the AlN/GaN superlattice, the COM and disc-detection methods are used to measure internal polarization-induced electric fields and we observed a reduction of the measured fields with increasing specimen thickness.

The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga$_2$O$_3$ has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main $\beta$-phase. Here, three polymorphs of Ga$_2$O$_3$, $\alpha$, $\beta$ and $\varepsilon$, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga$_2$O$_3$ polymorphs as materials at the heart of future electronic device generations., Comment: Updated manuscript version after peer review

InGaN/GaN nanowire arrays (NWAs) exhibit efficient photoluminescence (PL) which persists to temperatures of 200 °C and beyond. The PL characteristics exhibit chemical sensitivity when the NWAs are exposed to gases or liquids. Gas sensing tests on such arrays reveal both quenching and enhancing PL responses, depending on the kinds of gases chosen and NWA operation conditions employed. A characteristic observed for all kinds of analytes is that the concentration dependence of the PL response consistently follows Langmuir isotherms which are easy to interpret regarding adsorbate-specific adsorption energies. Another characteristic feature is that best-fitting adsorption energies do not reveal as species-dependent constants but rather follow linear dependencies on temperature with species-dependent slopes. We show that this behavior can be explained by a competition of test gases and background gases for common adsorption sites. A particularly interesting gas species is water vapor, which can form competing adsorbates with quenching and enhancing characteristics. Unlike oxidizing gas species (O2, NO2, and O3) and water vapor, reducing gas species do not exhibit an intrinsic ability to modify the PL yield. Reducing gas species, rather, are detected in an indirect manner by consuming quenching oxygen adsorbates and by forming enhancing H2O ones as these interact with oxygen species coadsorbed in reactive backgrounds of ambient or synthetic air.

TSE1 is the first solar thermal power plant operating in Southeast Asia. It was planned by Solarlite GmbH with support of Tiede- & Niemann GmbH, both German based. It is the first power plant with direct steam generation (DSG) concept and superheating in parabolic troughs. The solar field has a nominal power of 19 MWth driving a 5 MWel turbine by superheated steam at 30 bar and 330 °C. During 2010/2011 Solarlite built the solar field, while the later owner and operator Thai Solar Energy (TSE) from Bangkok built the power block, in Kanchanaburi, Thailand. TSE1 is being operated by TSE since January 2012 (Kruger et al., 2012). This publication is based on a study within the KanDis project, funded by the German Federal Ministry for Economic Affairs and Energy, in which an extensive database of records of almost 500 sensors installed in the power plant (solar field and power block) has been investigated. The data have been provided by Solarlite with a time resolution of about 1 min. Within the KanDis project, a stable operation could be demonstrated (Khenissi et al., 2015; Kruger et al., 2016). Even under the fluctuating irradiance conditions in the rainy season of Thailand, the turbine could be operated well and generate electricity. Evaluation of the operation data has helped to learn more about DSG behaviour. The TSE1 layout and the implemented control strategies were evaluated and strategies for improvement of TSE1 are suggested within this paper. From the experiences with the TSE1 power plant, conclusions could be drawn to improve the layout and control of future DSG plants.

We present a pH-sensitive AlGaN/GaN ion sensitive field-effect transistor (ISFET) with monolithically integrated Wheatstone bridge layout and demonstrate the possibility to identify and to compensate the influence of cross-sensitivities to environmental changes, such as temperature, without the need of an external electronic circuit. Based on this Wheatstone-approach low-drift penicillinase-modified AlGaN/GaN solution-gate field-effect transistors (PenFET) with an ultrathin Al2O3 gate coating were prepared and exhibit improved stability and sensitivity compared to PenFETs with conventional wet chemically oxidized gate surface over a course of 60 days as they yield a denser, more homogeneous enzyme layer on the gate area. Moreover, using the Wheatstone-approach can identify a loss of physisorbed enzymes as the dominant aging mechanism for shorter time-periods and stable operation conditions after a period of 10 days with an extracted average Michaelis constants of (20 ± 4) μM for the immobilized enzyme layer by quantitative evaluation applying a kinetic model. By application of the Wheatstone-approach for acetylcholinesterase-modified AlGaN/GaN solution-gate field-effect transistors (AcFET) as a model, we demonstrate that temperature-induced drift can be differentiated from substrate-induced signals and a substantial loss of immobilized enzymes over the course of 12 days after production can be identified.

The photoluminescence properties of InGaN/GaN nanowire arrays were used to probe the adsorption of H 2 O, O 2 , NO 2 and O 3 on III-nitride surfaces. Upon adsorption these gases either enhance (H 2 O) or quench (O 2 , NO 2 , O 3 ) the photoluminescence intensity which allows the related adsorption processes to be evaluated. An analysis of the experimental data in terms of the Langmuir Adsorption and Recombination model reveals that – at room temperature – the Langmuir energy of adsorption E ads increases in the order H 2 O, O 2 , NO 2 , and O 3 and that for each gas species E ads increases linearly with increasing surface temperature. We show that this behaviour can be explained by a competition of these air constituents for the same kind of Ga(In) adsorption sites. In contrast, exploratory experiments with H 2 and simple hydrocarbons reveal that these reducing species neither quench nor enhance the native photoluminescence response, which indicates that these do not compete for the same adsorption sites as the background air constituents.

We demonstrate that the complex adsorption behavior of H2O on InGaN/GaN nanowire arrays is directly revealed by their ambient-dependent photoluminescence properties. Under low-humidity, ambient-temperature, and low-excitation-light conditions, H2O adsorbates cause a quenching of the photoluminescence. In contrast, for high humidity levels, elevated temperature, and high excitation intensity, H2O adsorbates act as efficient photoluminescence enhancers. We show that this behavior, which can only be detected due to the low operation temperature of the InGaN/GaN nanowires, can be explained on the basis of single H2O adsorbates forming surface recombination centers and multiple H2O adsorbates forming surface passivation layers. Reversible creation of such passivation layers is induced by the photoelectrochemical splitting of adsorbed water molecules and by the interaction of reactive H3O+ and OH– ions with photoactivated InGaN surfaces. Due to electronic coupling of adsorbing molecules with photoactivated surf...

Surface treatment of the foreign substrate is a critical factor influencing heteroepitaxial catalyst-free growth of nanowires, their crystal quality, their diameter and their areal density. To this end, catalyst-free growth of GaN nanowires on Al2O3(0001) by plasma-assisted molecular beam epitaxy was achieved using the following substrate surface treatments: (a) deposition of a SixNy layer on nitridated Al2O3 surface, and (b) deposition of Si on bare Al2O3 surface. The nanostructure of GaN nanowires and GaN/Al2O3 interfaces was explored by quantitative high-resolution transmission electron microscopy and related analytical methods. Spontaneous growth of GaN nanowires was realized on the amorphous SixNy layer, while a discontinuous crystalline zone in contact with Al2O3 was identified as partially strained AlN. Subsequently, GaN nanowires were directly grown on top of Al2O3 among stress-free Si islands. The orientation relation of these islands with the substrate was the [112]( 1 ¯ 1 ¯ 1 )Si//[ 1 1 ¯ 00 ](0001)Al2O3, providing the minimum lattice misfit between the two structures. Increasing the Si deposition time a higher density of Si islands was realized, leading to non-coalesced nanowires of lower density and better structural quality. Hence, the presence of Si islands induced a mask-like effect on the nucleation of GaN nanowires that can be exploited for a controlled catalyst-free growth of nanowires.

The so called phase-coherence length ${l}_{\ensuremath{\varphi}}$ and its relation to the geometrical dimensions of a sample determine the electronic transport regime. Different approaches are established for extracting ${l}_{\ensuremath{\varphi}}$ from magnetotransport data of mesoscopic systems and need to be cross-checked by using experimental data on the same model system in order to ensure an overall consistent theoretical description. Suitable model systems for testing this consistency are single GaN:Ge nanowires. Their magnetoconductance at low temperatures exhibits universal conductance fluctuations as well as weak localization effects. We find that the values of ${l}_{\ensuremath{\varphi}}$ obtained from the established analysis of the magnitude of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) decrease with increasing measurement current, whereas the corresponding values of ${l}_{\ensuremath{\varphi}}$ determined by the analyses of the correlation field ${B}_{\text{C}}$ and the weak localization effect yield the same value for ${l}_{\ensuremath{\varphi}}$ independent of the measurement current used. We apply and modify the existing theoretical framework for bias-dependent differential conductance fluctuations, in order to explain the decrease of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) with increasing current density in our dc measurements. This leads to the same values of ${l}_{\ensuremath{\varphi}}$ independent of the analysis approach applied to the same set of data.

III-nitride semiconductors (AlGaN, GaN and InGaN) have received considerable attention in various fields ranging from high-frequency and high-temperature electronics [1] to LED lighting technologies [2]. [...]

We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of $5.3\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }\mathrm{MV}/\mathrm{cm}$ is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.

The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III–V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II–VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures., ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO coordinated project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program. S.M.S. acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”. The HAADF-STEM experiments were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon-Universidad de Zaragoza. A.F.i.M. thanks SNSF for funding through the NCCR QSIT. This work has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 654360 NFFA-Europe. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717 – ESTEEM3., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

Studies of ambient-pressure and high-pressure behaviour of photoluminescence (PL) as well as the phase transition pressure from wurtzite to cubic structure for a series of Zn 1−x Mg x O alloys as a function of the Mg-content x (0 ≤ x ≤ 0.34) are presented. The measured PL peak energy is shown to increase approximately linearly with increasing magnesium content from 3.36 eV to 4.11 eV, and the phase transition pressure from wurtzite to cubic structure decreases with increasing x from 12.6 GPa to 5.9 GPa. The phase transition is accompanied by a strong decrease of the near band-gap PL intensity. The value of the PL peak pressure coefficient d E PL /d p changes non-monotonically. It varies between 18.6 meV/GPa and 25.2 meV/GPa, and seems to depend on the layer thickness rather than on the layer composition, exhibiting strong scattering for several alloys with similar magnesium content. This behaviour can be due to the stabilizing influence of sapphire substrate, as is indicated both by the pressure coefficients and by the values of pressure of phase transition from wurtzite to rock salt structure in Zn 1−x Mg x O layers with the same or similar composition and different layer thicknesses.

The influence of nitrogen incorporation in high concentrations on the structural properties and morphology of SnO2–xNx thin films grown by chemical vapor deposition is studied. A decrease in crystallite size and a lattice expansion in SnO2–xNx films with increasing x are found by X-ray diffraction analysis and Raman spectroscopy. Substitutional nitrogen with a binding energy of 397.15 eV was detected by X-ray photoelectron spectroscopy, attributed to the N3− ion in the SnN bond. The increase of the N atomic concentration x in SnO2–xNx films from 0 to 7.9 at.% without phase separation with increasing NH3 flow rate during the deposition is accompanied by a decrease of O atomic concentration.

We report on the growth of cuprous oxide on MgO(001) by plasma-assisted molecular beam epitaxy. An increased copper to oxygen ratio enhances the cube-on-cube growth of the Cu 2 O crystallites compared to the Cu 2 O(110)||MgO(100) oriented crystallites. This correlates with the occurrence of a (3√2×√2)R45° surface reconstruction observed during growth. This reconstruction is known to be unstable under oxygen-rich conditions and stabilizes the Cu 2 O(001) surface compared to the Cu 2 O(110) surface for higher copper to oxygen ratios enhancing cube-on-cube growth. High resolution X-ray diffraction analysis of the resulting cube-on-cube Cu 2 O layers indicates partial relaxation by 39.7%. Investigations of the optical absorption and photoluminescence properties of the films reveal excitonic contributions comparable to those of single crystals.

The transport properties of Ge-doped single GaN nanowires are investigated, which exhibit a weak localization effect as well as universal conductance fluctuations at low temperatures. By analyzing these quantum interference effects, the electron phase coherence length was determined. Its temperature dependence indicates that in the case of highly doped nanowires electron-electron scattering is the dominant dephasing mechanism, while for the slightly doped nanowires dephasing originates from Nyquist-scattering. The change of the dominant scattering mechanism is attributed to a modification of the carrier confinement caused by the Ge-doping. The results demonstrate that the phase coherence length can be tuned by the donor concentration making Ge-doped GaN nanowires an ideal model system for studying the influence of impurities on quantum-interference effects in mesoscopic and nanoscale systems.

Despite the fact that metastable $\ensuremath{\alpha}$-Ga${}_{2}$O${}_{3}$ becomes increasingly interesting for applications in $e.g.$ high-power electronics, its basic material properties remain unclear. The authors employ plasma-assisted molecular-beam epitaxy to grow $r$-plane-oriented thin films of this birefringent semiconductor, as that crystal orientation allows the measurement of its anisotropic optical properties. The firm knowledge of these properties allows device design to proceed, including band-gap engineering by alloying with $\ensuremath{\alpha}$-In${}_{2}$O${}_{3}$ and $\ensuremath{\alpha}$-Al${}_{2}$O${}_{3}$.

An optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2–10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO2 coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO2/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO2 based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10−16 cm2 s−1 at a temperature of 100 °C. Temperature‐dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of Al2O3, which is known as a poor oxygen ion conductor within the analyzed temperature regime., We thank Herbert Over from the Institute of Physical Chemistry at the JLU Giessen for sharing his knowledge and experience and helping improve the manuscript. Financial support was provided by the DFG via the GrK (Research training group) 2204 “Substitute Materials for sustainable Energy Technologies.” M.C. thanks Ramón y Cajal program RYC‐2013‐12448. The authors also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and MAT2017‐83169‐R AEI/FEDER,UE. This article is based upon work from COST Action MP1402 “Hooking together European research in atomic layer deposition (HERALD),” supported by COST (European Cooperation in Science and Technology). S.M.S. acknowledges funding from “Programa Internacional de Becas ‘la Caixa”’‐Severo Ochoa.” ICN2 members acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017‐85087‐C3. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, grant no. SEV‐2013‐0295) and is funded by the CERCA Programme /Generalitat de Catalunya. Part of this work was performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. This received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 654360 NFFA‐Europe.

Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.

Neuderth, Paula et al., An experimental strategy for systematically assessing the influence of surface passivation layers on the photocatalytic properties of nanowire photoanodes by combining photocurrent analysis, photoluminescence spectroscopy and high resolution transmission electron microscopy with a systematic variation of sample structure and the surrounding electrolyte is demonstrated. Following this approach we can separate the impact on recombination and transport processes of photogenerated carriers. We apply this strategy to analyze the influence of ultra-thin TiO2, CeO2 and Al2O3 coatings deposited by atomic layer deposition on the photoelectrochemical performance of InxGa1−xN/GaN nanowire (NW) photoelectrodes. The passivation of surface states results in an increase of the anodic photocurrent (PC) by a factor of 2.5 for the deposition of 5 nm TiO2. In contrast, the PC is reduced for CeO2- and Al2O3-coated NWs due to enhanced defect recombination in the passivation layer or increased band discontinuities. Furthermore, photoelectrochemical oxidation of the InxGa1−xN/GaN NW photoelectrode is attenuated by the TiO2 layer and completely suppressed for a layer thickness of 7 nm or more. Due to efficient charge transfer from the InxGa1−xN NW core a stable TiO2-covered photoanode with visible light excitation is realized., Financial support is provided by the DFG via the GrK (Research training group) 2204 “Substitute Materials for sustainable Energy Technologies.” MC thanks Ram ́on y Cajal program RYC- 2013-12448. We also acknowledge nancial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496). This article is based upon work from COST Action MP1402 “Hooking together European research in atomic layer deposition (HERALD)”, supported by COST (European Cooperation in Science and Technology). SMS acknowledges funding from “Programa Internacional de Becas “la Caixa”- Severo Ochoa”. JA, MdlM. and SMS also acknowledge funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO e-TNT (MAT2014-59961-C2-2-R). ICN acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV- 2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. The atomic resolution HAADF-STEM microscopy was conducted in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de Aragon-Universidad de Zaragoza. JA and SMS thank them for offering access to their instruments and expertise. Part of the present work has been performed in the framework of Universitat Aut`onoma de Bar- celona Materials Science PhD program. RM gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG.

Metastable $ϵ$-gallium oxide is expected to have very good properties for high-power electronics, combining the large band gap and high breakdown field of $\ensuremath{\beta}$-Ga${}_{2}$O${}_{3}$ with a very high spontaneous polarization. Unfortunately, synthesizing $ϵ$-Ga${}_{2}$O${}_{3}$ is quite tricky, yet the authors have managed it. Introducing tin avoids the formation of volatile suboxides, which leadsto a widened growth window in which $ϵ$-gallium oxide can beformed. Furthermore, the growth mechanism that the authors revealcould be relevant for the growth of any binary oxide.

Penicillinase-modified AlGaN/GaN field-effect transistors (PenFETs) are utilized to systematically investigate the covalently immobilized enzyme penicillinase under different experimental conditions. We demonstrate quantitative evaluation of covalently immobilized penicillinase layers on pH-sensitive field-effect transistors (FETs) using an analytical kinetic PenFET model. This kinetic model is explicitly suited for devices with thin enzyme layers that are not diffusion-limited, as it is the case for the PenFETs discussed here. By means of the kinetic model it was possible to extract the Michaelis constant of covalently immobilized penicillinase as well as relative transport coefficients of the different species associated with the enzymatic reaction which, exempli gratia, give information about the permeability of the enzymatic layer. Based on this analysis we quantify the reproducibility and the stability of the analyzed PenFETs over the course of 33 days as well as the influence of pH and buffer concentration on the properties of the enzymatic layer. Thereby the stability measurements reveal a Michalis constant KM of (67 ± 13)μM while the chronological development of the relative transport coefficients suggests a detachment of physisorbed penicillinase during the first two weeks since production. Our results show that AlGaN/GaN PenFETs prepared by covalent immobilization of a penicillinase enzyme layer present a powerful tool for quantitative analysis of enzyme functionality.

We report on the control and modification of optical transitions in 40× GaN/AlN heterostructure superlattices embedded in GaN nanowires by an externally applied bias. The complex band profile of these multi-nanodisc heterostructures gives rise to a manifold of optical transitions, whose emission characteristic is strongly influenced by polarization-induced internal electric fields. We demonstrate that the superposition of an external axial electric field along a single contacted nanowire leads to specific modifications of each photoluminescence emission, which allows to investigate and identify their origin and to control their characteristic properties in terms of transition energy, intensity and decay time. Using this approach, direct transitions within one nanodisc, indirect transitions between adjacent nanodiscs, transitions at the top/bottom edge of the heterostructure, and the GaN near-band-edge emission can be distinguished. While the transition energy of the direct transition can be shifted by external bias over a range of 450 meV and changed in intensity by a factor of 15, the indirect transition exhibits an inverse bias dependence and is only observable and spectrally separated when external bias is applied. In addition, by tuning the band profile close to flat band conditions, the direction and magnitude of the internal electric field can be estimated, which is of high interest for the polar group III-nitrides. The direct control of emission properties over a wide range bears possible application in tunable optoelectronic devices. For more fundamental studies, single-nanowire heterostructures provide a well-defined and isolated system to investigate and control interaction processes in coupled quantum structures.

We present a study of GaN single-nanowire ultraviolet photodetectors with an embedded GaN/AlN superlattice. The heterostructure dimensions and doping profile were designed in such a way that the application of positive or negative bias leads to an enhancement of the collection of photogenerated carriers from the GaN/AlN superlattice or from the GaN base, respectively, as confirmed by electron beam-induced current measurements. The devices display enhanced response in the ultraviolet A ($\approx$ 330-360 nm) / B ($\approx$ 280-330 nm) spectral windows under positive/negative bias. The result is explained by correlation of the photocurrent measurements with scanning transmission electron microscopy observations of the same single nanowire, and semi-classical simulations of the strain and band structure in one and three dimensions., This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2017), copyright (C) American Chemical Society after peer review. To access the final edited and published work see http://doi.org/10.1021/acs.nanolett.7b01118

We demonstrate that the complex adsorption behavior of H

We demonstrate the direct analysis of polarization-induced internal electric fields in single GaN/Al0.3Ga0.7N nanodiscs embedded in GaN/AlN nanowire heterostructures. Superposition of an external electric field with different polarity results in compensation or enhancement of the quantum-confined Stark effect in the nanodiscs. By field-dependent analysis of the low temperature photoluminescence energy and intensity, we prove the [0001̅]-polarity of the nanowires and determine the internal electric field strength to 1.5 MV/cm.

We report on an all-optical sensor system that employs GaN/InGaN nanowire heterostructures (NWH) as optochemical transducers. When exposed to ultraviolet light, such NWHs emit green luminesce light. This photoluminescence (PL) is intense and temperature stable, and persists up to transducer temperatures well beyond 200 °C. When exposed to oxidizing gases such as O 3 , NO 2 , and O 2 the PL intensity decreases. At room temperature minimum detectable O 3 -, NO 2 -, and O 2 -concentrations are 50 ppb, 500 ppb and 100 ppm, respectively. Above their minimum detectable concentrations the O 3 , NO 2 and O 2 responses increase in a quasi-logarithmic manner up to a temperature-dependent saturation level. It is shown that the observed features can be explained within a model of Langmuir adsorption and surface recombination.

We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening.

We present a study of germanium as n-type dopant in wurtzite GaN films grown by plasma-assisted molecular-beam epitaxy, reaching carrier concentrations of up to 6.7 × 1020 cm−3 at 300 K, well beyond the Mott density. The Ge concentration and free carrier density were found to scale linearly with the Ge flux in the studied range. All the GaN:Ge layers present smooth surface morphology with atomic terraces, without trace of pits or cracks, and the mosaicity of the samples has no noticeable dependence on the Ge concentration. The variation of the GaN:Ge band gap with the carrier concentration is consistent with theoretical calculations of the band gap renormalization due to electron–electron and electron–ion interaction, and Burstein–Moss effect.

Inherent to the nanowire morphology is the exciting possibility of fabricating materials organized at the nanoscale in three dimensions. Composition and structure can be varied along and across the nanowire, as well as within coaxial shells. This opens up a manifold of possibilities in nanoscale materials science and engineering which is only possible with a nanowire as a starting structure. As the variation in composition and structure is accompanied by a change in the band structure, it is possible to confine carriers within the nanowire. Interestingly, this results in the formation of local two, one and zero-dimensional structures from an electronic point of view within the nanowire. This novel palette of nano-structures paves the way toward novel applications in many engineering domains such as lasers, high-mobility transistors, quantum information and energy harvesting. In the present review we summarize and give an overview on recent achievements in the design and growth of advanced quantum structures starting from nanowire templates. The quantum structures presented have been grown by molecular beam epitaxy and correspond to different confinement approaches: quantum wells (2D), quantum wires (1D) and quantum dots (0D).

A series of samples consisting of alternating stripes of ZnO grown by molecular-beam epitaxy (MBE) and radio-frequency (rf) sputtered Ga-doped ZnO stripes was laterally microstructured with a self-aligned pattern transfer method. We measured as a function of temperature the Seebeck coefficient S and the electrical resistivity ρ in-plane of the samples with the transport direction perpendicular to the stripe direction. Throughout the series the bar width and hence the number of interfaces was kept constant, but the interface profile was varied yielding different interface lengths and geometries. The dependence of S, ρ and the power factor S2/ρ on the interface length at room temperature were simulated using an empirical network model and it was demonstrated that the macroscopic transport coefficients are very sensitive to the interface region and that even this rather simple modelling yields useful information about the interface region.