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InP quantum dots (QDs) are the most competitive in terms of environmentally friendly QDs. However, the synthesis of InP QDs requires breakthroughs in low-cost and safe phosphorus precursors such as tri(dimethylamino)phosphine [(DMA)3P]. It is found that even if the oxygen is completely avoided, there are still oxidation state defects at the core/shell interface of InP QDs. Herein, the record-breaking (DMA)3P-based red InP QDs were synthesized with the assist of HF processing to eliminate the InPOxdefect and improve the fluorescence efficiency. The maximum photoluminescence quantum yield was 97.7%, which is the highest of the red InP QDs synthesized by the aminophosphine. The external quantum efficiency and brightness of the QD light-emitting diode device are also improved accordingly from 0.6% and 1276 cd·m–2to 3.5% and 2355 cd·m–2, respectively.

The impact of new control systems in Sweden's health care industry is examined. A traditional budget system is usually abandoned in favor of a system on which payments made to hospitals for services are based. Results show that the new system had both functional and dysfunctional consequences. The most important functional result is improvements in productivity. However, higher productivity requires reductions in capacity at particular production levels.

The Wiedemann–Franz law states that the charge conductance and the electronic contribution to the heat conductance are proportional. This sets stringent constraints on efficiency bounds for thermoelectric applications, which seek a large charge conduction in response to a small heat flow. We present experiments based on a quantum dot formed inside a semiconducting InAs nanowire transistor, in which the heat conduction can be tuned significantly below the Wiedemann–Franz prediction. Comparison with scattering theory shows that this is caused by quantum confinement and the resulting energy-selective transport properties of the quantum dot. Our results open up perspectives for tailoring independently the heat and electrical conduction properties in semiconductor nanostructures.

Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley–Queisser limit was demonstrated in hot-carrier devices based on InAs–InP–InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this Letter, we combine plasmonic nanoantennas with InAs–InP–InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon-energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of nonthermalized hot electrons when the photoexcited electron energy is above the barrier and by photothermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5–1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures.

In this work, arrays of predominantly relaxed InGaN platelets with indium contents of up to 18%, free from dislocations and offering a smooth top c-plane, are presented. The InGaN platelets are grown by metal–organic vapor phase epitaxy on a dome-like InGaN surface formed by chemical mechanical polishing of InGaN pyramids defined by 6 equivalent {101̅1} planes. The dome-like surface is flattened during growth, through the formation of bunched steps, which are terminated when reaching the inclined {101̅1} planes. The continued growth takes place on the flattened top c-plane with single bilayer surface steps initiated at the six corners between the c-plane and the inclined {101̅1} planes, leading to the formation of high-quality InGaN layers. The top c-plane of the as-formed InGaN platelets can be used as a high-quality template for red micro light-emitting diodes.

Radiation hard thin-film solar cell technologies are necessary in order to achieve a step forward in the specific power of solar arrays for space applications. In this article, we analyze the degradation of nanowire (NW) solar cells under high energy particles. GaAs NW solar cells have been irradiated with protons of 100 and 350 keV at different fluences. The radiation hardness of the NW solar cells in all the cases is remarkable in comparison with GaAs planar solar cells and prior literature. Design guidelines to optimize the specific power of NW solar cells for space applications by jointly increasing their efficiency and radiation hardness are presented.

High-performance photodetectors operating in the near-infrared (0.75–1.4 μm) and short-wave infrared (1.4–3.0 μm) portion of the electromagnetic spectrum are key components in many optical systems. Here, we report on a combined experimental and theoretical study of square millimeter array infrared photodetectors comprising 3 million n+–i–n+InP nanowires grown by MOVPE from periodically ordered Au seed particles. The nominal i-segment, comprising 20 InAs0.40P0.60quantum discs, was grown by use of an optimized Zn doping to compensate the nonintentional n-doping. The photodetectors exhibit bias- and power-dependent responsivities reaching record-high values of 250 A/W at 980 nm/20 nW and 990 A/W at 532 nm/60 nW, both at 3.5 V bias. Moreover, due to the embedded quantum discs, the photoresponse covers a broad spectral range from about 0.70 to 2.5 eV, in effect outperforming conventional single InGaAs detectors and dual Si/Ge detectors. The high responsivity, and related gain, results from a novel proposed photogating mechanism, induced by the complex charge carrier dynamics involving optical excitation and recombination in the quantum discs and interface traps, which reduces the electron transport barrier between the highly doped n+contact and the i-segment. The experimental results obtained are in perfect agreement with the proposed theoretical model and represent a significant step forward toward understanding gain in nanoscale photodetectors and realization of commercially viable broadband photon detectors with ultrahigh gain.

Space power systems require photovoltaics that are lightweight, efficient, reliable, and capable of operating for years or decades in space environment. Current solar panels use planar multijunction, III–V based solar cells with very high efficiency, but their specific power (power to weight ratio) is limited by the added mass of radiation shielding (e.g., coverglass) required to protect the cells from the high-energy particle radiation that occurs in space. Here, we demonstrate that III–V nanowire-array solar cells have dramatically superior radiation performance relative to planar solar cell designs and show this for multiple cell geometries and materials, including GaAs and InP. Nanowire cells exhibit damage thresholds ranging from ∼10–40 times higher than planar control solar cells when subjected to irradiation by 100–350 keV protons and 1 MeV electrons. Using Monte Carlo simulations, we show that this improvement is due in part to a reduction in the displacement density within the wires arising from their nanoscale dimensions. Radiation tolerance, combined with the efficient optical absorption and the improving performance of nanowire photovoltaics, indicates that nanowire arrays could provide a pathway to realize high-specific-power, substrate-free, III–V space solar cells with substantially reduced shielding requirements. More broadly, the exceptional reduction in radiation damage suggests that nanowire architectures may be useful in improving the radiation tolerance of other electronic and optoelectronic devices.

In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal–organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101̅1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In0.09Ga0.91N and 151 meV for In0.18Ga0.82N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In0.09Ga0.91N platelets to 1.95 eV on In0.18Ga0.82N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In0.18Ga0.82N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In0.09Ga0.91N platelets and red emission on In0.18Ga0.82N platelets.

One-dimensional nanostructure arrays can show fascinatingly different, tunable optical response compared to bulk systems. Here we study theoretically and demonstrate experimentally how to engineer the reflection and absorption of light in epitaxially grown vertical arrays of InAs nanowires (NWs). A striking observation is optically visible colors of the array, which we show can be tuned depending on the geometrical parameters of the array. Specifically, larger diameter NW arrays absorb light more effectively out to a longer wavelength compared to smaller diameter arrays. Thus, controlling the diameter provides a way to tune the optically observable color of an array. We also find that arrays with a larger amount of InAs material reflect less light (or absorb more light) than arrays with less material. On the basis of these two trends, InAs NW arrays can be designed to absorb light either much more or much less efficiently than a thin film of an effective medium containing the same amount of InAs as the NW array. The tunable absorption and low area filling factor of the NW arrays compared to thin film bode well for III-V photovoltaics and photodetection.

We demonstrate experimentally nonequilibrium transport in unipolar quasi-1D hot electron devices reaching the ballistic limit at room temperature. The devices are realized with heterostructure engineering in nanowires to obtain dopant- and dislocation-free 1D-epitaxy and flexible bandgap engineering. We show experimentally the control of hot electron injection with a graded conduction band profile and the subsequent filtering of hot and relaxed electrons with rectangular energy barriers. The number of electrons passing the barrier depends exponentially on the transport length with a mean-free path of 200–260 nm, and the electrons reach the ballistic transport regime for the shortest devices with 70% of the electrons flying freely through the base electrode and the barrier reflections limiting the transport to the collector.

GaN nanowires (NWs) are promising building blocks for future optoelectronic devices and nanoelectronics. They exhibit stronger piezoelectric properties than bulk GaN. This phenomena may be crucial for applications of NWs and makes their study highly important. We report on an investigation of the structure evolution of a single GaN NW under an applied voltage bias along polar [0001] crystallographic direction until its mechanical break. The structural changes were investigated using coherent X-ray Bragg diffraction. The three-dimensional (3D) intensity distributions of the NWs without metal contacts, with contacts, and under applied voltage bias in opposite polar directions were analyzed. Coherent X-ray Bragg diffraction revealed the presence of significant bending of the NWs already after metal contacts deposition, which was increased at applied voltage bias. Employing analytical simulations based on elasticity theory and a finite element method (FEM) approach, we developed a 3D model of the NW bending under applied voltage. From this model and our experimental data, we determined the piezoelectric constant of the GaN NW to be about 7.7 pm/V in [0001] crystallographic direction. The ultimate tensile strength of the GaN NW was obtained to be about 1.22 GPa. Our work demonstrates the power of in operando X-ray structural studies of single NWs for their effective design and implementation with desired functional properties.

The development of photovoltaics as a serious means of producing renewable energy has accelerated greatly in the last ten years, with prices for silicon-based solar cell systems dropping dramatically in the last few years. The next great opportunity for photovoltaics following this competitiveness in prices will be to enhance the cell and panel efficiencies. It is quite generally seen that the most viable platform on which this should be realized will be as augmented silicon solar cells, in which a top cell will be combined with the silicon bottom cell in a tandem configuration, by which the efficiency can be enhanced by a factor from 20% to 50%, depending on details of the approach. In this paper, we report on the status of one such approach, namely, with a top cell comprising III–V nanowires, connected to the bottom silicon cell in a two-terminal or four-terminal configuration. Among the most important opportunities, we show that a substrate-free growth, called Aerotaxy, offers a radical reduction in the total price picture. Besides the description of the key technical approaches, we also discuss the environmental issues.

The future of solid-state lighting can be potentially driven by applications of InGaN/GaN core–shell nanowires. These heterostructures provide the possibility for fine-tuning of functional properties by controlling a strain state between mismatched layers. We present a nondestructive study of a single 400 nm-thick InGaN/GaN core–shell nanowire using two-dimensional (2D) X-ray Bragg ptychography (XBP) with a nanofocused X-ray beam. The XBP reconstruction enabled the determination of a detailed three-dimensional (3D) distribution of the strain in the particular nanowire using a model based on finite element method. We observed the strain induced by the lattice mismatch between the GaN core and InGaN shell to be in the range from −0.1% to 0.15% for an In concentration of 30%. The maximum value of the strain component normal to the facets was concentrated at the transition region between the main part of the nanowire and the GaN tip. In addition, a variation in misfit strain relaxation between the axial growth and in-plane directions was revealed.

A survey has been conducted of past research on the various budgetary controls of Denmark, Sweden, Finland, and Norway. The goal was to determine the types of issues, problems and concerns that have provided the basis for research. Research perspectives which originated within the Nordic area are highlighted and examined. The major influences on research in the area were also examined.

Nanowire array solar cells show intrinsic light trapping and absorption enhancement properties due to the diffraction and optical resonances. Here, we report the effect of varying incidence angle on the performance of GaAs nanowire array solar cells. We provide experimental evidence that nanowire array solar cells are highly efficient at gathering diffuse or tilted incident light, even at very high incidence angles; hence, the performance of the nanowire solar cell is retained up to these high incident light angles. Specifically, the measured efficiency at an incidence angle of 60° is 95% of the efficiency at normal incidence. Moreover, our measurements show that a nonzero incidence angle is beneficial for wavelengths above 600 nm, which results in an efficiency improvement by 0.5% absolute points. This increase is so large that we even measure a small increase in absolute output power at 15° tilt, thus, more than fully compensating for the reduced incoming power over the cell with increasing tilt. We show how this gain arises from an enhanced absorption in the part of the nanowire with a high probability of charge extraction. Thus, nanowires show great promise for the delivery of high efficiency in practical nontracking positioning conditions, as well as under diffuse light illumination.

A GaAs nanowire array solar cell with an independently verified solar energy conversion efficiency of 15.3% and open-circuit voltage of 0.906 V under AM1.5g solar illumination at 1-sun intensity has been fabricated. This is the highest published efficiency for nanowire array solar cells and is twice the prior record for GaAs nanowire array solar cells. The solar cell has been fabricated by substrate-based epitaxy but is structurally compatible with substrate-less aerotaxy fabrication, providing a path to high-volume manufacturing. The short-circuit current of 21.3 mA/cm2 was generated with axial p-n junction GaAs cores covering 13% of the surface area, which is a volume of GaAs equivalent to a 370-nm-thick planar layer.

X-ray nanobeams are unique nondestructive probes that allow direct measurements of the nanoscale strain distribution and composition inside the micrometer thick layered structures that are found in most electronic device architectures. However, the method is usually extremely time-consuming, and as a result, data sets are often constrained to a few or even single objects. Here we demonstrate that by special design of a nanofocused X-ray beam diffraction experiment we can (in a single 2D scan with no sample rotation) measure the individual strain and composition profiles of many structures in an array of upright standing nanowires. We make use of the observation that in the generic nanowire device configuration, which is found in high-speed transistors, solar cells, and light-emitting diodes, each wire exhibits very small degrees of random tilts and twists toward the substrate. Although the tilt and twist are very small, they give a new contrast mechanism between different wires. In the present case, we image complex nanowires for nanoLED fabrication and compare to theoretical simulations, demonstrating that this fast method is suitable for real nanostructured devices.

We have investigated the electronic transport through 3 μm long, 45 nm diameter InAs nanowires comprising a 5 nm long InP segment as electronic barrier. After assembly of 12 nm long oligo(phenylene vinylene) derivative molecules onto these InAs/InP nanowires, we observed a pronounced, nonlinear I–Vcharacteristic with significantly increased currents of up to 1 μA at 1 V bias, for a back-gate voltage of 3 V. As supported by our model calculations based on a nonequilibrium Green Function approach, we attribute this effect to charge transport through those surface-bound molecules, which electrically bridge both InAs regions across the embedded InP barrier.

Using both synchrotron-based photoemission electron microscopy/spectroscopy and scanning tunneling microscopy/spectroscopy, we obtain a complete picture of the surface composition, morphology, and electronic structure of InP nanowires. Characterization is done at all relevant length scales from micrometer to nanometer. We investigate nanowire surfaces with native oxide and molecular adsorbates resulting from exposure to ambient air. Atomic hydrogen exposure at elevated temperatures which leads to the removal of surface oxides while leaving the crystalline part of the wire intact was also studied. We show how surface chemical composition will seriously influence nanowire electronic properties. However, opposite to, for example, Ge nanowires, water or sulfur molecules adsorbed on the exterior oxidized surfaces are of less relevance. Instead, it is the final few atomic layers of the oxide which plays the most significant role by strongly negatively doping the surface. The InP nanowires in air are rather insensitive to their chemical surroundings in contrast to what is often assumed for nanowires. Our measurements allow us to draw a complete energy diagram depicting both band gap and differences in electron affinity across an axial nanowire p-n junction. Our findings thus give a robust set of quantitative values relating surface chemical composition to specific electronic properties highly relevant for simulating the performance of nanoscale devices.

We present tunneling field-effect transistors fabricated from InP-GaAs heterostructure nanowires with an n-i-pdoping profile, where the intrinsic InP region is modulated by a top gate. The devices show an inverse subthreshold slope down to 50 mV/dec averaged over two decades with an on/off current ratio of approximately 107for a gate voltage swing (VGS) of 1 V and an on-current of 2.2 μA/μm. Low-temperature measurements suggest a mechanism of trap-assisted tunneling, possibly explained by a narrow band gap segment of InGaAsP.

Semiconductor nanowires are expected to be important components in future nano-electronics and photonics. Already a wide range of applications has been realized, such as high-performance field-effect transistors, bio/chemical sensors, diode logics and single-nanowire lasers. As nanowires have small cross-sections and large surface-to-bulk ratios, their properties can be significantly influenced by individual atomic-scale structural features, and they can have properties or even atomic arrangements with no bulk counterparts. Hence, experimental methods capable of directly addressing the atomic-scale structure of nanowires are highly desirable. One such method is scanning tunnelling microscopy (STM), which, by direct imaging of the atomic and electronic structure of surfaces has revolutionized the perception of nanoscale objects and low-dimensional systems. Here we demonstrate how combining STM with an embedding scheme allows us to image the interior of semiconductor nanowires with atomic resolution. Defect structures such as planar twin segments and single-atom impurities are imaged inside a GaAs nanowire. Further, we image an intriguing GaAs nanowire that is separated into two distinct nanocrystallites along the growth direction of the wire.

The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour–liquid–solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.

Along with rapidly developing nanotechnology, new types of analytical instruments and techniques are needed. Here we report an alternative procedure for electrical measurements on semiconductor nanowhiskers, allowing precise selection and visual control at close to atomic resolution. We use a combination of two powerful microscope techniques, scanning tunneling microscopy (STM) and simultaneous viewing in a transmission electron microscope (TEM). The STM is mounted in the sample holder for the TEM. We describe here a method for creating an ohmic contact between the STM tip and the nanowhisker. We examine three different types of STM tips and present a technique for cleaning the STM tip in situ. Measurements on 1-μm-tall and 40-nm-thick epitaxially grown InAs nanowhiskers show an ohmic contact and a resistance of down to 7 kΩ.

We have investigated the possibility of using carbon nanotubes as flexible, mobile electrical probes able to electrically contact nanometre-scaled objects. Configurations with metal electrodes evaporated on top of multi-, few- and single-wall carbon nanotubes have been studied. Scanning probe manipulation was used to create nano-mechanical switches by first cutting the various tubes, and then moving the parts back into electrical contact. We found multiwall tubes to be best suited as mobile probes, mainly due to their insensitivity to mechanical deformation. Finally, we present an example where the scanning probe manipulation technique has been used to electrically connect two carbon nanotubes to a 7 nm gold particle.

A novel technique has been developed to lithographically modify GaInAs/InP quantum well (QW) structures. 10 MeV ⁶⁹Ga³⁺ ions are implanted into a 40 nm InP/ 9 nm GaInAs/InP (100) substrate along the [100] direction. A 50 nm thick Au mask switches a fraction of the aligned ions into dechannelling trajectories. The dechannelled ions enhance the ion beam induced intermixing of the GaInAs QW with the surrounding InP cladding layers. Low-temperature photoluminescence (PL) was used to study bandgap modification of the GaInAs QW after implantation for a fluence range of 4×10¹¹-4×10^13 69Ga³⁺ ions cm^-2. A larger bandgap energy shift was observed with PL under the Au mask (28 meV) compared to areas not covered with the Au mask (12 meV). Furthermore, the integrated PL intensity was found to be lower in the masked areas, which is consistent with a greater defect concentration in the regions under the Au mask.

Gold nanoparticles are of great interest for various nanoelectronic applications, e.g., for making single electron transistors or very fine leads to molecular size entities. For this and other applications, it is important that all particles have controllable size and shape. In this paper, we describe the production of size-selected gold aerosol particles in the 20 nm range made by evaporation in a high-temperature tube furnace and subsequent size selection. To obtain spherical particles, it was necessary to reshape the particles at high temperature, which was investigated for temperatures between 25°C and 1200°C. High-resolution transmission electron microscopy showed that the degree of crystallinity became higher for higher reshaping temperature. During reshaping at high temperature, an anomalous charging behavior was discovered, whereby negatively as well as positively charged particles became multiply negatively charged. Possible mechanisms for explaining this thermally activated phenomenon are discussed.

Using spontaneous self-organization effects is an efficient way to produce nano-structures, as for instance quantum wires and quantum dots. This article is focused on the strain-induced self-organization, or “self-assembling” effect, producing quantum dots. Particularly the following aspects will be addressed: (i) the phenomenology of the 2D–3d morphology transition, (ii) the effects of materials choices and growth conditions on density, size and homogeneity of dots, and (iii) manipulations to get laterally aligned and vertically stacked dot structures.

A parametric study of creep crack growth in CT specimens taken out from weldments is performed by use of finite element simulations. The investigated parameters are different combinations of weldment constituent material creep properties and the starter notch position in the heat affected zone. Plane strain conditions are assumed and the C*integral together with a constraint parameter are used to characterise the creep crack growth rate.The results show that numerical simulations are needed, in addition to experimental results, in order to understand the behaviour of a growing creep crack in a cross-weld CT specimen.

We have produced size-selected gold particles with sizes below 30 nm by means of aerosol technology. The method allows for particles of many different materials to be produced with high purity and narrow size distribution, and it allows easy tranport and deposition of the particles onto any substrate. The particle quality has been investigated by transmission electron microscopy, revealing a high degree of crystallinity, especially after sintering at high temperatures.

The material creep properties of weldments, i.e.the minimum creep strain rate, the creep rupture strength and the creep ductility, do in general differ from those of the parent material. It is often assumed that if the weld material creep strength is higher than that of the parent, the life of the weldment will also be higher. This is not always true. In fact, when subjected to a prescribed displacement, e.g.a circumferential weldment in a pipe subjected to internal pressure, a creep hard weld develops a stress concentration which may result in a reduced life expectancy of the weldment system. Similarly, the properties of the heat affected zone often differ from those of the parent and weld material properties which may be one reason for early Type IV cracking. In addition, the weld preparation geometry, the width of the HAZ and other geometric parameters influence the stress distribution. The present paper summarizes some of the effects of material property variations in weldments and their impact on the creep life expectancy. It is shown that the stress concentrations resulting from material property mis-matching may be high enough to produce premature creep damage in the weldment, or will cause creep cracking starting from initial defects.

A large circumferentially oriented crack was discovered in a weld of a double T-piece in the main steam pipe of a Swedish power plant. The pipe and the weld were manufactured from 0.5Cr0.5Mo0.25V steel. In this paper a nominal creep life assessment and an assessment considering mis-matching are carried out for a defect free component and for a component with a crack. While the nominal prediction of a defect free component indicates a creep life of approximately 2200000 hours, a creep life of 550000 hours is obtained when mis-matching effects and enhanced axial stresses due to system loads are considered. If mis-matching is not taken into account, as opposed to if it is, the estimated incubation plus creep crack growth time is approximately 30–70% longer.

A method of analysis is presented for circular cylindrical shells under non-uniform external loads. The equations are valid for moderately large displacements and take secondary creep into account. Thermal effects and initial imperfections are included.

The results are presented of an experimental investigation of creep buckling of circular cylindrical shells. The test specimens, manufactured from an aluminum alloy similar to 24S, had radius to thickness ratios between 30 and 150 and length to radius ratios greater than 2. They were subjected to axial compression or bending at a temperature of 225 deg C (430 deg F) and at various stress levels. The critical time under a constant load was determined as a function of the stress level, the shell geometry, and the type of loading. It was found that the shells subjected to pure compression had a substantially shorter lifetime than those subjected to pure bending with the same maximum applied stress. The thickest test specimens failed through collapse into a “wrinkling” mode which for the pure compression case is axisymmetric, whereas the thinner cylinders buckled into a typical diamond pattern. In all cases, buckling occurred at one of the edges. The postbuckling configuration was found to depend not only on the geometry of the shell but also on the load level. For very low stress levels, even the thinner cylinders buckled in the short wave pattern (symmetric for compression). A comparison between the present experimental results and theoretical values of the critical time presented in earlier works showed that a fairly good estimate may be obtained for the case of axial compression, whereas the approximate theory for creep buckling under pure bending gives an unduly conservative result.

The absorption of light is at the core of photovoltaic applications. For many nanostructure-based devices, an assessment of the absorption in the nanostructures is complicated by a thick, opaque substrate that prohibits transmission measurements. Here, we show how a single reflection measurement can be used for approximating the amount of light absorbed in vertical semiconductor nanowire arrays.

Photoluminescence (PL) properties are reported for a set of m‐plane GaN films with Mg doping varied from mid 1018cm−3to above 1020cm−3. The samples were grown with MOCVD at reduced pressure on low defect density bulk GaN templates. The sharp line near bandgap bound exciton (BE) spectra observed below 50 K, as well as the broader donor–acceptor pair (DAP) PL bands at 2.9–3.3 eV give evidence of several Mg related acceptors, similar to the case of c‐plane GaN. The dependence of the BE spectra on excitation intensity as well as the transient decay behaviour demonstrate acoustic phonon assisted transfer between the acceptor BE states. The lower energy donor–acceptor pair spectra suggest the presence of deep acceptors, in addition to the two main shallower ones at about 0.23 eV. Similar spectra from Mg‐doped GaN nanowires (NWs) grown by MOCVD are also briefly discussed.

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Nanocrystals of indium phosphide, with diameters of around 10 nm, have been produced via an aerosol route. The method is based on the formation of monodisperse indium droplets and the subsequent reaction with phosphine at elevated temperature. The size of the final InP nanocrystal is self-limited by the size of the introduced size-selected In droplet. This size can be tuned carefully, thus this method allows a narrow size distribution of the nanocrystals. The kinetics of the reaction of In to produce InP depends on the temperature and the phosphine flow. Extensive high resolution transmission electron microscopy studies lead to a consistent picture of the process. Our approach opens the possibility for efficient production of size-selected semiconductor nanocrystals and it will allow new types of self-assembly and control of quantum dots.

A tungsten (W) grating was fabricated and embedded in GaAs by MOVPE (metal organic vapor phase epitaxy) with the aim of realizing novel applications of ultrafine metal electrodes within semiconductor nanodevices. A combination of W grating and a semiconductor single heterobarrier was used to control the vertical current through the structure based on an effective barrier height modulation controlled by the Schottky depletion around the metal. Transistor operation was observed at room temperature and the mode of operation discussed. Moreover, a gated resonant tunneling transistor was demonstrated at 20 K by integrating GaInP/GaAs double barriers and W grating, including a 1.4 ×1.4 µm2opening window, to form a vertical channel. The peak-to-valley current ratios were modulated by the gate bias, and fine features due to the lateral potential constriction were observed directly in the current-voltage characteristics.

Matrices of uncontacted, nano-scale, tungsten discs, embedded in GaAs by epitaxial overgrowth, have been characterised electrically. By varying the spacing between the discs, it is demonstrated that when the disc separation is smaller than 300 nm (for a doping level of 1016cm-3), the conductance drops by seven orders of magnitude and the material becomes semi-insulating. The presence of the metal-induced barrier is deduced from the temperature dependence of the current transport. These results are discussed in terms of overlapping Schottky depletion regions around the discs.

We have measured the photoluminescence of individual quantum dots grown by the Stranski-Krastanow growth mode. Studies of many individual quantum dots reveal that about one quantum dot in a thousand switches between two emission intensity levels on a time-scale of seconds. The switching is temperature and excitation power activated. For each set of experimental conditions, the statistics is consistent with random telegraph noise. By strong illumination it is possible to turn the switching off, after which the quantum dot emits continuously.

We have used the Schottky depletion around overgrown tungsten (W) features to define vertical conducting channels having submicron lateral extension. In the first example, epitaxially buried lattices of tungsten discs are used, which give rise to a semi-insulating behavior in GaAs. A designed opening of n×nvacant disc positions in this lattice defines a transport channel in the otherwise semi-insulating material. A double barrier resonant tunneling (DBRT) structure (GaInP/GaAs) is placed immediately below the disc lattice, and the current-voltage characteristics are investigated. In diodes with submicron openings (0.7 µm×0.7 µm), a fine structure including additional current peaks is found, which is not seen in larger diodes. The magnetic field dependence of the current peaks is investigated and discussed. As a second example, we report on the fabrication of gateable zero-dimensional (0D) resonant tunneling structures that are obtained using selective area growth in openings (d=300 nm) in an otherwise continuous W mask.

Epitaxial overgrowth of GaAs over nanometer-sized tungsten patterns is studied experimentally. The relationships between the quality of the overgrowth and geometrical parameters, such as crystallographic orientation, metal width, metal thickness, and grating period, are investigated. The overgrown structures are characterized using a scanning electron microscope at top surfaces and at cleaved edges, and by I-Vmeasurements of Schottky diodes formed between the metal grid and the overgrown GaAs. It is established that it is possible to obtain a high-quality overgrowth layer with a completely planarized growth front within 200 nm of growth.

Optical techniques play a significant role in studies of nano-structures. The electronic structures of quantum dots, for example, vary with the geometric sizes in an ensemble, resulting in broadened spectral lines. Recently, different forms of local spectroscopic techniques have been applied to investigate such inhomogeneously broadened emission lines. In this paper we report on three methods for local spectroscopy: cathodo-luminescence, luminescence induced by a scanning tunnel microscope and microphotoluminescence. Each of these techniques is shown to have the capacity to investigate single quantum dots, with linewidths in the range 40-1000 µeV. Besides demonstrating the possibility of imaging and spectroscopically studing individual dots, we also demonstrate the possibility of investigating single impurity atoms, in imaging as well as in emission spectroscopy modes.