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This paper reviews the state-of-the-art technologies, characterizations, materials (precursors and encapsulants), and challenges concerning multicolor and white light-emitting diodes (LEDs) based on carbon dots (CDs) as color converters. Herein, CDs are exploited to achieve emission in LEDs at wavelengths longer than the pump wavelength. White LEDs are typically obtained by pumping broad band visible-emitting CDs by an UV LED, or yellow–green-emitting CDs by a blue LED. The most important methods used to produce CDs, top-down and bottom-up, are described in detail, together with the process that allows one to embed the synthetized CDs on the surface of the pumping LEDs. Experimental results show that CDs are very promising ecofriendly candidates with the potential to replace phosphors in traditional color conversion LEDs. The future for these devices is bright, but several goals must still be achieved to reach full maturity.

The application of titanium dioxide (TiO2) in the photovoltaic field is gaining traction as this material can be deployed in doping-free heterojunction solar cells with the role of electron selective contact. For modeling-based optimization of such contact, knowledge of the titanium oxide defect density of states is crucial. In this paper, we report a method to extract the defect density through nondestructive optical measures, including the contribution given by small polaron optical transitions. The presence of both related to oxygen-vacancy defects and polarons is supported by the results of optical characterizations and the evaluation of previous observations resulting in a defect band fixed at 1 eV below the conduction band edge of the oxide. Solar cells employing pulsed laser deposited-TiO2 electron selective contacts were fabricated and characterized. The J-V curve of these cells showed, however, a S-shape, then a detailed analysis of the reasons of such a behavior was carried out. We use a model involving the series of a standard cell equivalent circuit with a Schottky junction in order to explain these atypical performances. A good matching between the experimental measurements and the adopted theoretical model was obtained. The extracted parameters are listed and analyzed to shade light on the reasons behind the low-performance cells.

We report the photocatalytic properties of ZnO and Cu2O thin films deposited on glass substrates at room temperature by DC sputtering and pulsed laser deposition. The photoactivity of the films was investigated through the degradation of rhodamine B (RhB) and methyl orange (MO) under solar light. In order to select the most suitable film of ZnO for the of RhB and MO degradation, the relationship between the characteristics (e.g. energy levels and defects concentration) of ZnO films and their effectiveness in the photocatalytic yield of RhB and MO been studied, where several films were deposited by using different oxygen partial pressures (PO2: 0.05–1.3mbar), while Cu2O films were grown under a pressure of 0.01mbar. The XRD patterns show that all ZnO films have (002) preferential orientation, and crystallite size increases from 73 to 122nm raising PO2. The gap Eg of ZnO (3.26 and 4.15eV) depends on PO2, and the films present photoluminescence emission in the UV–Vis-near IR region. On the basis of structural, optical and electrical characterizations of both films, a comparative study was carried out on the dyes degradation. Cu2O films exhibit a high photoactivity with MO (81.69%) under solar light (6h), whilst for RhB the best elimination rate (60.85%) was achieved with ZnO films deposited at 0.1mbar, which were also the ones exhibiting the highest PL peak intensity at the characteristic absorption wavelength of RhB (553nm).

Terahertz resonant nanoantennas have recently become a key tool to investigate otherwise inaccessible interactions of such long-wavelength radiation with nano-matter. Because of their high-aspect-ratio rod-shaped geometry, resonant nanoantennas suffer from severe loss, which ultimately limits their field localization performance. Here we show, via a quasi-analytical model, numerical simulations, and experimental evidence, that a proper tapering of such nanostructures relaxes their overall loss, leading to an augmented local field enhancement and a significantly reduced resonator mode volume. Our findings, which can also be extended to more complex geometries and higher frequencies, have profound implications for enhanced sensing and spectroscopy of nano-objects, as well as for designing more effective platforms for nanoscale long-wavelength cavity quantum electrodynamics.

Cu2O/ZnO:Al (AZO) and Cu2O/ZnO/AZO heterojunctions have been deposited on glass substrates by a unique three-step pulsed laser deposition process. The structural, optical, and electrical properties of the oxide films were investigated before their implementation in the final device. X-ray diffraction analysis indicated that the materials were highly crystallized along the c-axis. All films were highly transparent in the visible region with enhanced electrical properties. Atomic force and scanning electron microscopies showed that the insertion of a ZnO layer between the Cu2O and AZO films in the heterojunction enhanced the average grain size and surface roughness. The heterojunctions exhibited remarkable diode behavior and good rectifying character with low leakage current under reverse bias. The presence of the ZnO interlayer film significantly reduced the parasitic and leakage currents across the barrier, improved the quality of the heterostructure, made the energy band between AZO and Cu2O layers smoother, and eliminated the possibility of interface recombination, leading to much longer electron lifetime.

The use of transition metal oxides for the selective carrier contact in the crystalline silicon solar cells technology is rising to interest for the excellent optoelectrical properties of these materials whose implementation, however, can result in lousy performing cells due to an S-shaped electrical characteristic. In this paper, we fabricated solar cells showing S-shaped J-V curve and carried out an analysis of the reasons of such behavior using a model involving the series of a standard cell equivalent circuit with a Schottky junction in order to explain these atypical performances. A good matching between the experimental measurements and the adopted theoretical model was obtained. The extracted parameters are listed and analyzed to shade light on the reasons behind the low-performance cells.

The generation of structured light by means of metasurfaces is presented and the applications in the characterizations of polarization rotation and Pancharatnam-Berry phase are discussed.

One of the most recent applications of TiO2 thin films is as an oxide layer in memristors, electronic devices considered as one of the most promising nonvolatile memories and as possible building units for neuromorphic computing. This chapter aims to describe several fabrication ways, either (electro)chemical or physical methods, of TiO2 thin films and to highlight the relationship between method and layer properties. Some fundamentals on the mechanism of memristors’ operation, that is, resistive switching in oxide thin films, will be given, classifying the different types of devices based on the used electrode materials and underlying physicochemical processes. Finally, it will be discussed how the oxide characteristics influence the resistive switching phenomenon, that is, the performances of memristors.

Redox-based resistive switching memories (ReRAM) based on metal oxides are considered as the next generation non-volatile memories and building units for neuromorphic computing. Using different deposition techniques results however in different structural and electric properties, modulating the device performance. In this study HfO2 and Nb2O5 were prepared electrochemically by anodizing sputtering-deposited Hf and Nb in borate buffer solution. Photoelectrochemical measurements were used to study the solid state properties of the anodic oxides, such as band gap and flat band potential. In the case of anodic HfO2, detected photocurrent is ascribed to optical transitions between localized (generated by the presence of oxygen vacancies into the oxide) and extended states. Impedance measurements disclosed the formation of n-type Nb2O5 and insulating HfO2. Pt top electrode was deposited onto the metal/anodic oxide junctions to fabricate ReRAM cells. Whereas switching behaviour of Nb/anodic Nb2O5/Pt cells was not reliable, good endurance and retention performances were proved in the case of Hf/anodic HfO2/Pt cells, showing that electrochemical growth of the oxides can be a reliable way to fabricate solid electrolytes for resistive switching memories.

A cheap, compact and customisable characterisation system for memristor devices, working between ± 10 V, is presented. SPICE (Simulation Program with Integrated Circuit Emphasis) simulations are performed to verify the circuit feasibility and a proper software is developed to drive the system. The potentiality of the realised system is tested by performing several electrical measurements on both Cu/HfO2/Pt memristors and two-terminals commercial devices.

The structural and photoelectrical properties of Al-doped ZnO (AZO)/SiC/p-Si and AZO/SiC/n-Si heterojunctions, fabricated at low temperature by pulsed laser deposition, were investigated by means of a number of techniques. Raman analysis indicates that SiC layers have the cubic 3C-SiC phase, whilst X-ray diffraction measurements show that AZO films exhibit a hexagonal wurtzite structure, highly textured along the c-axis, with average crystallites size of 35.1 nm and lattice parameter c of 0.518 nm. The homogeneous and dense surface morphology observed by scanning electron microscopy was confirmed by atomic force microscopy images. Moreover, UV–Vis-NIR spectra indicated a high transmittance of SiC films in the region 550–2500 nm, about 80% transmittance of AZO films in the 450–1000 nm region, and optical band gaps in good agreement with literature. These results prove that pulsed laser deposition is a low-cost technique suitable to grow SiC and AZO films with excellent material properties. The effect of the Si doping on the current transport mechanisms in the heterojunctions was investigated by current-voltage measurements under dark and white light illumination. Both heterojunctions exhibit a diode behaviour and relatively low leakage current, with a noticeable superiority for the AZO/SiC/n-Si device also under illumination, with an illumination/dark ratio of about 400. Our results indicate that the AZO/SiC/p-Si heterojunctions, with higher values of ideality factor, series resistance and lower rectifying ratio, have a complex current transport compared to the diodes grown on n-type Si. Additionally, capacitance-voltage measurements and Mott-Schottky plot allowed to determine a built-in potential of 0.51 V for the Al/AZO/SiC/p-Si/Al device.

The general aim of this paper is that to further deepen and elaborate the results obtained in a previous study of the authors, in which the relationship between the characteristics of five different types of LED lamps and the humans non-image-forming reactions were investigated, by conducting a more detailed statistical analysis and by highlighting the neurological aspects.

A detailed structural investigation of the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films deposited on sapphire substrates by pulsed laser deposition was performed by in situ temperature-dependent X-ray diffraction (XRD) measurements. The structural results are correlated with those of infrared radiometry measurements in the SWIR (2.5-5 μm) and LWIR (8-10.6 μm) spectral ranges. The main results indicate a good agreement between XRD and optical analysis, therefore demonstrating that the structural transition from monoclinic to tetragonal phases is the dominating mechanism for controlling the global properties of the SMT transition. The picture that emerges is a SMT transition in which the two phases (monoclinic and tetragonal) coexist during the transition. Finally, the thermal hysteresis, measured for thin films with different thickness, showed a clear dependence of the transition temperature and the width of the hysteresis loop on the film thickness and on the size of the crystallites.

This chapter reviews the state of the art of materials, technologies, characterizations, process and challenges concerning hybrid white light‐emitting diodes (LEDs). Here, for a “hybrid LED” we mean a device based on a layer of organic phosphors (or a mix of inorganic and organic ones) pumped by a high‐energy inorganic LED. Light is emitted by a frequency down‐conversion (sometimes simply named color‐conversion) process. Benefits and weak spots of this technology are investigated with a special attention for the materials involved into the process of frequency down‐conversion, in order to envisage the future impact of the hybrid lighting technology among the well‐established inorganic ones.

A detailed structural investigation of the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films deposited on sapphire substrates by pulsed laser deposition was performed by in situ temperature-dependent X-ray diffraction (XRD) measurements. The structural results are correlated with those of infrared radiometry measurements in the SWIR (2.5-5 μm) and LWIR (8-10.6 μm) spectral ranges. The main results indicate a good agreement between XRD and optical analysis, therefore demonstrating that the structural transition from monoclinic to tetragonal phases is the dominating mechanism for controlling the global properties of the SMT transition. The picture that emerges is a SMT transition in which the two phases (monoclinic and tetragonal) coexist during the transition. Finally, the thermal hysteresis, measured for thin films with different thickness, showed a clear dependence of the transition temperature and the width of the hysteresis loop on the film thickness and on the size of the crystallites.

The Doppler frequency shift of sound or electromagnetic waves has been widely investigated in many different contexts and, nowadays, represents a formidable tool in medicine, engineering, astrophysics, and optics. Such effect is commonly described in the framework of the universal energy-momentum conservation law. In particular, the rotational Doppler effect has been recently demonstrated using light carrying orbital angular momentum. When a wave undergoes a cyclic adiabatic transformation of its Hamiltonian, it is known to acquire the so-called Pancharatnam–Berry (PB) phase. In this work, an experimental evidence of the direct connection between the high-order PB phase time evolution on the Poincaré sphere and the rotational Doppler frequency shift of light is provided. A metasurface operating at telecom wavelengths is employed to impose a total (spin and orbital) angular momentum (TAM) on the light wave, while two TAM converters ensure a closed cycle on the Poincaré sphere. By rotating one of the converters, a significant Doppler frequency shift is observed without variation of the output TAM. The proposed metasurface-based approach offers new advanced ways to engineer the frequency content of light.

In this work, a way to grow isolated and coalesced ZnO nanorods on p-GaN/sapphire structure is presented. Chemical bath deposition [1],[2] was used to grow ZnO nanorods of device-quality on a p-GaN/n-GaN/sapphire template, simply controlling the duration time of the growth process and the concentration of the nutrient solution in the bath. Several p-GaN templates were soaked in a nutrient solution, prepared with different concentration of zinc nitrate hexahydrate (Sigma-Aldrich, reagent grade 98%) and hexamethylenetetramine (Alfa Aesar, ACS 99%) in deionized water, while being heated at a temperature of 80 °C for a period varying from 8 to 25 hours; then, the samples were left in the solution to cool down naturally to room temperature. Increasing the duration of the process leads to compact and sound layers (instead of separated nanorods), as well as the concentration of the solution. Fig. 1 shows the top-view fieldemission scanning electron microscopy (FE-SEM) images of the samples, obtained for different growth time and 70 mM concentration. The first sample (grown for 8 h) exhibits a typical nanorod layout (Fig. 1.a). The nanorods are oriented along the (0001) direction of the substrate. After 15 h, most of the rods coalesce into a unique layer but the uniformity of the latter is broken by several “empty” areas where ZnO does not seem to be grown (Fig. 1.b). After 25 h, the ZnO appears as a sound compact layer with some defects on the surface (Fig. 1.c). In general, an increase of the concentration results in a better coalescence of the rods but too high concentrations stop the growth process. ZnO layer grown for 25 h in 100 mMconcentrated solution (Fig. 2.a) appears to be more compact than in the case of 70 mM concentration. The improved coalescence should be due to the increase of the nanorods diameter with the concentration, as reported in [3]. Concerning the 500 mM sample (Fig. 2.b), surprisingly only several nanoflowers and nanopillars appear to be deposited on the surface; in particular, the latter are not aligned along the (0001) sapphire plane but just placed on the surface with random orientations. Probably the kinetics of the chemical reactions is hindered by the oversaturation of the solution.

We report on carrier injection features in light-emitting diodes (LEDs) based on nonintentionally doped-ZnO/p-GaN heterostructures. These LEDs consist of a ZnO layer grown by chemical-bath deposition (CBD) onto a p-GaN template without using any seed layer. The ZnO layer (~1- $\mu \text{m}$ thickness) consists of a dense collection of partially coalesced ZnO nanorods, organized in wurtzite phase with marked vertical orientation, whose density depends on the concentration of the solution during the CBD process. Due to the limited conductivity of the p-GaN layer, the recombination in the n-region is strongly dependent on the spreading length of the holes, ${L}_{h}$ , coming from the p-contact. Moreover, the evaluation of ${L}_{h}$ is not easy and generally requires the design and the fabrication of several LED test patterns. We propose a simple and effective method to calculate ${L}_{h}$ , just based on simple considerations on ${I}$ – ${V}$ characteristics, and a way to improve the injection efficiency in the n region based on a noncircular electrode geometry. In particular, an interdigitated electrode structure is proved to be more efficient in terms of hole injection from n- to p-region.

Plasmonic nanoantennas (NAs) have received a growing attention in recent years due to their ability to confine light on sub-wavelength dimensions [1]. More recently, this property has been exploited in the terahertz (THz) frequency range (0.1–10 THz) for enhanced sensing and spectroscopy [2], as well as for more fundamental investigations [3]. These applications typically require high local electric fields that can be achieved by concentrating THz radiation into deeply sub-wavelength volumes located at the NAs extremities. However, the achievable near-field enhancement values are severely limited by the poor resonance quality factor of traditional rod-shaped THz NAs. Unlike what is commonly assumed in the infrared domain [4], here we show that an optimal NA tapering angle can be effectively introduced to obtain higher quality factors and, at least, twofold higher local near-field enhancement in comparison with standard (wire-like) dipolar THz NAs. To evaluate how the tapering angle affects the NA performance, a simplified quasi-analytical model was first developed. Each NA is considered as a truncated cone constituted by a sequence of gold cylinders of increasing radii, so that the effective refractive index of the surface mode propagating along the NA changes gradually along the main axis. Once the reflection coefficients for the surface mode at both extremities are retrieved [5], a NA can be interpreted as a Fabry-Perot resonator and its resonances can be analytically calculated. This model reveals a trade-off between large tapering angles (resulting in a low reflection coefficient at the large extremity) compared to small tapers (which are affected by high propagation losses for the surface mode), leading to an optimal taper angle. FEM-based simulations (COMSOL Multiphysics) were then used to confirm this prediction. 60-nm-thick gold tapered NA dimers were designed with 45-μm-long arms (in order to resonate at around 1 THz) and with their facing tips (100-nm-wide) separated by a 30 nm gap, thus realizing a bowtie geometry (Fig. 1a). We numerically investigated the near-field enhancement in the gap between the NAs, varying the tapering angle α from 0° to 10°.

Carbonyl-functionalized indenofluorene was electropolymerized with a high faradaic efficiency of 85% and the solid state properties of the resulting polymeric thin films were investigated. They displayed modular optical properties depending on their oxidation state. The approach used for inorganic semiconductors was applied to polyindeonofluorene derivative. Mott-Schottky analysis evidenced a switching from p-type to n-type electrical conduction, suggesting an ambipolar behaviour of the polymer. As an application, flexible organic memristors were fabricated and resistive switching properties were observed.

A way to grow and characterize isolated and coalesced ZnO nanorods on $p$ -GaN/sapphire structure is presented. Chemical bath deposition can be used to grow ZnO nanorods of device-quality, simply controlling the duration time of the growth process and the concentration of the nutrient solution in the bath. Increasing the duration of the process, as well as the concentration of the solution, leads to compact and sound layers instead of separated nanorods. However, too high concentrations stop the growth process. Light-emitting diodes fabricated on these ZnO-p-GaN heterostructure have a peak of electroluminescence at 400 nm and exhibit interesting electrical and optical properties. Optical power of 225 µW at 0.425 $A$ /cm2and a related wall-plug efficiency of 0.23% are obtained with 2 mm-diameter LEDs fabricated on a ZnO layer grown for 30 h in a solution of concentration 70 mM. The performances shown by these LEDs can be used as a way to test the quality of the ZnO growth.

We report on the properties of a poly(methyl methacrylate)-based coating used as a host for an organic dye in hybrid white light-emitting diodes. The device is composed by a pump source, which is a standard inorganic GaN/InGaN blue light-emitting diode (LED) emitting at around 450 nm, and a spin-coated conversion layer making use of Lumogen® F Yellow 083. Under prolonged irradiation, the coating exhibits significant bleaching, thus degrading the color rendering performance of the LED. We present experimental results that confirm that the local temperature rise of the operating diode does not affect the conversion layer. It is also proven that, during the test, the photostability of the organic dye is compromised, resulting in a chromatic shift from Commission Internationale de l’Eclairage (CIE) (x;y) coordinates (0.30;0.39) towards the color of the pump (0.15;0.04). Besides photodegradation of the dye, we address a phenomenon attributed to modification of the polymer matrix activated by the LED’s blue light energy as confirmed by ultraviolet–visible and Fourier-transform infrared spectroscopic analyses. Three methods for improving the overall stability of the organic coating are presented.

Licence CC BY-NC-ND The potentiality of anodic TiO2 as an oxide material for the realization of resistive switching memory cells has been explored in this paper. Cu/anodic-TiO2/Ti memristors of different sizes, ranging from 1 × 1 μm2 to 10 × 10 μm2 have been fabricated and characterized. The oxide films were grown by anodizing Ti films, using three different process conditions. Measured IV curves have shown similar asymmetric bipolar hysteresis behaviors in all the tested devices, with a gradual switching from the high resistance state to the low resistance state and vice versa, and a R_OFF/R_ON ratio of 80 for the thickest oxide film devices.

ZnO thin films were deposited by home-made DC sputtering of zinc target under mixed gases (Argon, Oxygen) plasma on glass substrates. Films were deposited by varying oxygen partial pressure (PO2) from 0.09 to 1.3 mbar in the deposition chamber, at a fixed substrate temperature of 100 °C. The samples were characterized by photoluminescence (PL), X-ray diffraction (XRD), optical transmissions (UV–vis), scanning electron microscopy (SEM) and electrical (Hall effect) measurements. The results indicate that by varying the oxygen pressure in the deposition chamber, the films show a precise and well defined photoluminescence emissions for each range of pressure covering almost the entire visible domain (UV, UV-Violet, Violet, Blue, and Red) with high intensities. Moreover, the deposited films have different defects levels. The XRD analysis indicates that the films are well grown along the c-axis peak, but with different crystalline quality. Optical measurements reveal a high transmission, up to 90%, in the spectral region between 400 and 2500 nm and a large variation of the optical band gap (3.16–4.34 eV). As an application of the deposited ZnO films, the photo-catalytic degradation of a synthetic solution of Rhodamine B (RhB) poured on a ZnO thin film was successfully achieved and an elimination rate of 38% was obtained after exposing the film to solar light for 3 h.

The paper presents the resistive switching of electroforming-free Ti/anodic- TiO 2 /Cu memristors. Anodic TiO 2 thin films were prepared by anodizing Ti layers. Microscale devices were fabricated by direct laser-assisted photolithography. Experimental results showed a bipolar and self-rectifying behavior of the devices, which could be useful for crossbar array configurations. Moreover, a gradual resistive switching of the devices in both directions was observed, indicating the presence of multi-level resistance states.

Among all the photovoltaic technologies developed so far, dye-sensitized solar cells are considered as a promising alternative to the expensive and environmentally unfriendly crystalline silicon-based solar cells. One of the possible strategies employed to increase their photovoltaic efficiency is to reduce the charge recombination at the cell conductive substrate through the use of a compact blocking layer. In this paper, we report on the fabrication and characterization of dye-sensitized solar cells employing niobium pentoxide (Nb 2 O 5 ) thin film blocking layer deposited through the pulsed laser deposition technique on conductive substrates. The careful selection of the optimal film thickness led to a 30% enhancement of the photoconversion efficiency with respect to reference cells fabricated without blocking layer. Open circuit voltage decay and electrochemical impedance spectroscopy techniques proved that the effective suppression of the charge recombination occurring at the substrate/electrolyte interface represents the main reason for the improvement of the photovoltaic efficiency.

The general aim of the experimental study, presented in this paper, was that to investigate the relationship between the characteristics of five different types of LED (Light Emitting Diode) lamps, which are nowadays those allowing the highest energy consumption reduction, and the humans non image-forming (NIF) reactions.

The precise determination of the polarization state of light is fundamental for a vast variety of applications in remote sensing, astronomy, optics and terahertz technology, to name just a few. Typically, polarization characterization is performed by using a combination of multiple optical devices such as beam splitters, polarizers, and waveplates. Moreover, to achieve high-precision, balanced photodetectors and lock-in amplifiers are employed, thus contributing to increasing system complexity. Here, a technique for polarization rotation measurements with a dynamic range of 180° and a sensitivity of about 10−2 degrees is realized using a properly designed metasurface. Such device generates a vector beam with an azimuthally-dependent polarization distribution, as a result of the superposition of two vortex beams carrying opposite orbital angular momenta (ℓ = ±30). After propagation through a linear polarizer, the spatial intensity profile of such a beam turns into 60 lobes. By tracking the displacement of only two of these lobes on a camera, the rotation of the input polarization state can be retrieved with high resolution. The proposed approach offers a new route toward the development of compact high-precision polarimeters and can also be exploited in quantum information processing, optical communications, as well as nonlinear and chiral optics.

Hafnium oxide and Nb doped HfO2 were grown by anodizing sputtering-deposited Hf and Hf-4at.%Nb. Photoelectrochemical characterization was carried out in order to estimate solid state properties such as band gap, flat band potential and electrons internal photoemission threshold energy as a function of thickness and composition of anodic oxides. Optical transitions at energy lower than the band gap value of the investigated anodic films were evidenced, and they are attributed to optical transitions involving localized states inside the band gap. Such states were located at 3.6 eV and 3.9 eV below the conduction band edge for the Nb free and Nb containing hafnium oxide, respectively. Impedance measurements were performed in a wide range of electrode potentials under anodic and cathodic polarization with respect to the flat band potential, confirming the formation of insulating oxides with high dielectric constant. The latter increases from 20 to 41 due to incorporation of Nb into the oxide.

In this work, we report on a new, efficient and low cost process of silicon carbide (SiC) powder purification intended to be used in photovoltaic applications. This process consists on the preparation of porous silicon carbide layers followed by a photo-thermal annealing under oxygen atmosphere and chemical treatment. The effect of etching time on impurities removal efficiency was studied. Inductively coupled plasma atomic emission spectrometry (ICP-AES) results showed that the best result was achieved for an etching time of 10 min followed by gettering at 900 °C during 1 h. SiC purity is improved from 3N (99.9771%) to 4N (99.9946%). Silicon carbide thin films were deposited onto silicon substrates by pulsed laser deposition technique (PLD) using purified SiC powder as target. Significant improvement of the minority carrier lifetime was obtained encouraging the use of SiC as a passivation layer for silicon.

We report studies of the effect of rapid thermal annealing (RTA) on the optical properties of a low-loss 1.3 mum saturable Bragg reflector (SBR), consisting of a GaInNAs/GaAs single quantum well embedded in an AlAs/GaAs Bragg reflector grown monolithically on a GaAs substrate. RTA gives rise to a blueshift of the photoluminescence (PL) peak (and therefore of the excitonic absorption peak) and an enhancement of PL intensity, while the reflectivity properties including peak reflectivity and bandwidth are not degraded. Temperature dependent photoluminescence measurements show that the RTA-induced blueshift of photoluminescence consists of two components: one originating from the increase of optical transition energies and another from the reduction of carrier localization. Time-resolved photoluminescence results at room temperature provide information about the recombination dynamics of carriers directly relevant to the application of the SBR in laser mode locking. Published version

We report the use of GaInNAs/GaAs material system for a range of 1.3 μm vertical-cavity devices namely VCSELs, VCSOAs, VECSELs and SESAMs. Using optical pumping, we demonstrate that up to 4 mW of 1290 nm output power can be fibre-coupled from a VCSEL. We also show that tayloring the VCSEL structure allows to produce a monolithic long-wavelength VCSOA with up to 16 dB of gain. We also report the first demonstration of a 1.3 μm VECSEL with more than 0.5 W of CW ouptut power. Finally, annealing effects on the properties of a GaInNAs SBR and modelocking of two Nd:doped solid state lasers using this element are described.

We report the unambiguous demonstration of controlled quantum well intermixing (QWI) in the technologically important GaInNAs/ GaAs 1.3 mum material system. QWI is a key technique to selectively modify the band gap of quantum wells, which has found broad application in semiconductor lasers and photonic integrated circuits (PICs). Extending such technology to GaInNAs/GaAs structures is highly desirable due to the technologically advantageous properties of this material system. Here, we investigate well-characterized GaInNAs quantum well material which has been annealed "to saturation" before QWI processing to allow unambiguous interpretation of results. After RTA at 700 degreesC for similar to 180 s, controlled shifts in band-gap at room temperature of up to 200 nm have been observed in sputtered SiO2-capped samples, whilst uncapped and PECVD SiO2-capped samples demonstrated negligible shift. This selective modification of the band gap has been confirmed by detailed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Analysis of composition profile by SIMS revealed that the QWI is due to the interdiffusion of In-Ga between the quantum wells and the barriers enhanced by the point defects generated during the sputtering process. Investigation of a series of samples of differing N concentrations will be presented, which provides extra information about the intrinsic properties of GaInNAs. (C) 2003 Elsevier B.V. All rights reserved.

Wet oxidation of GaAs/AlGaAs structures is an important technique in the processing of advanced devices such as vertical cavity surface emitting lasers (VCSELs). In one VCSEL application, the low-index and electrically-insulating AlxOy layers have been used to obtain high-reflectivity and broad bandwidth distributed Bragg reflector mirrors (DBRs). A further recent development has shown that combined lateral–vertical oxidation of intracavity AlGaAs layers can be used to tune the resonant wavelength of a semiconductor microcavity. The slow oxidation rate limits the lateral scale of practical wet oxidation to mesas structures of 50–100 μm in width. Therefore post-processing assessment of spectral changes requires microreflectivity measurement capability with high spatial resolution. In the following, we describe the fabrication and assessment of microcavity structures in the 1.3 μm range. The micro-reflectivity set-up consists of microscope-objective focussing of broadband light, combined with optics to relay the data to a spectrograph, and a CCD camera for alignment. This simple set-up allows the measurement of calibrated reflectivity for features down to a few 10's of μm in size over a large spectral range (600–1800 nm). We present microreflectivity measurements of wide-bandwidth oxidised DBRs, and most significantly, for the first time to our knowledge, of oxidation control of the resonant wavelength of a microcavity in the 1.3 μm range.

Rapid thermal annealing combined with SiO2 caps deposited on the surface of samples by different techniques is used to selectively disorder 1.3 mum GaInNAs/GaAs multiquantum wells which have been preannealed in situ to the stage of blueshift saturation. After thermal annealing under specific conditions, a shift in band gap of over 170 meV has been obtained in sputtered SiO2-capped samples, while uncapped and plasma enhanced chemical vapor deposited SiO2-capped samples demonstrated a negligible shift. Quantum well intermixing in sputtered SiO2-capped samples originates from enhanced compositional interdiffusion due to the generation of point defects by ion bombardment during the sputtering process. Secondary ion mass spectrometry has confirmed that the enhanced blueshift was caused by the interdiffusion of group III atoms (In and Ga) between the quantum wells and barriers. Detailed photoluminescence and excitation spectroscopy were performed to study the optical properties of both intermixed and nonintermixed samples. Published version

Zinc oxide nanorods have great potential for the realization of high efficiency heterostructure LEDs based on pdoped gallium nitride. In order to obtain a good confinement of the light, a well-aligned nanorod waveguiding structure is desirable. This paper reports on the fabrication of vertical zinc oxide nanorods using a solution-based growth process that does not require a seed layer. The nanorods obtained follow the crystalline growth direction of the GaN layer along the c-axis. Various results with different reagent concentrations are reported.

We report a multi-wavelength passive filter working in the 1.3 μm range, fabricated by a post-growth technique based on the combined lateral-vertical steam oxidation of AlGaAs layers within a microcavity. Wafers are photolithographically patterned and etched through the top to create mesa structures which are oxidized from both edges, and control of resonant wavelength is achieved through mesa width, thickness and compositional control of intracavity layers to be oxidized, and oxidation conditions. Microreflectivity measurements of the processed devices show that in this wavelength range it is possible using this approach to control the resonant wavelength over a range of about 52 nm.

The plume of a laser-ablated indium–tin-oxide target was investigated by optical emission spectroscopy. Atomic and ionic species of indium, tin and oxygen were observed; moreover, molecular bands of indium oxide were identified in the fluorescent spectra. The effects of the oxygen as a background gas and of the laser fluence on the behaviour of the ejected particles were studied with respect to the intensity of the emission and the delay time as a function of the observation distance from the target surface. The non-linear behaviour of the fluorescent species with the process conditions could infer spatial variations of the plume composition. The analysis demonstrates a plume expansion process where the ejected species interact with the molecules of the background gas by reactive collisions.

We performed optical emission spectroscopy to monitor the plasma produced during the ablation of indium-tin-oxide targets under different oxygen pressure conditions using a pulsed UV laser. Molecular bands of InO were identified in the fluorescent spectra produced by pulsed laser ablation. InO line monitoring allowed obtaining the optimal conditions for good-quality ITO film deposition. We demonstrated that it is possible to correlate InO line spectroscopic parameters with the conditions required to fabricate a high-conductivity and high-transparent ITO thin film. In particular, low resistivity (10-4 to 10-3 Ω cm) was obtained in films deposited at room temperature by regulating oxygen pressure into the chamber in order to keep the InO line amplitude at a predetermined value. © 2001 Elsevier Science B.V. All rights reserved.

Organic field-effect transistors were fabricated using an electrochemical route. The dielectric oxide was grown by anodization of a Ti:Si alloy, while 3,4-polyethylenedioxythiophene has been employed as a semiconducting polymer. OutputI-Vcharacteristics showed a transistor effect dependent on dielectric thickness. Fitting between I-V measurements and theoretical simulations in the triode region confirmed the presence of a conduction path through the polymer which degrades the electrical characteristics of the devices.

This work reports on the benefits and promising opportunities offered by white LED hybrid technology, based on a mixing perylene-based dyes in order to obtain a warm white light for frequency-down conversion. In a standard Ce:YAG-based white LED, the white light appears cold due to the weakness of red wavelength components in the emission spectrum. In order to obtain a warmer white, one possible solution is to add a red phosphor to the yellow one to move the chromatic coordinates properly, though the luminous efficiency drastically decreases due to the increased light absorption of the coating layer. It is generally believed that the low efficiency of warm white LEDs is the main issue today for LED-based lighting. Using photoluminescence of Lumogen® F Yellow 083, a perylene-based polymer dye commercialized by BASF, and adding a small quantity of another perylene-based dye, Lumogen® F Red 305 (BASF), we obtained high-efficiency warm white LEDs by yellow and red conversion from a standard 450 nm GaN/InGaN royal blue LED. Different weight proportions of dyes were dissolved in solutions with equal amounts of poly-methyl-methacrylate (PMMA) in ethyl acetate, then the LEDs were dip-coated in each solution and optically characterized. Record values of 8.03 lm of luminous flux and 116.11 lm/W of optical efficiency were achieved. Finally, the effects of both driving current, and pump wavelength on LED performances – such as chromatic coordinates, correlated color temperature, color rendering index (CRI), and optical efficiency – were investigated.

We report an investigation of selective quantum-well intermixing (QWI) in 1.3-μm GaInNAs/GaAs multi quantum wells by silica-cap-induced disordering processes. After thermal annealing under specific conditions, controlled shifts of band gap at room temperature of over 200 nm have been observed in sputtered SiO 2 -capped samples, while uncapped and SiO 2 -capped samples by plasma-enhanced chemical vapor deposition demonstrated negligible shift. This selective modification of the band gap in GaInNAs quantum wells has been confirmed by detailed photoluminescence and photoluminescence excitation spectroscopy, and by secondary ion mass spectrometry. The controlled tuning of the band gap of GaInNAs/GaAs by QWI is important for a wide range of photonic integrated circuits and advanced device applications.

A high efficiency white light emitting diode (LED) was fabricated by generation of frequency down-conversion from a GaN/InGaN blue LED. In place of conventional inorganic phosphors, a perylene-based dye was used for colour conversion. The resulting hybrid structure is analysed by focusing on the visual performance of the realised LEDs employing the most relevant photometric parameters of a light source. Preparation of the organic polymer is described as well. The thermal stability of the dye was investigated and a simple structure which avoids colour degradation is proposed.