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This study reports a cost-effective approach for the fabrication of CuO–ZnO nanocomposites with different morphological features via in situ and impregnation approaches of a controlled amount of copper to obtain optimum photocatalytic activity. The impregnation approach results in flower, sheet and coral-shaped CuO particles in junction with ZnO nanowires (NWs). On the other hand, the transformation of ZnO NWs to flower-like (very rough) and pillar-like CuO–ZnO nanostructures with CuO outgrowths has been observed with an in situ approach as a function of molar ratio [(CuNO3 : ZnNO3)]: hexamethylenetetrammine of 2 > 1 > 0.5 > 0.25. CuO–ZnO nanocomposites exhibited a slight red-shift in their band-gap values (3.2 to 2.8 eV) when increasing the copper content during the in situ approach. An increase (∼2 fold) in the photocatalytic efficiency for the degradation of methyl orange has been noticed with in situ grown CuO–ZnO composites on an Si substrate as compared to pristine ZnO NWs. The apparent rate constant (k) was found to be the highest for in situ fabricated CuO–ZnO nanocomposites (1.5 × 10−1 h−1) as compared to 2.6 × 10−2 h−1 for impregnated Cu1:Zn1, and least (1.8 × 10−2 h−1) for pristine ZnO NWs during methyl orange photooxidation. CuO–ZnO nanocomposites with rough exposed surface and CuO outgrowths provide a high active surface area and stable p–n junctioned interfacial contact for application in environmental purification.

International audience; (Tb, Eu)–co-doped ZnO films with about 3 at.% total doping rate were grown by magnetron sputtering on Si substrate. Post annealing treatments were performed at 973–1373 K in continuous nitrogen flow to investigate the transformation of microstructural and optical characteristics by means of X-ray diffraction, transmission electron microscopy, photoluminescence and electroluminescence. For annealing temperatures lower than 1073 K, segregation of Eu and Tb was observed mainly at the film/substrate junction. For temperatures higher than 1173 K, additional phases appeared, namely, Zn2SiO4 and rare earth silicates. For the highest temperature investigated (1373 K), only silica and rare earth silicates remained in the film due to Zn evaporation. PL measurements indicated a very intense Eu emission associated with the presence of rare earth silicate inclusions. Energy transfer from Tb towards Eu was evidenced in this secondary phase. At last, based on these preliminary works, a (Tb, Eu)–co-doped ZnO/Si electroluminescent structure was produced and showed very promising results paving the way for very thin ZnO based light emitting diodes.

Rhenium disulfide (ReS2), known in nature as the mineral rheniite, is a very interesting compound owing to its remarkable fundamental properties and great potential to develop novel device applications. Here we perform density functional theory (DFT) calculations to investigate the structural properties and compression behavior of this compound and also of the (Re,Mo)S-2 solid solution as a function of Re/Mo content. Our theoretical analysis is complemented with high-pressure X-ray diffraction (XRD) measurements, which have allowed us to reevaluate the phase transition pressure and equation of state of 1T-ReS2. We have observed the 1T-to-1T' phase transition at pressures as low as similar to 2 GPa, and we have obtained an experimental bulk modulus, B-0, equal to 46(2) GPa. This value is in good agreement with PBE+D3 calculations, thus confirming the ability of this functional to model the compression behavior of layered transition metal dichalcogenides, provided that van der Waals corrections are taken into account. Our experimental data and analysis confirm the important role played by van der Waals effects in the high-pressure properties of 1T-ReS2., J.I. and C.P. acknowledge the Spanish Ministry of Science, Innovation and Universities via Grant FIS2017-2017-83295-P. T.W. acknowledges financial support from Polish Ministry of Science and Higher Education via Diamond Grant no. D\2015 002645.

By using ZnO thin films doped with Ce, Tb or Eu, deposited via radiofrequency magnetron sputtering, we have developed monochromatic (blue, green and red, respectively) light emitting devices (LEDs). The rare earth ions introduced with doping rates lower than 2% exhibit narrow and intense emission peaks due to electronic transitions in relaxation processes induced after electrical excitation. This study proves zinc oxide to be a good host for these elements, its high conductivity and optical transparency in the visible range being as well exploited as top transparent electrode. After structural characterization of the different doped layers, a device structure with intense electroluminescence is presented, modeled, and electrically and optically characterized. The different emission spectra obtained are compared in a chromatic diagram, providing a reference for future works with similar devices. The results hereby presented demonstrate three operating monochromatic LEDs, as well as a combination of the three species into another one, with a simply-designed structure compatible with current Si technology and demonstrating an integrated red-green-blue emission.

The increasing need for efficient memories with integrated functionalities in a single device has led the electronics community to investigate and develop different materials for resistive switching (RS) applications. Among these materials, the well-known Si nanocrystals (NCs) have demonstrated to exhibit RS properties, which add to the wealth of phenomena that have been studied on this model material platform. In this work, we present ZnO/Si NCs/p-Si resistive switching devices whose resistance state can be electrically read at 0 V under the application of low-power monochromatic illumination. The presented effect is studied in terms of the inner structural processes and electronic physics of the device. In particular, the creation of conductive filaments through the Si NC multilayers induces a low-resistance path for photogenerated carriers to get extracted from the device, whereas in the pristine state charge extraction is strongly quenched due to the insulating nature of the NC-embedding SiO2 matrix. In addition, spectral inspection of the generated photocurrent allowed unveiling the role of Si NCs in the reported effect. Overall, the hereby shown results pave the way to obtain memories whose RS state can be read under low-power conditions.

Phosphorus- and boron-doped silicon nanocrystals (Si NCs) embedded in silicon oxide matrix can be fabricated by plasma-enhanced chemical vapour deposition (PECVD). Conventionally, SiH4 and N2O are used as precursor gasses, which inevitably leads to the incorporation of ≈10 atom % nitrogen, rendering the matrix a silicon oxynitride. Alternatively, SiH4 and O2 can be used, which allows for completely N-free silicon oxide. In this work, we investigate the properties of B- and P-incorporating Si NCs embedded in pure silicon oxide compared to silicon oxynitride by atom probe tomography (APT), low-temperature photoluminescence (PL), transient transmission (TT), and current–voltage (I–V) measurements. The results clearly show that no free carriers, neither from P- nor from B-doping, exist in the Si NCs, although in some configurations charge carriers can be generated by electric field ionization. The absence of free carriers in Si NCs ≤5 nm in diameter despite the presence of P- or B-atoms has severe implications for future applications of conventional impurity doping of Si in sub-10 nm technology nodes.

In this paper, we report on the photoresponse properties of a SiO2/Si/SiO2 multilayer (ML) system deposited at 500 °C and integrated into a metal-oxide-semiconductor structure (Al/ML/p-Si) without any further thermal annealing. High contents of elemental Si (Si0) revealed by the XPS measurements suggests the formation of amorphous Si-nanoclusters (a-Si ncls) within the ML. Dark and illuminated I–V curves of the device indicate its sensitivity to ultraviolet, visible and near infrared light under applied bias and at 0 V. In addition, we observed variation of the spectral response and the external quantum efficiency as a function of the electrode size. At 0 V, the device illuminated with a wavelength at 550 nm exhibits a maximum external quantum efficiency of 0.95% and a spectral response of 4.1 mA/W. This photoresponse has been ascribed to photon absorption in SiO2 sub-band states created by the a-Si ncls and defects formed by the excess Si inside the SiO2/Si/SiO2 multilayers.

In this work, the relation between the photovoltaic and resistive switching (RS) properties of metal-oxide-semiconductor devices containing Si nanocrystal (Si NC) superlattices is investigated. A first approximation concludes that the low resistance state achieved by the RS process allows for enhanced photogenerated carrier extraction when compared to the high resistance state and pristine devices. By using different current compliance values during the electroforming process, the low resistance state is further modified, improving its conductivity and the collection probability of photogenerated carriers. Conversion efficiency is enhanced by at least one and up to five orders of magnitude by applying different electroforming processes. In addition to promoting the RS properties in these devices, spectral response measurements demonstrate that Si NCs are partially responsible for the optical absorption, and that their contribution is maintained after electroforming. We thus conclude that the proposed methodology can improve the conversion efficiency of this and other multijunction solar cells or structures that also exhibit RS properties. Through RS, a dense network of conductive filaments is promoted in the insulating region, which reduces the travel distance of photocarriers for their collection., This work was financially supported by the Spanish Ministry of Economy, Industry and Competitiveness (TEC2016-76849-C2-1-R) and the German Research Foundation (ZA191/27-3 and ZA191/33-1). J.L.F. acknowledges financial support from the subprogram “Ayudas para la Formación de Profesorado Universitario” (FPU16/06257) from the Spanish Ministry of Education, Culture and Sports.

Altres ajuts: G. Vesio acknowledges the Spanish Government for his Ph.D. grant in the FPU program. Low-power, high-performance metal-insulator-metal (MIM) non-volatile resistive memories based on HfO2 high-k dielectric are fabricated using a drop-on-demand inkjet printing technique as a low-cost and eco-friendly method. The characteristics of resistive switching of Pt (bottom)/HfO2/Ag (top) stacks on Si/ SiO2 substrates are investigated in order to study the bottom electrode's interaction with the HfO2 dielectric layer and the resulting effects on resistive switching. The devices show low Set and Reset voltages, high ON/OFF current ratio, and relatively low switching current (~1 μA), which are comparable to the characteristics of current commercial CMOS memories. In order to understand the resistive switching mechanism, direct structural observation is carried out by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM) on cross-sectioned samples prepared by focused ion beam (FIB). In addition, electron energy loss spectroscopy (EELS) inspections discard a silver electro-migration effect.

In the last few years, the emergence of studies concerning the resistive switching (RS) phenomenon has resulted in the finding of a large amount of materials being capable of acting as an active layer in such devices, i.e., the layer where the change in resistance takes place. Whereas the normal operation consists of the electrical readout of the modified resistance state of the device after electrical writing, electro-photonic approaches seek the involvement of light in these devices, be it either for the active Set or Reset operations or the readout. We propose in this work silicon nanocrystal multilayers (Si NC MLs) as an active material for being used in RS devices, taking advantage of their outstanding optical properties. The resistance states of Si NC MLs were obtained by electrical excitation, whose readout is carried out by electrical and electro-optical means, thanks to a distinguishable electroluminescence emission under each state. To achieve this, we report on an adequate design that combines both the Si NC MLs with ZnO as a transparent conductive oxide, whose material properties ensure the device RS performance while allowing the electro-optical characterization. Overall, such an occurrence states the demonstration of a Si NCs-based electroluminescent RS device, which paves the way for their future integration into photonic integrated circuits.

The soaring global demand for flexible, wearable and transparent devices has created an urgent need for new fabrication technologies that are both cost-competitive and eco-friendly. Printed electronics holds the promise of enabling low-cost, scalable solutions exploiting the ability of innovative materials to be used as processed inks onto a large area substrate. In this article, we demonstrate the direct drop-on-demand inkjet printing technology as a viable method for the fabrication of fully-printed metal–insulator–metal capacitors on a flexible substrate (Kapton®), where the high-k hafnium oxide (HfO2) was selected as the dielectric. After a low-temperature annealing process, the deposited nanoparticle (NP)-based ink of HfO2 showed high homogeneity and good integrity of the printed thin film by microscopy and spectroscopy studies. The fully-printed capacitors were characterized by field-emission scanning and transmission electron microscopies. X-ray diffraction patterns, as well as Raman scattering and Fourier-transform infrared spectra, revealed the presence of a polycrystalline solid layer, without solvent organic ink remains. The bonding structure of the HfO2 layer and the interface with the Ag electrode was studied by X-ray photoelectron spectroscopy. The good performance of the thin film was proved by its relative permittivity, k = 12.6, and dielectric loss tangent, tan δ = 0.0125 at 1 MHz. Finally, the electrical current density–voltage and capacitance–voltage measurements have been studied in the frequency range from 10 kHz to 1 MHz. The obtained results indicate that MIM capacitors based on inkjet-printed flexible HfO2 NPs work properly within the ITRS 2016 roadmap requirements.

The fabrication of SiOx/SiO2 superlattices combined with thermal annealing enables the size and density control of Si quantum dots. The layered-arranged Si quantum dots represent a model system to systematically study the photonic and electronic properties of indirect gap quantum dots prepared in a CMOS compatible way. Hence, the model system is used to understand the interplay of absorption and recombination, the carrier kinetics and the electronic transport properties for matrix embedded Si quantum dots. The interplay of radiative and non-radiative recombination will be discussed for high quantum yield. Doping of Si quantum dots and the respective quantification is at the limit of the respective high resolution techniques but clearly show the effect of self-purification. Recent results using the Si quantum dots in memristor devices will be presented.

The achievement of an efficient all-Si electrically-pumped light emitter is a major milestone in present optoelectronics still to be fulfilled. Silicon nanocrystals (Si NCs) are an attractive material which, by means of the quantum confinement effect, allow attaining engineered bandgap visible emission from Si by controlling the NC size. In this work, SiO2-embedded Si NCs are employed as an active layer within a light-emitting device structure. It is demonstrated that the use of an additional thin Si3N4 interlayer within the metal-insulator-semiconductor device design induces an enhanced minority carrier injection from the substrate, which in turn increases the efficiency of sequential carrier injection under pulsed electrical excitation. This results in a substantial increase in the electroluminescence efficiency of the device. Here, the effect of this Si3N4 interlayer on the structural, optical, electrical, and electro-optical properties of a Si NC-based light emitter is reported, and the physics underlying these results is discussed.

The resistive switching properties of silicon-aluminium oxynitride (SiAlON) based devices have been studied. Electrical transport mechanisms in both resistance states were determined, exhibiting an ohmic behaviour at low resistance and a defect-related Poole−Frenkel mechanism at high resistance. Nevertheless, some features of the Al top-electrode are generated during the initial electroforming, suggesting some material modifications. An in-depth microscopic study at the nanoscale has been performed after the electroforming process, by acquiring scanning electron microscopy and transmission electron microscopy images. The direct observation of the devices confirmed features on the top electrode with bubble-like appearance, as well as some precipitates within the SiAlON. Chemical analysis by electron energy loss spectroscopy has demonstrated that there is an out-diffusion of oxygen and nitrogen ions from the SiAlON layer towards the electrode, thus forming silicon-rich paths within the dielectric layer and indicating vacancy change to be the main mechanism in the resistive switching.

Asymmetric (CuGaO2/ZnO/ZnMgO) and symmetric (ZnMgO/ZnO/ZnMgO) multiple quantum well (MQW) structures were successfully fabricated using pulsed laser deposition (PLD) and their comparison were made. Efficient room temperature photoluminescent (PL) emission was observed from these MQWs and temperature dependent luminescence of asymmetric and symmetric MQWs can be explained using the existing theories. A systematic blue shift was observed in both MQWs with decrease in the confinement layer thickness which could be attributed to the quantum confinement effects. The PL emission from asymmetric and symmetric MQW structures were blue shifted compared to 150 nm thick ZnO thin film grown by PLD due to quantum confinement effects.

In this work, the electroluminescence (EL) emission of zinc oxide (ZnO)/Si nanocrystals (NCs)-based light-emitting devices was studied under pulsed electrical excitation. Both Si NCs and deep-level ZnO defects were found to contribute to the observed EL. Symmetric square voltage pulses (50-μs period) were found to notably enhance EL emission by about one order of magnitude. In addition, the control of the pulse parameters (accumulation and inversion times) was found to modify the emission lineshape, long inversion times (i.e., short accumulation times) suppressing ZnO defects contribution. The EL results were discussed in terms of the recombination dynamics taking place within the ZnO/Si NCs heterostructure, suggesting the excitation mechanism of the luminescent centers via a combination of electron impact, bipolar injection, and sequential carrier injection within their respective conduction regimes.

A transparent and conducting ZnO:Er:Yb thin film with upconversion properties has been achieved after being annealed with continuous laser radiation just before the ablation point of the material. This work demonstrates that the laser energy preserves the conductivity of the film and at the same time creates an adequate surrounding for Er and Yb to produce visible upconversion at 660, 560, 520, and 480 nm under 980 nm laser excitation. The relation between the structural, electrical and upconversion properties is discussed. It is observed that the laser energy melts part of the material, which recrystallizes creating rare earth oxides and two different wurtzite structures, one with substitutional rare earths and oxygen vacancies (responsible for the conductivity) and the other without substitutional rare earth ions (responsible for the upconversion emission). (C) 2017 Elsevier B.V. All rights reserved.

The optical, electrical and electro-optical properties of metal-insulator-semiconductor (MIS) devices containing Si-rich silicon nitride (SRN) films are reported. The photoluminescence and optical absorption characterization evidences that optically active centers present a defective nature. As a consequence, a Poole-Frenkel type transport mechanism is considered, which is in agreement with the electrical results. The electro-optical performance of the devices has been found to be strongly modulated by the poly-Si electrode transmittance. After taking into account this contribution, EL emission has been studied, being the obtained spectra in accordance to the PL ones, which proofs that the same luminescent centers are also being excited electrically. The electrical response of the devices has been investigated under light excitation, showing induced photocurrent and photoconductivity, which makes SRN a good candidate material for photovoltaic applications.

We investigated picosecond carrier recombination in Si/SiO2 nanocrystal superlattices by ultrafast transient transmission, time-resolved photoluminescence, and Raman spectroscopy. The recombination is of multicarrier origin and it depends strongly on the nanoscale structure of the samples (e.g., crystallinity, percolation, and size distribution). Several recombination pathways were found, including Auger recombination, trapped-carrier Auger recombination, exciton–exciton recombination, and subsequent trapping in band tail states of amorphous silicon phase. The sample microscopic structure is determined using a single parameter, the stoichiometric parameter x, during the plasma-enhanced chemical-vapor deposition process. The percolated samples are hot candidates for all-silicon tandem photovoltaic solar cells in the future.

The electro-optical properties of devices containing Si-rich silicon nitride (SRN) thin films, deposited by means of low-pressure chemical-vapor deposition (LPCVD), are studied. After optically evidencing the presence of defects in our films, a Poole-Frenkel conduction mechanism has been considered, which is in agreement with electrical experiments. We found that the poly-Si layer strongly modulates the electroluminescence and light absorption of the devices. In addition, they showed photocurrent and photoconductivity under illumination, SRN becoming a promising material for photovoltaic applications.

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of an all-Si tandem solar cell with a band gap from 1.3-1.7 eV, tuneable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. Here we examined the luminescence and transport of Si NCs embedded in SiC. We observed luminescence that redshifts from 2.0 to 1.5 eV with increasing nominal NC size. Upon further investigation, we found that this redshift is to a large extent due to Fabry-Pérot interference. Correction for this effect allows an analysis of the spectrum emitted from within the sample. We also produced p-i-n solar cells and found that the observed I-V curves under illumination could be well-fitted by typical thin-film solar cell models including finite series and parallel resistances, and a voltage-dependent current collection function. A minority carrier mobility-lifetime product on the order of 10-10 cm2/V was deduced, and a maximum open-circuit voltage of 370 mV achieved.

Antimony trisulfide (Sb2S3), found in nature as the mineral stibnite, has been studied under compression at room temperature from a joint experimental and theoretical perspective. X-ray diffraction and Raman scattering measurements are complemented with ab initio total-energy, lattice-dynamics, and electronic structure calculations. The continuous changes observed in the volume, lattice parameters, axial ratios, bond lengths, and Raman mode frequencies as a function of pressure can be attributed to the different compressibility along the three orthorhombic axes in different pressure ranges, which in turn are related to the different compressibility of several interatomic bond distances in different pressure ranges. The structural and vibrational properties of Sb2S3 under compression are compared and discussed in relation to isostructural Bi2S3 and Sb2Se3. No first-order phase transition has been observed in Sb2S3 up to 25 GPa, in agreement with the stability of the Pnma structure in Bi2S3 and Sb2Se3 previously reported up to 50 GPa. Our measurements and calculations do not show evidence either for a pressure-induced second-order isostructural phase transition or for an electronic topological transition in Sb2S3. © 2016 American Chemical Society., This work has been performed under financial support from Spanish MINECO under Projects MAT2013-46649-C4-2/3-P and MAT2015-71070-REDC. This publication is fruit of “Programa de Valoración y Recursos Conjuntos de I+D+i VLC/CAMPUS” and has been financed by the Spanish Ministerio de Educación, Cultura y Deporte, as part of “Programa Campus de Excelencia Internacional” through Projects SP20140701 and SP20140871.

The optical and photovoltaic properties of Si NCs/SiC multilayers (MLs) are investigated using a membrane-based solar cell structure. By removing the Si substrate in the active cell area, the MLs are studied without any bulk Si substrate contribution. The occurrence is confirmed by scanning electron microscopy and light-beam induced current mapping. Optical characterization combined with simulations allows us to determine the absorption within the ML absorber layer, isolated from the other cell stack layers. The results indicate that the absorption at wavelengths longer than 800 nm is only due to the SiC matrix. The measured short-circuit current is significantly lower than that theoretically obtained from absorption within the ML absorber, which is ascribed to losses that limit carrier extraction. The origin of these losses is discussed in terms of the material regions where recombination takes place. Our results indicate that carrier extraction is most efficient from the Si NCs themselves, whereas recombination is strongest in SiC and residual a-Si domains. Together with the observed onset of the external quantum efficiency at 700–800 nm, this fact is an evidence of quantum confinement in Si NCs embedded in SiC on device level.

We offer a complete structural and optical study of samples containing silicon nanocrystals (Si-NCs) embedded in SiO2/SiON multilayers, varying the oxynitride layer thickness from 2.5 to 7 nm. Using energy-filtered transmission electron microscopy we have determined the size distribution of the precipitated Si-nanoaggregates. Raman scattering measurements were used to investigate the Si-NC size and crystalline quality. By combining both techniques, the nanoaggregate crystalline degree has been evaluated, with values around 50% for all the samples. Photoluminescence spectroscopy has shown a blueshift of the emission at smaller NC sizes, presenting the sample with Si-NCs of 3.9 nm the best emission properties.

Asymmetric (CuGaO2/ZnO/ZnMgO) and symmetric (ZnMgO/ZnO/ZnMgO) multiple quantum well (MQW) structures were successfully fabricated using pulsed laser deposition (PLD) and their comparison were made. Efficient room temperature photoluminescent (PL) emission was observed from these MQWs and temperature dependent luminescence of asymmetric and symmetric MQWs can be explained using the existing theories. A systematic blue shift was observed in both MQWs with decrease in the confinement layer thickness which could be attributed to the quantum confinement effects. The PL emission from asymmetric and symmetric MQW structures were blue shifted compared to 150 nm thick ZnO thin film grown by PLD due to quantum confinement effects.

We investigate the optical properties of high-quality Si nanocrystals $(\mathrm{NCs})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers under high hydrostatic pressure with Raman scattering and photoluminescence (PL) measurements. The aim of our study is to shed light on the origin of the optical emission of the Si $\mathrm{NCs}/\mathrm{Si}{\mathrm{O}}_{2}$. The Si NCs were produced by chemical-vapor deposition of Si-rich oxynitride $(\mathrm{SRON})/\mathrm{Si}{\mathrm{O}}_{2}$ multilayers with 5- and 4-nm SRON layer thicknesses on fused silica substrates and subsequent annealing at 1150 \ifmmode^\circ\else\textdegree\fi{}C, which resulted in the precipitation of Si NCs with an average size of 4.1 and 3.3 nm, respectively. From the pressure dependence of the Raman spectra we extract a phonon pressure coefficient of $8.5\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa}$ in both samples, notably higher than that of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(5.1\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/\mathrm{GPa})$. This result is ascribed to a strong pressure amplification effect due to the larger compressibility of the $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. In turn, the PL spectra exhibit two markedly different contributions: a higher-energy band that redshifts with pressure, and a lower-energy band which barely depends on pressure and which can be attributed to defect-related emission. The pressure coefficients of the higher-energy contribution are $(\ensuremath{-}27\ifmmode\pm\else\textpm\fi{}6)$ and $(\ensuremath{-}35\ifmmode\pm\else\textpm\fi{}8)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa}$ for the Si NCs with a size of 4.1 and 3.3 nm, respectively. These values are sizably higher than those of bulk $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}(\ensuremath{-}14\phantom{\rule{0.16em}{0ex}}\mathrm{meV}/\mathrm{GPa})$. When the pressure amplification effect observed by Raman scattering is incorporated into the analysis of the PL spectra, it can be concluded that the pressure behavior of the high-energy PL band is consistent with that of the indirect transition of Si and, therefore, with the quantum-confined model for the emission of the Si NCs.

Electron beam evaporation was employed in order to fabricate Al- and Tb-codoped Si oxide multilayers via the delta-doping approach. This methodology permits the control of the rare-earth (RE) separation along the growth direction with nanometric resolution. To investigate the control of the RE separation in the growth direction, different SiO 2 thicknesses were studied. After deposition, the samples were submitted to different annealing processes for 1 h in N 2 , at temperatures ranging from 700 to 1100 °C. Photoluminescence experiments reveal narrow emissions ascribed to Tb 3+ ions in all samples, with an intensity variation depending on the oxide thickness and annealing temperature. In addition, the incorporation of Al under different spatial configurations produced an enhancement of more than one order of magnitude in the photoluminescence intensity, in respect to the best sample without Al. Finally, time-resolved measurements were carried out in order to determine the 5 D 4 → 7 F 5 transition dynamics, obtaining a decay time of ~1.6 ms ascribed to the Tb 3+ ions.

Si NCs embedded in a wide band gap material are promising candidates for all-Si tandem solar cells thanks to the possibility of tuning the NC band gap by varying their size, offered by the NC quantum confinement. The high temperature treatment required for Si crystallization represents a concern for the integration of this material in a tandem solar cell production route. The limiting step for solid phase crystallization is grain nucleation. A high temperature nucleation seeding followed by grain growth at a lower temperature is therefore a promising method to reduce the thermal budget. To this aim, we report on different annealing routes pursued to decrease the thermal budget: in all cases the germs of the c-Si nucleation are created by a high temperature (1150 °C) spike, then the crystallization follows at different temperatures, above and below the 3C-SiC solid phase epitaxial growth temperature (800 °C). The results are discussed in terms of the structural features produced by each process: the time required for complete dehydrogenation, mandatory to avoid disruptive hydrogen evolution during the high temperature spike; Si and SiC crystallization; defect nucleation in c-SiC, correlated to the rate of temperature rise during the annealing; role of the c-Si/SiC interface energy in affecting the growth process. The photovoltaic properties of the resulting structures are estimated based on the measured optical absorption, electronic transitions, and electrical transport.

The effect of the oxide barrier thickness (tSiO2) reduction and the Si excess ([Si]exc) increase on the electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO2 superlattices (SLs) is investigated. The active layers of the metal–oxide–semiconductor devices were fabricated by alternated deposition of SRON and SiO2 layers on top of a Si substrate. The precipitation of the Si excess and thus formation of Si nanocrystals (NCs) within the SRON layers was achieved after an annealing treatment at 1150 °C. A structural characterization revealed a high crystalline quality of the SLs for all devices, and the evaluated NC crystalline size is in agreement with a good deposition and annealing control. We found a dramatic conductivity enhancement when the Si content is increased or the SiO2 barrier thickness is decreased, due to a larger interaction of the carrier wavefunctions from adjacent layers. EL recombination dynamics were studied, revealing radiative recombination decay times of the order of tens of microseconds. Lower lifetimes were found at higher [Si]exc, attributed to exciton confinement delocalization, whereas intermediate barrier thicknesses present the slowest decay. The electrical-to-light conversion efficiency increases monotonously at thicker barriers and smaller Si contents. We ascribe these effects mainly to free carriers, which enhance carrier transport through the SLs while strongly quenching light emission. Finally, the combination of the different results led us to conclude that tSiO2 ∼ 2 nm and [Si]exc from 12 to 15 at% are the ideal structure parameters for a balanced electro-optical response of Si NC-based SLs.

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by annealing of $a\ensuremath{-}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}:\mathrm{H}$ precursors is studied using temperature-dependent current-voltage measurements supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all temperatures and the temperature dependence of conductivity shows that the materials behave as disordered semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films are $n$ type, and films with Si NCs exhibit up to ${10}^{3}$ times higher conductivity (up to $0.1\phantom{\rule{0.16em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) after exposure to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen.

Transparent and conducting ZnO:Er:Yb thin films with visible up-conversion (660-nm emission under 980-nm excitation) were fabricated by RF magnetron sputtering. The as-deposited films were found to be transparent and conducting and the activation of the Er ions in these films to produce up-conversion luminescence was achieved by different post-deposition annealing treatments in air, vacuum or by laser annealing using a Nd:YVO4 laser. The structural, electrical and optical properties and the up-conversion efficiency of these films were found to be strongly influenced by the annealing method, and a detailed study is reported in this paper. It has been demonstrated that, although the air annealing was the most efficient in terms of up-conversion, laser annealing was the only method capable of activating Er ions while preserving the electrical conductivity of the doped films. It has been shown that a minimum energy was needed in laser annealing to optically activate the rare earth ions in the ZnO host material to produce up-conversion. Up-converting and transparent conducting ZnO:Er:Yb films with an electrical resistivity of 5 x 10(-2) cm and transparency 80% in the visible wavelength range has been achieved by laser annealing. (C) 2015 Elsevier B.V. All rights reserved.

In this work we apply low-loss electron energy loss spectroscopy (EELS) to probe the structural and electronic properties of single silicon nanocrystals (NCs) embedded in three different dielectric matrices (SiO2, SiC and Si(3)N(4)). A monochromated and aberration corrected transmission electron microscope has been operated at 80 kV to avoid sample damage and to reduce the impact of radiative losses. We present a novel approach to disentangle the electronic features corresponding to pure Si-NCs from the surrounding dielectric material contribution through an appropriate computational treatment of hyperspectral datasets. First, the different material phases have been identified by measuring the plasmon energy. Due to the overlapping of Si-NCs and dielectric matrix information, the variable shape and position of mixed plasmonic features increases the difficulty of non-linear fitting methods to identify and separate the components in the EELS signal. We have managed to solve this problem for silicon oxide and nitride systems by applying multivariate analysis methods that can factorize the hyperspectral datacubes in selected regions. By doing so, the EELS spectra are re-expressed as a function of abundance of Si-NC-like and dielectric-like factors. EELS contributions from the embedded nanoparticles as well as their dielectric surroundings are thus studied in a new light, and compared with the dielectric material and crystalline silicon from the substrate. Electronic properties such as band gaps and plasmon shifts can be obtained by a straightforward examination. Finally, we have calculated the complex dielectric functions and the related electron effective mass and density of valence electrons.

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells. Refereed/Peer-reviewed

The electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO 2 superlattices are studied for different silicon excess and layer thicknesses. The precipitation and crystallization of the Si excess present within the SRON layers is induced by a post-deposition annealing treatment, in order to form Si nanocrystals (Si-NCs). The electrical characterization performed in dark conditions allowed for deducing the charge transport mechanism through the superlattice structure, found to follow the Poole-Frenkel law. In addition, the EL investigation revealed the correlation between EL excitation and transport mechanisms, suggesting that impact ionization of high-energy conduction electrons dominates the whole frame. The reduction of the SiO 2 barrier thickness and the increase in the Si excess were found to enhance the carrier transport through the superlattices due to the reduction of the electrons mean free path, which, in turn, modifies the EL properties.

Ordered silicon nanocrystals in silicon carbide are produced by Plasma Enhanced Chemical Vapor Deposition by means of the multilayer approach followed by annealing at 1100 °C. The crystallization is verified by Raman scattering, X-ray diffraction, Transmission Electron Microscopy, and UV–vis spectroscopy. The conditions for the periodic structure to survive the high temperature annealing and for the SiC barrier to confine the Si crystal growth are examined by energy-filtered transmission electron microscopy and X-ray reflection. The final layout appears to be strongly influenced by the structural features of the as-deposited multilayer. Threshold values of Si-rich carbide sublayer thickness and Si-to-C ratio are identified in order to preserve the ordered structure. The crystallized fraction is observed to be correlated with the total silicon volume fraction. The constraints are examined through the use of ab-initio calculations of matrix-embedded silicon nanocrystals, as well as in terms of existing models for nanocrystal formation, in order to establish the role played by the interface energy on nanocrystal outgrowth, residual amorphous fraction, and continuous crystallization. A parameter space of formation of ordered Si nanocrystals is proposed. The diffusivity of carbon in the crystallized material is evaluated, and estimated to be around 10–16 cm2/s at 1100 °C.

In this work we present a structural, optical and electrical characterization of Si x C 1− x /SiC multilayer systems with different silicon content. After the deposition process, an annealing treatment was carried out in order to induce the silicon nanocrystals formation. By means of energy-filtered transmission electron microscopy (EFTEM) we observed the structural morphology of the multilayers and the presence of crystallized silicon nanoprecipitates for samples annealed up to 1100 °C. We discuss the suitability of optical techniques such as Raman scattering and reflectance and transmittance ( R&T ) for the evaluation of the crystalline fraction of our samples at different silicon excess ranges. In addition, the combination of R&T measurements with simulation has proved to be a useful instrument to confirm the structural properties observed by EFTEM. Finally, we explore the origin of the extremely high current density revealed by electrical measurements, probably due to the presence of an undesired defective SiC y O z ternary compound layer, already supported by the structural and optical results. Nevertheless, the variation of the electrical measurements with the silicon amount indicates a small but significant contribution from the multilayers.

For good performance of photonic devices whose working principle is based on the enhancement of electromagnetic fields obtained by confining light into dielectric resonators with dimensions in the nanometre length scale, a detailed knowledge of the optical mode structure becomes essential. However, this information is usually lacking and can only be indirectly obtained by conventional spectroscopic techniques. Here we unraveled the influence of wire size, incident wavelength, degree of polarization and the presence of a substrate on the optical near fields generated by cavity modes of individual hexagonal ZnO nanowires by combining scanning near-field optical microscopy (SNOM) with electrodynamics calculations within the discrete dipole approximation (DDA). The near-field patterns obtained with very high spatial resolution, better than 50 nm, exhibit striking size and spatial-dispersion effects, which are well accounted for within DDA, using a wavevector-dependent dipolar interaction and considering the dielectric anisotropy of ZnO. Our results show that both SNOM and DDA simulations are powerful tools for the design of optoelectronic devices able to manipulate light at the nanoscale. Fil: Goñi, Alejandro R.. Consejo Superior de Investigaciones Científicas. Instituto de Ciencia de los Materiales de Barcelona; España Fil: Güell, Frank. Universidad de Barcelona; España Fil: Pérez, Luis Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Lopez-Vidrier, Julian. Universidad de Barcelona; España Fil: Ossó, J. Oriol. Universidad de Barcelona; España Fil: Coronado, Eduardo A.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Morante, Joan R.. Universidad de Barcelona; España

The effect of the annealing temperature and the SiO2 barrier thickness of silicon nanocrystal (NC)/SiO2 superlattices (SLs) on their structural and optical properties is investigated. Energy-filtered transmission electron microscopy (TEM) revealed that the SL structure is maintained for annealing temperatures up to 1150 °C, with no variation on the nanostructure morphology for different SiO2 barrier thicknesses. Nevertheless, annealing temperatures as high as 1250 °C promote diffusion of Si atoms into the SiO2 barrier layers, which produces larger Si NCs and the loss of the NC size control expected from the SL approach. Complementary Raman scattering measurements corroborated these results for all the SiO2 and Si-rich oxynitride layer thicknesses. In addition, we observed an increasing crystalline fraction up to 1250 °C, which is related to a decreasing contribution of the suboxide transition layer between Si NCs and the SiO2 matrix due to the formation of larger NCs. Finally, photoluminescence measurements revealed that the emission of the superlattices exhibits a Gaussian-like lineshape with a maximum intensity after annealing at 1150 °C, indicating a high crystalline degree in good agreement with Raman results. Samples submitted to higher annealing temperatures display a progressive emission broadening, together with an increase in the central emission wavelength. Both effects are related to a progressive broadening of the size distribution with a larger mean size, in agreement with TEM observations. On the other hand, whereas the morphology of the Si NCs is unaffected by the SiO2 barrier thickness, the emission properties are slightly modified. These observed modifications in the emission lineshape allow monitoring the precipitation process of Si NCs in a direct non-destructive way. All these experimental results evidence that an annealing temperature of 1150 °C and 1-nm SiO2 barrier can be reached whilst preserving the SL structure, being thus the optimal structural SL parameters for their use in optoelectronics.

We use Raman scattering to investigate the size distribution, built-in strains and the crystalline degree of Si-nanoclusters (Si-nc) in high-quality Si-rich oxynitride/SiO2 multilayered samples obtained by plasma enhanced chemical vapor deposition and subsequent annealing at 1150 °C. An initial structural characterization of the samples was performed by means of energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction (XRD) to obtain information about the cluster size and the presence of significant amounts of crystalline phase. The contributions to the Raman spectra from crystalline and amorphous Si were analyzed by using a phonon confinement model that includes the Si-nc size distribution, the influence of the matrix compressive stress on the clusters, and the presence of amorphous Si domains. Our lineshape analysis confirms the existence of silicon precipitates in crystalline state, in good agreement with XRD results, and provides also information about the presence of a large compressive stress over the Si-nc induced by the SiO2 matrix. By using the Raman spectra from low temperature annealed samples (i.e., before the crystallization of the Si-nc), the relative scattering cross-section between crystalline and amorphous Si was evaluated as a function of the crystalline Si size. Taking into account this parameter and the integrated intensities for each phase as extracted from the Raman spectra, we were able to evaluate the degree of crystallization of the precipitated Si-nc. Our data suggest that all samples exhibit high crystalline fractions, with values up to 89% for the biggest Si-nc. The Raman study, supported by the EFTEM characterization, indicates that this system undergoes a practically abrupt phase separation, in which the precipitated Si-nanoclusters are formed by a crystalline inner part surrounded by a thin amorphous shell of approximately 1-2 atomic layers. © 2014 AIP Publishing LLC., The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No: 245977, under the project title NASCEnT. The present work was supported by the Spanish national project LEOMIS (TEC2012-38540-C02-01) and MAT2010-16116.

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as...

In this invited talk, we will report our latest advances in the field of nanostructured silicon-based light emitters with special emphasis in those that are rare earth doped. We have recently shown that it is possible to push external quantum efficiencies over the 10% by fine tuning the material composition and by using a new device concept that we have called hot electron engineering. Most of the rare earth amount introduced in the host material must be optically active. This can be accomplished by using a deposition method that introduces the rare earth during film growth and by a suitable passivating annealing procedure that: i) does not promote rare earth precipitation and clustering; ii) brings the rare earth to the 3+ oxidation state and iii) reduces non radiative defects. Additionally, for electroluminescent devices the matrix composition must be engineered so that it allows conduction-band transport in contrast with defective systems in which transport is hopping-like. The hot electron engineering concept relies on the fact that the rare earth excitation process in Si-based dielectric matrices proceeds mainly via impact excitation by hot electrons. Thus, devising suitable structures for which electrons can be accelerated to average energies that are resonant to the rare earth excitation energies, will improve both efficiency and maximum power emission. Additionally, driving the device with pulsed polarization and short duty cycle will improve device efficiency and lifetime. Finally, the structures must allow at the same time significant light extraction efficiency. This can be accomplished by improving the antireflective properties of the whole stack and/or by nanostructuring the ITO or polysilicon electrode. We will be show silicon oxides, nitrides and oxynitrides as hosts for the rare-earth species and, additionally, we will display the advantages and drawbacks of introducing small percentages of carbon and aluminum. The rare-earth species incorporated in those matrices are Er, Ce, Tb, Eu and Nd and their light emission efficiency depend in a complex way on matrix composition, material processing, device structure and device polarization. The simplest hot electron engineering structures are bilayers and trilayers with silicon oxide as accelerator. More complex structures like multilayers and superlattices will be also introduced and optimal designs will be proposed. Some reliability issues and potential solutions for them will be also presented. We acknowledge existing collaborations in this field with teams of the McMaster University at Hamilton (Canada), Institut de Microelectrònica de Barcelona (Spain), Nanophotonics Technology Center of Valencia (Spain), Instituto de Óptica of Madrid (Spain), Ion Beam Institute at Rossendorf (Germany), IMTEK at the University of Freiburg (Germany) and CIMAP-CNRS at Caen (France).

Charge transport and electroluminescence mechanisms in Si-rich Si oxynitride/silicon oxide (SRON/SiO2) superlattices deposited on p-type Si substrate are reported. The superlattice structures were deposited by plasma-enhanced chemical-vapor deposition and subsequently annealed at 1150 °C to precipitate and crystallize the Si excess into Si nanocrystals. The dependence of the electrical conduction on the applied voltage and temperature was found to be well described by a Poole-Frenkel transport mechanism over a wide voltage range. On the other hand, the observed dependence of the electroluminescence on the SRON layer thickness is a clear proof of quantum confinement and was attributed to an excitonic radiative recombination taking place in the confined states within the Si quantum dots. A model is proposed based on thermal hopping of electrons between the quantum dots acting as trap states (Poole-Frenkel). A correlation between carrier transport and electroluminescence has been established considering impa...

[EN] The resistive switching properties of ITO/ZnO/p-Si devices have been studied, which present well-defined resistance states with more than five orders of magnitude difference in current. Both the high resistance state (HRS) and the low resistance state (LRS) were induced by either sweeping or pulsing the voltage, observing some differences in the HRS. Finally, the charge transport mechanisms dominating the pristine, HRS, and LRS states have been analyzed in depth, and the obtained structural parameters suggest a partial re-oxidation of the conductive nanofilaments and a reduction of the effective conductive area., This work was financially supported by the Spanish Ministry of Economy and Competitiveness (Project Nos. TEC2012-38540-C02-01 and TEC2016-76849-C2-1-R). O.B. also acknowledges the subprogram "Ayudas para Contratos Predoctorales para la Formacion de Doctores" of the Spanish Ministry of Economy and Competitiveness for economical support. X.P., C.L., and C.G. are grateful to C. Frilay for his expertise in the maintenance of the sputtering kit used for the growth of the ZnO films.

Light emitting Al-Tb/SiO2 nanomultilayers (NMLs) for optoelectronic applications have been produced and characterized. The active layers were deposited by electron beam evaporation onto crystalline silicon substrates, by alternatively evaporating nanometric layers of Al, Tb, and SiO2. After deposition, all samples were submitted to an annealing treatment for 1 h in N2 atmosphere at different temperatures, ranging from 700 to 1100 °C. Transmission electron microscopy confirmed the NML structure quality, and by complementing the measurements with electron energy-loss spectroscopy, the chemical composition of the multilayers was determined at the nanoscopic level. The average composition was also measured by X-ray photoelectron spectroscopy (XPS), revealing that samples containing Al are highly oxidized. Photoluminescence experiments exhibit narrow emission lines ascribed to Tb3+ ions in all samples (both as-deposited and annealed ones), together with a broadband related to SiO2 defects. The Tb-related emission intensity in the sample annealed at 1100 °C is more than one order of magnitude higher than identical samples without Al. These effects have been ascribed to the higher matrix quality, less SiO2 defects emitting, and a better Tb3+ configuration in the SiO2 matrix thanks to the higher oxygen content favored by the incorporation of Al atoms, as revealed by XPS experiments.