Photoinduced IR absorption in (La(1-x)Sr(x)Mn)(1-\delta)O3: changes of the anti-Jahn-Teller polaron binding energy with doping

Photoinduced IR absorption was measured in (La(1-x)Sr(x)Mn)(1-\delta)O3. A midinfrared peak centered at ~ 5000 cm$^{-1}$ was observed in the x=0 antiferromagnetic sample. The peak diminishes and softens as hole doping is increased. The origin of the photoinduced absorption peak is atributted to the photon assisted hopping of anti-Jahn-Teller polarons formed by photoexcited charge carriers, whose binding energy decreases with increasing hole doping. The shape of the peak indicates that the polarons are small.Comment: 5 pages, 3 figures, submitted to PR

2021 roadmap on lithium sulfur batteries

Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK's independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space

Processing parameters for ZnO-based thick film varistors obtained by screen printing

Thick film varistors based on the ZnO-Bi2O3-Sb2O3 system have been prepared by screen printing on dense alumina substrates. Different processing parameters like the paste viscosity, burn out and sintering cycles, green and sintered thickness, have been studied to improve the processing of ZnO-based thick film varistors. Starting powders were pre-treated in two different ways in order to control both the Bi-rich liquid phase formation and the excessive volatilization of Bi2O3 during sintering due to the high area/volume ratio of the thick films. Significant changes have been observed in the electrical properties related to the different firing schedule and selection of the starting powders.<br><br>Se han preparado varistores basados en el sistema ZnO-Bi2O3-Sb2O3 en forma de lámina gruesa sobre sustratos de alúmina densa. Diferentes parámetros del procesamiento como la viscosidad de la pasta, los ciclos de calcinación y sinterización y el espesor en verde y sinterizado han sido estudiados para mejorar el procesamiento de los varistores basados en ZnO preparados en forma de lámina gruesa. Los polvos de partida fueron pretratados de dos formas diferentes con el objetivo de controlar la formación de la fase líquida rica en bismuto y la excesiva volatilización de Bi2O3 durante la sinterización debida a la alta relación área-volumen de las láminas gruesas. Se han observado cambios significativos en las propiedades eléctricas relacionadas con los diferentes ciclos de calcinado y con la selección de los polvos de partida

Microscopic-scale investigation of the degradation of InGaN-based laser diodes submitted to electrical stress

This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by microcathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i.e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with subbandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defects involved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements

Degradation of InGaN/GaN laser diodes investigated by micro-cathodoluminescence and micro-photoluminescence

We present an investigation of the degradation of InGaN/GaN laser diodes grown on a GaN substrate. The results indicate that: (i) Ageing induces a significant increase in the threshold current (Ith) of the lasers, which is attributed to an increase in non-radiative recombination; (ii) Ith increase is correlated to a decrease in the micro-cathodoluminescence signal measured (after the removal of the top metallization) in the region under the ridge; (iii) micro-photoluminescence measurements indicate that constant current stress increases non-radiative recombination within the quantum wells (and not only within the barriers), and induces an increase in the emission wavelength of the degraded region. VC 2013 AIP Publishing LLC

Microscopic-scale investigation of the degradation of InGaN-based laser diodes submitted to electrical stress

This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by micro-cathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i. e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with sub-bandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defects involved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements

A combined µ-Cathodoluminescence and µ-Photoluminescence Investigation of the Degradation of InGaN/GaN Laser Diodes

Despite the excellent potential of InGaN laser diodes (LDs), the reliability of these devices is still limited by a number of factors. The parametric degradation (i.e. the gradual increase in threshold current, Ith, induced by electrical stress) has been ascribed to an increase in non-radiative recoInbination within the active region [ToIniya2010, Meneghini2010], or to a decrease in the injection efficiency [Marona2008]; however, from the experiments presented so far in the literature (mostly based on EL measurements), it is difficult to distinguish between these two hypothesis, since both mechanisms produce siInilar effects on the EL characteristics of LDs. The aiIn of this paper is to contribute to the understanding of the degradation of InGaN LDs, by presenting the first study based on combined micro-CL and micro-PL measurements, carried out on untreated and aged devices. Results strongly suggest that degradation originates froIn an increase in non-radiative recoInbination within the QWs, and provide information on the geometry and si7:e of the degraded regions. The results of the stress tests indicate that: (i) when submitted to constant current stress, the LDs show a gradual increase in Ith; Ith increase has a power law dependence on time, which is consistent with previous literature studies, and suggests that degradation proceeds through a diffusion process; (ii) the Ith increase is well correlated to the decrease in sub-threshold emission, indicating that that degradation is related to an increase in the losses within the active region. To understand the physical origin of degradation, we removed the ridge Inetallization fronl a set of untreated and aged lasers, and we characteri7:ed the devices by means of f.LCL measurements (Ebeam=20 keY). Monochromatic CL maps (measured at 430 nm, i.e. at the QW peak) indicate that stress induces a significant decrease in the radiative efficiency of the LDs; this occurs only in the region close to the ridge, i.e. in the area where the current density (and heat dissipation) is maximum (>4 kA/cm2) during stress. From CL measurements it is difficult to understand if degradation occurs only in the barriers (where carriers are mostly generated with electron beaIn excitation), or also in the QWs. To give an answer to this question, we carried out resonant micro-PL mapping(380 nm excitation): also in this case results demonstrated the existence of a significant darkening of the region close to the ridge, thus confirming that stress induced a worsening of the radiative efficiency of the QWs (and not only in the barriers). In sUInInary, results strongly suggest that the degradation of the LDs is related to an increase in non-radiative recombination, rather than to a variation in injection efficiency, and occurs also in the QWs, and not only in the barriers: in fact, by It-PL analysis, we measured a significant darkening of the ridge region even with sub-bandgap excitation, i.e. by directly generating carriers only in the QWs

Microscopic-scale investigation of the degradation of InGaN-based laser diodes submitted to electrical stress

This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by microcathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i.e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with subbandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defectsinvolved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements

2021 roadmap on lithium sulfur batteries

Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK's independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space