Due to the weak thermal and chemical stability of organic resins which are used for conventional white LEDs to embed phosphors, inorganic color converters such as phosphor ceramics and phosphor-in-glasses are currently being used to replace conventional color converters based on organic materials, especially for high power and high brightness applications. In this paper we report on the study of sintered glass ceramics based on low melting glass in which commercial YAG:Ce3+ phosphors are embedded. A low Tg is necessary to avoid high temperature sintering which can damage the optical properties of the embedded phosphors. Two different types of glass have been studied: borosilicate and tellurite. The compositions have been optimized in terms of stability, sintering efficiency and thermal conductivity. Selected samples were optical charterized using a GaN high power multimode 450 nm Laser Diode, with a maximum output power of 1.6 W at 1.5 A.
We investigated both the effect of high irradiation density and high operating temperature, as well as their color-rendering index. The sintered glass ceramic based on borosilicate glass showed better high power stability because of its higher thermal conductivity.
Within this paper, we summarize some of the degradation mechanism that still affect GaN-based optoelectronic devices. The most common source of the degradation is the creation of lattice defects, which lower the optical efficiency due to their role as non-radiative recombination centers, as proven in the case of UV-B LEDs. The local generation of defects is not the only possibility, with diffusion of impurities (possibly hydrogen from the p-side) being shown to be the limiting factor in the case of green laser diodes. Under extreme bias conditions, such as the EOS events, the robustness of the current carriers and spreading structures is critical, as shown by failure of bonding wires, metal lines and vias in white LEDs. In every optoelectronic device photons themselves possess an energy at least equal to the bandgap, and can be an additional source of degradation that cannot be eliminated.
With this work we report on the design, development and testing of near UV LED-based systems for oxygen gas sensing. The design and developed system is an optoelectronic setup based on 405 nm LEDs which excites and measures the photoluminescence emitted from a porphyrin based luminophor. By means of an accurate optical and optoelectronic setup, the system is able to operate without the need of avalanche photodiodes, thus resulting in a compact and low energy structure. The optical setup is specifically designed to maximize both the LED light exciting the luminophor and converted light acquired from the sensor.
We built a multi-channel led starlight simulator capable to reproduce the radiation of stars of F, G, K and M spectral types in the wavelength range 365-940 nm. This range overlaps the photosynthetic active interval allowing us to use the simulator for biological experiments under radiation and atmospheric conditions close as much as possible to those expected on extrasolar planets. The simulator is a laboratory tool which is part of the “Atmosphere in a Test Tube” project, aimed to study the photosynthetic efficiency of bacteria under alien environmental conditions and their possible impact on the atmosphere of the host planet. This paper describes the software developed to control the simulator. We begin by presenting a conceptual overview of the instruments and then illustrating the top-level requirements and the architecture of the control software. Finally, we give a description of the graphical user interface.
We report on the design and study of solid state laser sources for lighting applications. While LEDs are affected by droop, limiting efficacy at higher currents, a possible solution is represented by solid state laser lighting, where a blue (450nm) laser is exciting a luminescent material thus achieving white light. With this work we designed and tested several LARP (Laser-Activated-Remote-Phosphors) test structures, both diffused lighting and focused applications will be discussed. Results indicates that good efficiency are achievable, without any sensible droop also at high injection currents. Phosphors have also been subjected to thermal stability tests up to 550°C.
This paper reports on an extensive investigation on the degradation mechanisms that may limit the long term reliability of heterogeneous III-V/Silicon DBR laser diodes for integrated telecommunication applications in the 1.55 μm window. The devices under test, aged for up to 500 hours under different bias conditions, showed a gradual variation of both optical (L-I) and electrical (I-V, C-V) characteristics. In particular, the laser diodes exhibited an increase in the threshold current, a decrease of the turn-on voltage and an increase in the apparent charge density within the space-charge region, which was extrapolated from C-V measurements. For longer stress times, these two latter processes were found to be well correlated with the worsening of the optical parameters, which suggests that degradation occurred due to an increase in the density of defects within the active region, with consequent decrease in the non-radiative (SRH) lifetime. This conclusion is also supported by the fact that during stress the apparent charge profiles indicated a re-distribution of charge within the junction. A preliminary investigation on the physical origin of the defects responsible for degradation was carried out by DLTS measurements, which revealed the presence of five different deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap was found to be compatible with an interface defect located between the In<sub>0.53</sub>Al<sub>x</sub>Ga<sub>0.47-x</sub>As SCH region and the InP layer.
This paper demonstrates that when InGaN LEDs are submitted to a constant reverse bias, they can show a time-dependent breakdown, that leads to the catastrophic failure of the devices. By submitting green and blue LEDs to constant voltage stress in the range between -40 V and -60 V we demonstrate that: (i) under reverse bias conditions, current is focused on localized paths, whose positions can be identified by electroluminescence measurements, and that originate from the presence of extended defects; (ii) during a constant voltage stress, the reverse current of the LEDs gradually increases; (iii) for longer stress times, all devices show a time-dependent breakdown; (iv) time-to-failure has an exponential dependence on stress voltage, and is Weibull-distributed.
GaN-HEMTs with p-GaN gate have recently demonstrated to be excellent normally-off devices for application in power conversion systems, thanks to the high and robust threshold voltage (V<sub>TH</sub>>1 V), the high breakdown voltage, and the low dynamic Ron increase. For this reason, studying the stability and reliability of these devices under high stress conditions is of high importance. This paper reports on our most recent results on the field- and time-dependent degradation of GaN-HEMTs with p-GaN gate submitted to stress with positive gate bias. Based on combined step-stress experiments, constant voltage stress and electroluminescence testing we demonstrated that: (i) when submitted to high/positive gate stress, the transistors may show a negative threshold voltage shift, that is ascribed to the injection of holes from the gate metal towards the p-GaN/AlGaN interface; (ii) in a step-stress experiment, the analyzed commercial devices fail at gate voltages higher than 9-10 V, due to the extremely high electric field over the p-GaN/AlGaN stack; (iii) constant voltage stress tests indicate that the failure is also time-dependent and Weibull distributed. The several processes that can explain the time-dependent failure are discussed in the following.
With this work we report on the design of an LED based star simulator. The simulator is the result of a cooperation
between the Italian National Astrophysics Institute and LightCube SRL, a University of Padova (Italy) R&D spin-off.
The simulator is designed to achieve a luminous output customizable both in spectrum and in intensity. The core of the
system is a 25 channels independent LED illuminator specifically designed to replicate the spectral emission of the
desired star. The simulated star light intensity can also be carefully tuned to achieve the correct illuminance at a specific
distance from the star.
This paper critically reviews the most relevant failure modes and mechanisms of InGaN LEDs for lighting application. At chip level, both the epitaxial heterostructure and the ohmic contacts may be affected. This may result in: (i) the formation of defects within the active region, resulting in the increase of non-radiative recombination and leakage current, (ii) the reduction of the injection efficiency consequent to increased trap-assisted tunneling, (iii) the degradation of contact resistance with increase of forward voltage. Package-related failures – not described in this paper - include
(iv) thermally-activated degradation processes, affecting the yellow phosphors, the plastic package or the encapsulating materials and (v) darkening of the Ag package reflective coating, the latter due to chemical reaction with contaminants as Cl or S. In order to enucleate and study the different physical failure mechanisms governing device degradation, single quantum well (SQW) blue LEDs, InGaN laser structures and commercially-available white LEDs to high temperature and/or high current density have been submitted to accelerated testing at high temperature and high current density.
The thermal droop (reduction of the optical power when the temperature is increased) is a phenomenon that strongly
limits the efficiency of InGaN-based light-emitting diodes. In this paper we analyze the role of Shockley-Read-Hall
(SRH) recombination and of the electron blocking layer (EBL) in the process by using numerical simulations and
literature data. The benefic impact of EBL suggests that carrier escape from the quantum wells gives a significant
contribution to the thermal droop, therefore we review some of the mechanisms described in the literature (thermionic
emission, phonon-assisted tunneling, thermionic trap-assisted tunneling). Since no formulation is able to fit the behavior
of the measured SQW devices, we develop a new model based on two phonon-assisted tunneling steps through a
defective state, extended in order to take into account zero-field emission. By using experimental data, material constants
from the literature and only two fitting parameters the model is able to reproduce the experimental behavior.
We discuss some of the key issues to be addressed along the way to complement, and possibly to replace, the standard semiclassical Boltzmann picture with genuine quantum approaches for the simulation of carrier transport and recombination in GaN-based LEDs, with the goal of gradually removing the fitting parameters presently required by semiempirical "quantum corrections" and to better understand the processes responsible for the efficiency droop. As examples of augmented semiclassical models, we present a three-step description of trap-assisted tunneling, especially relevant below the optical turn-on, and a carrier-density-dependent estimate of the phonon-assisted capture rate from bulk states to quantum wells (QWs). Moving to genuine quantum models, we solve the semiconductor Bloch equations to calculate the gain/absorption spectra of AlGaN/GaN QWs, and we discuss our first simulations of spatially and energetically resolved currents across the active region of a single-QW LED based on the nonequilibrium Green’s function approach.
In the last years, a lot of extrasolar planets have been discovered in any direction of the Galaxy. More interesting, some of them have been found in the habitable zone of their host stars. A large diversity of spectral type, from early types (A) to colder ones (M), is covered by the planetary system host stars. A lot of efforts are done in order to find habitable planets around M stars and indeed some habitable super earths were found. In this framework, “Atmosphere in a Test Tube”, a project started at Astronomical observatory of Padua, simulates planetary environmental condition in order to understand how and how much the behavior of photosynthetic bacteria in different planetary/star scenarios can modify the planet atmosphere. The particular case of an habitable planet orbiting a M dwarf star is under study for the time being. The irradiation of an M star, due to its lower surface temperature is very different in quality and quantity by the irradiation of a star like our Sun. We would like to describe the study of feasibility of a new kind of tunable led stellarlight simulator capable to recreate the radiation spectrum of M type stars (but with the potential to be expanded even to F, G, K star spectra types) incident on the planet. The radiation source is a multiple LED matrix cooled by means of air fan technology. In order to endow it with modularity this device will be composed by a mosaic of circuit boards arranged in a pie-chart shape, on the surface of which will be welded the LEDs. This concept is a smart way in order to replace blown out pieces instead of changing the entire platform as well as implement the device with new modules suitable to reproduce other type of stars. The device can be driven by a PC to raise or lower the intensity of both each LED and the lamp, in order to simulate as close as possible a portion of the star spectrum. The wavelength intervals overlap the limits of photosynthetic pigment absorption range (280-850 nm), while the range of the radiation source will be between 365 nm and 940 nm. The reason why we chose a higher outer limit is that M stars have the emission peak at about 1000 nm and we want to study the effects of low-light radiation on bacterial vitality. The innovative concept behind this radiative source is the use of the LED components to simulate the main stellar absorption lines and to make this a dynamic-light. Last but not least the use of LED is crucial to keep the device compact and handy. This device could help us to better understand the link between radiation and NIR-photosynthesis and could find applications in the field of photobioreactors as a test bench for the choice of the wavelength to be used in order to maximize the production rate. Other fields of application are the microscopy light sources field and the yeasts growth sector.
This paper reviews the properties of the defects which limit the performance and the reliability of LEDs based on InGaN. More specifically we discuss: (i) the origin and properties of the defects responsible for SRH recombination; (ii) the role of defects in favoring the degradation of InGaN-based LEDs. Original data are compared to previous literature reports to provide a clear understanding of the topic.
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.
Color temperature, intensity and blue spectrum of the light affects the ganglion receptors in human brain stimulating the human nervous system. With this work we review different methods for obtaining tunable light emission spectra and propose an innovative white LED lighting system. By an in depth study of the thermal, electrical and optical characteristics of GaN and GaP based compound semiconductors for optoelectronics a specific tunable spectra has been designed. The proposed tunable white LED system is able to achieve high CRI (above 95) in a large CCT range (3000 - 5000K).
The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: <i>(i)</i> a significant increase in the photoluminescence efficiency of the devices; <i>(ii)</i> a decrease in the parasitic emission bands located between 380 nm and 400 nm; <i>(iii)</i> an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples.
With this work we report on the performance and degradation mechanism of commercially available remote phosphors (RP) for SSL. Thermal analysis indicates that phosphors can reach temperatures above 60°C during operation at an ambient temperature of 25°C when subjected to an optical power of 346 mW/cm2. We also demonstrate that temperature is a strong driving force for the degradation. Results indicate a gradual reduction in luminous flux output and a decrease of correlated color temperature as a consequence of stress. We demonstrate that the degradation rate is strongly correlated with stress temperature with an activation energy of 1.36 eV for a TTF of 70%.
Recent studies demonstrated that degradation of InGaN-based laser diodes is due to an increase in non-radiative
recombination rate within the active layer of the devices, due to the generation of defects.
The aim of this paper is to show - by DLTS - that the degradation of InGaN-based laser diodes is strongly correlated to
the increase in the concentration of a deep level located within the active region. The activation energy of the detected
deep level is 0.35-0.45 eV. Hypothesis on the nature of this deep level are presented in the paper.
With this work we propose an innovative method for the analysis of the reliability of LED and Laser devices and
systems. The basic idea of the proposed method is the separation of the different degradation forces that lead to the
decrease of LED performances during ageing. By using a specific reliability analysis procedure it is possible to
separately evaluate the effects of the single accelerating factors: temperature, current intensity, applied signal waveform,
voltage overstress, optical and mechanical solicitation. To individually determine the degradation kinetics it is
fundamental to separate the effects of temperature and current. For these reasons we carried out iso-currents reliability
tests, where several devices have been stressed with the same current at different junction temperatures, and iso-thermal
stresses, where junction temperature is instead constant for different applied currents. The result of the analysis will be a
multivariable law that relates the several degradation parameters in the form of degradation kinetics. This will allow the
estimation of the devices lifetime for a very wide operating conditions region. During degradation an extensive set of
measurements have been carried out at fixed steps in the form of photometric, optical, electrical, capacitive, mechanical
and thermal characterization. The combination of these results allows the understanding of what degradation
mechanisms are taking place and therefore it is a fundamental tool to improve system reliability. Degradation has also
been studied by analyzing catastrophic damages by means of failure analysis; the failure investigation is useful for the
catastrophic damage: melting of bonding wire, contacts evaporation, facet melting (for laser diodes).
With this paper we describe recent results on the physical mechanisms responsible for the gradual degradation
of GaN-based laser diodes and Light-Emitting Diodes (LEDs). The results described in the following were obtained by
means of an extensive electrical and optical characterization of laser diodes and LEDs submitted to accelerated stress
conditions. The experimental evidence described within this paper demonstrate that: <i>(i)</i> during stress, the threshold
current of laser diodes can significantly increase, possibly due to a diffusion-related process; <i>(ii)</i> slope efficiency of laser
diodes does not significantly change as a consequence of stress; <i>(iii)</i> LED samples - with the same epitaxial structure of
laser diodes - show a significant decrease in optical power during stress time; degradation is more prominent at low
measuring current levels, suggesting that it is due to the increase in non-radiative recombination; <i>(iv)</i> the worsening of
the optical characteristics of LEDs and laser diodes is significantly correlated to the increase in the defect-related current
components. Results described within this paper strongly support the hypothesis that the degradation of laser diodes and
LEDs submitted to stress at high current densities (>4 kA/cm<sup>2</sup>) is due to the increase in the concentration of defects
within the active layer of the devices, activated by the high flux of accelerated carriers through the quantum-well region.
Over the last years, important efforts have been done in order to understand the degradation mechanisms of GaN-based
LEDs submitted to forward-bias stress tests. On the other hand, only little work has been done to understand the
degradation of LEDs submitted to reverse-bias stress. However, this topic is of high interest, since (i) the reverse-bias
robustness of the LEDs is strongly correlated to their stability under Electrostatic Discharge (ESD) events and (ii) the
analysis of the reverse-bias degradation can provide important information on the role of high electric fields and reverse
current in limiting the reliability of the LEDs.
Therefore the aim of this paper is to describe a detailed investigation on the reverse-bias degradation of GaN-based
LEDs. The results described in this paper indicate that: (i) under reverse bias, LEDs can show a weak luminescence
signal, due to the recombination of carriers injected in the quantum-wells; (ii) reverse-bias stress can induce the
degradation of the electrical characteristics of the LEDs (increase in reverse-current, decrease in breakdown voltage),
due to the generation of point defects in proximity of pre-existing defective regions. (iii) Furthermore, our tests indicate
that the defective regions responsible for reverse-current conduction can constitute weak points with respect to ESD
events: ESD failures are determined by the shortening of the junction in proximity of one of the defective sites
responsible for reverse-current conduction.