Diego Barbisan

Analysis of the physical processes responsible for the degradation of deep-ultraviolet light emitting diodes

This paper reports an extensive analysis of the degradation of deep-ultraviolet light-emitting diodes submitted to dc stress test. The study was carried out by means of combined electrical and optical characterization techniques. Results described in the paper indicate that the following: (i) stress can induce a significant decrease in the optical power emitted by the devices; (ii) optical power decrease is more prominent at low measuring current levels, thus suggesting that degradation is related to an increase in the concentration of defects; (iii) stress induces a significant increase in the green-yellow parasitic emission of the devices; and (iv) stress causes a localized increase in the apparent charge distribution profiles in the active region of the devices. Experimental evidence collected within this work suggest that degradation is due to an increase in the defectiveness in the active layer of the devices, with subsequent worsening of their radiative efficiency.

Study and Development of a Fluorescence Based Sensor System for Monitoring Oxygen in Wine Production: The WOW Project

The importance of oxygen in the winemaking process is widely known, as it affects the chemical aspects and therefore the organoleptic characteristics of the final product. Hence, it is evident the usefulness of a continuous and real-time measurements of the levels of oxygen in the various stages of the winemaking process, both for monitoring and for control. The WOW project (Deployment of WSAN technology for monitoring Oxygen in Wine products) has focused on the design and the development of an innovative device for monitoring the oxygen levels in wine. This system is based on the use of an optical fiber to measure the luminescent lifetime variation of a reference metal/porphyrin complex, which decays in presence of oxygen. The developed technology results in a high sensitivity and low cost sensor head that can be employed for measuring the dissolved oxygen levels at several points inside a wine fermentation or aging tank. This system can be complemented with dynamic modeling techniques to provide predictive behavior of the nutrient evolution in space and time given few sampled measuring points, for both process monitoring and control purposes. The experimental validation of the technology has been first performed in a controlled laboratory setup to attain calibration and study sensitivity with respect to different photo-luminescent compounds and alcoholic or non-alcoholic solutions, and then in an actual case study during a measurement campaign at a renown Italian winery.

Characterization and endurance study of aluminate/silicate/garnet/nitride phosphors for high-performance SSL

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%.

ESD characterization of multi-chip RGB LEDs

Abstract In this paper we present an extensive analysis of the failure mechanisms of RGB (multi-chip) LEDs submitted to ESD testing: the tests have been carried out on several commercially available LEDs of three different suppliers. In order to better understand the failure mechanisms, we have submitted LEDs to ESD tests under reverse and forward bias condition separately, by means of a Transmission Line Pulser (TLP). The experimental results indicate that: (i) red LEDs (based on AlInGaP) have an higher ESD robustness with respect to green and blue samples (based on InGaN), both under reverse and under forward bias test; (ii) TLP negative pulses with a current smaller than the failure threshold can induce a decrease of the leakage current in GaN-based LEDs, due to a partial annihilation of defective paths responsible for reverse conduction; (iii) typical failure mechanism of devices is represented by a catastrophic event, with short-circuiting of the junction. Moreover, some of the analyzed red LEDs had shown “soft” failure, with gradual increase of the leakage current and corresponding decrease of the optical power, even without a catastrophic damage.

Control software for the Multi-Channel Led starlight simulator

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.

Adaptive multi-wavelength LED star simulator for space life studies

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.

Effects and exploitation of tunable white light for circadian rhythm and human-centric lighting

After an introduction on the effects of light on human wellness, circadian rhythm and perception of objects with this work we propose an innovative white LED lighting system able to smoothly change CCT (Correlated Color Temperature) while sensibly changing the spectral power density in the blue region (460-480 nm). Designing methodology, photometrical, electrical, optical and thermal results will be discussed. The proposed tunable white LED system is able to achieve high CRI (Color Rendering Index) (above 90) in a large CCT range (2800 - 5500 K), thus allowing a high comfort light in different conditions. Results from thermal design indicate a maximum junction temperature of 85°C at 700 mA, with an equivalent thermal resistance junction to ambient of 1.6 K/W for the series of LEDs used in the light engine. The optical emission has been designed by means of a color mixing chamber and a parabolic reflector thus achieving a good color uniformity at different emission angles and 18° beam divergence. The Light engine is engineered to be efficient (above 85 lm/W) and cost effective while working with a standard commercially available constant current LED driver. The specific control gear can be programmed to control the intensity and CCT of several lamps connected together.

Defects in GaN-based LEDs: impact on internal quantum efficiency and on reliability

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.

A tunable integrated system to simulate colder stellar radiation

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.

Thermal, optical, and electrical engineering of an innovative tunable white LED light engine

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).