Modeling the electrical characteristics of InGaN/GaN LED structures based on experimentally-measured defect characteristics

Defects can significantly modify the electro-optical characteristics of InGaN light-emitting diodes (LEDs); however, modeling the impact of defects on the electrical characteristics of LEDs is not straightforward. In this paper, we present an extensive investigation and modeling of the impact of defects on the electrical characteristics of InGaN-based LEDs, as a function of the thickness of the quantum well (QW). First, we demonstrate that the density of defects in the active region of III-N LEDs scales with increasing thickness of the InGaN QW. Since device layers with high indium content tend to incorporate more defects, we ascribed this experimental evidence to the increased volume of defects-rich InGaN associated to thicker InGaN layers. Second, we demonstrate that the current-voltage characteristics of the devices are significantly influenced by the presence of defects, especially in the sub turn-on region. Specifically, we show that the electrical characteristics can be effectively modeled in a wide current range (from pA to mA), by considering the existence of trap-assisted tunneling processes. A good correspondence is obtained between the experimental and simulated electrical characteristics (I-V), by using-in the simulation-the actual defect concentrations/activation energies extracted from steady-state photocapacitance, instead of generic fitting parameters

Defects in III-N LEDs: experimental identification and impact on electro-optical characteristics

III-N light-emitting-diodes (LEDs) are subject of intense investigations, thanks to their high efficiency and great reliability. The quality of the semiconductor material has a significant impact on the electro-optical performance of LEDs: for this reason, a detailed characterization of defect properties and the modeling of the impact of defects on device performance are of fundamental importance. This presentation addresses this issue, by discussing a set of recent case studies on the topic; specifically, we focus on the experimental characterization of defects, and on the modeling of their impact on the electro-optical characteristics of the devices

Public participation in the implementation of the Natura 2000 network in Italy: the stakeholders’ experiences

Public participation in the implementation of the Natura 2000 network in Italy: the stakeholders’ experiences. Natura 2000 is an ecological network of protected areas identified by the Habitats Directive (92/43/EEC) and the Birds Directive (79/409/EC) - recently replaced by the Directive 2009/147/EC - in order to ensure the long-term protection of endangered species in their natural habitats in European Union (EU) territory. EU Member States are responsible for developing and implementing the procedures defined by Habitats Directive, but there are no specific recommendations about participation of stakeholders and local community in the decision-making process. Consequently, each country has adopted a different participatory strategy taking into account the principles of integration approach. The integration approach is based on combining human activities and nature conservation purposes in the same area or, at least, in areas in close proximity. In Italy, the implementation of Natura 2000 network was developed at local level (Regions and Autonomous Provinces) using different approaches and procedures. Starting from these considerations, the aim of the study was to analyse the stakeholders’ involvement process during the implementation of Habitats Directive in Italy and the management of Natura 2000 sites. This study was realized using three main criteria with the respective indicators: (1) inclusiveness of participatory process; (2) democracy of participatory process; (3) cooperation and conflicts during the implementation process of Natura 2000 network. The data were collected through the administration by email of a semi-structured questionnaire to 56 stakeholders divided in four main groups of interest (public administrations, universities and research centres, environmental associations, private organizations). The results of the survey show that the participatory process was characterized by a low level of inclusiveness despite the existence of some best practices at local level. In many cases, the decisions were taken by a low number of stakeholders - mostly public administrations - while the individuals of local community were only informed. Conversely, the results highlights as positive aspect the increasing level of trust among stakeholders after the Natura 2000 network implementation process

Management of Natura 2000 sites in Italy: An exploratory study on stakeholders’ opinions

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An interdisciplinary research project aimed at understanding and controlling blooms of cyanobacteria in Lake Ledro (eastern Italian Alps)

In the last years, blooms of Planktothrix rubescens were observed in several lakes undergoing re-oligotrophication. Lake Ledro (zmax=48 m) is a mesotrophic, hardwater lake (650 m a.s.l.) located in SW Trentino (Eastern Italian Alps). It is an impacted lake exploited for hydropower production. In the 70s and the 80s the lake suffered from eutrophication, but in the past two decades water quality improved most likely as consequence of the introduction of a waste water treatment plant. Despite this, the cyanobacteria contribution increased and a very intense bloom of Planktothrixrubescens was observed during the 2009 fall and subsequent winter and spring. In an attempt to solve this problem, the Autonomous Province of Trento funded a multidisciplinary research project to study the environmental and biotic factors promoting the development of Planktothrix populations. The annual nutrient load from the catchment area was calculated from water balance and surface and groundwater nutrient inputs, with the identification of the possible sources of nutrients. Environmental variables were acquired on discrete samples and in situ by multiparameter probes and instrumentations fixed on the principal inlet, in piezometers and on a platform, equipped with meteorological 59 instrumentation, moored to the lake bottom. A hydrodynamic and water quality model was adopted to simulate water quality conditions in the lake during scenarios of decreased phosphorus loading and a combined modification of parameters like hydroelectric withdrawal and depth of the intake. Paleolimnological investigations have been conducted aimed at the definition of reference conditions and, through cyanobacterial pigments, at the evaluation of secular fluctuations of Planktothrix in relation to climatic variability, human impact and lake basin management. During stratification, Planktothrix mainly developed in 2-4 m thick water layers located just below the euphotic depth (10-18 m in summer), with biovolumes ranging between 4 and 12 mm3 l-

Modeling the electrical characteristics of InGaN/GaN LED structures based on experimentally-measured defect characteristics

Defects can significantly modify the electro-optical characteristics of InGaN light-emitting diodes (LEDs); however, modeling the impact of defects on the electrical characteristics of LEDs is not straightforward. In this paper, we present an extensive investigation and modeling of the impact of defects on the electrical characteristics of InGaN-based LEDs, as a function of the thickness of the quantum well (QW). First, we demonstrate that the density of defects in the active region of III-N LEDs scales with increasing thickness of the InGaN QW. Since device layers with high indium content tend to incorporate more defects, we ascribed this experimental evidence to the increased volume of defects-rich InGaN associated to thicker InGaN layers. Second, we demonstrate that the current-voltage characteristics of the devices are significantly influenced by the presence of defects, especially in the sub turn-on region. Specifically, we show that the electrical characteristics can be effectively modeled in a wide current range (from pA to mA), by considering the existence of trap-assisted tunneling processes. A good correspondence is obtained between the experimental and simulated electrical characteristics (I-V), by using-in the simulation-the actual defect concentrations/activation energies extracted from steady-state photocapacitance, instead of generic fitting parameters 2021 IOP Publishing Ltd

Deep defects in InGaN LEDs: modeling the impact on the electrical characteristics

Deep defects have a fundamental role in determining the electro-optical characteristics and in the efficiency of InGaN light-emitting diodes (LEDs). However, modeling their effect on the electrical characteristics of the LED is not straightforward. In this paper we analyze the impact of the defects on the electrical characteristics of LEDs: we analyze three single-quantum-well (SQW) InGaN/GaN LED wafers, which differ in the density of defects. Through steady-state photocapacitance (SSPC) and light-capacitance-voltage measurements, the energy levels of these deep defects and their concentrations have been estimated. By means of a simulation campaign, we show that these defects have a fundamental impact on the current voltage characteristic of LEDs, especially in the sub turn-on region. The model adopted takes into consideration trap assisted tunneling as the main mechanism responsible for current leakage in forward bias. For the first time, we use in simulations the defect parameters (concentration, energy) extracted from SSPC. In this way, we can reproduce with great accuracy the current-voltage characteristics of InGaN LEDs in a wide current range (from pA to mA). In addition, based on SSPC measurements, we demonstrate that the defect density in the active region scales with the QW thickness. This supports the hypothesis that defects are incorporated in In-containing layers, consistently with recent publications

UV LED reliability: degradation mechanisms and challenges

The market for UV LEDs is experiencing a rapid growth, also driven by the need for effective and efficient disinfection systems. Before UV LEDs can be widely accepted by the market, they need to demonstrate a high reliability, with lifetimes of several thousands of hours. Several physical processes may limit the reliability of UVB and UVC LEDs, resulting in a loss in efficiency during long term operation.This paper aims at discussing the most relevant processes that can lead to the degradation of UVB and UVC LEDs, with focus on: (i) instability of the electrical properties, which may result in gradual changes in the turn-on voltage of the devices during long-term operation. (ii) The generation of defects within the active region of the devices, with consequent increase in the Shockley- Read-Hall non-radiative recombination rate. Optical spectroscopy is found to be very effective for the identification of deep (midgap) traps during operation of the devices. (iii) trap states near the junction, with consequent impact on trap-assisted-tunneling of the current-voltage characteristics. (iv) the propagation of point defects, especially impurities, and accumulation of charges at heterointerfaces, that can reduce the carrier injection efficiency, thus leading to a decrease in the emitted optical power