1,720,976 research outputs found
Analysis of defects and physical mechanisms that limit the ESD robustness of Light Emitting Diodes
Full text linkThis thesis reports the main results obtained from the Ph.D. research activity of the candidate. The activity was focused on the study of defects and physical mechanisms that limit the ESD robustness of Light Emitting Diodes (LEDs). In particular, most of the research activity was mainly focused on the analysis of GaN-based LEDs, which are the basis for the realization of blue and UV emitters, and white LEDs based on phosphor conversion.
After an initial overview on the most important theoretical concepts necessary for the understanding of physical results, four main sections can be identified in this thesis, concerning the presentation of research activity:
- First, we report an extensive analysis of the defect-related localized emission processes occurring in InGaN/GaN-based light emitting diodes at low reverse and forward-bias conditions. The analysis is based on combined electrical characterization and spectrally and spatially resolved electroluminescence (EL) measurements. Results of this analysis show that: (i) under reverse bias, LEDs can emit a weak luminescence signal, which is directly proportional to the injected reverse current. Reverse-bias emission is localized in submicrometer-size spots; the intensity of the signal is strongly correlated to the threading dislocation (TD) density, thus suggesting that TDs are preferential paths for leakage current conduction. (ii) Under low forward-bias conditions, the intensity of the EL signal is not uniform over the device area. Spectrally resolved EL analysis of green LEDs identifies the presence of localized spots emitting in the yellow spectral region, whose origin is ascribed to localized tunneling occurring between the quantum wells and the barrier layers of the diodes, with subsequent defect-assisted radiative recombination.
- Afterwards, we propose an extensive study of the electroluminescence characteristics of InGaN-based LEDs with color-coded structure, i.e. with a triple quantum well structure in which each quantum well has a different indium content, in order to analyze the carrier distribution inside the quantum wells of the active region. The analysis is based on combined electroluminescence measurements and two-dimensional simulations, carried out at different current and temperature levels. Results indicate that: (i) the efficiency of each of the quantum wells strongly depends on device operating conditions (current and temperature); (ii) at low current and temperature levels, only the quantum well closer to the p-side has a significant emission; (iii) emission from the other quantum wells is favored at high current levels. The role of carrier injection, hole mobility, carrier density and non-radiative recombination in determining the relative intensity of the quantum wells is also discussed.
- At this point, we propose the results obtained from the analysis of physical mechanisms that limit the ESD robustness of GaN-based LEDs. The analysis was carried out on several LED families with different ESD robustness. Each of analyzed sample family is characterized by two different parameters: the failure rate measured after the application of a single ESD pulse, named First level failure F1, and the failure rate measured after the application of a second ESD pulse, named Second level failure F2. After an initial electro-optical characterization, we have analyzed the LEDs by means of slow capacitance transient, deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements. The experimental results show that: (i) the overall junction capacitance is strongly correlated to the First level failure F1, thus suggesting also a correlation between the maximum junction electric field and the First level failure F1 of LEDs; (ii) the amplitude of capacitance transients, related to trapping phenomena, is strongly correlated to the Relative failure parameter, which is defined as the ratio Second level failure F2/First level failure F1. Thus, the presence of defects in the LED structures can influence the ESD robustness measured after the application of two consecutive ESD pulses; (iii) the correlation between trapping and Relative failure is confirmed by both DLOS and DLTS measurements.
- To conclude the study of physical mechanisms that limit the ESD robustness of Light Emitting Diodes (LEDs), we present an ESD characterization carried out on commercially available LEDs. In particular, we present an extensive analysis of the failure mechanisms of RGB (multichip) LEDs submitted to ESD testing: the tests have been carried out on several commercially available LEDs of four 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 indicates 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. However, 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. Finally, also the temperature dependence of the ESD robustness of GaN-based devices has been studied.
Useful information on the research activity can be also found in the papers co-authored by the candidate and listed in the next section
Invisible treatment of an asymmetric adult class II malocclusion related to unilateral upper lateral incisor agenesis with a straight wire lingual appliance, mini-screw and premolar extraction: A case report
No full textThis case report describes a complex full-step class II malocclusion with unilateral upper lateral incisor agenesis in an adult patient treated with lingual straight-wire appliance and premolar extraction, with the two-fold aim of obtaining ideal occlusal relationship and smile aesthetic improvement. In view of this, it underlines how an appropriate treatment strategy, including extraction choice and anchorage control during space closure, is needed to achieve the planned results with a completely invisible lingual appliance combined with aesthetic veneers
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Degradation analysis of Violet high power LEDs
No full textWith this work we report on the degradation of GaN 420nm high brightness LEDs. Devices have been
subjected to constant current stress at 350mA at different temperatures. Results indicate that different ageing
mechanisms take place: i) a steep variation in electrical and capacitance characteristics after the first few hours of
operation, probably related to a reduction of activated doping in the p-type region caused by the flow of current ii) a
substantial increase of the optical power in the low forward bias current region iii) a decrease of OP in the high current
injection regime
Electrical, spectral and thermal analysis of yellow luminescent dots in InGaN green LEDs
No full textWith this paper we present an extensive analysis of localized emission process that occur at
low current densities in InGaN/GaN LEDs. The study was carried out by means of spatially
resolved EL measurements performed at different temperatures and currents. Results indicate that,
(i) when green LEDs are biased at low current densities, localized yellow-emitting spots can be
identified; (ii) the presence of yellow-emitting dots is strongly correlated to the existence of
strong tunneling components in the I-V curves; (iii) yellow-emitting dots (YEDs) can be detected
by EL measurements, and not by PL investigation; (iv) localized yellow luminescence is strongly
thermally-activated, with activation energy equal to 0.64 eV. The presence of YEDs is ascribed to
defect-related recombination of electrons that tunnel to the barrier layers
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