analysis and reliability study of luminescent materials for white lighting

In this work, we report on the characterization and reliability/stability study of phosphorescent materials for lighting applications. More specifically, we investigated (a) phosphors directly deposited over light-emitting diodes (LED) chip, (b) remote phosphor (RP) solutions encapsulated in plastic medium for LED lighting, and (c) phosphors without binder for extreme high-intensity laser diode white lighting. The optical and thermal properties of phosphors were studied to develop a sample based on a mix of phosphor compounds in order to achieve different correlated color temperatures (CCT) and high color rendering index (CRI) LEDs. Thermal properties of cerium-doped YAG (Yttrium Aluminum Garnet) phosphor materials were evaluated in order to study thermal quenching. A maximum phosphor operating temperature of 190–200 °C was found to cause a sensible efficiency degeneration. Reduced efficiency and Stokes shift also caused a localized temperature increase in the photoluminescent materials. In the case of remote phosphors, heat did not find a low thermal resistance path to the heatsink (as occurred through the GaN LED chip for direct phosphor-converted devices) and thermal analysis indicated that material temperature might therefore increase to values in excess of 60 °C when a radiation of 435 mW/cm2 hit the sample template. Reliability was also investigated for both plastic-encapsulated materials and binder-free depositions. Pure thermal reliability study indicated that phosphors encapsulated in polycarbonate material were stable up to temperature of approximately 100 °C, while binder-free phosphor did not show any sensible degradation up to temperatures of 525 °C

Inactivating SARS-CoV-2 Using 275 nm UV-C LEDs through a Spherical Irradiation Box: Design, Characterization and Validation

We report on the design, characterization and validation of a spherical irradiation system for inactivating SARS-CoV-2, based on UV-C 275 nm LEDs. The system is designed to maximize irradiation intensity and uniformity and can be used for irradiating a volume of 18 L. To this aim: (i) several commercially available LEDs have been acquired and analyzed; (ii) a complete optical study has been carried out in order to optimize the efficacy of the system; (iii) the resulting prototype has been characterized optically and tested for the inactivation of SARS-CoV-2 for different exposure times, doses and surface types; (iv) the result achieved and the efficacy of the prototype have been compared with similar devices based on different technologies. Results indicate that a 99.9% inactivation can be reached after 1 min of treatment with a dose of 83.1 J/m2

GaN-based power devices: Physics, reliability, and perspectives

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semicon- ductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spon- taneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high- voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench- structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main proper- ties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics

III-N optoelectronic devices: understanding the physics of electro-optical degradation

III-N optoelectronic devices are of great interest for many applications. Visible emitters (based on InGaN) are widely used in the lighting, display and automotive fields. Ultraviolet LEDs (based on AlGaN) are expected to be widely used for disinfection, medical treatments, surface curing and sensing. Photodetectors and solar cells based on InGaN are also of interest, thanks to their great robustness and wavelength tunability. III-N semiconductors are expected to be robust, thanks to the wide bandgap (allowing high temperature operation) and to the high breakdown field (favoring the robustness against electrostatic discharges and electrical overstress). However, InGaN- and AlGaN-based devices can show a significant degradation when submitted to long-term ageing. Several driving forces can contribute to the worsening of the electrical and optical characteristics, including the operating temperature, the current, and the rate of non-radiative recombination in the quantum wells. The goal of this paper is to discuss the physics of degradation of III-V devices, by presenting a set of recent case studies, evaluated in our laboratories

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

Physical Origin of the Optical Degradation of InAs Quantum Dot Lasers

We present an extensive analysis of the physical mechanisms responsible for the degradation of 1.3-μm InAs quantum dot lasers epitaxially grown on Si, for application in silicon photonics. For the first time, we characterize the degradation of the devices by combined electro-optical measurements, electroluminescence spectra, and current-voltage analysis. We demonstrate the following original results: when submitted to a current step-stress experiment: 1) QD lasers show a measurable increase in threshold current, which is correlated to a decrease in slope efficiency; 2) the degradation process is stronger, when devices are stressed at current higher than 200 mA, i.e., in the stress regime, where both ground-state and excited-state emission are present; and 3) in the same range of stress currents, an increase in the defect-related current components is also detected, along with a slight decrease in the series resistance. Based on the experimental evidence collected within this paper, the degradation of QD lasers is ascribed to a recombination-enhanced defect reaction (REDR) process, activated by the escape of electrons out of the quantum dots

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

Reliability of III-V laser diodes and LEDs for lighting and telecommunication applications

The research in optoelectronics has recently shown impressive advancements, thanks to the development of disruptive technologies, such as Gallium Nitride LEDs and lasers - that are enabling novel lighting devices - and Silicon photonics - that will change the paradigm in broadband data transmission. Industrial laboratories and academic researchers are working towards the improvement of device performance and reliability. This goal is complicated by the complex nature of optoelectronic devices and systems: a deep knowledge on material properties, device characteristics and system optimization is needed to identify the physical mechanisms that limit the lifetime of the devices, and to address possible solutions. The aim of this thesis is to report on a detailed study of the physical factors that limit the performance and reliability of innovative optoelectronic technologies based on nitrides and arsenides. The investigation was conducted by means of purposely planned Accelerated Life Test (ALT) experiments and short-term stress tests on selected state-of-the-art devices, which let us identify the main physical processes responsible for the long-term degradation of the devices, and pinpointed the weaknesses of modern LEDs when temporary operated outside their Safe Operating Area (SOA) as a consequence of Electrical Over-Stress (EOS) events. With regard to Gallium Nitride (GaN)-based white and visible monochromatic LEDs, we found that under constant near-breakdown reverse bias the devices exhibit a time-dependent degradation phenomenon, which promotes the increase of the leakage current of the device, eventually leading to its catastrophic failure. The peculiar failure statistics, as well as the electro-luminescence and leakage-current data acquired during stress, could be interpreted by considering that highly-depleted GaN under reverse bias may behave like a partially-leaking dielectric that degrades over time due to a defect percolation process, similar to the dielectric degradation-driven breakdown process affecting Silicon MOS devices. On the other hand, high forward-current short-term step-stress on high-power LEDs revealed that sustained operation at driving currents above the maximum rating of the devices can rapidly induce failure in correspondence of a major current injection point, dependent on the specific chip structure, as a consequence of the localized power dissipation and temperature reached due to extreme current crowding effects and to the degradation of the conductivity of extended device regions. A clear dependence of the failure mode on the chip structure and device layout could also be found during the investigation on the effects of single short EOS events of increasing amplitude on high-power white LEDs. The experimental data helped identifying the best LED design to be employed in an electrically critical environment, also showing that EOS-related reliability issues tend to arise more from extrinsic elements of the LED system rather than from the semiconductor chip. Unlike High-Brightness (HB) state-of-the-art LEDs, whose main reliability concern is represented by EOS events, cost-effective mid-power LEDs for lighting applications were found to suffer from gradual degradation processes impacting in the long term on both the electrical an optical characteristics of the device. Moreover, the results of the ALTs highlighted the role of the plastic package in the degradation of the optical properties of the emitted light. The long-term reliability of mid-power LEDs was further investigated at system level by performing a lifetime analysis of commercial LED bulbs employing these devices as primary light sources. The increased complexity of the system under stress negatively impacted on the stability over time of the luminous performances of the luminaries, which was severely affected by the degradation of extrinsic elements like the diffusing dome or the current driver. In Part II of this work, our system-level analysis continued with an extensive investigation on the reliability of blue-emitting phosphors for near-UV laser excitation, as part of a research project performed in collaboration with the New Energy and Industrial Technology Development Organization (NEDO), Japan. By means of a series of pure thermal stress experiments and stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of the phosphors under extreme and more conservative operating conditions. In particular, while the phosphors demonstrated good stability during pure thermal treatment in air up to 300 °C, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the photo-luminescence, which was attributed to the thermally-induced autoionization of the Eu2+ optically active centers. By means of different material characterization techniques, evidence of this degradation process was also found on samples stressed under moderate 3 W/mm2 – 405 nm optical excitation. This indicated that the optically (and thermally) induced ionization of the optically-active species is the most critical degradation process for this family of phosphorescent material. The operating limits of an improved second generation luminescent material were also investigated by means of short term stress under 405 nm optical excitation. The experimental data showed that, for a given deposition condition, a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state photo-luminescence performances and degradation of the material were found to take place, which suggested that the material was being operated in an unuseful excitation regime, mainly limited by the thermal management capabilities of the carrier substrate employed for our experimental purposes rather than from intrinsic properties of the phosphor. Part III of this thesis is devoted to the analysis of the degradation processes of heterogeneous III-V/Silicon infrared (IR) laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed (i) an increase in the threshold current, (ii) a decrease in the turn-on voltage, and (iii) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects towards the active region of the devices. In order to further investigate the physical origin of the diffusing defects, capacitance Deep-Level Transient Spectroscopy (C-DLTS) analysis was performed. The results indicate the presence of several 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 characteristic of the quaternary material employed to grow the active region of the device.L'ultimo decennio di ricerca nell'ambito dei dispositivi optoelettronici ha contribuito allo sviluppo e alla successiva introduzione sul mercato di rivoluzionarie tecnologie, sia in ambito illuminotecnico, grazie agli innovativi sistemi di illuminazione basati su LED e laser in Nitruro di Gallio, che in ambito telecomunicazioni, dove la Silicon photonics promette di stravolgere l'odierno approccio alla trasmissione a larga banda. Innumerevoli laboratori di ricerca universitari e privati concorrono nell'incrementare le performance e l'affidabilità dei dispositivi. Tale obbiettivo, tuttavia, è ostacolato dalla complessa natura dei dispositivi optoelettronici e, ancor più, dei sistemi su di essi basati. Una conoscenza approfondita dei materiali, delle proprietà dei dispositivi e dell'ottimizzazione a livello di sistema risultano essere indispensabili per l'identificazione dei meccanismi fisici che limitano il tempo di vita utile dei dispositivi, e per lo sviluppo di possibili soluzioni. Scopo di questa tesi è descrive i risultati di un'estesa analisi finalizzata ad identificare i processi fisici che limitano l'affidabilità e le prestazioni degli innovativi sistemi optoelettronici basati su semiconduttori III-V. Per tale scopo sono stati realizzati specifici esperimenti di degrado accelerato e stress a breve termine su dispositivi allo stato dell'arte, che hanno permesso di individuare i processi fisici responsabili del degrado, graduale o catastrofico, cui possono essere soggetti i moderni dispositivi optoelettronici durante la propria vita operativa. Nello specifico, le analisi condotte su LED ad emissione nel visibile basati su Nitruro di Gallio (GaN) hanno innanzitutto rivelato che la prolungata polarizzazione del dispositivo con tensioni inverse vicine al valore critico di breakdown è accompagnata dall'instaurarsi di un processo di degrado tempo-dipendente che induce l'aumento della corrente di leakage del LED, portandolo eventualmente alla failure catastrofica. La peculiare distribuzione statistica del tempo di failure, unita all'andamento temporale di corrente e segnale di elettro-luminescenza durante lo stress in inversa, sono stati interpretati supponendo che il Nitruro di Gallio, portato in svuotamento spinto dalle elevate tensioni inverse, possa comportarsi come un dielettrico con perdite che degrada nel tempo a causa della percolazione di difetti, con un processo del tutto simile al breakdown tempo-dipendente del dielettrico presente nei MOS in Silicio. Lo studio sugli effetti a breve termine di elevate correnti di polarizzazione su LED in GaN ad alta potenza ha invece evidenziato una localizzazione della failure dei dispositivi in prossimità dei principali punti di iniezione di corrente, specificatamente associabili alla particolare struttura del dispositivo in analisi. Tale tipologia di stress si è inoltre rivelata altamente dannosa anche per la struttura epitassiale del dispositivo, che, a causa delle elevate temperature e densità di corrente in gioco, ha mostrato incrementi di resistività localizzati. Una più marcata dipendenza delle modalità e delle condizioni di failure dalla struttura e dal layout superficiale è emersa dall'analisi degli effetti di eventi di overstress di breve durata su LED bianchi ad alta potenza. I risultati sperimentali hanno portato all'identificazione della tipologia di LED più adeguata da adottare in ambienti soggetti a disturbi elettrici, mostrando inoltre come le problematiche relative ad eventi di overstress elettrico siano determinate più dalle debolezze di elementi estrinseci del LED che dal chip di semiconduttore. L'attività di ricerca ha poi evidenziato come per LED bianchi a media potenza in condizioni operative limite i processi di degrado graduale delle caratteristiche ottiche ed elettriche del dispositivo, ed in particolar modo relativi al package plastico, possano risultare deleteri per l'affidabilità a lungo termine della sorgente allo stato solido. L'analisi affidabilistica su tale tipologia di LED è stata poi estesa a livello di sistema, andando ad investigare i meccanismi di degrado che interessano le comuni lampadine a bulbo basate per l'appunto su LED bianchi a media potenza. La maggiore complessità del sistema illuminante sotto stress ha impattato negativamente sulle performance a lungo termine del bulbo, la cui affidabilità è risultata essere per lo più limitata da elementi estrinseci al LED, quali la cupola diffusiva oppure il driver di corrente. Nella seconda parte di questa tesi sono state analizzate le proprietà affidabilistiche di uno di questi elementi estrinseci: i fosfori. In collaborazione con l'organizzazione per lo sviluppo delle nuove energie e delle tecnologie industriali del Giappone (NEDO), è stata condotta un'estesa ricerca sull'affidabilità di fosfori ad emissione nel blu per eccitazione nel vicino UV tramite sorgenti a stato solido, in particolare laser. Attraverso una serie di stress termici e/o sotto fascio ottico ad elevate intensità, è stato possibile identificare il principale meccanismo fisico responsabile del degrado delle performance di fotoluminescenza del materiale fosforescente in esame. Nello specifico, correlando quest'ultimo fenomeno con le variazioni delle proprietà chimico-fisiche del materiale, è stato possibile identificare nell'autoionizzazione otticamente e/o termicamente indotta dei centri otticamente attivi di Eu2+ la principale causa di degrado del fosforo. La stress ottico a breve termine su pigmento luminescente di seconda generazione ha poi permesso di identificare un valore limite di intensità per il pompaggio ottico continuo. Oltre tale soglia, le performance del materiale luminescente calano per effetto congiunto della perdita di efficienza dovuta alle alte temperature di esercizio e al degrado non reversibile del materiale stesso. Nella terza parte di questa tesi vengono analizzati i meccanismi di degrado di diodi laser IR ibridi basati su semiconduttori III-V e Silicio progettati per sistemi di telecomunicazione e di interconnessione integrati. Esperimenti di degrado accelerato condotti a corrente costante hanno permesso l'identificazione di diversi processi di degrado graduale, tra i quali: (i) l'aumento della corrente di soglia, (ii) la decrescita della tensione di turn-on del diodo (iii) e l'incremento della concentrazione di carica apparente in prossimità della regione attiva. Sulla base della correlazione tra tali processi, è stata formulata l'ipotesi secondo la quale l'origine del degrado ottico del dispositivo risieda nell'aumento del rate di ricombinazione non radiativa, possibilmente dovuto alla diffusione di difetti verso la regione attiva. Al fine di investigare l'origine della specie diffondente sono state impiegate tecniche di analisi spettroscopica che hanno permesso l'identificazione di diversi livelli trappola all'interno del dispositivo, il principale dei quali, localizzato 0.43 eV al di sopra della banda di valenza, è risultato essere compatibile con un difetto d'interfaccia caratteristico del materiale semiconduttore utilizzato per la crescita della regione attiva dei dispositivi in esame