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Vaccari, Simone (2014) Analysis of defects and physical mechanisms that limit the ESD robustness of Light Emitting Diodes. [Ph.D. thesis]

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Abstract (english)

This 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.

Abstract (italian)

La seguente tesi riporta i principali risultati ottenuti dall’attività di ricerca di Dottorato del candidato. L’attività è stata focalizzata sullo studio dei difetti e dei meccanismi fisici the limitano la robustezza alle scariche elettrostatiche (ESD) dei diodi emettitori di luce (LED). In particolare, la maggior parte dell’attività di ricerca è stata principalmente focalizzata sull’analisi dei LED basati su nitruro di gallio (GaN), che sono la base per la realizzazione di emettitori blu e UV e di LED bianchi basati sulla conversione dei fosfori.
Dopo una panoramica iniziale dei concetti teorici più importanti necessari per la comprensione dei risultati fisici, in questa tesi possono essere identificate quattro sezioni principali che riguardano la presentazione dell’attività di ricerca:

- In primo luogo riportiamo un’estesa analisi dei processi di emissione localizzati legati ai difetti, che si verificano nei diodi emettitori di luce basati sulla struttura InGaN/GaN. L’analisi è basata su una caratterizzazione elettrica combinata con misure di elettroluminescenza (EL) risolte spettralmente e spazialmente. I risultati di questa analisi mostrano che: (i) in condizioni di polarizzazione inversa i LED possono emettere un debole segnale di luminescenza, che è direttamente proporzionale alla corrente inversa iniettata. L’emissione in polarizzazione inversa è localizzata in spot di dimensione submicrometrica; l’intensità del segnale è fortemente correlata alla densità di threading dislocation (TD), suggerendo quindi che le threading dislocation sono percorsi preferenziali per la conduzione della corrente di leakage. (ii) In condizioni di bassa polarizzazione diretta, l’intensità del segnale EL non è uniforme sull’area del dispositivo. L’analisi EL risolta spettralmente dei LED verdi identifica la presenza di spot localizzati che emettono nella regione spettrale gialla, la cui origine è stata attribuita a tunneling localizzato che si verifica tra le buche quantiche e gli strati di barriera dei diodi, con successiva ricombinazione radiativa assistita da difetti.

- Successivamente proponiamo uno studio esteso delle caratteristiche di elettroluminescenca dei LED basati su InGaN con struttura color-coded, cioè una struttura a tripla buca quantica nella quale ciascuna buca quantica ha un contenuto di indio differente, allo scopo di analizzare la distribuzione di portatori all’interno delle buche quantiche della regione attiva. L’analisi è basata su misure di elettroluminescenza combinate con simulazioni bidimensionali, eseguite a differenti livelli di corrente e temperatura. I risultati indicano che: (i) l’efficienza di ciascuna delle buche quantiche dipende fortemente dalle condizioni operative del dispositivo (corrente e temperatura); (ii) a bassi livelli di corrente e temperatura solo la buca quantica più vicina al lato p ha un’emissione significativa; (iii) l’emissione dalle altre buche quantiche è favorita ad elevati livelli di corrente. Sarà anche discusso il ruolo dell’iniezione dei portatori, della mobilità delle lacune, della densità di portatori e della ricombinazione non radiativa nel determianre l’intensità relativa delle buche quantiche.

- A questo punto proponiamo i risultati ottenuti dall’analisi dei meccanismi fisici che limitano la robustezza alle scariche elettrostatiche (ESD) dei LED basati su GaN. L’analisi è stata eseguita su numerose famiglie di LED con differenti robustezze ESD. Ciascuna delle famiglie di campioni analizzate è caratterizzata da due differenti parametri: il tasso di failure misurato dopo l’applicazione di un singolo impulso ESD, denominato First level failure F1, e il tasso di failure misurato dopo l’applicazione di un secondo impulso ESD, denominato Secondo level failure F2. Dopo un’iniziale caratterizzazione elettro-ottica, abbiamo analizzato i LED per mezzo di misure di transienti capacitivi lenti, deep level optical spectroscopy (DLOS) e deep level transient spectroscopy (DLTS). I risultati sperimentali mostrano che: (i) la capacità di giunzione complessiva è fortemente correlata al First level failure F1, suggerendo quindi anche una correlazione tra il massimo campo elettrico di giunzione e il First level failure F1 dei LED; (ii) l’ampiezza dei transienti capacitivi, legata a fenomeni di intrappolamento, è fortemente correlata al parametro Relative failure, che è definito come il rapporto Second level failure F2/First level failure F1. Quindi la presenza di difetti nelle strutture LED può influenzare la robustezza ESD misurata dopo l’applicazione consecutiva di due impulsi ESD; (iii) la correlazione tra il trapping e il Relative failure è confermata sia dalle misure di DLOS, sia da quelle DLTS.

- Per concludere lo studio dei meccanismi fisici che limitano la robustezza ESD dei diodi emettitori di luce (LED), presentiamo una caratterizzazione ESD eseguita su dei LED disponibili commercialmente. In particolare presentiamo un’estesa analisi dei meccanismi di failure dei LED RGB (multichip) sottoposti a test ESD: i test sono stati eseguiti su numerosi LED disponibili commercialmente di quattro differenti produttori. Allo scopo di comprendere meglio i meccanismi di failure, abbiamo sottoposto i LED a test ESD in condizioni di polarizzazione inversa e diretta separatamente, per mezzo di un Transmission Line Pulser (TLP). I risultati sperimentali indicano che: (i) i LED rossi (basati su AlInGaP) hanno una robustezza ESD più alta rispetto ai campioni verdi e blu (basati su InGaN), sia nei test in polarizzazione inversa, sia in quelli in polarizzazione diretta; (ii) impulsi TLP negativi con una corrente inferiore alla soglia di failure possono indurre una diminuzione della corrente di leakage nei LED basati su GaN, a causa di un parziale annientamento dei percorsi difettosi responsabili per la conduzione inversa; (iii) il tipico meccanismo di failure dei dispositivi è rappresentato da un evento catastrofico, con cortocircuitazione della giunzione. Tuttavia, alcuni dei LED rossi analizzati hanno mostrato “soft” failure, con graduale aumento della corrente di leakage ed una corrispondente diminuzione della potenza ottica, anche in assenza di un danno castastrofico. Infine, è stata studiata anche la dipendenza dalla temperatura della robustezza ESD dei dispositivi basati su GaN.

Utili informazioni sull’attività di ricerca possono essere trovate negli articoli in cui ha collaborato il candidato ed elencati nella successiva sezione.

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EPrint type:Ph.D. thesis
Tutor:Zanoni, Enrico
Supervisor:Meneghini, Matteo
Ph.D. course:Ciclo 26 > Scuole 26 > INGEGNERIA DELL'INFORMAZIONE > SCIENZA E TECNOLOGIA DELL'INFORMAZIONE
Data di deposito della tesi:29 January 2014
Anno di Pubblicazione:28 January 2014
Key Words:Difetti, robustezza ESD, scariche elettrostatiche, LED, Diodi emettitori di luce. Defects, ESD robustness, electrostatic discharges, LED, Light-emitting diodes.
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-INF/01 Elettronica
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria dell'Informazione
Codice ID:6540
Depositato il:03 Nov 2014 13:20
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