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Reliability and failure mechanisms of GaN HEMT devices suitable for high-frequency and high-power applications
Full text linkThis thesis reports the main reliability results and failure mechanisms analysis on Gallium Nitride High Electron Mobility Transistors (GaN-HEMTs) obtained during the three years of PhD activity. The activity has been focused (i) on the main reliability issues of GaN HEMTs for both high frequency applications, like telecommunication or satellite applications, and high power applications, like high-power switching, (ii) on the failure analysis study of the critical device degradation under high electric-field bias conditions, (iii) and on the deep analysis of few parasitic effects that influence the static and dynamic behaviour of this technology. The work has followed the ending of the European project KorriGaN and few research collaborations with European research centers and private companies, exploiting the possibility of understanding what critical issues are actually considered the main threat of each specific application, with the opportunity of using the acquired knowledge on the other GaN transistor's operating fields.
The first part of the thesis reports all the activity performed inside the last task of the reliability sub-project of the European project KorriGaN (Key ORganisation for Research on Integrated circuit in GaN technology), called Task-Force project. The purpose was to define the most critical working conditions over the last set of devices developed with the knowledge acquired along the project, and to study the failure mechanisms that reduce the main performances on short and long time period, on devices with different substrate quality. OFF-state reliability tests have shown the improved robustness of the technology with critical failure voltages beyond 100V, much better than older devices developed during the previous batches of KorriGaN project, underlining the great influence of a good device processing which can overcome poor quality of the epitaxial or the substrate layers. Looking at the possible dominant failure mechanisms in on-state conditions, and the related degradation accelerating factors of this robust technology, further analysis have highlighted a failure mechanism accelerated by hot-electron effects in the on-state medium-term reliability, with a negligible effect of the temperature. This result is one of the first reported study where the hot electrons clearly accelerate the performance degradation of GaN-HEMT devices.
Similar reliability activities have been performed within the research collaboration with the European Space Agency. In particular, the work has been accomplished both with the activity performed inside the Padova microelectronic laboratories and with 5 cumulative months of placement at the ESA-ESTEC research center in the Netherlands. The studies on a quite stable technology of GaN-HEMT devices suitable for space applications have highlighted a good stability of the main static and RF performances with temperature. From the storage and the reliability analysis, the following results have been observed: good thermal stability of all the main parameters up to a quite critical temperature (T=350°C), beyond which diode degradation happens even during the first hours of test; optimal robustness capabilities above 100V of drain voltage during short-term tests at ambient temperature and at higher temperatures; stable behaviour of the space-designed devices even during high junction-temperature long-term DC stress.
Following previous activities carried out on older devices of KorriGaN project, the next part of the thesis describes a deep analysis on the off-state reliability of GaN devices. Many efforts have been spent to better understand the failure mechanisms involved on the critical gate degradation of previous technologies of AlGaN/GaN HEMTs, due to some open questions that literature has still not completely explained. Despite the common acceptance of the last years on the "critical voltage" definition, some different results have been obtained testing the gate reverse-bias behaviour of GaN HEMTs, suggesting a failure mechanism correlated with the initial defectivity of the device under test. Further analysis with fixed negative gate bias have shown the same failure mechanism even below the estimated critical voltage. Failure analysis tools, guided by the electroluminescence emission images (EL), have been used for a deeper investigation on the physical evolution of the damage, following few failure analysis studies reported in the literature. Results have allowed to verify the presence of pre-existent defects and only sometimes to identify the appearance of stress-induced defects, highlighting the big difficulty and sometimes the impossibility to locate material cracks with nanometer-scale size on the semiconductor, at least at the early stages of the device degradation.
On the last part of the thesis, some deep investigations on parasitic effects have been reported, with a special focus on the kink effect and on the current collapse effect. These activities have been performed inside the "Preliminary deep levels characterization in materials and test structures" work-package of the European project MANGA (Manufacturable GaN). Studying devices coming from old technologies, it has been possible to correlate the presence of the kink effect with a parasitic EL increase of the emission spectrum in the range of the yellow-red band, even correlated with a yellow broad-band peak founded during cathodoluminescence measurements (CL). These results well support a previous work that assumes the channel electrons interaction with two parasitic levels inside the GaN energy-gap as the origin of the kink effect. Studying AlGaN/GaN devices with different composition and doping of the barrier and the buffer layer respectively, it has been possible to correlate the presence of iron doping in the GaN buffer layer with an increase of the current collapse effect at the high electric-field quiescent points, identifying the activation energy of the trap responsible of this dynamic degradation. Furthermore, the combined use of Secondary Ion Mass Spectroscopy measurements (SIMS) and numerical simulations have allowed to provide a better physical demonstration of the experimental trend, confirming the measured results obtained on both the current collapse measurements and the trap activation-energy analysis.Questa tesi riassume i principali risultati ottenuti nello studio dell'affidabilità e dei principali meccanismi di guasto nei transistor ad alta mobilità basati su Nitruro di Gallio (GaN-HEMT). L'attività di ricerca dei tre anni di dottorato è stata incentrata (i) sulle principali problematiche affidabilistiche degli HEMT su GaN adatti sia alle applicazioni ad alta frequenza, come il campo delle telecomunicazioni o satellitare, sia alle applicazioni ad alta potenza, come il campo degli switch per alta potenza, (ii) sull'analisi fisica del degrado causato dall'applicazione di alti campi elettrici, (iii) e sull'analisi approfondita di alcuni effetti parassiti che influenzano le caratteristiche statiche e dinamiche di tale tecnologia. Il lavoro ha seguito le ultime fasi del progetto europeo KorriGaN e alcune collaborazioni con centri di ricerca europei e aziende private, integrando la possibilità di capire quali possano essere le problematiche attualmente considerate più critiche in ogni specifica applicazione degli HEMT su GaN, con l'opportunità di trasferire le conoscenze acquisite anche all'interno degli altri campi operativi.
La prima parte della tesi riassume tutta l'attività svolta all'interno del progetto Task-Force, attività conclusiva del settore affidabilità del progetto europeo KorriGaN (Key ORganisation for Research on Integrated circuit in GaN technology). Scopo del progetto era individuare le condizioni di funzionamento più critiche nell'ultimo set di dispositivi sviluppati a partire dai risultati ottenuti durante i precedenti anni di progetto, dando particolare importanza ai meccanismi di guasto responsabili della riduzione delle principali prestazioni nel breve e nel medio periodo, e ai diversi comportamenti indotti dalle diverse qualità dei substrati utilizzati nella crescita di tali dispositivi. I test di affidabilità a canale chiuso hanno dimostrato un significativo miglioramento della robustezza rispetto ai dispositivi sviluppati nei precedenti anni di progetto, con tensioni critiche di rottura oltre i 100V. Tali risultati sono stati confermati in tutti i wafer, indipendentemente dalla qualità del substrato, sottolineando come un buon processing dei dispositivi possa completamente mascherare la scarsa qualità dell'epitassia o dei substrati utilizzati. A canale aperto invece, l'analisi dei principali meccanismi di guasto e dei fattori di accelerazione del degrado ha mostrato un particolare meccanismo di degradazione accelerato dagli elettroni caldi (hot electrons) presenti all'interno del canale, con una trascurabile influenza della temperatura di test. Questo risultato è uno dei primi che mostra chiaramente l'influenza degli elettroni caldi nei meccanismi di degrado degli HEMT basati su Nitruro di Gallio.
Analoghi studi su affidabilità e meccanismi di guasto sono stati eseguiti all'interno dell'attività di collaborazione con l'Agenzia Spaziale Europea. In particolare, l'attività è stata sviluppata sia all'interno dei laboratori di microelettronica di Padova, sia presso il centro di ricerca dell'Agenzia Spaziale ESA-ESTEC in Olanda, per un periodo complessivo di mobilità di 5 mesi. Gli studi sono stati eseguiti su una tecnologia di dispositivi ormai abbastanza consolidata, adattata alle esigenze specifiche delle applicazioni satellitari. I risultati hanno mostrato una buona stabilità delle principali perfomance DC e RF dei dispositivi al variare delle temperatura, e una buona stabilità nei test di storage fino ai 350°C, temperatura critica alla quale i diodi di gate cominciano rapidamente a degradare. Una significativa affidabilità è stata inoltre rilevata sia nei test a breve termine, con ottima stabilità oltre i 100V di tensione di drain a temperatura ambiente e ad alte temperature, sia nei test a lungo termine, eseguiti sui dispositivi designati per l'applicazione spaziale con test ad elevate temperature di giunzione.
La successiva parte della tesi tratta un'analisi approfondita dell'affidabilità a canale chiuso dei transistor su GaN, seguendo precedenti lavori volti allo stesso scopo. Molti studi sono stati eseguiti per comprendere meglio quali siano i meccanismi di guasto coinvolti nella degradazione del gate delle precedenti tecnologie di GaN-HEMT, a causa di alcune questioni ancora irrisolte sulle quali la letteratura non ha ancora dato una completa chiarificazione. Nonostante la definizione di "critical voltage" (tensione critica) ormai comunemente accettata, alcuni test sul comportamento del gate in polarizzazione inversa hanno evidenziato risultanti contrastanti, suggerendo un diverso meccanismo di guasto correlato alla difettosità iniziale del campione. Altre analisi in polarizzazione costante hanno mostrato lo stesso meccanismo di guasto a tensioni di gate ben al di sotto della tensione critica. In seguito, su alcuni campioni sono state eseguite approfondite indagini di guasto per meglio comprendere l'evoluzione fisica del meccanismo di rottura, seguendo alcuni studi recentemente riportati in letteratura. Guidati dalle misure di elettroluminescenza (EL) precedentemente ottenute, tali analisi hanno permesso di verificare la presenza di difetti pre-esistenti e solo in certi casi di identificare la comparsa di alcuni difetti indotti dallo stress, evidenziando l'enorme difficoltà e talvolta l'impossibilità di localizzare dei difetti con dimensioni di scala nanometrica nei diversi strati di semiconduttore (cracks), almeno durante le prime fasi di degrado del campione.
L'ultima parte della tesi riporta alcune indagini approfondite su specifici effetti parassiti presenti negli HEMT su GaN, in particolar modo analizzando l'effetto kink e il collasso di corrente (current collapse). Questi studi sono stati svolti all'interno del progetto europeo MANGA (Manufacturable GaN), nel settore dedicato all'indagine dei livelli energetici responsabili degli effetti parassiti. Studiando alcuni dispositivi appartenenti a tecnologie meno recenti, è stato possibile correlare la presenza dell'effetto kink con un aumento inusuale dello spettro di elettro-luminescenza nel range della banda rosso-gialla, a sua volta correlato con un largo picco di emissione nel giallo rilevato durante misure di catodo-luminescenza negli stessi dispositivi. Tali studi confermano un precedente lavoro in cui si assume che l'effeto kink sia originato dall'interazione degli elettroni nel canale con due livelli energetici parassiti presenti all'interno dell'energy gap del GaN. Studiando invece alcuni dispositivi HEMT basati sull'eterostruttura AlGaN/GaN, ma con differenti composizioni dello strato barriera e drogaggio dello strato buffer, è stato possibile correlare la concentrazione del ferro, usato come drogante all'interno del buffer, con un incremento del current collapse nei punti di polarizzazione a maggior campo elettrico, identificando poi l'energia di attivazione della trappola responsabile di tale degrado delle caratteristiche dinamiche. L'uso combinato di misure SIMS (Secondary Ion Mass Spectroscopy) e simulazioni numeriche hanno permesso di dare una dimostrazione fisica dell'effetto osservato nelle misure di laboratorio, confermando sia i risultati ottenuti in termini di collasso di corrente, sia la valutazione sperimentale dell'energia di attivazione della trappola
Nanoparticles at the fluid interface
No full textInternational audienceGold nanoparticles (NPs) coated with stimuli-responsive polymer transfer from water to oil and from oil to water across the planar interfaces when environmental parameters are slightly changed. The oil to water transfer occurs when the temperature is reduced below 5 °C, while they transfer from salty water to oil when the environmental temperature returns to room temperature. The water-to-oil particle transfer after warming is dictated by the ionic strength of the aqueous phase, which is directly associated with the ionic strength response of the polymer brushes anchored on the NPs. In contrast, the temperature onset for the oil-to-water NP transfer after cooling is weakly correlated with the transition temperature of the polymer brushes in either of the bulk phases. The transfer mechanism disparity for the two directions during NP crossing of oil-water interfaces result from an intricate interplay of interfacial interactions and transitions. In this talk, experimental investigations and some modeling will be presented in order to elucidate the NP transfer across fluid interfaces. References Bidirectional Nanoparticle Crossing of Oil-Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer. Stocco Antonio, Chanana Munish, Su Ge, Cernoch Peter, Binks Bernard, Wang Dayang: Angewandte Chemie / Angew Chem Int Ed, vol. 51 p.9647 (2012) Acknowledgements Special thanks go to Helmuth Möhwald, Munish Chanana (MPIKG), and all colleagues involved in this project
Nanoparticles at the fluid interface
No full textInternational audienceGold nanoparticles (NPs) coated with stimuli-responsive polymer transfer from water to oil and from oil to water across the planar interfaces when environmental parameters are slightly changed. The oil to water transfer occurs when the temperature is reduced below 5 °C, while they transfer from salty water to oil when the environmental temperature returns to room temperature. The water-to-oil particle transfer after warming is dictated by the ionic strength of the aqueous phase, which is directly associated with the ionic strength response of the polymer brushes anchored on the NPs. In contrast, the temperature onset for the oil-to-water NP transfer after cooling is weakly correlated with the transition temperature of the polymer brushes in either of the bulk phases. The transfer mechanism disparity for the two directions during NP crossing of oil-water interfaces result from an intricate interplay of interfacial interactions and transitions. In this talk, experimental investigations and some modeling will be presented in order to elucidate the NP transfer across fluid interfaces. References Bidirectional Nanoparticle Crossing of Oil-Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer. Stocco Antonio, Chanana Munish, Su Ge, Cernoch Peter, Binks Bernard, Wang Dayang: Angewandte Chemie / Angew Chem Int Ed, vol. 51 p.9647 (2012) Acknowledgements Special thanks go to Helmuth Möhwald, Munish Chanana (MPIKG), and all colleagues involved in this project
Nanoparticles at the fluid interface
No full textInternational audienceGold nanoparticles (NPs) coated with stimuli-responsive polymer transfer from water to oil and from oil to water across the planar interfaces when environmental parameters are slightly changed. The oil to water transfer occurs when the temperature is reduced below 5 °C, while they transfer from salty water to oil when the environmental temperature returns to room temperature. The water-to-oil particle transfer after warming is dictated by the ionic strength of the aqueous phase, which is directly associated with the ionic strength response of the polymer brushes anchored on the NPs. In contrast, the temperature onset for the oil-to-water NP transfer after cooling is weakly correlated with the transition temperature of the polymer brushes in either of the bulk phases. The transfer mechanism disparity for the two directions during NP crossing of oil-water interfaces result from an intricate interplay of interfacial interactions and transitions. In this talk, experimental investigations and some modeling will be presented in order to elucidate the NP transfer across fluid interfaces. References Bidirectional Nanoparticle Crossing of Oil-Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer. Stocco Antonio, Chanana Munish, Su Ge, Cernoch Peter, Binks Bernard, Wang Dayang: Angewandte Chemie / Angew Chem Int Ed, vol. 51 p.9647 (2012) Acknowledgements Special thanks go to Helmuth Möhwald, Munish Chanana (MPIKG), and all colleagues involved in this project
GaN HEMT Reliability: From Time Dependent Gate Degradation to On-state Failure Mechanisms
No full textIn this paper, we compare degradation modes and failure mechanisms of different AlGaN/GaN HEMT technologies. We present data concerning reverse-bias degradation of GaN-based HEMTs, which results in a dramatic increase of gate leakage current, and present a time-dependent model for gate degradation. Some of the tested technologies demonstrated to be immune from this failure mechanism up to drain-gate voltages in excess of 100 V. When this was case, the main failure mode consisted of drain current degradation during on-state tests, resulting from charge trapping in the gate-drain access region attributed to hot-electron effects. Finally, the use of diagnostic techniques such as electroluminescence microscopy and Deep Level Transient Spectroscopy for the identification of failure modes and mechanisms of GaN-based HEMTs is reviewed. Concerning reverse-bias degradation of GaN-based HEMTs, we demonstrate that, (i) when submitted to reverse-gate stress, HEMTs can show both recoverable and permanent degradation. (ii) recoverable degradation consists in the decrease in gate current and threshold voltage, which are ascribed to the simultaneous trapping of negative charge in the AlGaN layer, and of positive charge close to the AlGaN/GaN interface. (iii) permanent degradation consists in the generation of parasitic leakage paths. Time-dependent analysis suggests that permanent degradation can be ascribed to a defect generation and percolation process. Results supports the existence of a time to breakdown for HEMT degradation, which significantly depends on the stress voltage level. On the contrary, AlGaN/GaN technologies which were found to be resistant to gate degradation (off-state critical voltage larger than 100 V for a 0.25 um gate device) were submitted to on-state tests at different gate and drain voltage levels. All tests showed a non-recoverable degradation of electrical parameters (drain saturation current, threshold voltage and on-state resistance) and electroluminescence signal EL, with a strong dependence on the EL value of the bias point, and a negligible dependence of temperature. Once verified that EL intensity represents a reliable estimate of channel hot electron effects, we attributed the degradation to hot electron trapping in the gate-drain access region. Using EL intensity as a measure of the stress acceleration factor, we derived an acceleration law for GaN HEMT hot electron degradation similar to the one already demonstrated for GaA
Degradation mechanisms in AlGaN/GaN HEMTs submitted to off and on-state stress conditions
No full textThis paper describes a deep investigation of the degradation mechanisms induced by off-state and on-state stress in
AlGaN/GaN HEMTs. Concerning reverse-bias degradation, results underline that the exposure to reverse-bias stress can
induce (i) a recoverable change in the gate current due to the accumulation of negative charges under the gate, (ii) and a
permanent degradation of gate characteristics due to the generation of vertical parasitic leakage paths through the
AlGaN layer. Further analysis of the kinetics of this degradation mechanism, correlated with time-resolved
Electroluminescence (EL) measurements, allowed to define a model that explains the time-dependence of the
phenomenon and the role of gate voltage as accelerating factor, providing an interpretation for both recoverable and
permanent modifications of the main device characteristics induced by reverse-bias stress.
On the other hand, on devices that have shown an improved robustness against the reverse-bias gate degradation, we
investigated the origin of the degradation under on-state stress. In this case, results obtained with a combined electrical
and optical analysis, showed that on-state stress may induce a significant decrease in drain saturation current and
electroluminescence (EL) signal, with a degradation rate that strongly depends on the EL intensity measured before
stress, which is representative of the presence of hot-electrons in the channel. On-state degradation can be ascribed to a
decrease in the electric-field, due to the trapping of electrons within the barrier or at the surface induced by hotelectrons.
Therefore, using the EL signal as measure of the stress accelerating factor, it was possible to derive an
acceleration law for hot-electron degradation on GaN HEMTs
Breakdown Walkout induced by reverse bias stress in AlGaN/GaN HEMTs
No full textWe report in this paper the effect of the reverse bias stress in AlGaN/GaN HEMTs, observing the current trend during the stress and
its correlation with the emission intensity. The increase of the breakdown voltage after the stress, with the other parameters
variations, can be related with a charge trapping phenomena in the gate-to-drain channel region
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
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