<p>InAlN thin films and InAlN/GaN heterostructures have been intensively studied over recent years due to their applications in a variety of devices, including high electron mobility transistors (HEMTs). However, the quality of InAlN remains relatively poor with basic material and structural characteristics remain unclear.</p><p>Molecular beam epitaxy (MBE) is used to synthesize the materials for this research, as MBE is a widely used tool for semiconductor growth but has rarely been explored for InAlN growth. X-ray photoelectron spectroscopy (XPS) is used to determine the electronic and chemical characteristics of InAlN surfaces. This tool is used for the first time in application to MBE-grown InAlN and heterostructures for the characterization of surface oxides, the bare surface barrier height (BSBH), and valence band offsets (VBOs).</p><p>The surface properties of InAlN are studied in relation to surface oxide characteristics and formation. First, the native oxide compositions are studied. Then, methods enabling the effective removal of the native oxides are found. Finally, annealing is explored for the reliable growth of surface thermal oxides.</p><p>The bulk properties of InAlN films are studied. The unintentional compositional grading in InAlN during MBE growth is discovered and found to be affected by strain and relaxation. The optical characterization of InAlN using spectroscopy ellipsometry (SE) is also developed and reveals that a two-phase InAlN model applies to MBE-grown InAlN due to its natural formation of a nanocolumnar microstructure. The insertion of an AlN interlayer is found to mitigate the formation of this microstructure and increases mobility of whole structure by fivefold.</p><p>Finally, the synthesis and characterization of InAlN/GaN HEMT device structures are explored. The density and energy distribution of surface states are studied with relationships to surface chemical composition and surface oxide. The determination of the VBOs of InAlN/GaN structures with different In compositions are discussed at last.</p> / Dissertation

Zugl.: Kassel, University, Diss., 2006. / Download lizenzpflichtig.

Zugl.: Kassel, Univ., Diss., 2006

Stuttgart, Univ., Diss., 2001.

PhD Thesis / Recently, gallium nitride high electron mobility transistor [GaN HEMT] has evolved as a promising device in the field of power electronics. It has excellent material qualities such as high bandgap, high saturation velocity, and good thermal stability which is expected to give superior device performances compared to its Si counterparts. One of the major challenges in GaN technology is to achieve enhancement operation (or normally off mode) due to the presence of its inherent two-dimensional electron gas[2DEG]. Among many methods developed to realize this, pGaN HEMT has emerged as the most encouraging technique for power GaN technology due to its high threshold voltage and good reliability. However, one of the major issues in pGaN HEMTs is that it suffers from high gate leakage current which limits their device performance. In this thesis, we have made a detailed study of the gate leakage process in pGaN HEMTs in terms of modeling, TCAD simulations, and alternative methods being used to reduce gate leakage in pGaN devices. A numerical model has been developed to model the gate leakage in pGaN HEMTs as a function of gate bias and temperature. This model is validated against 5 devices with different contact metals, geometries, and process conditions. A single model with a consistent set of parameters can fit the experimental data for all these 5 devices without the need to invoke multiple mechanisms to explain the gate leakage process. The numerical model relied on some simplifications, such as ignoring series resistance, using the compact diode model, and using a simplified expression to describe trap-assisted tunneling. Using commercial TCAD simulations, can address these limitations since the simulator computes the electric field distribution throughout the structure. Furthermore, using TCAD some of the trap levels have been identified which accounts for leakage at low bias. We were able to calibrate our TCAD simulations against published data for the drain current and then used the calibrated simulation environment to accurately simulate gate leakage using parameters that closely correspond to the physical phenomena described, including interface trap parameters, which we identify with known trap levels in GaN. Finally, we have examined different strategies that have been implemented so far to reduce leakage current. The pGaN layer is important in the whole device operation. Its doping concentration and thickness affect the leakage characteristics. Three modified structures have been studied through TCAD simulations which decrease gate leakage current. In each case, we used our calibrated TCAD model to study the impact on the drain current as well as the leakage current. Our results closely fit published experimental results and therefore provide confidence on the simulated dependence of leakage and drive current behavior on process modifications. The specific results, and our model overall, are expected to be of benefit to device designers in optimizing device structures for leakage while maintaining the required drive current. / Thesis / Doctor of Philosophy (PhD)

No description available.

Ce manuscrit présente les résultats d’une analyse exhaustive des mécanismes physiques qui limitent les performances et la fiabilité des transistors à haute mobilité d’électrons (HEMT) sur nitrure de gallium (GaN). En particulier : • Les phénomènes de dégradation à fort champ électrique des HEMT sur GaN sont analysés en comparant les données expérimentales avec les résultats de simulations physiques. Des stresses DC de 150 heures ont été effectués en conditions de canal ouvert et de pincement. Les effets des dégradations qui ont caractérisé ces deux types de stresses sont les suivants: une chute de courant DC de drain, une amplification des effets de gate-lag, et une diminution du courant inverse de grille. Les simulations physiques indiquent que la génération simultanée de piéges de surface (et/ou barrière) et de volume peut expliquer tous les modes de dégradation décrits plus haut. Les mesures expérimentales ont également montré que le stress en canal ouvert a causé une chute de la transconductance seulement pour de fortes valeurs de la tension VGS, alors que le stress au pincement a provoqué une chute de transconductance uniforme pour toutes les valeurs de VGS. Ce comportement peut être reproduit par la simulation physique pourvu que, dans le cas de stress a canal ouvert, on considère que les piéges s’accumulent au long d’une vaste région qui s’étend latéralement du bord de la grille vers le contact de drain, tandis que, dans le cas du stress au pincement, on considère que la génération des pièges ait lieu dans une portion plus petite de la zone d’accès à proximité de la grille et qu’elle soit accompagnée par une grande dégradation des paramètres de transport du canal. Enfin on propose que les électrons chauds et l’augmentation de la contrainte par le champ électrique soient à l’origine des dégradations observées après les stresses a canal ouvert et au pincement respectivement. • Les piéges dans les HEMT sur GaN ont été caractérisés en utilisant les techniques de DLTS et leur comportement associé de charge/décharge est interprété à l’aide des simulations physiques. Sous certaines conditions de polarisation, les piéges du buffer peuvent produire de faux signaux de piéges de surface, c'est-à-dire, le même type de signaux I-DLTS et ICTS attribués généralement aux piéges de surface. Clarifier cet aspect est très important à la fois pour les tests de fiabilité et pour l’optimisation des dispositifs, car il peut provoquer une identification erronée du mécanisme de dégradation, et par conséquent induire une mauvaise correction des procédés technologiques. • Les mécanismes physiques qui provoquent l’effondrement du courant RF dans les HEMT sur GaN sont analysés par le biais de mesures expérimentales et de simulations physiques. Ce travail propose les conditions suivantes : i) les piéges du buffer aussi bien que ceux de surface peuvent contribuer à l’effondrement du courant RF à travers un mécanisme identique qui impliquerait la capture et l’émission des électrons provenant de la grille; ii) la passivation de la surface diminue considérablement l’effondrement du courant RF par la réduction du champ électrique en surface et la diminution qui en découle de l’injection d’ électrons de la grille vers les pièges ; iii) pour des densités de piéges de surface inférieures à 9 × 1012 cm-2 , des barrières de potentiel superficiels dans l’ordre de 1-2 eV peuvent coexister avec des piéges de surface ayant des énergies plus faibles et qui causent l’effondrement du courant RF caractérisé par des constantes de temps relativement courtes. • Les effets de l’effondrement du courant dans les HEMT sur GaN sont étudiés en utilisant les résultats de mesures expérimentales et de simulations physiques. D’après les mesures pulsées, les dispositifs employés montrent un gate-lag considérable et un drain-lag négligeable qui peuvent être attribués à la présence de piéges de surface et de buffer respectivement. / This thesis reports the results of an extensive analysis of the physical mechanisms that limit the performance and reliability of gallium nitride (GaN) based High Electron Mobility Transistors (HEMT). In particular: • High electric field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate–source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively. • Traps are characterized in AlGaN-GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce ‘‘false’’ surface-trap signals, i.e. the same type of current-mode DLTS (I DLTS) or gate-lag signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process. • The physical mechanisms underlying RF current collapse effects in AlGaN-GaN high electron mobility transistors are studied by means of measurements and numerical device simulations. This work suggests the following conclusions: i) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; ii) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; iii) for surface-trap densities lower than 9 × 1012 cm-2, surface-potential barriers in the 1–2 eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants. • Current collapse effects are investigated in AlGaN/GaN HEMTs by means of measurements and numerical device simulations. According to pulsed measurements, the adopted devices exhibit a significant gate-lag and a negligible drain-lag ascribed to the presence of surface and buffer traps, respectively. Furthermore, illumination of the devices with two specific wavelengths can result in either a recovering of current collapse or a decrease in the gate current. On the other hand, numerical device simulations suggest that the kink effect can be explained by electron trapping into barrier traps and the subsequent electron emission after a critical electric-field value is reached. / Questa tesi riporta i risultati ottenuti da un’ampia analisi dei meccanismi fisici che limitano le prestazioni e l’affidabilità dei transistor ad alta mobilità di elettroni (HEMT) al nitruro di gallio (GaN). In particolare: • I fenomeni di degradazione ad alto campo elettrico nei GaN/AlGaN/GaN HEMT sono analizzati confrontando i dati sperimentali con i risultati delle simulazioni numeriche. Sono stati effettuati stress DC di 150 ore in condizioni di canale aperto e chiuso. Gli effetti di degradazione che hanno caratterizzato entrambi i tipi di stress sono i seguenti: una caduta nella corrente DC di drain, un’amplificazione degli effetti di gate lag, e una diminuzione della corrente inversa di gate. Le simulazioni numeriche indicano che la generazione simultanea di trappole in superficie (e/o barriera) e buffer può spiegare tutti i suddetti modi di degradazione. Le misure sperimentali hanno mostrato inoltre che lo stress a canale aperto ha causato una caduta della tranconduttanza solo ad alte tensioni VGS, mentre lo stress a canale chiuso ha provocato una caduta della transconduttanza uniforme a tutte le tensioni VGS. Questo comportamento può essere riprodotto con le simulazioni se, nel caso di stress a canale aperto, si assume che le trappole si accumulano lungo un’ampia regione che si estende lateralmente dal bordo di gate verso il contatto di drain, mentre, nel caso di stress a canale chiuso, si suppone che la generazione delle trappole abbia luogo in una più stretta porzione della zona di accesso vicino al bordo di gate e che sia accompagnata da una degradazione significativa dei parametri di trasporto del canale. Infine si propone che gli elettroni caldi del canale e l’aumento di strain indotto dal campo elettrico siano alla base delle degradazioni osservate dopo gli stress a canale aperto e chiuso rispettivamente. • Le trappole in AlGaN-GaN HEMTs sono caratterizzate usando le tecniche di DLTS e il relativo comportamento di carica/scarica é interpretato con l’aiuto delle simulazioni numeriche. Sotto particolari condizioni di polarizzazione, le trappole di buffer possono produrre falsi segnali da trappole di superficie, ossia lo stesso tipo di segnali I-DLTS e forma d’onda di gate lag attribuiti generalmente alle trappole di superficie. Chiarire questo aspetto è molto importante sia per le prove di affidabilità che per l’ottimizzazione dei dispositivi, in quanto può provocare una errata identificazione del meccanismo di degradazione, portando ad azioni correttive sbagliate nell’ottimizzazione del processo tecnologico. • I meccanismi fisici che originano il collasso di corrente RF negli HEMT AlGaN-GaN sono analizzati usando misure sperimentali e simulazioni numeriche. Questo lavoro suggerisce le seguenti condizioni: i) sia le trappole di superficie che quelle di buffer possono contribuire al collasso di corrente RF tramite un simile meccanismo fisico che coinvolge la cattura e l’emissione di elettroni provenienti dal gate; ii) la passivazione della superficie diminuisce fortemente il collasso della corrente RF tramite la riduzione del campo elettrico in superficie e la conseguente diminuzione dell’iniezione di elettroni dal gate alle trappole; iii) per densità di trappole di superficie minori di 9 × 1012 cm-2 , barriere di potenziale superficiale di 1-2 eV possono coesistere con trappole di superficie aventi energie relativamente basse e che provocano effetti di collasso di corrente RF caratterizzati da costanti di tempo relativamente corte. • Gli effetti di collasso di corrente negli HEMT AlGaN-GaN sono studiati usando i risultati delle misure sperimentali e delle simulazioni numeriche. Basandosi sulle misure delle caratteristiche d’uscita impulsate, i dispositivi utilizzati mostrano un evidente gate-lag e un trascurabile drain-lag, attribuiti alla presenza di trappole di superficie e buffer rispettivamente.

Doctor en Ingeniería Eléctrica / Este trabajo presenta el diseño, construcción y caracterización de dispositivos para receptores radioastronómicos en las bandas Q (30-50GHz) y W (80-110GHz). Por un lado, el dispositivo desarrollado para la banda Q es de interés para la banda 1 del telescopio argentino-brasileño LLAMA (Long Latin American Array). Por otro lado, los componentes de banda W pueden ser utilizados en la banda 3 de LLAMA o en posibles mejoras para el Telescopio Austral de Ondas Milimétricas (SMWT) que es mantenido por nuestro grupo. Para la banda Q, se diseñó y construyó un amplificador de bajo ruido utilizando un esquema hibrido de integración. Se integró un transistor de alta movilidad electrónica (HEMT) y un circuito integrado monolítico de microondas (MMIC) obtenido comercialmente. Con este diseño una temperatura de ruido inferior a 20 K y una ganancia superior a 30 dB pueden ser obtenidas. En la banda W se desarrollaron varios componentes. En primer lugar, se empaquetaron amplificadores comerciales MMIC de las compañías OMMIC y HRL. Al ser medidos a 15K estos amplificadores de bajo ruido alcanzaron temperaturas de ruido menores a 100K y ganancias superiores a 17 dB. Dada su disponibilidad comercial se determinó que son apropiados para ser utilizados como segundo amplificador en un receptor. Segundo, utilizando diodos Schottky comerciales, se fabricaron mezcladores sub-armónicos que cubren la banda W extendida. Las técnicas de desarrollo han variado desde el uso de componentes discretos hasta el diseño de MMICs para reducir el tamaño de los mezcladores. Los componentes mencionados previamente han sido ensamblados en un módulo compacto que puede ser utilizado en la etapa de mezcla de frecuencias. Este módulo posee una temperatura de ruido menor a 800 K y ganancia superior a 2dB a temperatura ambiente. Finalmente, se construyó una antena de ranura cuyo perfil ha sido optimizado para mejorar sus principales características (reflexiones, ancho de banda, polarización cruzada y simetría de haz). Entre las antenas de su tipo, esta es la única que posee un perfil optimizado lo que ha permitido obtener el mejor funcionamiento alcanzado hasta el momento. / Este trabajo ha sido parcialmente financiado por el Proyecto Gemini-Conicyt 32130023, Centro Basal de Astronomía y Tecnologías Afines (CATA), "Programa de Formación de Capital Humano Avanzado" de CONICYT y el Comité Mixto ESO-Chile

Zugl.: Kassel, Univ., Diss., 2008

Zugl.: Kassel, Universiẗat, Diss., 2009.