Electroabsorption and electrorefraction were studied in GaAs/AlGaAs and InP/InAsP multiple quantum well waveguides. Measurements of changes of the absorption coefficient and the refractive index with wavelength and bias voltage were made. Switching ratios of up to 18 dB were obtained for the GaAs/AlGaAs material. The Kramers-Kronig relation was used to calculate the theoretical phase shifts from the absorption coefficient data. / Thesis / Master of Engineering (ME)

The growth optimization and fabrication of 980nm quantum well (QW) lasers is presented. Photoluminescence (PL) spectroscopy is used to determine the optimized growth conditions for the QWs. The results are presented for optimization of both growth temperature and group V overpressure. Broad area lasers, with active regions grown at and around optimized QW growth conditions, are fabricated and characterized under pulsed conditions. These results are used to determine the optimum growth conditions for a ridge waveguide (RWG) laser structure. Once grown, RWG lasers are fabricated and characterized under continuous wave (CW) conditions. External quantum efficiencies as high as 71 % and cavity losses as low as 5.2 cm-1 are achieved. / Thesis / Master of Engineering (ME)

Yes / This work develops a novel dynamic load modulation Power Amplifier (PA) circuity that can provide an optimum compromise between linearity and efficiency while covering multiple cellular frequency bands. Exploiting monolithic microwave integrated circuits (MMIC) technology, a fully integrated 1W Doherty PA architecture is proposed based on 0.1 µm AlGaAs/InGaAs Depletion- Mode (D-Mode) technology provided by the WIN Semiconductors foundry. The proposed wideband DPA incorporates the harmonic tuning Class-J mode of operation, which aims to engineer the voltage waveform via second harmonic capacitive load termination. Moreover, the applied post-matching technique not only reduces the impedance transformation ratio of the conventional DPA, but also restores its proper load modulation. The simulation results indicate that the monolithic drive load modulation PA at 4 V operation voltage delivers 44% PAE at the maximum output power of 30 dBm at the 1 dB compression point, and 34% power-added efficiency (PAE) at 6 dB power back-off (PBO). A power gain flatness of around 14 ± 0.5 dB was achieved over the frequency band of 23 GHz to 27 GHz. The compact MMIC load modulation technique developed for the 5G mobile handset occupies the die area of 3.2. / This research was funded by the European Regional Development Fund (FEDER), through COMPETE 2020, POR ALGARVE 2020, Fundação para a Ciência e a Tecnologia (FCT) under i-Five Project (POCI-01-0145-FEDER-030500). This work is also part of the POSITION-II project funded by the ECSEL joint Undertaking under grant number Ecsel-345 7831132-Postitio-II-2017-IA. This work is supported by FCT/MCTES through national funds and when applicable co-funded EU funds under the project UIDB/50008/2020-UIDP/50008/2020. The authors would like to thank the WIN Semiconductors foundry for providing the MMIC GaAs pHEMT PDKs and technical support. This work is supported by the Project TEC2017-88242-C3-2-R- Spanish Ministerio de Ciencia, Innovación e Universidades and EU-FEDER funding.

The rapid development of wireless applications has created a demand for low-cost, compact, low-power hardware solutions. This demand has driven efforts to realize fully integrated, "single-chip" systems. While substantial progress had been made in the integration of many RF and baseband processing elements through the development of new technologies and refinements of existing technologies, progress in the area of fully monolithic filters has been limited due to the losses (low Qs) associated with integrated passive elements in standard IC processes. The work in this thesis focuses on the development low-loss, Q-enhanced LC filters in GaAs E/D-SAGFET technology. This thesis presents a methodology for designing Q-enhanced LC resonators and low-loss, monolithic LC filters based on these resonators. The first phase of this work focused on the Q-enhancement of LC resonator structures using FET-based active negative resistance circuits. Three passive resonators were designed, fabricated, and measured to determine their loss and frequency response. Furthermore, six Q-enhanced resonators were designed, fabricated, and measured to compare the performance of various negative resistance circuit designs. In the second phase of this work, four of these Q-enhanced resonator designs were used to implement fully-integrated second-order Butterworth bandpass filters. Each filter was designed for a 60 MHz, -3 dB bandwidth centered at 1.88 GHz, corresponding to the North American PCS transmit band. The best filter design achieves 0 dB of passband insertion loss while consuming 16 mA of current from a 3 V source (48 mW). Passband gain (up to 15 dB) can be achieved with increased bias current before instability is encountered. The filter provides more than 30 dB of rejection at 1.7 and 2 GHz and more than 70 dB of rejection below 1.5 GHz. In the filter passband, the noise figure is 12 dB and the output 1 dB compression point is -18 dBm. These Q-enhanced LC filters have potential application as image-reject filters in GaAs integrated transceiver designs. / Master of Science

Nous avons proposé d'étudier le potentiel de l'outil d'épitaxie HVPE (Hydride Vapour Phase Epitaxy) pour la croissance de nanofils de GaAs. La morphologie nanofil permet au matériau épitaxié de libérer les contraintes dans le cas de l’hétéroépitaxie et de mettre en œuvre des procédés de croissance sur des substrats à faible coût comme les substrats de silicium. Dans ce contexte, j’ai effectué la croissance auto-catalysée de nanofils et des nanoobjets de GaAs par HVPE sur substrat silicium. La HVPE utilise des molécules de GaCl synthétisées à l'intérieur du réacteur dans la zone en amont du substrat à haute température (T> 700 °C). La décomposition du GaCl est beaucoup plus difficile entre 600 °C et 700 °C. Dans ce manuscrit des calculs thermodynamiques des constantes d’équilibre de formation du gallium liquide sont donnés et analysés. Les rapports de flux atomiques III/V obtenus sont de 11 à 222, bien plus élevés que les rapports utilisés dans procédés MBE et MOVPE. Ce travail expérimental a été couplé à un travail de modélisation théorique. Une étude de la faisabilité d’utiliser un substrat constitué de nanofils de GaAs comme électrode dans une pile microbienne a également été initiée. / III-V semiconductor nanowires exhibit excellent electrical and optical properties in laterally confined geometry which is very promising for monolithic integration of photonic nanodevices on silicon substrates. Hydride Vapor Phase Epitaxy (HVPE) process growth was therefore developed in this thesis for the growth of GaAs nanowires. This report is organized into two chapters.The first chapter introduces the state-of-the-art of self-catalyzed GaAs nanowires and nano-structures on silicon substrate. We have demonstrated the growth of self - catalyzed GaAs nanowires by HVPE on un-patterned Si (111) substrates at a low temperature of 600 °C with extremely high GaCl/AsH3 flow ratios. A model that explains well the experimental findings was developed. The second part proposes the design of a Microbial Fuel Cell (MFC) prototype based on GaAs nanowire samples. A MFC prototype based on GaAs nanowire and substrate was developed.

In this thesis, a detailed study of the electrical and optical characterisations of Heterojuction Bipolar Transistors (HBTs) for optical detection is presented. By comparing both DC and optical characterisations between In0.49Ga0.51P/GaAs Single Heterojuction Bipolar Transistors (SHBTs) and Double Heterojuction Bipolar Transistors (DHBTs), the advantages of using the DHBT as a short wavelength detector are shown. Phenomena related to the base region energy band bending in the DHBT caused by a self-induced effective electric field is discussed and its effects on the performance of the device are elaborated. The use of an eye diagram has been employed to provide requisite information for performance qualification of SHBT/DHBT devices. These give a more detailed understanding compared to conventional S-parameters method. A detailed comparison of In0.49Ga0.51P/GaAs SHBT and DHBT performance using an eye diagram as a functional tool by adopting a modified T-shaped small signal equivalent circuit are given. By adopting this modified T-shaped small signal equivalent circuit, the use of In0.49Ga0.51P/GaAs Double Heterojuction Phototransistors (DHPT) as a short wavelength photodetector is analysed. It is therefore shown that an eye diagram can act as a powerful tool in HBTs/HPTs design optimisations, for the first time in this work. In order to predict the spectral response (SR) and optical characterisations of GaAs-based HPTs, a detailed theoretical absorption model is also presented. The layer dependence of an optical flux absorption profile, along with doping dependent absorption coefficients are taken into account for the optical characterisation prediction. With the aim of eliminating the limitation of current gain as a prerequisite, analytical modelling of SR has been developed by resolving the continuity equation and applying realistic boundary conditions. Then, related physical parameters and a layer structure profile are used to implement simulations. A good agreement with the measured results of the Al0.3Ga0.7As/GaAs HPT is shown validating the proposed theoretical model.

No description available.

Le manuscrit traite de l'épitaxie en phase vapeur par la méthode aux hydrures (HVPE) de micro- et nanofils Ga(In)N et GaAs. La HVPE est une méthode de croissance originale qui utilise des précurseurs en éléments III chlorés permettant des vitesses de croissance importantes. La croissance sélective de réseaux de microfils GaN sur des substrats silicium avec et sans couche tampon d'AlN, masqués par un diélectrique, est étudiée. Nous montrons que sans la couche tampon, la nucléation de plusieurs fils par ouverture a lieu. Par contre, l'emploi d'une couche tampon d'AlN entre le masque diélectrique et le substrat silicium permet la synthèse de réseaux de fils de grande qualité cristalline et optique par HVPE. Une étude théorique et expérimentale de la croissance d'InGaN par HVPE est effectuée. Les résultats indiquent qu'avec un précurseur pour l’élément indium de type InCl, la synthèse d'InGaN est très difficile ; mais, qu'elle est en revanche facilitée par l'emploi d'un précurseur de type InCl 3 . Nous démontrons la croissance de nanofils GaN/AlN coeur/coquille sur substrat saphir plan c en une seule étape. Un mécanisme original mixte VLS-VS est proposé en guise d'explication. La stabilité de la phase Zinc-Blende de nanofils GaAs, ultra-longs et de diamètre 10 nm, obtenus par un procédé VLS catalysé Au, est démontrée pour la première fois expérimentalement et est expliquée grâce à un modèle thermodynamique et cinétique de nucléation. / The manuscript deals with the growth of Ga(In)N and GaAs micro- and nanowires by hydride vapor phase epitaxy (HVPE). HVPE is an original growth process with very high growth rates. This particular feature is due to the use of chloride molecules as element III precursors. Selective area growth of arrays of GaN microwires on silicon substrates covered by a dielectric mask, with or without an intermediate AlN buffer layer, is studied. We show that without the AlN buffer layer, nucleation of many wires in a single opening cannot be prevented. On the other hand, with an intermediate AlN buffer layer between the silicon substrate and the dielectric mask, the growth of arrays of microwires with high crystalline and optical properties is achieved. A theoretical and experimental study of the growth of InGaN is carried out. Results show that with InCl as indium precursor, synthesis of InGaN is difficult, but the use of InCl 3 precursors makes it easier. The growth of core/shell GaN/AlN nanowires on c-sapphire substrates in a single step process is demonstrated. A mixed VLS/VS growth mechanism is proposed as explanation. The stability of the Zinc-Blende phase in ultra-long and 10 nanometers in diameter GaAs nanowires grown by Au-assisted VLS is experimentally demonstrated for the first time. This is successfully explained by a nucleation model involving thermodynamic and kinetic considerations.

L’intégration du GaAs sur Si est un des défis majeurs des 40 dernières années puisqu’elle permettrait de combiner les nombreux avantages du Si, dont notamment son bas coût, avec les propriétés de haute mobilité et de gap direct du GaAs. Les cellules photovoltaïques multi-jonctions à base de matériau III-V permettent d’obtenir les plus hauts rendements de conversion photovoltaïque. Cependant, leur coût de fabrication élevé est un aspect limitatif de leur utilisation. Nous nous sommes intéressés ici à une étude préliminaire visant à réaliser leur intégration sur substrat Si. In fine, l’objectif sera la réalisation de cellules tandems GaAs/Si et GaAs/Ge sur substrat Si. L’intégration du GaAs et du Ge sur Si conduit cependant à la formation de dislocations et de fissures du fait de leurs désaccords de maille et de leurs différences de coefficient d’expansion thermique respectifs. De plus, du fait de la différence de polarité entre le GaAs et le Si, cette intégration conduit également à la formation de domaine d’anti-phase. Nous présentons dans cette étude un procédé d’intégration permettant à la fois l’élimination de ces défauts et le passage du courant entre le matériau épitaxié et le Si. Ce procédé est basé sur l’utilisation d’ouvertures de tailles nanométriques dans une silice fine, qui nous permet ainsi de réaliser la croissance du GaAs sur Si sous forme de cristaux, par épitaxie latérale à partir de nano-germes de GaAs ou de Ge. Pour ce faire, nous utilisons l’épitaxie par jet chimique sans gaz vecteur qui est une technique de croissance permettant une bonne sélectivité. La croissance sera tout d’abord étudiée dans des ouvertures aléatoires, facilement réalisées in-situ sous ultravide, puis dans des ouvertures localisées de tailles fixées. Ces dernières sont obtenues suite à une procédure longue et complexe qui repose sur des étapes de nettoyage chimique, d’enrésinement, de lithographie électronique, de développement et de gravure ionique réactive. Nous présenterons les résultats de la croissance directe de cristaux de GaAs dans les ouvertures sur Si (001) et Si (111), et également à partir de nano-germes de Ge. Ce procédé d’intégration a permis l’élimination des trois types de défauts précédemment indiqués, et nous avons obtenu de très bons résultats notamment lors de l’intégration dans les ouvertures localisées sur Si (111). Nous verrons que la morphologie des nano-germes de Ge peut toutefois être problématique lors de la reprise d’épitaxie du GaAs. La possibilité de passage du courant par effet tunnel à travers la silice fine sera ensuite vérifiée et le dopage des cristaux de GaAs avec du Si sera également présenté. / GaAs on Si integration is one of the major challenges of the last 40 years as it would allow to combine Si advantages, like its low cost, with GaAs high mobility and direct bandgap. Multi-junction photovoltaic cells based on III-V materials have the highest photovoltaic conversion efficiencies. However, their high manufacturing cost is a limiting aspect of their use. This is why we have made a preliminary study aiming at realizing their integration on Si substrate. In fine, the objective will be the realization of tandem solar cells made of GaAs/Si and GaAs/Ge on Si substrate. However, GaAs and Ge integrations on Si lead to dislocations and cracks formations because of their respective differences of lattices parameters and thermal expansion coefficients. Moreover, because of the difference of polarity between GaAs and Si, this integration also leads to anti-phase domain formation. We present in this study an integration process allowing both these defects elimination and current passage between the epitaxial material and Si. This process is based on the use of nanoscale openings in a thin silica, which allows us to carry out GaAs crystals growth on Si by lateral epitaxy from GaAs or Ge nano-seeds. To do this, we use chemical beam epitaxy which is a growth technique allowing good selectivity. Firstly, the growth will be studied inside randomly dispersed openings, which are easily made in situ under ultra-high vacuum, and then inside localized openings with fixed sizes. These are obtained after a long and complex procedure including chemical cleaning, resist spin-coating, electronic lithography, development and reactive ion etching. We will present GaAs crystals direct growth inside openings on Si (001) and (111), and also from Ge nano-seeds. This integration process allowed the elimination of the three types of defects previously mentioned, and we have obtained very good results especially for the integration inside localized openings on Si (111). We will see that Ge nano-seeds morphology can however be problematic during the GaAs lateral epitaxy. In addition, the current passage by tunnel effect through the thin silica will be verified and the GaAs crystals doping with Si will also be presented.

Electromagnetic radiation in the frequency range between about 100 GHz and 5 THz can be used for spectroscopy and microscopy, but it is also promising for security screening and even wireless communication. In the present thesis a planar photoconducting large-area THz radiation source is presented. The device exhibits outstanding properties, in particular high THz field strength and generation efficiency and large spectral bandwidth with short THz pulse length. The THz emission is based on acceleration and deceleration of photoexcited carriers in semiconductor substrates. A metallic interdigitated structure at the surface of semi-insulating GaAs provides the electrodes of an Auston switch. In a biased structure photoexcited charge carriers are accelerated. Hence electromagnetic waves are emitted. An appropriately structured second metallization, electrically isolated from the electrodes, prevents destructive interference of the emitted waves. The structure investigated here combines several advantages of different conventional photoconducting THz sources. First, it provides high electric acceleration fields at moderate voltages owing to the small electrode separation. Second, the large active area in the mm2 range allows excitation by large optical powers of some mW. Optical excitation with near-infrared femtosecond lasers is possible with repetition rates in the GHz range. The presented results point out the excellent characteristics regarding the emitted THz field strength, average power, spectral properties, and easy handling of the interdigitated structure in comparison to various conventional emitter structures. Various modifications of the semiconductor substrate and the optimum excitation conditions were investigated. In the second part of this thesis the dynamic conductivity of GaAs/AlxGa1-xAs superlattices in an applied static electric field was investigated with time-resolved THz spectroscopy. The original goal was to explore whether the predicted effect of gain of electromagnetic radiation at THz frequencies is present in such structures. Superlattice samples were grown according to the experimental requirements, which include high specific resistance and sufficient THz transparency. The characterization of the superlattices by Fourier transform infrared spectroscopy and photoluminescence spectroscopy confirms the pronounced miniband properties of the bandstructure. Furthermore indications of Bloch oscillations were found by transport measurements. However, we could not measure a change of the dynamic conductivity when the electric field was toggled. Specific reasons for this and related experiments of other groups are discussed. / Elektromagnetische Strahlung im Frequenzbereich zwischen etwa 100 GHz und 5 THz wird für verschiedene Anwendungen wie Spektroskopie und Mikroskopie genutzt, kann aber auch für Sicherheitstechnik oder sogar Datenübertragung interessant sein. In der hier vorgestellten Forschungsarbeit wird eine großflächige fotoleitende THz-Strahlungsquelle beschrieben, die sich durch eine große THz-Feldstärke und große spektrale Bandbreite auszeichnet. Die THz-Emission basiert auf der Beschleunigung und Verzögerung fotogenerierter Ladungen in Halbleitersubstraten. Eine metallische Interdigitalstruktur auf der Oberfläche von semi-isolierendem GaAs bildet die Elektroden eines Fotoschalters. Ist an diese Struktur eine Spannung angeschlossen, werden optisch generierte Ladungsträger beschleunigt und strahlen elektromagnetische Wellen ab. Eine geeignet strukturierte und isolierte zweite Metallisierung verhindert destruktive Interferenzen der abgestrahlten Wellen. Die vorgeschlagene Struktur vereinigt dabei die Vorteile verschiedener herkömmlicher fotoleitender THz-Quellen. Einerseits ermöglicht der kleine Elektrodenabstand große elektrische Felder zur Beschleunigung fotogenerierter Ladungen schon bei moderaten Spannungen. Andererseits kann die große aktive Fläche von einigen mm2 mit großen optischen Leistungen im Bereich einiger mW angeregt werden. Die optische Anregung mit Nahinfrarot-Femtosekunden- Lasern kann mit Wiederholraten bis in den GHz-Bereich geschehen. Bedingt durch die Eigenschaften der Anregungspulse entstehen kurze spektral breite THz-Pulse. Die vorliegenden Ergebnisse verdeutlichen die hervorragenden Eigenschaften der Interdigitalstruktur im Vergleich zu verschiedenen herkömmlichen Geometrien bezüglich der Feldstärke der abgestrahlten Wellen, der mittleren Leistung und der spektralen Eigenschaften. Dabei ist die Struktur sehr einfach zu handhaben. Es wurden verschiedene Modifikationen des Substrates und die optimalen Bedingungen der optischen Anregung untersucht. Der zweite Teil dieser Arbeit behandelt die Erforschung der dynamische Leitfähigkeit von GaAs/AlxGa1-xAs-Übergittern in Abhängigkeit von einem elektrischen Feld mit Hilfe der zeitaufgelösten THz-Spektroskopie. Es sollte geklärt werden, ob der vorhergesagte Effekt der Verstärkung elektro-magnetischer Strahlung in solchen Strukturen möglich ist. Dazu wurden Übergitterproben gemäß den experimentellen Anforderungen hergestellt. Zu den Vorgaben gehört ein hoher spezifischer Widerstand und ausreichende Transparenz im THz-Bereich. Die Charakterisierung der Übergitter mit Fotolumineszenz- und Fourier-Transformations-IR-Spektroskopie bestätigte die ausgeprägten Minibandeigenschaften der Bandstruktur. Hinweise auf Bloch-Oszillationen wurden durch Ladungstransportmessungen gefunden. Dennoch war eine Änderung der dynamischen Leitfähigkeit beim Schalten des elektrischen Feldes nicht messbar. Gründe dafür und ähnliche Experimente anderer Gruppen werden diskutiert.