Ce travail a été réalisé au Centre de Recherche en Nanofabrication et Nanocaractérisation (CRN[indice supérieur 2]) de l'Université de Sherbrooke. Il porte sur l'étude, la réalisation et la caractérisation de structures métal - isolant - semiconducteur (MIS) sur nitrure de gallium. Le nitrure de gallium (GaN) est un matériau semiconducteur de la famille III-V à large bande interdite directe, ayant des propriétés électriques, physiques et mécaniques très intéressantes. Il a été découvert depuis plus de quatre décennies. Les difficultés de son élaboration, les problèmes d'inefficacités du dopage p et les densités élevées des défauts cristallins dans les couches épitaxiées, ont constitué pendant longtemps des obstacles majeurs au développement de la technologie GaN [I. Akasaki, 2002]. Il a fallu attendre jusqu'au début des années 1990 pour voir apparaître des couches de meilleure qualité et pour améliorer l'efficacité du dopage p [J.Y. Duboz, 1999]. Cet événement a été l'étape majeure qui a révolutionné la technologie à base de GaN et a permis d'amorcer son intégration dans le milieu industriel. Depuis, la technologie à base de ce matériau ne cesse de progresser à un rythme exponentiel. Il se présente aujourd'hui comme un matériau de choix pour la réalisation de dispositifs électroniques de puissances et de hautes fréquences pouvant fonctionner dans des milieux hostiles. Grâce à sa bande interdite directe et son pouvoir d'émission à faible longueur d'onde, il est aussi très convoité pour la réalisation de dispositifs optoélectroniques de hautes performances en émission ou en détection, tels que les DELs, Lasers ou les photodétecteurs. Malgré l'avancé rapide qu'a connu le GaN, certains aspects de ce matériau restent encore mal maîtrisés, tels que la réalisation de contacts ohmiques de bonne qualité ou encore le contrôle des interfaces métal/GaN et isolant/GaN. Les hétérostructures isolant/GaN sont caractérisées par une forte densité d'états de surface (D[indice inférieur it]). Ce phénomène, aussi rapporté sur GaAs et sur la plupart des matériaux III-V, induit l'ancrage du niveau de fermi au centre de la bande interdite. Il constitue l'un des freins majeurs au développement d'une technologie MIS (MOS) fiable sur GaN. À travers ce document, nous rapportons les résultats des travaux entrepris pour la réalisation de capacités MIS, de contacts ohmiques et de diodes Schottky sur les deux types de substrat GaN et p-GaN. Le diélectrique utilisé comme couche isolante pour les structures MIS est le Nitrure de Silicium (Si[indice inférieur x]N[indice inférieur y]) déposé par PECVD. Ces travaux constituent une introduction aux procédés de microfabrication sur nitrure de gallium, aux difficultés liées aux effets de surface dans le GaN et aux étapes de préparation chimique en vue de minimiser la densité de charges d'état à l'interface métal/GaN et diélectrique/GaN. La première partie du document est dédiée à la caractérisation optique et électrique des substrats GaN utilisés par étude de spectroscopie de photoluminescence (PL) et par étude Schottky.

The objective of this PhD research was to design and implement novel microfluidic radio-frequency (RF) structures on multilayer organic substrates for cooling and tuning purposes. The different designs were implemented to target applications up to C-band (4 GHz – 8 GHz) frequencies. The system-on-package (SoP) solution adopted throughout this work is well adapted for such designs where there is a need to integrate the functionality of different sub-components into a single hybrid fully packaged system. The first part of the thesis is dedicated to the study of a specific liquid cooling scheme using integrated microchannels on organics placed beneath different types of heat sources. A 1 W gallium nitride (GaN) die was cooled using this method and an analysis is presented regarding the cases where the coolant is static or dynamic inside the microchannel. The second part of the thesis deals with microfluidically reconfigurable microstrip RF circuits, mainly bandpass filters and power amplifiers (PAs). The microfluidic tuning technique is based on the change in the effective dielectric constant that the RF signal “sees” when traveling above two microchannels with different fluids. This technique was used to shift the frequency response of an L-band microstrip bandpass filter by replacing DI water with acetone inside a 60 mil micro-machined cavity. This technique was also used to design reconfigurable matching networks which constitute the main part of the proposed tunable GaN-based PA for S- and C-band applications. The final part of the thesis expands the previous results by combining both cooling and tuning in a single RF design. To prove the concept, cooling and tuning microchannels were integrated into a single package to cool a GaN-based PA and tune its frequency response at the same time from 2.4 GHz to 5.8 GHz and vice versa.

Les diodes électroluminescentes (LEDs) bleues ou blanches actuelles sont constituées de couches épitaxiales planaires, essentiellement à base de GaN. Sans autres opérations technologiques, la réflexion totale interne aux interfaces réduit le nombre de photons extraits à quelques pourcents du nombre de photons émis. Cette thèse s'intéresse à un concept en rupture : les LEDs à nanofils GaN ou ZnO. Plus précisément elle vise à préciser l'intérêt de ces couches pour l'augmentation du rendement d'extraction. Nous nous sommes plus particulièrement intéressés à trois types de couches (taille des fils, arrangement), chacune mettant en jeu un processus d'extraction différent. La première géométrie, basée sur des fils relativement gros (rayons 50-100nm minimun) et distants vise à profiter des résonances ou du guidage optique pouvant exister au sein de chaque fil pour canaliser l'émission spontanée. Les coefficients de couplage de la couche active sur ces modes ainsi que la réflectivité des modes guidés en bout de fils ont notamment été évalué numériquement en fonction de la taille des fils. La seconde approche, issue de l'étude goniométrique de couches de fils MBE sur substrat Silicium, vise à profiter des propriétés d'indice effectif des couches de fils sublongueurs d'onde pour éviter le phénomène de réflexion totale à l'origine des faibles rendements d'extraction des couches planaires. Le modèle anisotrope numérique développé montre qu'un rendement d'extraction proche de 70% est envisageable sur ce type de couche épitaxiée sur Silicium. Enfin la troisième approche, plus prospective, vise à initier une étude sur l'émission dans des réseaux de fils agencés périodiquement.

Ce travail concerne l'intégration, par épitaxie sous jets moléculaires (EJM), de matériaux nitrures d'éléments III (en particulier GaN) sur des substrats et couches tremplins à base d'oxyde de zinc (ZnO). L'objectif était la réalisation et l'étude d'hétérostructures nitrures de type puits quantiques (PQs) (Al,Ga)N/GaN et (In,Ga)N/GaN, en vue d'évaluer leurs potentialités pour la réalisation de diodes électroluminescentes (LEDs). En particulier, deux orientations cristallographiques ont été étudiées : le plan " polaire " (0001) (dit plan C) et le plan " non polaire " (11-20) (dit plan A). Les couches de GaN orientées suivant le plan A (11-20), " a-GaN ", ont été épitaxiées sur des tremplins de (Zn, Mg)O (11-20) / saphir (10-12) réalisés par EJM. L'anisotropie de la morphologie de surface, de la microstructure cristalline, ainsi que de l'émission optique des couches de a-GaN, a été mise en évidence. Une série d'échantillons de PQs de a-(GaN/Al0.2Ga0.8N) avec des épaisseurs de puits différentes a été fabriquée, et l'absence d'effet Stark quantique confiné dans ces hétérostructures a été établie. Des procédés de croissance de GaN sur des substrats de ZnO massifs d'orientation A, " a-ZnO ", et C, " c-ZnO ", ont également été développés. En particulier, des couches de GaN (0001), " c-GaN ", avec une polarité Ga- ou N- ont été épitaxiées sur la face O de substrats c-ZnO. Les mécanismes de détermination de la polarité ont été analysés. Des LEDs bleues contenant une zone active constituée de PQs (In, Ga)N / GaN ont été réalisées sur des substrats c-ZnO. Des puissances de sortie atteignant 40 µW à 20 mA et 0,1 mW à 60 mA ont été mesurées. Enfin, des PQs (In, Ga)N / GaN ont été fabriqués sur substrats a-ZnO et comparés à des PQs fabriqués sur c-ZnO avec des conditions de croissance équivalentes. Les résultats indiquent une concentration en In plus importante dans le cas des PQs épitaxiés sur c-ZnO et une polarisation de l'émission de PL suivant la direction <1-100> dans le cas des PQs épitaxiés sur a-ZnO.

This dissertation details the study of band bending in n- and p-type GaN samples with a Kelvin probe utilizing different illumination geometries, ambients (air, oxygen, vacuum 10-6 mbar), and sample temperatures (77 – 650 K). The Kelvin probe, which is mounted inside an optical cryostat, is used to measure the surface potential. Illumination of the GaN surface with band-to-band light generates electron-hole pairs, which quickly separate in the depletion region due to a strong electric field caused by the near-surface band bending. The charge that is swept to the surface reduces the band bending and generates a surface photovoltage (SPV). Information about the band bending can be obtained by fitting the SPV measurements with a thermionic model based on the emission of charge carriers from bulk to surface and vice versa. The band bending in freestanding n-type GaN templates has been evaluated. The Ga-polar and N-polar surfaces exhibit upward band bending of about 0.74 and 0.57 eV, respectively. The surface treatment also plays a major role in the SPV behavior, where the SPV for mechanical polished surfaces restores faster than predicted by a thermionic model in dark. When measuring the photoluminescence (PL) signal, the PL from mechanically polished surfaces was about 4 orders of magnitude smaller than the PL from chemically mechanically polished surfaces. The PL and SPV behaviors were explained by the presence of a large density of defects near the surface, which quench PL and aid in the restoration of the SPV via electron hopping between defects. Temperature-dependent SPV studies have also been performed on doped n- and p-type GaN samples. In Si-doped n-type GaN, the estimated upward band bending was about 1 eV at temperatures between 295 and 500 K. However, in p-type GaN, the downward band bending appeared to increase with increasing temperature, where the magnitude of band bending increased from 0.8 eV to 2.1 eV as the temperature increased from 295 to 650 K. It appears that heating the p-type GaN samples allows for band bending values larger than 1 eV to fully restore. Pre-heating of samples was of paramount importance to measure the correct value of band bending in p-type GaN. The slope of the dependence of the SPV on excitation intensity at low temperatures was larger than expected; however, once the temperature exceeded 500 K, the slope began to reach values that are in agreement with a thermionic model.

The development of gallium nitride (GaN) light emitting devices has reached extraordinary echelons. As such, the characterization and analysis of the behavior of GaN materials is essential to the advancement of GaN technology. In this thesis, the effect of temperature on the optical and electrical properties of p-type GaN is investigated. The GaN samples used in this work were grown by various methods and studied by Kelvin probe and photoluminescence (PL) techniques. Specifically, the surface photovoltage (SPV) behavior and PL data were analyzed at different temperatures and illumination intensities. Using the SPV results, we show that p-type GaN exhibits n-type conductivity at low temperatures (80 K). If the sample is heated beyond a characteristic temperature, TC, the conductivity reverts to p-type. This temperature of conversion can be tuned by varying the illumination intensity. We explain this conductivity conversion using a simple, one-acceptor phenomenological model. Temperature-dependent PL measurements taken on Mg-doped p-type GaN layers show abrupt and tunable thermal quenching of the PL intensity. This effect is explained by a more complex model but with the same assertions, that the system must undergo a change in conductivity at low temperatures and under UV illumination. It is necessary to understand the observed behaviors, since the implications of such could have an effect on the performance of devices containing p-type GaN materials.

Because the surface plays an important role in the electrical and optical properties of GaN devices, an improved understanding of surface effects should help optimize device performance. In this work, atomic force microscopy (AFM) and related techniques have been used to characterize three unique sets of n-type GaN samples. The sample sets comprised freestanding bulk GaN with Ga polar and N polar surfaces, epitaxial GaN films with laterally patterned Ga- and N-polar regions on a common surface, and truncated, hexagonal GaN microstructures containing Ga-polar mesas and semipolar facets. Morphology studies revealed that bulk Ga-polar surfaces treated with a chemical-mechanical polish (CMP) were the flattest of the entire set, with rms values of only 0.4 nm. Conducting AFM (CAFM) indicated unexpected insulating behavior for N-polar GaN bulk samples, but showed expected forward and reverse-bias conduction for periodically patterned GaN samples. Using scanning Kelvin probe microscopy, these same patterned samples demonstrated surface potential differences between the two polarities of up to 0.5 eV, where N-polar showed the expected higher surface potential. An HCl cleaning procedure used to remove the surface oxide decreased this difference between the two regions by 0.2 eV. It is possible to locally inject surface charge and measure the resulting change in surface potential using CAFM in conjunction with SKPM. After injecting electrons using a 10 V applied voltage between sample and tip, the patterned polarity samples reveal that the N-polar regions become significantly more negatively charged as compared to Ga-polar regions, with up to a 2 eV difference between charged and uncharged N polar regions. This result suggests that the N-polar regions have a thicker surface oxide that effectively stores charge. Removal of this oxide layer using HCl results in significantly decreased surface charging behavior. A phenomenological model was then developed to fit the discharging behavior of N-polar GaN with good agreement to experimental data. Surface photovoltage (SPV) measurements obtained using SKPM further support the presence of a thicker surface oxide for N polar GaN based on steady state and restoration SPV behaviors. Scanning probe microscopy techniques have therefore been used to effectively discriminate between the surface morphological and electrical behaviors of Ga- vs. N-polar GaN.

There is an understanding of surface photovoltage (SPV) behavior for GaN, yet little is known about the SPV behavior for AlGaN. In this work, a Kelvin probe was used to measure the SPV for p-type AlGaN. Very slow SPV transients were found in AlGaN, which could not be explained with a simple thermionic model. A possible explanation of this behavior is the segregation of impurities to the surface, which causes significant reduction of the depletion region width (down to 2 nm), with carrier tunneling and hopping becoming the dominant mechanisms responsible for the SPV transients. To verify this assumption, the near-surface defective region (about 40 nm) has been removed through the ICP-RIE process. After the etching, the SPV transients became fast and increased in magnitude by about 0.6 eV. By using the thermionic model, band bending was estimated to be -1 eV.

This dissertation is focused on the research efforts to develop the growth, processing, and modeling technologies for GaN-based Heterojunction Field Effect Transistors (HFETs). The interest in investigating GaN HFETs is motivated by the advantageous material properties of nitride semiconductor such as large band gap, large breakdown voltage, and high saturation velocity, which make it very promising for the high power and microwave applications. Although enormous progress has been made on GaN transistors in the past decades, the technologies for nitride transistors are still not mature, especially concerning the reliability and stability of the device. In order to improve the device performance, we first optimized the growth and fabrication procedures for the conventional AlGaN barrier HFET, on which high carrier mobility and sheet density were achieved. Second, the AlInN barrier HFET was successfully processed, with which we obtained improved I-V characteristics compared with conventional structure. The lattice-matched AlInN barrier is beneficial in the removal of strain, which leads to better carrier transport characteristics. Furthermore, new device structures have been examined, including recess-gate HFET with n+ GaN cap layer and gate-on-insulator HFET, among which the insertion of gate dielectrics helps to leverage both DC and microwave performances. In order to depict the microwave behavior of the HFET, small signal modeling approaches were used to extract the extrinsic and intrinsic parameters of the device. An 18-element equivalent circuit model for GaN HFET has been proposed, from which various extraction methods have been tested. Combining the advantages from the cold-FET measurements and hot-FET optimizations, a hybrid extraction method has been developed, in which the parasitic capacitances were attained from the cold pinch-off measurements while the rest of the parameters from the optimization routine. Small simulation error can be achieved by this method over various bias conditions, demonstrating its capability for the circuit level design applications for GaN HFET. Device physics modeling, on the other hand, can help us to reveal the underlying physics for the device to operate. With the development of quantum drift-diffusion modeling, the self-consistent solution to the Schrödinger-Poisson equations and carrier transport equations were fulfilled. Lots of useful information such as band diagram, potential profile, and carrier distribution can be retrieved. The calculated results were validated with experiments, especially on the AlInN layer structures after considering the influence from the parasitic Ga-rich layer on top of the spacer. Two dimensional cross-section simulation shows that the peak of electrical field locates at the gate edge towards the drain, and of different kinds of structures the device with gate field-plate was found to efficiently reduce the possibility of breakdown failure.

Steady-state and transient surface photovoltages in undoped GaN are studied in various environments (air, nitrogen, oxygen, vacuum) at room temperature and 400 K with a Kelvin probe attached to an optical cryostat. The results are explained within a phenomenological model accounting for the accumulation of photo-generated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Mechanisms of surface photovoltage are discussed in detail. Photoadsorption and photodesorption of negatively charged species will either increase or decrease the surface potential and thus band bending. Oxygen is the assumed species responsible for the SPV changes in air ambient during continuous UV illumination. This variation in SPV will be confirmed with photoluminescence measurements.