Certains domaines d'applications tels que l'aérospatial, l'automobile ou le forage de haute profondeur peuvent nécessiter la visualisation de certains paramètres physiques dans des environnements hostiles. Les capteurs microélectroniques basés sur le silicium y atteignent souvent leurs limites, qui sont qualifiées de conditions " sévères ". Ce travail se base principalement sur l'étude de solutions de capteurs mécaniques fonctionnant en conditions sévères. Le principe de ces capteurs repose sur l'exploitation de transistors de mesures HEMT à base de nitrures III-V (III-N), à la fois piézoélectriques et semiconducteurs, qui reste stable en conditions sévères. La compréhension des interactions entre physique des semiconducteurs et physique des matériaux ainsi que la caractérisation de structures possibles pour la détection mécanique représentent les principaux enjeux de ce sujet de thèse. La modélisation mécanique analytique et numérique des structures étudiées a permis d'appréhender le comportement de structures piézoélectriques multicouches. Le couplage de ce modèle électromécanique avec un modèle électronique du capteur a permis d'établir la faisabilité du principe de détection ainsi que la linéarité de la réponse du capteur. La caractérisation des prototypes réalisés en cours de thèse ont corroboré la linéarité du capteur tout en faisant apparaître l'influence de nombreux effets parasites réduisant sa sensibilité à savoir les effets de résistance parasites et de piézorésistances variables.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

The design, fabrication, and characterizations of the spin-polarized AlxGa1-xN/GaN HEMT structure have been achieved for spintronic application. By band calculation within linear combination of atomic orbitals and two-band k·p methods, the theoretical spin-splitting energy and minimum-spin-splitting surface of wurtzite structure have been investigated as a function of the Fermi wavevector with various strain-relaxations. Base on these results, the design of host material of the nonballistic spin-FET has also been proposed. By optimizing the Al composition and n2DEG, the Fermi surface of two-dimensional electron gas is supposed to reach the minimum-spin-splitting surface to produce resonant spin-lifetime. Because the high quality AlxGa1-xN/GaN HEMT structure is necessary for realizing the spin-FET, the influence of the growth conditions on the polarity and structure quality of the GaN epilayer have been studied on the sample grown by plasma-assisted molecular beam epitaxy. Ga-polar AlGaN/GaN heterostructures on c-Al2O3 has been realized by growing over the Al-rich AlN nucleation layer. And the reduction of interface roughness and threading dislocation scatterings of the electrons in two-dimensional electron gas has also been achieved by growing GaN epilayer under slightly Ga-rich condition. Furthermore, the effect of different types of threading dislocation on the electron mobility of the AlxGa1-xN/GaN HEMT structure has been investigated as well. At low temperature, the electron mobility of two-dimensional electron gas in AlGaN/GaN heterostructures is majorly scattered by the edge type dislocation rather than the screw type. The designs of proposed host material for spin-FETs have been realized through growing high quality spin-polarized AlxGa1-xN/GaN HEMT structures with various Al composition (x= 0.191 ¡V 0.397) grown on c-Al2O3 by metalorganic vapor phase epitaxy. The high mobility (10682 cm2/Vs at 0.4 K), flat interface (surface roughness < 0.5 nm), and high quality HEMT provide a good environment to study the spin-splitting energy. To investigate the spin-splitting energy as functions of the Fermi wavevector, the Shubnikov-de Haas measurements were performed. A large spin-splitting energy (10.76 meV) has been fabricated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.14 ¡Ñ 108 m-1 for the host material of the Datta-Das spin-FET. And for the first time, the minimum-spin-splitting surface has been experimentally generated in Al0.390Ga0.61N/GaN HEMT structure with kf = 8.33 ¡Ñ 108 m-1 for the host material of the nonballistic spin-FET.

Heterostructures of the materials system GaN/AlGaN/GaN were grown by molecular beam epitaxy on 6H-SiC substrates and high electron mobility transistors (HEMTs) were fabricated. For devices with large gate periphery an air bridge technology was developed for the drain contacts of the finger structure. The devices showed DC drain currents of more than 1 A/mm and values of the transconductance between 120 and 140 mS/mm. A power added efficiency of 41 % was measured on devices with a gate length of 1 µm at 2 GHz and 45 V drain bias. Power values of 8 W/mm were obtained. Devices with submicron gates exhibited power values of 6.1 W/mm (7 GHz) and 3.16 W/mm (25 GHz) respectively. The rf dispersion of the drain current is very low, although the devices were not passivated. / Heterostrukturen im Materialsystem GaN/AlGaN/GaN wurden mittels Molekularstrahlepitaxie auf 6H-SiC-Substraten gewachsen und High-Electron-Mobility-Transistoren (HEMTs) daraus hergestellt. Für Bauelemente mit großer Gateperipherie wurde eine Air-Bridge-Technik entwickelt, um die Drainkontakte der Fingerstruktur zu verbinden. Die Bauelemente zeigten Drainströme von mehr als 1 A/mm und Steilheiten zwischen 120 und 140 mS/mm. An Transistoren mit Gatelängen von 1 µm konnten Leistungswirkungsgrade (Power Added Efficiency) von 41 % (bei 2 GHz und 45 V Drain-Source-Spannung) sowie eine Leistung von 8 W/mm erzielt werden. Bauelemente mit Gatelängen im Submikrometerbereich zeigten Leistungswerte von 6,1 W/mm (7 GHz) bzw. 3,16 W/mm (25 GHz). Die Drainstromdispersion ist sehr gering, obwohl die Bauelemente nicht passiviert wurden.

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) offer numerous benefits for both direct current (DC) and radio frequency (RF) power technology due to their combination of large band gap, high electrical breakdown field, high peak and saturation carrier velocity, and good stability at high temperatures. In particular, AlGaN/GaN heterostructures are of great interest because of the unique conduction channel that develops as a result of the spontaneous and piezoelectric polarization that occurs in these layers. This channel is a vertically confined plane of free carriers that is often called a 2 dimensional electron gas (or 2DEG). Although these devices have shown an improvement in performance over previous heterostructures, reliability issues are a concern because of the high temperatures and electric fields that develop during operation. Therefore, characterizing electrical and thermal profiles within AlGaN/GaN HEMTs is critical for understanding the various factors that contribute to device failures. Little research has been performed to model and characterize these devices under RF bias conditions, and is therefore of great interest. Under pulsed conditions, a single cycle consists of an “on-state” period where power is supplied to the device and self-heating occurs, followed by an “off-state” period where no power is supplied to the device and the device cools. The percentage of a single cycle in which the device is powered is called the duty cycle. In this work, we present a coupled electro-thermo-mechanical finite-element model for describing the development of temperature, stress, and strain profiles within AlGaN/GaN HEMTs under DC and AC power conditions for various duty cycles. It is found that bias conditions including source-to-drain voltage, source-to-gate voltage, and pulsing frequency directly contribute to the electro-thermo-mechanical response of the device, which is known to effect device performance and reliability. The model is validated by comparing numerical simulations to experimental electrical curves (Ids-Vds) and experimental strain measurements performed using scanning joule expansion microscopy (SJEM). In addition, we show how the operating conditions (bias applied and AC duty cycle) impact the thermal profiles of the device and outline how the stress in the device changes through a pulsed cycle due to the changing thermal and electrical profiles. Qualitatively, the numerical model has good agreement across a broad range of bias conditions, further validating the model as a tool to better understand device performance and reliability.

Visible-blind and solar-blind ultraviolet photodetectors based on GaN/AlGaN were designed, fabricated, and characterized for commercial and military applications. High performance back-illuminated solar-blind MSM achieved external quantum efficiency of ~48%. The dark current of 40x40μm MSM was less than the instrument measurement limitation of 20fA for a bias <100V. No photoconductive gain was observed. With an n-type doped high-Al ratio "window" Al₀.₆Ga₀.₄N layer, back-illuminated solar-blind p-i-n photodiode achieved a quantum efficiency of ~55% at zero-bias. Absorption edge study of both MSM and p-i-n photodetectors, based on device spectral responses, resulted in a performance comparison of MSMs and p-i-ns, as the solar-blind photodetection requires a sharp solar-blind rejection. Photoconductive detectors and avalanche photodetectors, with the internal gain advantage, have been discussed as well. A 30μm diameter GaN avalanche photodiode achieved a gain >23, with a dark current less than 100pA. The breakdown showed a positive temperature coefficient of 0.03 V/K that is characteristic of avalanche breakdown. SiC APDs, as candidates for visible-blind applications, have been designed, fabricated and characterized. An avalanche gain higher than 10⁵, with a dark current less than 1nA, showed the potential of SiC APD replacing PMTs for high sensitivity visible-blind UV detection. A silicon-based optical receiver has been presented in the Appendix. With the photodiode internal avalanche gain ~4, a sensitivity ~-6.9dBm at 10Gbps has been achieved. / text

Nous avons étudié les propriétés structurales et optiques d'hétérostructures de nitrures d'éléments III. Les croissances ont été réalisées par épitaxie par jets moléculaires assistée plasma d'azote. L'étude a été en particulier axée sur des structures émettant dans l'UV, à des longueurs d'onde sub-300 nm. Pour atteindre cette longueur d'onde à l'aide de couches bidimensionnelles, il est nécessaire d'utiliser des alliages ternaires d'AlGaN à forte teneur en Al (supérieure à 50 %). Nous avons donc tout d'abord corrélé les résultats de techniques de caractérisation aux conditions de croissance. Ainsi, il est possible de contrôler en partie l'homogénéité de l'alliage grâce à la température de croissance et au rapport de flux métal/azote. Nous avons ensuite tenté d'appliquer ces résultats à des hétérostructures à puits quantiques AlGaN/AlGaN. D'autre part, nous avons réalisé des alliages digitaux (super-réseaux à courte période de puits quantiques GaN/AlN). Ceux-ci se sont montré une alternative intéressante aux couches 2D pour la réalisation de dispositifs émetteurs de lumière. Enfin, une part importante du manuscrit s'attache à l'étude des effets d'un recuit après la croissance et à haute température. Cette étude a été menée sur des couches d'AlGaN ainsi que sur des boites quantiques GaN/AlN. Pour les couches d'AlGaN, nous avons constaté une augmentation de l'inhomogénéité des couches après un recuit à 950 °C. Pour les boîtes quantiques GaN, nous avons constaté dès 1000 °C, un décalage vers le bleu de la luminescence. A 1300-1500 °C, des images TEM ont clairement montré la diffusion de Ga dans la matrice AlN au-dessus des boîtes quantiques.

Certains domaines d'applications tels que l'aérospatial, l'automobile ou le forage de haute profondeur peuvent nécessiter la visualisation de certains paramètres physiques dans des environnements hostiles. Les capteurs microélectroniques basés sur le silicium y atteignent souvent leurs limites, qui sont qualifiées de conditions " sévères ". Ce travail se base principalement sur l'étude de solutions de capteurs mécaniques fonctionnant en conditions sévères. Le principe de ces capteurs repose sur l'exploitation de transistors de mesures HEMT à base de nitrures III-V (III-N), à la fois piézoélectriques et semiconducteurs, qui reste stable en conditions sévères. La compréhension des interactions entre physique des semiconducteurs et physique des matériaux ainsi que la caractérisation de structures possibles pour la détection mécanique représentent les principaux enjeux de ce sujet de thèse. La modélisation mécanique analytique et numérique des structures étudiées a permis d'appréhender le comportement de structures piézoélectriques multicouches. Le couplage de ce modèle électromécanique avec un modèle électronique du capteur a permis d'établir la faisabilité du principe de détection ainsi que la linéarité de la réponse du capteur. La caractérisation des prototypes réalisés en cours de thèse ont corroboré la linéarité du capteur tout en faisant apparaître l'influence de nombreux effets parasites réduisant sa sensibilité à savoir les effets de résistance parasites et de piézorésistances variables.

abstract: With the high demand for faster and smaller wireless communication devices, manufacturers have been pushed to explore new materials for smaller and faster transistors. One promising class of transistors is high electron mobility transistors (HEMT). AlGaAs/GaAs HEMTs have been shown to perform well at high power and high frequencies. However, AlGaN/GaN HEMTs have been gaining more attention recently due to their comparatively higher power densities and better high frequency performance. Nevertheless, these devices have experienced truncated lifetimes. It is assumed that reducing defect densities in these materials will enable a more direct study of the failure mechanisms in these devices. In this work we present studies done to reduce interfacial oxygen at N-polar GaN/GaN interfaces, growth conditions for InAlN barrier layer, and microanalysis of a partial InAlN-based HEMT. Additionally, the depth of oxidation of an InAlN layer on a gate-less InAlN/GaN metal oxide semiconductor HEMT (MOSHEMT) was investigated. Measurements of electric fields in AlGaN/GaN HEMTs with and without field plates are also presented. / Dissertation/Thesis / Doctoral Dissertation Physics 2018