En optoélectronique, l'avènement des technologies basées sur les nitrures du groupe III permettront une diminution de la consommation d'électricité dédiée à l'éclairage. Le nitrure d'aluminium, AlN, est particulièrement attractif pour la fabrication de diodes électroluminescentes UV. L'objectif de ce travail de thèse est d'étudier la faisabilité d'un procédé de croissance de monocristaux d'AlN à haute température à partir d'une phase gazeuse chlorée (HTCVD). Les dépôts d'AlN sont réalisés à partir de NH3 et AlCl3 synthétisé in-situ par chloruration de l'aluminium métallique par le chlore. Une étude thermodynamique préliminaire est réalisée afin de mettre en évidence les possibilités mais aussi les limites d'utilisation des matériaux en testant leurs compatibilités. Les modélisations thermodynamiques du système Al-N-Cl-H permettent aussi de montrer les mécanismes chimiques globaux et certains paramètres propices à l'élaboration d'AlN par CVD. Une étude paramétrique expérimentale du procédé est menée afin de comprendre l'influence des conditions opératoires sur la vitesse de croissance, la morphologie de surface et l'état cristallin des dépôts d'AlN. Des conditions propices à l'élaboration de couches épitaxiées sont également recherchées. Les dépôts polycristallins et épitaxiés réalisés sont étudiés structuralement et leurs propriétés sont caractérisées. Une étude des mécanismes de croissance et des caractéristiques des polycristaux élaborés à forte vitesse est menée. La morphologie, la structure, les propriétés et les défauts des couches épitaxiées sont étudiées.

Éléments essentiels des systèmes de télécommunication depuis environ trente ans, les dispositifs à ondes élastiques de surface offrent également l'opportunité, de par leur sensibilité aux conditions environnementales et leur caractère passif, de réaliser des capteurs sans-fil autonomes (sans électronique ni source d'énergie embarquées), configuration particulièrement intéressante pour la mesure à haute température. Nos travaux se sont focalisés sur le choix des matériaux pouvant constituer de tels capteurs, au niveau des deux éléments les constituant : le substrat piézoélectrique et les électrodes métalliques. Ces dernières sont généralement constituées de platine, de par l'inertie chimique particulièrement importante de ce métal noble. Nos travaux nous ont permis d'attribuer la dégradation de ces électrodes, généralement observée aux alentours de 700°C, à un phénomène spécifique des films minces, dénommé agglomération, nous conduisant par la suite à envisager et à tester des solutions plus efficaces. En ce qui concerne le substrat, nos efforts ont porté sur la structure bicouche AlN/Saphir, prometteuse pour de telles applications, mais encore peu étudiée. Après l'optimisation des paramètres de dépôt du film mince par pulvérisation réactive magnétron, permettant d'obtenir une couche épitaxiée, nous nous sommes intéressés à la résistance à l'oxydation de l'AlN à haute température dans l'air. L'utilisation croisée de la diffraction des rayons X, de l'ellipsométrie et de la spectroscopie de masse des ions secondaires nous a permis de montrer que l'on peut envisager l'emploi de cette structure bicouche pour les applications visées à des températures allant jusqu'à 700°C / Surface acoustic waves devices are key components of telecommunication systems for more than thirty years or so. Because they are passive and very sensitive to external conditions, they also offer the possibility to make autonomous wireless sensors (electronic-less and battery-less), which could be particularly interesting in high-temperature environments. Our work was focused on the choice of materials allowing the fabrication of such sensors for both parts of the device: piezoelectric substrate and metallic electrodes. The latter are generally made of platinum because of the great chemical inertness of this noble metal. Our work allowed us to attribute their degradation, starting around 700°C on, to a phenomenon called agglomeration which is very specific to thin films. This result led us to consider and test more efficient solutions. Regarding the substrate, we mainly studied AlN/Sapphire bilayer structure, promising for such applications but not really studied yet. After the optimization of the deposition parameters of the thin film, realized by reactive magnetron sputtering, leading to the epitaxial quality, we studied the strength of AlN to oxidation under high temperature in air atmosphere. Results given by X-ray diffraction, ellipsometry and secondary ion mass spectroscopy converged to show that AlN/Sapphire structure is a good candidate for such applications at temperatures up to 700°C

L'excitation et la propagation des ondes de cisaillement dans les dispositifs à ondes acoustiques de surface SAW à base de nitrure d'aluminium, en milieux liquides, nécessitent l'inclinaison de l'axe c dans la structure hexagonale. Le but de cette étude était de déposer des couches minces d'AlN à axe c incliné par la technique de pulvérisation magnétron sans aucune modification du dispositif de pulvérisation, le substrat et la cible n'ayant subi ni inclinaison ni décalage. Il a été possible, par une approche basée seulement sur la variation des paramètres de croissance, de déposer des couches minces piézoélectriques avec un angle d'inclinaison de 13°±2° dans des conditions de haute pression (0.8 Pa) et basse température (300°C). Une couche de SiO2 a été également déposée afin de favoriser la croissance inclinée des grains et par conséquent de celle des colonnes. Les couches déposées présentent une grande homogénéité de l'épaisseur sur 75% d'un substrat de silicium de 3 pouces. Après la détermination des paramètres optimaux de « croissance inclinée », nous avons réalisé un dispositif à onde acoustique de surface fonctionnant en mode de cisaillement avec lequel nous avons démontré la possibilité d'exciter les ondes de cisaillement dans un dispositif de type AlN/Si02/Si à 486.2 MHz avec une vitesse de propagation d'environ 5835m/s et un facteur de couplage électromécanique de 0.014%. La réponse électrique est fort intéressante si on tient compte du faible couplage électromécanique dû au substrat utilisé. / The excitation and propagation, in liquid media, of shear waves in surface acoustic wave (SAW) devices based aluminum nitride (AlN) require the inclination of the c axis in the hexagonal structure. The purpose of this study was to deposit tilted c-axis AlN thin films by magnetron sputtering technique without any modification of the deposition system. The search approach was based only on the optimisation of deposition parameters. Substrate and the target were not inclined or shifted. It has been possible through an approach based solely on changes in growth parameters, to deposit thin piezoelectric layers with an inclination angle of 13 ° ± 2 ° under conditions of high pressure (0.8 Pa) and low temperature ( 300 ° C). A thin layer of SiO2 was also introduced to enhance the growth of tilted grains and therefore the columns. The deposited layers have a homogeneous thickness of 75% of a silicon substrate of 3 inches. After determining the optimal parameters leading to growth AlN film with tilted c-axis, we achieved a SAW device and hence demonstrate the ability to excite shear waves in AlN/Si02 /Si SAW structure. The performed device operate at 486.2 MHz corresponding to an acoustic velocity of about 5835m/s and an electromechanical coupling coefficient of 0.014%. The obtained electrical response is very interesting if we take into account the low electromechanical coupling of the structure due to the used substrate.

Os objetivos gerais deste trabalho foram estudar o comportamento de densificação e as propriedades mecânicas do nitreto de alumínio (AlN) com aditivo de sinterização. Os objetivos específicos foram estudar: 1) o efeito do teor de Y2O3 na densificação do AlN; 2) o efeito da adição de carbono na densificação do AlN com CaO; 3) o efeito da adição simultânea de Y2O3 e CaO na densificação do AlN; 4) o efeito da porosidade nas propriedades elásticas do AlN com Y2O3 ou CaO; e 5) o efeito do teor de Y2O3 ou CaO nas propriedades mecânicas do AlN. As amostras foram sinterizadas sob atmosfera inerte em várias temperaturas (1100-2000ºC) e analisadas por determinação de densidade, difração de raios X e microscopia eletrônica de varredura. As propriedades determinadas à temperatura ambiente foram: constantes elásticas pelo método de pulso-eco ultra-sônico, resistência à flexão biaxial, dureza Vickers e tenacidade à fratura pelos métodos Indentation Fracture (KIc-IF) e Indentation Strength (KIc-IS). O AlN com Y2O3 apresentou significativa densificação por sinterização no estado sólido, mas completa densificação ocorreu somente por sinterização via fase líquida. A densificação do AlN não foi influenciada pelo aumento de 0,5% a 4% de Y2O3, pois a temperatura de formação da fase líquida (~ 1725ºC) não variou nesta faixa de teor de aditivo. Porém, a adição de carbono levou à formação de segundas-fases refratárias no AlN com 4% de Y2O3, o que atrasou sua densificação devido ao aumento da temperatura de formação da fase líquida. A adição de carbono não mudou a temperatura de formação da fase líquida do AlN com 4% de CaO sinterizado acima de 1650ºC, mas sua densificação também atrasou devido à diminuição da fração de fase líquida e ao aprisionamento de gás nos poros fechados. A adição simultânea de CaO e Y2O3 diminuiu significativamente a temperatura de sinterização do AlN em relação às adições individuais destes aditivos. O AlN com mistura de CaO e Y2O3 atingiu quase completa densificação a partir de 1650ºC devido à formação de fase líquida com composição próxima da fase CaYAl3O7, que apresentou boas características de molhamento e espalhamento nesta temperatura de sinterização. A adição de carbono mudou a composição das segundas-fases de CaYAl3O7 para CaYAlO4 e, então, para YAM (2Y2O3.Al2O3) no AlN com mistura de CaO e Y2O3, o que atrasou sua densificação em baixas temperaturas de sinterização (< 1700ºC) devido à alta viscosidade e/ou baixa molhabilidade da fase CaYAlO4 e à alta refratariedade da fase YAM. As propriedades elásticas, bem como a dureza Vickers do AlN foram influenciadas pela porosidade, mas não foram influenciadas pelo tipo (CaO e Y2O3) e teor (0,5% a 8% em massa) de aditivo de sinterização. A resistência 7 à flexão foi influenciada pelo tipo e teor de aditivo de sinterização, sendo observado um teor crítico de aditivo, acima do qual esta propriedade diminuiu significativamente e, então, tendeu a permanecer constante em função do teor de aditivo. Não se observou correlação entre tenacidade à fratura e resistência à flexão. Os valores de KIc-IS, que não variaram com o tipo e teor de aditivo de sinterização, foram considerados mais consistentes do que os valores de KIc-IF. / The general objectives of this work were to study the densification behavior and the mechanical properties of aluminum nitride (AlN) with sintering aid. The specific objectives studied were: 1) the effect of Y2O3 content on the densification of AlN, 2) the effect of carbon addition on the densification of AlN with CaO, 3) the effect of simultaneous addition of Y2O3 and CaO on the densification of AlN; 4) the effect of porosity on elastic properties of AlN with CaO or Y2O3, and 5) the effect of Y2O3 or CaO content on the mechanical properties of AlN. The samples were sintered under inert atmosphere at various temperatures (1100ºC-2000ºC) and analyzed by density measurement, X-ray diffraction and scanning electron microscopy. The properties measured at room temperature were: elastic constants by the ultrasound pulse-echo method, biaxial flexural strength, Vickers hardness and fracture toughness by the Indentation Fracture (IF) and Indentation Strength (IS) methods. AlN with Y2O3 presented significant densification by solid-state sintering, but full densification occurred only by liquid-phase sintering. The densification of AlN was not influenced by the increase of 0.5% to 4% Y2O3, since the liquid-phase formation temperature (~ 1725ºC) did not change in this range of additive content. However, the addition of carbon to AlN with 4% Y2O3 led to formation of refractory second-phases, which delayed the densification due to an increase of the liquid-phase formation temperature. The addition of carbon did not change the liquid-phase formation temperature of AlN with 4% CaO sintered above 1650ºC, but its densification delayed significantly due to the diminution of liquid-phase fraction and trapped gas in closed pores. The simultaneous addition of CaO and Y2O3 significantly diminished the sintering temperature of AlN in relation to the individual additions of these additives. AlN with simultaneous addition of CaO and Y2O3 achieved almost full densification at 1650ºC due to formation of liquid-phase with composition close to CaYAl3O7 phase, which had good wetting and spreading characteristics at this temperature. The addition of carbon changed the composition of second-phases from CaYAl3O7 to CaYAlO4 and then to YAM (2Y2O3.Al2O3) in AlN with simultaneous addition of CaO and Y2O3, delaying its densification at low sintering temperatures (< 1700ºC) due to the high viscosity and/or low wettability of CaYAlO4 phase and high refractoriness of YAM phase. The elastic properties, as well as Vickers hardness were influenced by the increase of porosity, but were not influenced by type (CaO and Y2O3) and content (0.5% to 8% in mass) of sintering aid. Flexure strength was influenced by the type and content of sintering aid. It was observed a critical additive content, 9 above which the strength decreased significantly and then tended to remain almost constant as a function of additive content. No relationship was observed between fracture toughness and flexure strength. The KIc-IS values, which did not vary with type and content of sintering aid, were found to be more consistent than the KIc-IF values.

Ce travail a consisté en la croissance (par épitaxie à jets moléculaires) et la caractérisation de nanostructures à base de semiconducteurs nitrures (GaN, AlN et alliages) afin de développer de composants optoélectroniques avancés basés sur les transitions intrabandes pour la prochaine génération de systèmes de télécommunications à très haut débit. Une première série de résultats concerne la croissance de couches minces de nitrures, notamment celle des alliages d'AlGaN. D'après notre étude, la croissance de couches dont la fraction molaire d'Al reste en deçà de 35 % nécessite la présence d'un excès de Ga. Au delà, il est nécessaire d'utiliser l'In en tant que surfactant ou bien de réaliser des superalliages GaN/AlN. Des études du dopage Si de ce type de structures ont été menées. Nous avons ensuite étudié des structures à multicouches de puits quantiques GaN/AlN dopées Si. Celles-ci présentent les pics d'absorption ISB polarisée p à des longueurs d'onde de la gamme des télécommunications à température ambiante. L'effet de divers paramètres de croissance et de design a été étudié. L'analyse des caractérisations de ces échantillons a permis d'évaluer le champ électrique interne ainsi que l'offset de bande de conduction entre le GaN et l'AlN de nos structures. Concernant la synthèse des structures multicouches de boîtes quantiques GaN/AlN dopées Si, nous avons adapté la technique de croissance de ces structures pour minimiser la taille de boîtes, et ce, de manière à ce que leur absorption intrabande puisse atteindre les longueurs d'onde des télécommunications optiques. L'énergie du pic d'absorption des boîtes peut être ajustée en modifiant la quantité de GaN dans les boîtes, la température de croissance, et le temps d'interruption de croissance. Enfin, les résultats obtenus sur la réalisation de composants sont développés. Nous nous sommes focalisés sur les dispositifs basés sur l'absorption (photodétecteurs à puits et à boites quantiques, modulateurs électro-optiques) et l'émission de lumière infrarouge dans la gamme de longueurs d'onde des télécommunications. Des résultats prometteurs ont été obtenus sur l'ensemble de ces composants, ils forment une première étape vers la fabrication de composants pour les télécommunications à base de semiconducteurs nitrures.

<p>GaN quantum dots grown by molecular beam epitaxy are examined by micro-photoluminescence. The exciton and biexciton emission are identified successfully by power-dependence measurement. With two different samples, it can be deduced that the linewidth of the peaks is narrower in the thicker deposited layer of GaN. The size of the GaN quantum dots is responsible for the binding energy of biexciton (E^b_XX); E^b_XXdecreases with increasing size of GaN quantum dots. Under polarization studies, polar plot shows that emission is strongly linear polarized. In particular, the orientation of polarization vector is not related to any specific crystallography orientation. The polarization splitting of fine-structure is not able to resolve due to limited resolution of the system. The emission peaks can be detected up to 80 K. The curves of transition energy with respect to temperature are S-shaped. Strain effect and screening of electric field account for  blueshift of transition energy, whereas Varshni equation stands for redshifting. Both blueshifting and redshifting are compensated at temperature ranging from 4 K to 40 K.</p>

Today’s MEMS technology allows manufacturing of miniaturized, low power sensors that sometimes exceeds the performance of conventional sensors. The pressure sensor market today is dominated by MEMS pressure sensors. In this thesis two different pressure sensor techniques are studied. The first concerns ways to improve the sensitivity in the most commonly occurring pressure sensor, namely such based on the piezoresistive technique. Since the giant piezoresistive effect was observed in silicon nanowires, it was assumed that a similar effect could be expected in nano-thin silicon films. However, it turned out that the conductivity was extremely sensitive to substrate bias and could therefore be controlled by varying the backside potential. Another important parameter was the resistivity time drift. Long time measurements showed a drastic variation in the resistance. Not even after several hours of measurement was steady state reached. The drift is explained by hole injection into the buried oxide as well as existence of mobile charges. The piezoresistive effect was studied and shown to be of the same magnitude as in bulk silicon. Later research has shown the existence of such an effect where the film thickness has to be less than around 20 nm.  The second area that has been studied is the pressure sensitivity of in acoustic resonators. Aluminium nitride thin film plate acoustic resonators (FPAR) operating at the lowest-order symmetric (S0), the first-order asymmetric (A1) as well as the first-order symmetric (S1) Lamb modes have been theoretically and experimentally studied in a comparative manner. The S0 Lamb mode is identified as the most pressure sensitive FPAR mode. The theoretical predictions were found to be in good agreement with the experiments. Additionally, the Lamb modes have been tested for their sensitivities to mass loading and their ability to operate in liquids, where the S0 mode showed good results. Finally, the pressure sensitivity in aluminium nitride thin film bulk wave resonators employing c- and tilted c-axis texture has been studied. The c-axis tilted FBAR demonstrates a substantially higher pressure sensitivity compared to its c-axis oriented counterpart.

In recent years there has been a huge increase in the growth of communication systems such as mobile phones, wireless local area networks (WLAN), satellite navigation and various other forms of wireless data communication that have made analogue frequency control a key issue. The increase in frequency spectrum crowding and the increase of frequency into microwave region, along with the need for minimisation and capacity improvement, has shown the need for the development of high performance, miniature, on-chip filters operating in the low to medium GHz frequency range. This has hastened the need for alternatives to ceramic resonators due to their limits in device size and performance, which in turn, has led to development of the thin film electro-acoustics industry with surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters now fabricated in their millions. Further, this new technology opens the way for integrating the traditionally incompatible integrated circuit (IC) and electro-acoustic (EA) technologies, bringing about substantial economic and performance benefits. In this thesis the compatibility of aluminium nitride (AlN) to IC fabrication is explored as a means for furthering integration issues. Various issues have been explored where either tailoring thin film bulk acoustic resonator (FBAR) design, such as development of an improved solidly mounted resonator (SMR) technology, and use of IC technology, such as chemical mechanical polishing (CMP) or nickel silicide (NiSi), has made improvements beneficial for resonator fabrication or enabled IC integration. The former has resulted in major improvements to Quality factor, power handling and encapsulation respectively. The later has provided alternative methods to reduce electro- or acoustomigration, reduced device size, for plate waves, supplied novel low acoustic impedance material for high power applications and alternative electrodes for use in high temperature sensors. Another method to enhance integration by using the piezoelectric material, AlN, in the IC side has also been explored. Here methods for analysing AlN film contamination and stoichiometry have been used for analysis of AlN as a high-k dielectric material. This has even brought benefits in knowledge of film composition for use as a passivation material with SiC substrates, investigated in high power high frequency applications. Lastly AlN has been used as a buried insulator material for new silicon-on-insulator substrates (SOI) for increased heat conduction. These new substrates have been analysed with further development for improved performance indicated. / wisenet

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.