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Ce travail de thèse consiste à étudier l'instrumentation de la sonde de Kelvin (KFM) sur un microscope à force atomique (AFM) commercial et ensuite à caractériser les surfaces et composants à base nitrure de gallium (GaN). Le potentiel de surface Vs, entre une pointe métallique et un matériau semiconducteur dépend de la différence de travaux de sortie des deux matériaux, des concentrations en dopants et des états de surface du semiconducteur. La technique KFM permet d'obtenir cette information à une échelle nano ou micrométrique. Ce projet a consisté à développer ce mode de mesure à partir de microscopes AFM commerciaux. L'étude de l'instrumentation a permis de montrer la présence de couplages parasites qui entachent d'erreur la mesure de Vs. Une stratégie est alors proposée pour permettre la mesure de Vs tout en s'affranchissant de ces effets parasites. Cette technique est ensuite appliquée à la caractérisation de structures à base de GaN. L'intérêt pour ce matériau semiconducteur à large bande interdite est croissant en électronique de puissance, par exemple pour la réalisation d'émetteurs électroniques de puissance. Pour étudier les propriétés électriques de ce matériau, nous avons réalisé une référence de potentiel qui nécessite le développement de contacts ohmiques sur le GaN de type n et p. A partir des valeurs de Vs mesurées par KFM, nous en déduisons la densité de charges de surface et une estimation de la densité d'états de surface du GaN. Enfin, nous avons étudié par KFM les effets de traitements de surface sur des structures MIS à base de GaN de type n, ainsi que les effets de différentes passivations sur des transistors HEMT à base d'AIGaN/GaN. / The purpose of the thesis is to study GaN materials and devices with an atomic force microscopy in Kelvin Force Mode. The contact potential difference between a metal tip and a semiconducting material depends on the work function difference between the materials, the concentration of dopants, and the density of acceptor or donor surface states. KFM techniques provide this information at the nano- or micrometer scale. ln a first step, we have developed KFM measurement procedures on commercial microscopes in order to extract fully quantitative measurements of surface potentials. We have evidenced instrumental capacitive cross talks, for example between the electrostatic excitation and the microscope photodiode, which act as parasitic terms in the measurement of surface potentials, and need to be properly taken into account in order to get reliable measurements of contact potential differences. ln a second step we have studied the electrical properties of GaN surfaces, this material being of strong interest for power electronic applications such as electron emitters. To get a potential reference for KFM measurements, ohmic contacts on n and p-type GaN have been achieved. The KFM characterization of the layers shows surface-state induced band-bending at the oxidized GaN surface. From the values of surface potentials, we calculate the density of charge and estimate the density of surface states. We finally study the effects of surface treatments on n-GaN-MIS structures, as weIl as different types of passivation used in AlGaN/GaN HEMTs.

The Kelvin Probe Force Microscope is a type of scanning probe instrument that is used to discern the different work functions of a sample. A sharp probe at the end of a cantilever is lowered onto a substrate where electrostatic forces, caused by the difference in work function cause the cantilever to oscillate at the modulated frequency. Using this instrument, high resolution images can be obtained, mapping the surface electronic characteristics. However, developments of this instrument have generally been limited to obtaining higher resolution images as well as reducing noise in the output, limiting the widespread appeal of this expensive instrument. There exist many applications where extremely cheap, low footprint and easy-to-use Kelvin Probe Force Microscopes would be beneficial. In order to cheaply produce this microscope in batch, a post-processed CMOS-MEMS device is utilized. The polysilicon resistors act as a strain gauge such that a conventional optical system will not have to be employed. The ability to use integrated bimorph actuators on chip allow for movement of the cantilever without the employment of large piezoelectric stages with creep effects. Embedded electronics can be fabricated with the CMOS process alongside the MEMS device, allowing full integration of an on board amplifier and read out system. In general, a large table top system can be minimized onto the size of a <1 mm2 area, a microcontroller and a computer. In this work, a Kelvin Probe Force Microscope is designed, fabricated and validated. A MEMS device was designed following similar characteristics of a generic cantilever beam. The stiffness, length, resonant frequency, and other tip characteristics can be mimicked with careful design. The resultant designs were fabricated using a CMOS-MEMS process. In order to obtain a sharper tip with modified characteristics, various methods were employed; such as gallium-aluminum alloy tip formation as well as electroless plating onto the tip of the device. Finally, the resultant device is tested against a sample. It was seen that the MEMS device followed similar characteristics of the conventional microscope itself, validating the equations that define the method. Bimorph actuators were tested to show movement, allowing the integration of the cantilever with the XYZ-stage. Work function changes are observed while scanning different materials. It is shown throughout the course of this thesis, that a true Kelvin Probe Force Microscope can be designed, fabricated and validated using CMOS-MEMS technology.

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No description available.

In this thesis I will discuss the development, construction and testing of a sub-Kelvin Q-band electron paramagnetic resonance (EPR) spectrometer and optically detected magnetic resonance (ODMR) of wide bandgap semiconductors at Q-band. The sub-Kelvin EPR spectrometer was developed to be integrated into a standard commercial system. Characterisation of the cryogenics and microwave components of the spectrometer will be discussed as well as the design and adaptations made to enable EPR experiments to be performed below 1 K. A waveguide thermal break design, previously only used in detectors for the cosmic microwave background radiation, was optimised using ANSYS High frequency structure simulator (HFSS) to operate at Q-band and was built and tested in this spectrometer. The sub-Kelvin EPR spectra of Cr3+ in Al2O3 and [Cr12O9(OH)3(O2CCMe3)15] were obtained. The resonant cavity of the spectrometer was also successfully tested at room temperature, with a pulsed microwave bridge, paving the way for further development of the system to enable sub-Kelvin pulsed EPR. A home-built Q-band ODMR spectrometer was used to investigate the wide band gap semiconductors ZnO and InGaN/GaN multiple quantum wells (MQWs). The ZnO was a natural crystal, and used to characterise the spectrometer. ODMR from a green c-plane InGaN/GaN MQW was studied to probe the method of carrier localisation in the QW. It was shown qualitatively that the ODMR results support current theories for carrier localisation mechanisms, but the noise on these measurements needs to be reduced to provide quantitative support.

Le microscope à force atomique (AFM) est un outil puissant pour l'étude des systèmes biologiques. Dans cette thèse, je me suis intéressé à la corrélation de quatre types de microscopies : la microscopie à force atomique, la microscopie optique à fluorescence à haute résolution, la microscopie électronique, et la microscopie à sonde de Kelvin (KFM). La corrélation des trois premières microscopies a donné naissance à l'approche CLAFEM (Correlative Light Atomic Force Electron Microscopies). Cette technique m'a permis de détecter des organites intracellulaires, tels que l'appareil de Golgi et des mitochondries. Des comètes d'actine dues à l'infection d'une cellule par Shigella flexneri, et le site d'entrée de Yersinia pseudotuberculosis ont été imagés avec cette approche. Parallèlement à cette partie expérimentale, j'ai développé un logiciel, appelé pyAF, pour analyser des courbes de forces et pour corréler les différents types de microscopies. Dans une deuxième partie, j'ai effectué des mesures des propriétés électriques par KFM et exploré la possibilité d'utiliser celle-ci en milieu liquide. L'étude des propriétés électriques en KFM a été effectuée sur des virus de la mosaïque du tabac à l'air, en utilisant des leviers conventionnels. L'utilisation de sondes Kolibri (résonateurs à quartz oscillant à 1 MHz) à l'air et en liquide a été explorée. / The atomic force microscope (AFM) is a powerful tool for the study of biological systems. In this work, I was interested in the correlation of four types of microscopies: the atomic force microscopy, the high resolution fluorescence microscopy, the electron microscopy and the kelvin force microscopy (KFM). The correlation of the three first types of microscopies gave birth to the CLAFEM approach (Correlative Light Atomic Force Electron Microscopies). This technique allowed me to detect intracellular organelles, like the Golgi apparatus and mitochondria. Actin tails due to the infection of cells by the Shigella flexneri bacterium, and the entry site of Yersinia pseudotuberculosis bacteria were imaged with this approach. In parallel to this experimental part, I developed a software, called pyAF, for the analysis of force curves and the correlation of the different types of microscopies. In a second part, I measured electrical properties by KFM and explored the possibility to use KFM in liquid. Electrical properties of tobacco mosaic viruses were studied in air, using conventional cantilevers. I also used a new type of probe, called Kolibri, which is a quartz resonator oscillating at 1 MHz, in air and in liquid.

Les convertisseurs thermoélectriques conventionnels, exploitant l’effet Seebeck, sont constitués de matériaux massifs. Ce principe de conversion est fortement limité par les propriétés antagonistes de tout matériau massif à savoir la conductivité électrique et la conductivité thermique. De manière alternative, une technique de conversion directe basée sur l’émission thermoélectronique a été développée et révèle un rendement élevé à haute température. Le principe d’injection thermoélectronique est fortement limité par le travail de sortie, barrière énergétique que doit surmonter un électron pour être extrait de l’électrode. Un moyen de contourner cette difficulté est de fonctionnaliser la surface des électrodes avec un matériau à faible travail de sortie, typiquement 1eV. Le potassium et le césium ainsi que leurs oxydes ont été identifiés par la littérature comme matériaux candidats pour la fonctionnalisation de surface des électrodes réduisant considérablement leur travail de sortie, entre 0.4eV et 0.9eV. Leur utilisation a permis d’élargir le domaine de température pour lequel le convertisseur d’énergie thermoélectronique est efficace, s’approchant davantage du rendement limite de conversion de Carnot. / One main challenge associated with solid-state thermoelectric materials is to combine electron-crystal electrical conductivity and phonon-glass thermal properties that are difficult to conciliate. A markedly different principle of thermoelectric conversion is based on the micro thermionic generator that exploits electron injection and heat rejection across two electrodes, hot and cold, immersed in vacuum, a promising pathway to new electronic devices. To properly operate thermionic emission converters near room temperature, highly efficient emitting electrodes must be integrated. This naturally motivates the development and characterization of thin films that feature a very low work function lower than 1eV. Even better than alkalis like K and Cs, their oxides have been reported to produce work functions as low as 0.4 and 0.9eV respectively. The work function of the electrodes of the micro thermionic generator has been effectively reduced and the efficiency is much closer than the Carnot limit.

The Kelvin measurement is a sensitive and label-free method based on work function measurements. Work function, the minimum energy required to extract an electron from a metallic material, can be shifted by ionic charges and dipoles present on the surface. The scanning Kelvin nanoprobe (SKN), a probe-based microscopic imaging device, was used in the detection of work function changes induced by surface-immobilized oligonucleotide / DNA microarrays. The scanning Kelvin nanoprobe was able to study DNA microarrays smaller than 100 µm in size, produced with solution concentrations lower than 10 µmol/L. The limit of detection was estimated to be 15 ng DNA. Better than ± 10% relative variation was achieved for replicate spots. It was observed that higher surface densities of immobilized DNA molecules produced greater work function changes than lower surface densities. Surface saturation with increasing solution concentrations was observed as well. Also, longer strands of DNA produced greater work function changes than shorter strands. Statistical analysis of the results confirmed that non-complementary DNA strands could be differentiated from complementary strands by the Kelvin measurement. Single base mismatches on the complementary DNA strands were also detected by the Kelvin measurement. Different substrate materials were tested in the search for reliable and inexpensive sample slides with satisfactory DNA immobilization efficiency. Materials such as silicon wafers, gold-coated glass slides, gold-coated stainless steel slides, and gold compact discs (CD) were tested. A surface property comparison of gold-coated glass slides and compact discs was made by atomic force microscopy (AFM), and revealed very different microscopic features. The effect of cleaning on gold-coated glass slides was examined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Technical improvements were made to the SKN equipment progressively. Several revisions to the tip holder design have been employed for better electromagnetic shielding, enhanced robustness and easier tip change. An older signal generator was replaced with a professional PC audio card to provide more stable signal and more convenient on-screen fine tuning, also at a reduced cost. The Labview-based controlling program has also been improved through multiple iterations.

The Kelvin measurement is a sensitive and label-free method based on work function measurements. Work function, the minimum energy required to extract an electron from a metallic material, can be shifted by ionic charges and dipoles present on the surface. The scanning Kelvin nanoprobe (SKN), a probe-based microscopic imaging device, was used in the detection of work function changes induced by surface-immobilized oligonucleotide / DNA microarrays. The scanning Kelvin nanoprobe was able to study DNA microarrays smaller than 100 µm in size, produced with solution concentrations lower than 10 µmol/L. The limit of detection was estimated to be 15 ng DNA. Better than ± 10% relative variation was achieved for replicate spots. It was observed that higher surface densities of immobilized DNA molecules produced greater work function changes than lower surface densities. Surface saturation with increasing solution concentrations was observed as well. Also, longer strands of DNA produced greater work function changes than shorter strands. Statistical analysis of the results confirmed that non-complementary DNA strands could be differentiated from complementary strands by the Kelvin measurement. Single base mismatches on the complementary DNA strands were also detected by the Kelvin measurement. Different substrate materials were tested in the search for reliable and inexpensive sample slides with satisfactory DNA immobilization efficiency. Materials such as silicon wafers, gold-coated glass slides, gold-coated stainless steel slides, and gold compact discs (CD) were tested. A surface property comparison of gold-coated glass slides and compact discs was made by atomic force microscopy (AFM), and revealed very different microscopic features. The effect of cleaning on gold-coated glass slides was examined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Technical improvements were made to the SKN equipment progressively. Several revisions to the tip holder design have been employed for better electromagnetic shielding, enhanced robustness and easier tip change. An older signal generator was replaced with a professional PC audio card to provide more stable signal and more convenient on-screen fine tuning, also at a reduced cost. The Labview-based controlling program has also been improved through multiple iterations.