Composition Analysis of III-Nitrides at the Nanometer Scale: Comparison of Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography

International audienc

Investigation of p-type doping in GaN nanostructures by correlation between atom probe tomography and off-axis electron holography

La thèse porte sur l’étude du dopage de type p, à base de Mg, dans des nanostructures de GaN, dans le but de relier la distribution spatiale du dopant à son activité électrique grâce à la corrélation entre sonde atomique tomographique (APT) et holographie électronique hors axe optique réalisée dans un microscope électronique en transmission (TEM). L'APT est une technique de caractérisation qui repose sur l'évaporation par effet de champ des atomes de surface d'un échantillon, permettant l'analyse en termes de morphologie et de composition, en trois dimensions et à l'échelle atomique. L'holographie électronique hors axe optique fournit des cartographies du potentiel électrostatique introduit par les dopants actifs électriquement. Dans un premier temps, les conditions expérimentales ont dues être optimisées pour chacune des techniques, incluant la préparation des échantillons, les conditions de mesure ainsi que le traitement des données, de façon à obtenir des données fiables et les plus quantitatives possibles. Une analyse soigneuse et détaillée des artefacts et des erreurs qu’ils introduisent est rapportée. Il a en particulier été montré que réaliser les expériences d’holographie in-situ à haute température (400 °C) grâce à un porte-objet chauffant permettait d’augmenter très significativement le signal lié au dopage et ainsi accroitre la sensibilité de la mesure. Dans un deuxième temps, ces deux méthodes d’analyse ont été corrélées pour étudier d’une part l’influence de la température de croissance en MOCVD, d’autre part celle de la concentration nominale en dopants dans des nanostructures dédiées GaN. Nous avons pu confirmer grâce à l’APT l’existence de précipités riches en Mg dès que la concentration nominale excède environ 3E19 cm-3, dont la densité augmente avec la concentration nominale et diminue avec la température de croissance. Leur présence diminue la concentration en dopants potentiellement actifs situés dans la matrice en dehors de ces précipités. Néanmoins, les résultats obtenus par holographie, appuyés par des simulations numériques, indiquent que ces précipités n’auraient pas un rôle prépondérant dans la variation du potentiel électrostatique en fonction de la concentration nominale en dopants même pour des concentrations en Mg qui s’élèvent à 2E20cm-3.The aim of the thesis is to develop a methodology for the investigation of Mg which acts as p-type doping in GaN. We relate the spatial distribution of the dopants with their electrical activity which is achieved by coupling two complementary approaches, Atom Probe Tomography (APT) and Off-axis electron holography. These measurements have also been combined with high-resolution electron microscopy (HR-(S)TEM) for the structural characterization. APT is a unique characterization technique, based on the field effect evaporation of individual atoms of a needle shape sample, allowing the analysis of nano-devices both in terms of morphology and composition in three dimensions at the atomic scale. Off-axis electron holography uses an electron biprism to form an interference pattern from which the electrostatic potential arising from the active dopants can be determined. In this work the experimental procedure has been optimized for both techniques including specimen preparation, the microscope parameters and data treatment to recover accurate information about the position and activity of the dopants. For the holography measurements, a careful analysis of the artifacts that are present in these specimen has been performed to understand the effects of specimen preparation and charging under electron irradiation. We have performed these experiments at high temperature in-situ in the TEM (400 °C) as this increases the ionized dopant concentrations and reduces the artifacts that are present in our measurements. Having developed the methodology, these two techniques are then used to study the effect of temperature and dopant concentrations on the growth of Mg-doped GaN by MOCVD. We have been able to show by APT the existence of precipitates of Mg which are present from a concentration of 3E19 cm-3 whose size and density depends on the growth temperature and the total nominal dopant concentration. Their presence reduces the concentration of dopants that are potentially active in the specimens. However, the measurements of active dopants by holography combined with simulations suggest that the presence of these precipitates do not dominate the electrical properties of the material and that even in very highly doped specimens up to 2E20cm-3 the total active dopant concentrations are still higher than expected from previously published studies. The correlation between these techniques will provide valuable information to improve the Mg activation GaN which is currently a big issue for device manufacture

Etude du dopage de type p dans des nanostructures de GaN par corrélation entre sonde atomique tomographique et holographie électronique hors axe optique

The aim of the thesis is to develop a methodology for the investigation of Mg which acts as p-type doping in GaN. We relate the spatial distribution of the dopants with their electrical activity which is achieved by coupling two complementary approaches, Atom Probe Tomography (APT) and Off-axis electron holography. These measurements have also been combined with high-resolution electron microscopy (HR-(S)TEM) for the structural characterization. APT is a unique characterization technique, based on the field effect evaporation of individual atoms of a needle shape sample, allowing the analysis of nano-devices both in terms of morphology and composition in three dimensions at the atomic scale. Off-axis electron holography uses an electron biprism to form an interference pattern from which the electrostatic potential arising from the active dopants can be determined. In this work the experimental procedure has been optimized for both techniques including specimen preparation, the microscope parameters and data treatment to recover accurate information about the position and activity of the dopants. For the holography measurements, a careful analysis of the artifacts that are present in these specimen has been performed to understand the effects of specimen preparation and charging under electron irradiation. We have performed these experiments at high temperature in-situ in the TEM (400 °C) as this increases the ionized dopant concentrations and reduces the artifacts that are present in our measurements. Having developed the methodology, these two techniques are then used to study the effect of temperature and dopant concentrations on the growth of Mg-doped GaN by MOCVD. We have been able to show by APT the existence of precipitates of Mg which are present from a concentration of 3E19 cm-3 whose size and density depends on the growth temperature and the total nominal dopant concentration. Their presence reduces the concentration of dopants that are potentially active in the specimens. However, the measurements of active dopants by holography combined with simulations suggest that the presence of these precipitates do not dominate the electrical properties of the material and that even in very highly doped specimens up to 2E20cm-3 the total active dopant concentrations are still higher than expected from previously published studies. The correlation between these techniques will provide valuable information to improve the Mg activation GaN which is currently a big issue for device manufacture.La thèse porte sur l’étude du dopage de type p, à base de Mg, dans des nanostructures de GaN, dans le but de relier la distribution spatiale du dopant à son activité électrique grâce à la corrélation entre sonde atomique tomographique (APT) et holographie électronique hors axe optique réalisée dans un microscope électronique en transmission (TEM). L'APT est une technique de caractérisation qui repose sur l'évaporation par effet de champ des atomes de surface d'un échantillon, permettant l'analyse en termes de morphologie et de composition, en trois dimensions et à l'échelle atomique. L'holographie électronique hors axe optique fournit des cartographies du potentiel électrostatique introduit par les dopants actifs électriquement. Dans un premier temps, les conditions expérimentales ont dues être optimisées pour chacune des techniques, incluant la préparation des échantillons, les conditions de mesure ainsi que le traitement des données, de façon à obtenir des données fiables et les plus quantitatives possibles. Une analyse soigneuse et détaillée des artefacts et des erreurs qu’ils introduisent est rapportée. Il a en particulier été montré que réaliser les expériences d’holographie in-situ à haute température (400 °C) grâce à un porte-objet chauffant permettait d’augmenter très significativement le signal lié au dopage et ainsi accroitre la sensibilité de la mesure. Dans un deuxième temps, ces deux méthodes d’analyse ont été corrélées pour étudier d’une part l’influence de la température de croissance en MOCVD, d’autre part celle de la concentration nominale en dopants dans des nanostructures dédiées GaN. Nous avons pu confirmer grâce à l’APT l’existence de précipités riches en Mg dès que la concentration nominale excède environ 3E19 cm-3, dont la densité augmente avec la concentration nominale et diminue avec la température de croissance. Leur présence diminue la concentration en dopants potentiellement actifs situés dans la matrice en dehors de ces précipités. Néanmoins, les résultats obtenus par holographie, appuyés par des simulations numériques, indiquent que ces précipités n’auraient pas un rôle prépondérant dans la variation du potentiel électrostatique en fonction de la concentration nominale en dopants même pour des concentrations en Mg qui s’élèvent à 2E20cm-3

Three-dimensional measurement of Mg dopant distribution and electrical activity in GaN by correlative atom probe tomography and off-axis electron holography

International audienceThe distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 x 10 19 cm-3. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 x 10 20 cm-3 , despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential

Correlative investigation of Mg doping in GaN layers grown at different temperatures by atom probe tomography and off-axis electron holography

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Dopant radial inhomogeneity in Mg-doped GaN nanowires

International audienceUsing atom probe tomography, it is demonstrated that Mg doping of GaN nanowires grown by Molecular Beam Epitaxy results in a marked radial inhomogeneity, namely a higher Mg content in the periphery of the nanowires. This spatial inhomogeneity is attributed to a preferential incorporation of Mg through the m-plane sidewalls of nanowires and is related to the formation of a Mg-rich surface which is stabilized by hydrogen. This is further supported by Raman spectroscopy experiments which give evidence of Mg-H complexes in the doped nanowires. A Mg doping mechanism such as this, specific to nanowires, may lead to higher levels of Mg doping than in layers, boosting the potential interest of nanowires for light emitting diode applications

A scalable membrane electrode assembly architecture for efficient electrochemical conversion of CO2 to formic acid

Abstract The electrochemical reduction of carbon dioxide to formic acid is a promising pathway to improve CO2 utilization and has potential applications as a hydrogen storage medium. In this work, a zero-gap membrane electrode assembly architecture is developed for the direct electrochemical synthesis of formic acid from carbon dioxide. The key technological advancement is a perforated cation exchange membrane, which, when utilized in a forward bias bipolar membrane configuration, allows formic acid generated at the membrane interface to exit through the anode flow field at concentrations up to 0.25 M. Having no additional interlayer components between the anode and cathode this concept is positioned to leverage currently available materials and stack designs ubiquitous in fuel cell and H2 electrolysis, enabling a more rapid transition to scale and commercialization. The perforated cation exchange membrane configuration can achieve >75% Faradaic efficiency to formic acid at <2 V and 300 mA/cm2 in a 25 cm2 cell. More critically, a 55-hour stability test at 200 mA/cm2 shows stable Faradaic efficiency and cell voltage. Technoeconomic analysis is utilized to illustrate a path towards achieving cost parity with current formic acid production methods