Low loss GaN waveguides for visible light on Si substrates

International audienceIn this work, we present the realization and the characterization of an optical waveguide made of AlN and GaN layers grown by MBE on Si(111) substrate. For the fundamental mode at 633nm, the propagation losses are in the order of 2 dB/cm, which is a good number for SC waveguides at this wavelength. The propagation losses dramatically increase with the mode order. A careful comparison of measurements and modeling of the complete structure allows identifying the part of the losses due to absorptionin the Si substrate, and showing that propagation losses could be further reduced by using well chosen SOI substrates

Experimental demonstration of a Two-Dimensional Hole Gas (2DHG) in a GaN/AlGaN/GaN based heterostructure by optical spectroscopy

The polarization discontinuity across interfaces in polar nitride-based heterostructures can lead to the formation of two-dimensional electron and hole gases. In the past, the observation of these electron and hole gases has been achieved through various experimental techniques, most often by electronic measurements but occasionally by optical means. However, the occurrence of a two-dimensional hole gas has never been demonstrated optically. The objective of this article is to demonstrate, thanks to the combination of various optical spectroscopy techniques coupled to numerical simulations, the presence of a two-dimensional hole gas in a GaN/AlGaN/GaN heterostructure. This is made possible thanks to a GaN/AlGaN/GaN heterostructure displaying a micrometer-thick AlGaN layer and a GaN cap thicker than in conventional GaN-based HEMTs structures. The band structure across the whole heterostructure was established by solving self-consistently the Schr\"odinger and Poisson equations and by taking into account the experimentally determined strain state of each layer. Continuous and quasi-continuos photoluminescence reveal the presence of a broad emission band at an energy around 50 meV below the exciton emission and whose energy blueshifts with increasing excitation power density, until it is completely quenched due to the complete screening of the internal electric field. Time-resolved measurements show that the emission arising from the two-dimensional hole gas can be assigned to the recombination of holes in the potential well with electrons located in the top GaN as well as electron from the bottom AlGaN, each of them displaying different decay times due to unequal electric fields. Besides the optical demonstration of a two-dimensional hole gas in a nitride-based heterostructure, our work highlights the optical recombination processes involved in the emission from such a hole gas

Extreme ultraviolet detection using AlGaN-on-Si inverted Schottky photodiodes

We report on the fabrication of aluminum gallium nitride (AlGaN) Schottky diodes for extreme ultraviolet (EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid imagers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detectio

Height and body-mass index trajectories of school-aged children and adolescents from 1985 to 2019 in 200 countries and territories: a pooled analysis of 2181 population-based studies with 65 million participants

Summary Background Comparable global data on health and nutrition of school-aged children and adolescents are scarce. We aimed to estimate age trajectories and time trends in mean height and mean body-mass index (BMI), which measures weight gain beyond what is expected from height gain, for school-aged children and adolescents. Methods For this pooled analysis, we used a database of cardiometabolic risk factors collated by the Non-Communicable Disease Risk Factor Collaboration. We applied a Bayesian hierarchical model to estimate trends from 1985 to 2019 in mean height and mean BMI in 1-year age groups for ages 5–19 years. The model allowed for non-linear changes over time in mean height and mean BMI and for non-linear changes with age of children and adolescents, including periods of rapid growth during adolescence. Findings We pooled data from 2181 population-based studies, with measurements of height and weight in 65 million participants in 200 countries and territories. In 2019, we estimated a difference of 20 cm or higher in mean height of 19-year-old adolescents between countries with the tallest populations (the Netherlands, Montenegro, Estonia, and Bosnia and Herzegovina for boys; and the Netherlands, Montenegro, Denmark, and Iceland for girls) and those with the shortest populations (Timor-Leste, Laos, Solomon Islands, and Papua New Guinea for boys; and Guatemala, Bangladesh, Nepal, and Timor-Leste for girls). In the same year, the difference between the highest mean BMI (in Pacific island countries, Kuwait, Bahrain, The Bahamas, Chile, the USA, and New Zealand for both boys and girls and in South Africa for girls) and lowest mean BMI (in India, Bangladesh, Timor-Leste, Ethiopia, and Chad for boys and girls; and in Japan and Romania for girls) was approximately 9–10 kg/m2. In some countries, children aged 5 years started with healthier height or BMI than the global median and, in some cases, as healthy as the best performing countries, but they became progressively less healthy compared with their comparators as they grew older by not growing as tall (eg, boys in Austria and Barbados, and girls in Belgium and Puerto Rico) or gaining too much weight for their height (eg, girls and boys in Kuwait, Bahrain, Fiji, Jamaica, and Mexico; and girls in South Africa and New Zealand). In other countries, growing children overtook the height of their comparators (eg, Latvia, Czech Republic, Morocco, and Iran) or curbed their weight gain (eg, Italy, France, and Croatia) in late childhood and adolescence. When changes in both height and BMI were considered, girls in South Korea, Vietnam, Saudi Arabia, Turkey, and some central Asian countries (eg, Armenia and Azerbaijan), and boys in central and western Europe (eg, Portugal, Denmark, Poland, and Montenegro) had the healthiest changes in anthropometric status over the past 3·5 decades because, compared with children and adolescents in other countries, they had a much larger gain in height than they did in BMI. The unhealthiest changes—gaining too little height, too much weight for their height compared with children in other countries, or both—occurred in many countries in sub-Saharan Africa, New Zealand, and the USA for boys and girls; in Malaysia and some Pacific island nations for boys; and in Mexico for girls. Interpretation The height and BMI trajectories over age and time of school-aged children and adolescents are highly variable across countries, which indicates heterogeneous nutritional quality and lifelong health advantages and risks

Worldwide trends in underweight and obesity from 1990 to 2022: a pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults

Background Underweight and obesity are associated with adverse health outcomes throughout the life course. We estimated the individual and combined prevalence of underweight or thinness and obesity, and their changes, from 1990 to 2022 for adults and school-aged children and adolescents in 200 countries and territories. Methods We used data from 3663 population-based studies with 222 million participants that measured height and weight in representative samples of the general population. We used a Bayesian hierarchical model to estimate trends in the prevalence of different BMI categories, separately for adults (age ≥20 years) and school-aged children and adolescents (age 5–19 years), from 1990 to 2022 for 200 countries and territories. For adults, we report the individual and combined prevalence of underweight (BMI <18·5 kg/m2) and obesity (BMI ≥30 kg/m2). For schoolaged children and adolescents, we report thinness (BMI <2 SD below the median of the WHO growth reference) and obesity (BMI >2 SD above the median). Findings From 1990 to 2022, the combined prevalence of underweight and obesity in adults decreased in 11 countries (6%) for women and 17 (9%) for men with a posterior probability of at least 0·80 that the observed changes were true decreases. The combined prevalence increased in 162 countries (81%) for women and 140 countries (70%) for men with a posterior probability of at least 0·80. In 2022, the combined prevalence of underweight and obesity was highest in island nations in the Caribbean and Polynesia and Micronesia, and countries in the Middle East and north Africa. Obesity prevalence was higher than underweight with posterior probability of at least 0·80 in 177 countries (89%) for women and 145 (73%) for men in 2022, whereas the converse was true in 16 countries (8%) for women, and 39 (20%) for men. From 1990 to 2022, the combined prevalence of thinness and obesity decreased among girls in five countries (3%) and among boys in 15 countries (8%) with a posterior probability of at least 0·80, and increased among girls in 140 countries (70%) and boys in 137 countries (69%) with a posterior probability of at least 0·80. The countries with highest combined prevalence of thinness and obesity in school-aged children and adolescents in 2022 were in Polynesia and Micronesia and the Caribbean for both sexes, and Chile and Qatar for boys. Combined prevalence was also high in some countries in south Asia, such as India and Pakistan, where thinness remained prevalent despite having declined. In 2022, obesity in school-aged children and adolescents was more prevalent than thinness with a posterior probability of at least 0·80 among girls in 133 countries (67%) and boys in 125 countries (63%), whereas the converse was true in 35 countries (18%) and 42 countries (21%), respectively. In almost all countries for both adults and school-aged children and adolescents, the increases in double burden were driven by increases in obesity, and decreases in double burden by declining underweight or thinness. Interpretation The combined burden of underweight and obesity has increased in most countries, driven by an increase in obesity, while underweight and thinness remain prevalent in south Asia and parts of Africa. A healthy nutrition transition that enhances access to nutritious foods is needed to address the remaining burden of underweight while curbing and reversing the increase in obesit

Heterogeneous contributions of change in population distribution of body mass index to change in obesity and underweight NCD Risk Factor Collaboration (NCD-RisC)

From 1985 to 2016, the prevalence of underweight decreased, and that of obesity and severe obesity increased, in most regions, with significant variation in the magnitude of these changes across regions. We investigated how much change in mean body mass index (BMI) explains changes in the prevalence of underweight, obesity, and severe obesity in different regions using data from 2896 population-based studies with 187 million participants. Changes in the prevalence of underweight and total obesity, and to a lesser extent severe obesity, are largely driven by shifts in the distribution of BMI, with smaller contributions from changes in the shape of the distribution. In East and Southeast Asia and sub-Saharan Africa, the underweight tail of the BMI distribution was left behind as the distribution shifted. There is a need for policies that address all forms of malnutrition by making healthy foods accessible and affordable, while restricting unhealthy foods through fiscal and regulatory restrictions

Proprietes electroniques et optiques des heterostructures metal/semiconducteur

SIGLEINIST T 71044 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Etude des propriétés optoélectroniques et de transport dans les hétérostructures AlGaN/GaN pour la réalisation de détecteurs UV

L'objet de cette thèse est l'étude des hétérostructures GaN/(Al,Ga)N afin de réaliser des détecteurs UV. Les difficultés de croissance ainsi que les propriétés fondamentales du matériau sont présentées dans le chapitre 1 de manière à constituer les outils de conception des dispositifs à réaliser. Au chapitre 2, les problèmes de fluctuation d'alliage et de localisation de porteurs dans les alliages ternaires (Ga,In)N et (Al,Ga)N sont abordés. Le rôle du champ éléctrique dans les puits et les limites constituées par les fluctuations statistiques d'alliage sont alors mis en évidence. Un modèle permettant d'expliquer certains comportements non idéaux comme l'absorption sous le gap et sa non-linéarité par rapport au flux incident est également élaboré. Enfin les effets photopersistants dans les structures typiques de transistors à effet de champ sont étudiées. Les deux derniers chapitres concernent l'étude de démonstrateurs...This work investigates GaN/(Al,Ga)N heterostructures for achieving Uv photodetectors. In the first chapter, the difficulties in growing these structures are presented. Their fundamental properties are studied in order to understand their limits and develop tools to predict the behavior of devices. Alloy fluctuations and localisation in (Ga,In)N and (Al,Ga)N materials are investigated in the second chapter. The effect of the piezoelectric field and the spontaneous polarization in quantum wells is shown. In bulk materials, the sharpness of band edge absorption is proved to be close to statistic alloy broadening. A model to explain no-ideal behaviours as subbandgap absorption and nonlinearity of response versus optical power is developed. The effects of deep donors in the barrier of field effect transistors on persistent photoconductibilty are also shown...ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Relaxation des contraintes dans les hétérostructures épaisses (Al,Ga)N (une piste originale pour la réalisation de diodes électroluminescentes à cavité résonante)

L étude des différents modes de relaxation des contraintes a montré que dans les nitrures d éléments III orientés suivant l axe [0001], celle-ci ne peut pas s opérer par le glissement de dislocations traversantes. Pour les films soumis à une contrainte extensive, la relaxation des contraintes fait intervenir la fissuration du film puis l introduction de dislocations d interface à partir des bords des fissures. La caractérisation d hétérostructures fissurées (Al,Ga)N / GaN par microscopie électronique en transmission (MET et diffraction des rayons X (DRX), a été mise en évidence. Les propriétés de croissance latérale de l épitaxie en phase vapeur à base d organométalliques (EPVOM), permettent dans certaines conditions de croissance de cicatriser les fissures. Nous avons ainsi obtenu des films d AIO,20Ga0,80N épais, relaxés et de bonne qualité cristalline. Ces pseudo-substrats d (Al,Ga)N ont été utilisés pour effectuer la croissance pseudomorphe de miroirs de Bragg (Al,Ga)N/GaN. La caractérisation de ces structures a été réalisée aussi bien en ce qui concerne l état de contraintes que les propriétés optiques et électriques. Une méthode de mesure de la résistivité des miroirs de Bragg compatible avec une technologie planaire a notamment été développée. Enfin, des diodes électroluminescentes à cavité résonante (DEL-CR) ont été réalisées afin de valider la méthode proposée pour la croissance de films épais d (Al,Ga)N.In [0001]-oriented III-nitrides, the glide of threading dislocations is inefficient to relax the misfit stress. When films are grown under tensile stress, the plastic relaxation occurs through the film cracking and the introduction of misfit dislocations from the crack edges. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) have been used in order to characterize cracked (Al,Ga)N/GaN heterostructures. A cooperative mechanism between cracking and ductile relaxation has been outlined. The relaxation rate strongly depends in the (Al,Ga)N/GaN film thickness. By combining the lateral growth of (Al,Ga)N and the stress relaxation, craks have been healed and high quality (Al,Ga)N films have been grown by metal-organic vapour phase epitaxy (MOVPE). Coherent growth of (Al,Ga)N/GaN Bragg mirrors has been carried out of those thick relaxed (Al,Ga)N films. Their optical and electrical properties as well as their stress have been investigated. A resistivity measurement method has been developed to comply with the planar technology. Resonant cavity light emitting diodes (RCLED have been realized to demonstrate the benefit of this new (Al,Ga)N growth method.NICE-BU Sciences (060882101) / SudocSudocFranceF