Laboratoire d'Économie Mathématique et de Microéconomie Appliquée

Improvements in transmission electron microscopy have transformed nanobeam electron diffraction into a simple and powerful technique to measure strain. A Si0.69Ge0.31 layer, grown onto a Si substrate has been used to evaluate the precision and accuracy of the technique. Diffraction patterns have been acquired along a ⟨110⟩ zone axis using a FEI-Titan microscope and have been analyzed using dedicated software. A strain precision of 6×10−4 using a probe size of 2.7 nm with a convergence angle of 0.5 mrad has been reached. The bidimensional distortion tensor in the plane perpendicular to the electron beam has been obtained.

The shape of c-oriented GaN nanostructures is found to be directly related to the crystal polarity. As evidenced by convergent beam electron diffraction applied to GaN nanostructures grown by metal-organic vapor phase epitaxy on c-sapphire substrates: wires grown on nitridated sapphire have the N-polarity ([0001¯]) whereas pyramidal crystals have Ga-polarity ([0001]). In the case of homoepitaxy, the GaN wires can be directly selected using N-polar GaN freestanding substrates and exhibit good optical properties. A schematic representation of the kinetic Wulff’s plot points out the effect of surface polarity.

Graphene is the first engineering electronic material, which is purely two-dimensional: it consists of two exposed sp2-hybridized carbon surfaces and has no bulk. Therefore, surface effects such as contamination by adsorbed polymer residues have a critical influence on its electrical properties and can drastically hamper its widespread use in devices fabrication. These contaminants, originating from mandatory technological processes of graphene synthesis and transfer, also impact fundamental studies of the electronic and structural properties at the atomic scale. Therefore, graphene-based technology and research requires “soft” and selective surface cleaning techniques dedicated to limit or to suppress this surface contamination. Here, we show that a high-density H2 and H2-N2 plasmas can be used to selectively remove polymeric residues from monolayer graphene without any damage on the graphene surface. The efficiency of this dry-cleaning process is evidenced unambiguously by a set of spectroscopic and microscopic methods, providing unprecedented insights on the cleaning mechanisms and highlighting the role of specific poly-methyl-methacrylate residues at the graphene interface. The plasma is shown to perform much better cleaning than solvents and has the advantage to be an industrially mature technology adapted to large area substrates. The process is transferable to other kinds of two-dimensional material and heterostructures.

We demonstrate that state-of-the-art off-axis electron holography can be used to map active dopants in silicon nanowires as thin as 60 nm with 10 nm spatial resolution. Experiment and simulation demonstrate that doping concentrations of 10(19) and 10(20) cm(-3) can be measured with a detection threshold of 10(18) cm(-3) with respect to intrinsic silicon. Comparison of experimental data and simulations allows an estimation of the charge density at the wire-oxide interface of -1 x 10(12) electron charges cm(-2). Off-axis electron holography thus offers unique capabilities for a detailed analysis of active dopant concentrations in nanostructures.

We develop a model of a two‐sided asset market in which trades are intermediated by dealers and are bilateral. Dealers compete to attract order flow by posting the terms at which they execute trades—which can include prices, quantities, and execution speed—and investors direct their orders toward dealers who offer the most attractive terms. We characterize the equilibrium in a general setting, and we illustrate theoretically and numerically how the model can account for several important trading patterns in over‐the‐counter markets, which do not emerge from existing models.

We experimentally investigate the influence of AlN buffer growth on the nucleation and the polarity of a self-organized assembly of GaN nanowires (NWs) grown on Si. Two complementary growth mechanisms for AlN buffer deposited on Si are demonstrated. Both emphasize the aggregation of Si on the AlN surface and the growth of large cubic crystallites, namely, AlN pedestals. Further growths of GaN NWs assembly reveal that the GaN 2D layer found at the bottom of the NW assembly is the result of the coalescence of Ga-polar pyramids, whereas AlN pedestals are observed as preferential but not exclusive NW nucleation sites. NWs are N-polar or exhibit inversion domains with a Ga-polar core/N-polar shell structure. This suggests that N-polarity is a necessary condition to trigger NW self-organized nucleation due to a different facets energy hierarchy between the Ga- and the N-polar sides.

The properties of group III-Nitrides (III-N) such as a large direct bandgap, high melting point, and high breakdown voltage make them very attractive for optoelectronic applications. However, conventional epitaxy on SiC and sapphire substrates results in strained and defective films with consequently poor device performance. In this work, by studying the nucleation of GaN on graphene/SiC by MOVPE, we unambiguously demonstrate the possibility of remote van der Waals epitaxy. By choosing the appropriate growth conditions, GaN crystals can grow either in-plane misoriented or fully epitaxial to the substrate. The adhesion forces across the GaN and graphene interface are very weak and the micron-scale nuclei can be easily moved around. The combined use of x-ray diffraction and transmission electron microscopy demonstrate the growth of stress-free and dislocation-free crystals. The high quality of the crystals was further confirmed by photoluminescence measurements. First principles calculations additionally highlighted the importance of the polarity of the underlying substrate. This work lays the first brick towards the synthesis of high quality III-N thin films grown via van der Waals epitaxy.

Although zinc oxide is a promising material for the fabrication of short wavelength optoelectronic devices, p-type doping is a step that remains challenging for the realization of diodes. Out of equilibrium methods such as ion implantation are expected to dope ZnO successfully provided that the non-radiative defects introduced by implantation can be annealed out. In this study, ZnO substrates are implanted with nitrogen ions, and the extended defects induced by implantation are studied by transmission electron microscopy and x-ray diffraction (XRD) before and after annealing at 900 °C. Before annealing, these defects are identified to be dislocation loops lying either in basal planes in high N concentration regions, or in prismatic planes in low N concentration regions, together with linear dislocations. An uniaxial deformation of 0.4% along the c axis, caused by the predominant basal loops, is measured by XRD in the implanted layer. After annealing, prismatic loops disappear while the density of basal loops decreases and their diameter increases. Moreover, dislocation loops disappear completely from the subsurface region. XRD measurements show a residual deformation of only 0.05% in the implanted and annealed layer. The fact that basal loops are favored against prismatic ones at high N concentration or high temperature is attributed to a lower stacking fault energy in these conditions. The coalescence of loops and their disappearance in the subsurface region are ascribed to point defect diffusion. Finally, the electrical and optical properties of nitrogen-implanted ZnO are correlated with the observed structural features.

The state of the lateral surface plays a great role in the physics of silicon nanowires. Surprisingly, little is known about the phenomena that occur during growth on the facets of the wires. We demonstrate here that the size and shape of the facets evolve with the exposure time and the radial growth speed. Depending on the chemistry of the surface, either passivated by chlorine or decorated by gold clusters, the radial growth speed varies and the evolution of the facets is enhanced or impeded. If the radial growth speed is high enough, the faceting of the wire can change from top to bottom due to the exposure time difference. Three types of faceting are exposed, dodecagonal, hexagonal, and triangular. An evolution model is introduced to link the different faceting structures and the possible transitions.

We combine quantitative analyses of Z-contrast images with composition analyses employing atom probe tomography (APT) correlatively to provide a quantitative measurement of atomic scale interfacial intermixing in an InAs/GaSb superlattice (SL). Contributions from GaSb and InAs in the Z-contrast images are separated using an improved image processing technique. Correlation with high resolution APT composition analyses permits an examination of interfacial segregation of both cations and anions and their incorporation in the short period InAs/GaSb SL. Results revealed short, intermediate, and long-range intermixing of In, Ga, and Sb during molecular beam epitaxial growth and their distribution in the SL.

Cu-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> direct hybrid bonding is considered as one of the most promising approaches for matching the needs of three dimensional integrated circuits (3D-IC). In this paper we present the results of a complete morphological, electrical and reliability study conducted on four-layer copper structures realized by Cu-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> direct hybrid bonding. Ultra-fine 7 μm pitch, 3 μm × 3 μm pads daisy chains with up to 30 160 connections are bonded with submicron accuracy, matching the interconnection needs in upcoming 3D circuits. A focus is paid on the influence of the temperature of annealing on the bonding quality and electrical performances. Thanks to SEM, FIB/SEM tomography, AFM, TEM and TEM-EDX analysis, the morphology of the bonded structures is described. Electrical parametric tests exhibit very low resistance structures, with high functional yields and low variability. State-of-the-art contact resistivity is obtained for 200 and 400 °C treatments. Unbiased HAST, temperature cycling and temperature storage tests demonstrate excellent reliability performances. Finally, an electromigration resistance comparative study is conducted on bonded copper lines with TaN/Ta and TiN diffusion barriers.

Classical decision theory postulates that choices proceed from subjective values assigned to the probable outcomes of alternative actions. Some authors have argued that opposite causality should also be envisaged, with choices influencing subsequent values expressed in desirability ratings. The idea is that agents may increase their ratings of items that they have chosen in the first place, which has been typically explained by the need to reduce cognitive dissonance. However, evidence in favor of this reverse causality has been the topic of intense debates that have not reached consensus so far. Here, we take a novel approach using Bayesian techniques to compare models in which choices arise from stable (but noisy) underlying values (one-way causality) versus models in which values are in turn influenced by choices (two-way causality). Moreover, we examined whether in addition to choices, other components of previous actions, such as the effort invested and the eventual action outcome (success or failure), could also impact subsequent values. Finally, we assessed whether the putative changes in values were only expressed in explicit ratings, or whether they would also affect other value-related behaviors such as subsequent choices. Behavioral data were obtained from healthy participants in a rating-choice-rating-choice-rating paradigm, where the choice task involves deciding whether or not to exert a given physical effort to obtain a particular food item. Bayesian selection favored two-way causality models, where changes in value due to previous actions affected subsequent ratings, choices and action outcomes. Altogether, these findings may help explain how values and actions drift when several decisions are made successively, hence highlighting some shortcomings of classical decision theory.

Recent progress in quantum dot (QD) sensitized solar cells has demonstrated the possibility of low-cost and efficient photovoltaics. However, the standard device structure based on n-type materials often suffers from slow hole injection rate, which may lead to unbalanced charge transport. We have fabricated efficient p-type (inverted) QD sensitized cells, which combine the advantages of conventional QD cells with p-type dye sensitized configurations. Moreover, p-type QD sensitized cells can be used in highly promising tandem configurations with n-type ones. QDs without toxic Cd and Pb elements and with improved absorption and stability were successfully deposited onto mesoporous NiO electrode showing good coverage and penetration according to morphological analysis. Detailed photophysical charge transfer studies showed that high hole injection rates (108 s−1) observed in such systems are comparable with electron injection in conventional n-type QD assemblies. Inverted solar cells fabricated with various QDs demonstrate excellent power conversion efficiencies of up to 1.25%, which is 4 times higher than the best values for previous inverted QD sensitized cells. Attempts to passivate the surface of the QDs show that traditional methods of reduction of recombination in the QD sensitized cells are not applicable to the inverted architectures.

High resolution imaging and electron tomography are used to link nanoscale morphology with electrochemical activity in highly active Pt₃Ni₇nanostructured thin film catalysts, revealing the critical role of catalyst conditioning.

Novel behavior has been observed at the interface of LaAlO3/SrTiO3 heterostructures such as two dimensional metallic conductivity, magnetic scattering and superconductivity. However, both the origins and quantification of such behavior have been complicated due to an interplay of mechanical, chemical and electronic factors. Here chemical and strain profiles near the interface of LaAlO3/SrTiO3 heterostructures are correlated. Conductive and insulating samples have been processed, with thicknesses respectively above and below the commonly admitted conductivity threshold. The intermixing and structural distortions within the crystal lattice have been quantitatively measured near the interface with a depth resolution of unit cell size. A strong link between intermixing and structural distortions at such interfaces is highlighted: intermixing was more pronounced in the hetero-couple with conductive interface, whereas in-plane compressive strains extended deeper within the substrate of the hetero-couple with the insulating interface. This allows a better understanding of the interface local mechanisms leading to the conductivity.

Abstract We develop a New Monetarist model in continuous time where agents trade continuously in competitive markets and infrequently in pairwise meetings. Agents can produce and consume both in flows over time intervals and in discrete quantities at points in time. We detail the methodology to solve individual optimisation problems and characterise the full set of perfect foresight equilibria. We illustrate the role of continuous time and the tractability of our approach with three applications related to monetary policy: (i) forward guidance with policy announcements; (ii) aggregate demand management and firm entry; (iii) open market operations with partially liquid assets.

Dimensionality crossover is a classical topic in physics. Surprisingly, it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was confirmed experimentally based on micron-sized flux-closure dots.

This paper examines wealth accumulation among couple-headed households and investigates changes in within-household inequality over time and across couple statuses. Going beyond previous research that mostly studies wealth accumulation within marriages by comparing married with unmarried individuals, we consider the legal statuses of couples (cohabitation, civil union, and marriage) and property regimes (community and separate property). We apply multivariate regression analysis to high-quality longitudinal data from the French wealth survey (2015-2018) and find no differences in net worth accumulation between couples' legal statuses when property regimes are not accounted for. However, couples with a separate property regime accumulate more wealth than couples with a community property regime, and married couples with a separate property regime drive this association. Our results show that the gender wealth gap is larger for couples with a separate property regime, but it is partially compensated by accumulated wealth. Our results highlight the importance of legal statuses and property regimes in explaining the dynamics of between- and within-household inequality in France, specifically within a context of increasingly diversified marital trajectories. Supplementary Information: The online version contains supplementary material available at 10.1007/s10680-022-09632-5.

GaN quantum dots doped with Tm atoms and embedded in AlN have been characterized by high-angle annular dark-field imaging using a scanning transmission electron microscope. Direct visualization of individual Tm atoms in AlN layers has been achieved. We have found that besides being present in GaN dots, Tm atoms also tend to segregate at AlN barriers. The Tm distribution is related to the capping mechanism of the dots with AlN. A visibility coefficient based on locally integrated, rather than peak, intensities is introduced to determine quantitatively the number of Tm atoms in a given atomic column. Experimental and simulated images show that this visibility presents a reduced sensitivity to the defocus or to the position of the Tm atom within the thin lamella.

This paper analyzes assortative mating and its contribution to inequality in France. We first provide descriptive evidence on the statistical association in several socio‐economic attributes of partners. Second, we assess the contribution of assortative mating to earnings inequality between couples. We provide a new method for assessing the contribution of assortative mating to inequality in couple’s potential earnings, that accounts for selection bias arising from labor force participation. Our results indicate a strong degree of assortative mating in France. The correlation in earnings is around 0.17 for annual earnings, around 0.35 for full‐time equivalent earnings and up to 0.49 when using multi‐year average earnings. Assortative mating tends to increase inequality among couples. For annual earnings, the effect accounts for 3 to 9 percent of measured inequality. The effect of assortative mating on household potential earnings is much larger and amounts to 10 to 20 percent for observed inequality.