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In this paper, we studied the electrical characteristic of Schottky diodes based on gold contact on nitridated GaAs substrates. The used (100) GaAs substrate is n-type with concentration of Nd = 4.9 × 1015 cm–3. Nitridation process was performed using a N2 glow discharge source (GDS) creating N atomic species. A ultra-thin film with a thickness of 2.2 nm GaN is performed on GaAs surface. In order to study the electric characteristics under illumination, we use of a He–Ne laser of 1 mW power and 632.8 nm wavelength. The current–voltage (I–V) of the Au/GaN/GaAs structures was investigated at room temperature. The saturation current IS, the series resistance RS and the mean ideality factor n are, respectively, equal to 4.46 × 10–07 A, 172 Ohm, 1.4 in the dark and to 5.64 × 10–07 A, 148 Ohm, 1.21 under illumination. To analyze these results, a 1D-simulation code of forward and reverse current–voltage characteristics versus the critical parameters of a nitridated GaAs Schottky diode is implemented. The algorithm is based on the solution of the system composed by Poisson’s and continuities equations. In this calculation, we take into account the existence of the GaN layer and the localization of traps states in the perturbed interface. We have considered the W-shaped and U-shaped distribution of traps states in the band gap. The effects of the doping concentration of GaAs, the traps states density, light intensities and the work function of gold $${{\phi }_{m}}$$ are investigated. By fitting the experimental curve, we can deduce the values of the traps states and the exact value of the work function of gold, and consequently we validate the developed model.

The aim of the present work is to perform Attenuated Total Reflectance-Fourier Transform Infra-Red spectroscopy (ATR-FTIR) investigations during N2 plasma treatment of GaAs surfaces at 500 °C inside an UHV chamber using the high intensity light of SOLEIL synchrotron at SMIS beamline. Moreover this work uses the complementarities of Density Functional Theory (DFT) normal-mode frequency calculations with ATR-FTIR measurements to assist in the assignment of vibrations involving bonds to nitrogen atoms, especially by rendering the GaN layer partially amorphous (by removing the crystal symmetry constraint, using Car Parrinello dynamics to simulate annealing in geometry optimization). s and p polarizations were used to study the evolution of the different GaN vibrations during the nitridation of GaAs for two surface orientations (0 0 1) and (1 1 1)B. Results show that normal stretch vibrations are always detected at the same frequency matching the DFT calculated values very well, whereas other vibration modes (bridge symmetric and asymmetric stretches for (0 0 1) GaAs and tripod symmetric and asymmetric stretches for (1 1 1)B GaAs) induced by the non-crystallinity of the GaN layer exhibit variable frequencies. Furthermore evolutions of these frequencies with the nitridation time provide much information on bond creations, especially at the very beginning of the process.

Two kinds of N2 plasma source, ECR (electron cyclotron resonance) and GDS (glow discharge source) generating mostly N-radical atoms and N-cationic species respectively, were used to grow a thin nitride layer on a GaAs(100) substrate. It was found that this nitridation followed by annealing at 620 °C permits the crystallization of the nitride layer. Pyramidal Zinc Blende GaN nanostructures (zb-GaN) with four facets were obtained using GDS plasma. Surprisingly, a planar and pure wurtzite structure (w-GaN) was obtained using the ECR source. This w-GaN structure shows low photoluminescence intensity and a biaxial tensile strain due to lattice mismatch. Accordingly, the operator can select which phase is formed, simply by switching plasma source. The valence band discontinuity ΔEv has been determined to be 1.74 eV for the zb-GaN/GaAs and w-GaN/GaAs junctions by X-ray photoelectron spectroscopy. As a consequence the conduction bands of the GaAs substrate and the elaborated GaN thin layer are aligned for a zb-GaN/GaAs junction giving efficient electron transport at the zb-GaN/GaAs interface. For w-GaN/GaAs junction, the conduction band discontinuity ΔEc is 0.23 eV inducing an electron confinement in the GaAs(100) which can be an effective way to improve the electronic or optical properties of GaAs devices.

A non-destructive new imaging technique called Multi-Mode Elastic Peak Electron Microscopy (MM-EPEM), hypersensitive to surface chemistry and with an in-depth resolution of one atomic monolayer was developed. This method consists on performing several MM-EPEM images containing n × n pixels associated to an intensity of the elastic backscattered electrons by varying the incident electron energy in the range 200–2000 eV. This approach allows obtaining depth sampling information of the analyzed structures. Furthermore, MM-EPEM is associated with Monte-Carlo simulations describing the electron pathway in materials in order to obtain very precise quantitative information, for instance the growth mode and the organization of ultra-thin layers (2D materials) or nanoparticules. In this work, we used this new method to study the deposition of very small amount of gold down to one monolayer. Example of 3D reconstruction is also provided.

The high density of interface and surface states that cause the strong Fermi pinning observed on GaAs surfaces can be reduced by depositing GaN ultra-thin films on GaAs. To further improve this passivation, it is necessary to investigate the nitridation phenomena by identifying the distinct steps occurring during the process and to understand and quantify the growth kinetics of GaAs nitridation under different conditions. Nitridation of the cleaned GaAs substrate was performed using N 2 plasma source. Two approaches have been combined. Firstly, an AR-XPS (Angle Resolved X-ray Photoelectron Spectroscopy) study is carried out to determine the chemical environments of the Ga, As and N atoms and the composition depth profile of the GaN thin film which allow us to summarize the nitridation process in three steps. Moreover, the temperature and time treatment have been investigated and show a significant impact on the formation of the GaN layer. The second approach is a refined growth kinetic model which better describes the GaN growth as a function of the nitridation time. This model clarifies the exchange mechanism of arsenic with nitrogen atoms at the GaN/GaAs interface and the phenomenon of quasi-saturation of the process observed experimentally.

The correction function of a hemispherical analyzer (HSA) is determined for quantitative interpretations of electron spectroscopy. In this way, electron elastic images are performed using a scanning electron gun. This new method allowed the determination of the analysis area A(EK) of a HSA for the first time. An important result is the dependence of this analysis area on the electron kinetic energy EK. Indeed, results show that A(EK) varies as EK−1.2 regardless of the spectrometer configuration. This parameter is different from the so-called transmission function and must be taken into account for quantitative interpretation. Moreover, the transmission function T(EK) is also determined in this work and varies as a power function EKx where x is a fitting parameter which depends only on the width in the energy dispersive direction of the hemisphere entrance slit. These two apparatus functions are then validated thanks to XPS measurements by comparing results obtained on two different Ag surfaces. Then a general methodology to use these functions is given.

International audience; This study reports a synthesis by the sol-gel method of stable and size-controlled ZnO quantum dots (QDs) embedded into a SiO2 matrix in the diameter range of 2.7-5.5 nm with strong visible emission in the blue-green region. X-ray diffraction and selected area electron diffraction techniques evidence a wurtzite structure of the prepared ZnO QDs. Transmission electron microscopy investigation highlights that the ZnO nanoparticles produced in methanol solution have a nonuniform shape. Addition of tetraethyl ortho-silicate (TEOS) in the solution at different times blocks the nucleation of the QDs and allows the control of the QD size due to a SiO2 capping. Moreover, Fourier transformed infrared analysis and X-ray photoelectron spectroscopy (XPS) were used to show the ZnO QD composition and to confirm their incorporation into the SiO2 matrix by the creation of a Zn-O-Si crossing link. A strong UV band and a weak visible band are revealed at room temperature by photoluminescence (PL) measurement for pure ZnO nanoparticles, whereas, when TEOS is added, the UV band is quenched and a strong visible emission in the range 400-650 nm is enhanced. PL spectra fitting, using Gaussian curves, shows three possible transitions that induce the visible emission. This PL study is correlated to the quantitative XPS study combined with a simple model describing the nanoparticles giving the relative amount of species involved in this emission mechanism. These QDs are then used to manufacture humidity sensors. Indeed, electrical measurements show a high sensitivity to the relative humidity. Moreover, a remarkable enhancement of the sensor performance is observed when the ZnO QD size decreases.

An in situ method based on the combination between XPS and MM-EPES techniques is developed in order to study the growth mode and to determine the nanostructure surface of a deposited ultra thin film on solid substrate by determining both the thickness and the covering. In this way, XPS and MM-EPES modeling are done by adapting a simple approach of the surface organization. Then this method is used to study a gold film deposition on an oxidized silicon substrate. This study leads to determine accurately the surface organization and thus the growth mode. Moreover, the obtained results were validated by performing microscopic measurement by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). AFM and SEM results show a good agreement with those determined by the combination of XPS and MM-EPES techniques. Moreover, experiment shows that this method is able to determine surface parameters when the microscopy techniques do not give any information in the case of a small quantity of deposited Au.

This work attempts to characterize the Au/GaN/GaAs Schottky diode. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses (0.7–2.2 nm). We propose a study of the electrical quality of the components after the elaboration of the Au/GaN/GaAs systems; first without annealing and the second with annealing at 620 °C. Analysis of the current voltage I – V and capacitance voltage C – V characteristics of the Au/heated GaN/GaAs and Au/GaN/GaAs samples with 2.2 nm of GaN thickness allows the determination of the electrical parameter variations. Then, the ideality factor increases after the annealing at 620 °C and becomes equal to 2.86 and 2.77 for the 5 min and 30 min of nitridation, respectively. The calculated states density N ss shows less defects and traps in the Au/GaN/GaAs structure (not heated). It is seen that the electrical parameters of the Au/heated GaN/GaAs diode are significantly different from the conventional Au/GaAs Schottky diode. The improvement of the parameters may be attributed to the passivation of the GaAs surface with the formation of the GaN interfacial layer.

Many key technologies, to overcome some crucial steps, are needed for space missions. One of them concerns the re-entry: when the vehicle enters the atmosphere layer, its high velocity relative to the ground will generate significant friction and an increase in pressure. The kinetic energy of the object will be converted into heat energy and heat the gas forming a shock wave. Temperatures depend on the initial velocity, the atmosphere composition and its pressure. The gas is ionized and become plasma, it will therefore transfer its heat to the body of the probe not only by convection but also by radiation. To protect itself, the vehicle has a heat shield that can withstand extreme phenomena encountered during the descent. Additional constraints will impose a particular geometry and a heavy shield. So the goal is to have the lightest possible and effective protections to allow the probe to maximize its payload. For this, one of the key parameters is to know the behavior of the plasma and radiation produced during re-entry into the atmosphere. This thesis is positioned in this area of ​​study: experiments to create a re-entry plasma are intended to be observed by a spectrometer., La mise en œuvre de missions spatiales demande à développer de nombreuses technologies clés afin de surmonter certaines étapes cruciales. L’une d’entre elle concerne la rentrée atmosphérique : lorsque le véhicule pénètre dans la couche d’atmosphère, sa vitesse relative très grande par rapport au sol va engendrer des frottements importants ainsi qu’une augmentation de pression. L’énergie cinétique de l’objet va donc être transformée en énergie thermique et chauffer le gaz en formant une onde de choc. Les températures dépendent de la vitesse initiale, de la composition de l’atmosphère et de sa pression. Le gaz ainsi chauffé va s’ioniser et devenir un plasma, il va donc transférer sa chaleur au corps de la sonde non seulement par convection mais aussi par rayonnement. Afin de se protéger, le véhicule dispose d’un bouclier thermique pouvant résister aux phénomènes extrêmes rencontrés lors de la descente. Les contraintes supplémentaires vont imposer une géométrie particulière et un lourd bouclier. Ainsi l’objectif est d’avoir les protections les plus légères et efficaces possibles afin de permettre à la sonde de maximiser son emport en équipement scientifique. Pour cela, un des paramètres clés est de connaître le comportement et le rayonnement du plasma produit lors de la rentrée dans l’atmosphère. Cette thèse se positionne dans ce domaine d’étude: les expériences montées et réalisées ont pour objectif d’observer par moyens spectroscopiques un plasma correspondant à celui rencontré par les sondes spatiales.