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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.

The energy loss function (ELF) is a key parameter for the calculations of energy losses undergone by electrons in matter. It is often the only input in the calculations performed within the models based on the semi-classical dielectric response theory. Its perfect knowledge is thus of primordial importance. To evaluate the ELF, it is usual to consider as a model an expansion in Drude-Lindhard (DL) type oscillators with fixed values of the strength, width, energy and dispersion for the various oscillators. However, for materials that are characterized by a single sharp oscillator as aluminium or indium, it has been shown [Phys. Rev. B 46 (1992) 2486] that the damping parameter that corresponds in the DL model to the width of the oscillator increases for decreasing incident electron energy. To emphasize this effect, we perform in this work systematic calculation of the ELF for an indium target and for incident electron energies between 200 and 2000-eV. The ELF is determined by comparing REELS (reflection electron energy loss spectroscopy) experimental inelastic electron scattering cross sections with cross sections calculated within the semi-classical dielectric response model, which is implemented in the QUEELS-e(k,ω)-REELS software (Quantitative analysis of Electron Energy Losses at Surfaces) [Surf. Interface Anal. 36 (2004) 824]. We also perform measurements and calculations for varying incident and exit angles of the electron, namely for angles to the surface normal between 15o and 75o, to check the validity of our results for all geometries. Our results show that the damping parameter of indium in the DL model for electron energy of 200-eV is three times larger than for energy of 2000-eV. Copyright © 2014 John Wiley & Sons, Ltd.

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.

International audience; X-ray photoelectron spectroscopy (XPS) was used to monitor the chemical changes resulting from irradiation (> 295 nm) in air of poly(3-hexylthiophene) (P3HT), polymer which is a good candidate for photovoltaic applications. The formation of carbonyl moieties and the stepwise oxidation of sulphur atoms were characterised. The oxidation and the cleavage of the hexyl side-chain was monitored. It is also shown that sulfur was first converted into sulfoxides, then into sulfones and finally into sulfinate esters. The formation of these ultimate degradation products provoked a disruption of π-conjugation in P3HT, leading to diminished visible absorbance. Based on these results, a mechanism of P3HT photooxidation is proposed. Comparison of XPS data with previously reported infrared and UV-visible spectral analysis showed that the information provided by these techniques is completely consistent.

The Multi-Mode Elastic Peak Electron Spectroscopy (MM-EPES) analysis is confined to incoherent electron elastic scattering and the use of variable primary energy. This experimental method is very sensitive to the surface region of the sample. However, for quantitative interpretation, the MM-EPES method needs jointly a Monte Carlo (MC) computer simulation of electron trajectories in the solid. In the present work, we proposed a new approach to calculate the percentage ηe of elastic reflected electrons by the surface of a sample. This simulation takes into account the surface effects (i.e. surface plasmon), and the atoms arrangement in the substrate. The concept of the surface excitation parameter (SEP) is also presented. Computer simulations were performed on the three low index single crystals of Cu, Au, Si and Ag. The results confirm that the distribution of substrate atoms, according to the crystallographic structure, influences the intensity measured by EPES. A simple prediction formula was proposed to calculate ηe for elastic electrons entering in a Retarding Field Analyzer (RFA) spectrometer which is the apparatus giving experimentally numerical values of the percentage ηe.

O2SF6 plasma effects on processed GaAs surfaces have been investigated. The influence of plasma parameters such as composition, power and exposure time has been studied. The microwave plasma treatment efficiency has been studied by surface depth profiling (cycles consisting of XPS measurements followed by a slight etching) coupled with an original modelling calculation. We have pointed out that SF6 addition in plasma increases the oxidative rule of oxygen by increasing the total oxide thickness and that the quantity of the different oxidative and passivating species does not limit the oxidative mechanism, in the O2:SF6 ratio range 80:20 to 40:60. The increase in the plasma power leads to a change in the composition of the outer region of the oxide layer with an increase in the gallium content on the surface, and to an increase in the total oxide layer. The exposure time to the plasma increases also these two phenomena.

The aim of the present study is to demonstrate the feasibility to form well-ordered nanoholes on InP(1 0 0) surfaces by low Ar + ion sputtering process in UHV conditions from anodized aluminum oxide (AAO) templates. This process is a promising approach in creating ordered arrays of surface nanostructures with controllable size and morphology. To follow the Ar + ion sputtering effects on the AAO/InP surfaces, X-ray photoelectron spectroscopy (XPS) was used to determine the different surface species. In 4d and P 2p core level spectra were recorded on different InP(1 0 0) surfaces after ions bombardment. XPS results showed the presence of metallic indium on both smooth InP(1 0 0) and AAO/InP(1 0 0) surfaces. Finally, we showed that this experiment led to the formation of metallic In dropplets about 10 nm in diameter on nanoholes patterned InP surface while the as-received InP(1 0 0) surface generated metallic In about 60 nm in diameter.

This article investigates the growth of InN layers on (1 0 0) InP in ultra-high vacuum using a glow discharge source (GDS). Auger electron spectroscopy (AES) was used to understand the different steps of the nitridation process with the analysis of In-MNN, N-KLL and P-LMM transitions. A modeling of Auger signals using a stacked layers model allows us to confirm that four monolayers of indium nitride are created on (1 0 0) InP. InN layers grown on (1 0 0) InP were studied optically by photoluminescence (PL) spectroscopy versus the excitation power and the sample temperature (10–300 K). Results show broad spectral band energy close to the lowest reported InN bandgap. The temperature dependence of the PL peak energy showed a S-shaped behavior (decrease–increase–decrease). The results suggest that the InN-related emission is significantly affected by the change in carrier dynamics with increasing temperature: the effect can be due to the large exciton localization effects.

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.

III–V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 °C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment.

Electron spectroscopy (AES and XPS) and photoluminescence analysis (PL) were used to study porous layers elaborated by electrochemical etching of p-GaAs and n-InP substrates. Surface characterizations have been performed on the as received porous substrates, ions bombarded and nitridated porous layers. Porous surfaces of GaAs(1 0 0) and InP(1 0 0) have been compared after different treatments: Argon ion bombardment and nitridation with a glow discharge source (GDS) in UHV. These processes have been followed by Auger measurements (AES). The effects of the bombardment and the nitridation of porous InP were studied. The PL investigations of the porous GaAs layer show visible and infrared photoluminescence bands. Quantum confinement in the nanocrystallites of GaAs formed by electrochemical etching can explain the luminescence of the porous GaAs layers.

EPES (elastic peak electron spectroscopy) allows measuring the percentage of elastic backscattered electrons η e from a surface excited by primary electrons. However, this method must be combined with Monte Carlo simulations to get quantitative information. After a brief description of the algorithm used in this work (named MC2), we focused on the adaptation of this simulation for nanoporous surfaces (named MC2-NP). The theoretical results obtained put in evidence the dependence of η e versus pore diameter (d), depth of the pores (h) and covering rate (CR) of the pores on the surface. Results obtained on surfaces having cylinder-shaped and cone-shaped holes with nanometer dimensions are presented too. To validate theoretical results obtained with MC2-NP, silicon(111) nanoporous surfaces have been prepared with an anodized aluminum oxide (AAO) template and by argon ion bombardment in an UHV chamber. Uniform nanohole arrays were formed as a replica of ordered lattice pattern of the template. Then EPES experimental measurements have been performed on planar and nanoporous Si(111) surfaces using a retarding field analyzer (RFA). The experimental results put in evidence that the percentage of the elastically backscattered electrons is influenced by the patterning of the surface. Then comparing values of η e obtained experimentally with those obtained with MC2-NP simulations, we show the sensitivity of the EPES method for studying nanoporous surfaces. In this way, we expect fast estimation of nanohole's dimensions by in-situ MM-EPES (Multi-Mode EPES) without other techniques such as for example scanning electron microscopy.

In previous reports, we have investigated on the influence of current transport mechanism on the sensitivity of InP Schottky and pseudo-Schottky diodes, for nitrogen dioxide detection. In this paper, the influence of the metal nature and the structure of the rectifying contact on the sensing layer are highlighted. More especially, we will focus on the behavior of these Schottky structures during and after their exposures to nitrogen dioxide (NO2) and ozone (O3). In this context, two types of structures have been studied: the simple Schottky diodes and the more elaborated structures called pseudo-Schottky. These diodes are realized with Au or Pd as metals. First, we will describe samples preparation and electrical characterization of the two types of structures. Then, we will compare sensor responses of these different structures under NO2 and O3. Finally, to better understand the action of these gases on the sensitive layer (the Schottky contact), X-rays photoelectron spectroscopy (XPS) is performed to follow the evolution of the metallic layer. The poisoning effect of active sites, which appears after NO2 and O3 expositions, is illustrated by comparative study of the active layer before and after exposure.

The Curie temperature of Mn5Ge3 has been successfully enhanced by carbon doping. In this context the diffusion of a carbon thin film in Mn5Ge3 has been studied at room temperature. A value of the diffusivity of about D = 2.4 × 10-23 ± 0.5 × 10-23 m2 s-1 is reported. This value is in the typical range of interstitial diffusion coefficients. Moreover Auger Ge and Mn peaks shifts and Ge3d core level have been investigated to get some details on the reactivity of carbon atoms in Mn5Ge3. Mn Auger transitions display a shift of 4 eV whereas Ge transitions do not. Similarly Ge3d core level does not contain C related contribution but presents a Mn one. These observations confirmed the fact that carbon atoms are not inert species for Mn. It suggests that a ternary Ge-Mn-C alloy could occur and should be taken into account when doping the Mn5Ge3 with carbon. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience

We report the first synthesis of GaAs nanowires (NWs) by Au-assisted vapor-liquid-solid (VLS) growth in the novel hydride vapor phase epitaxy (HVPE) environment. Forty micrometer long rodlike monocrystalline GaAs nanowires exhibiting a cubic zinc blende structure were grown in 15 min with a mean density of 10(6) cm(-2). The synthesis of such long figures in such a short duration could be explained by the growth physics of near-equilibrium HVPE. VLS-HVPE is mainly based on solidification after direct and continuous feeding of the arsenious and GaCl growth precursors through the Au-Ga liquid catalyst. Fast solidification (170 microm/h) is then assisted by the high decomposition frequency of GaCl. This predominant feeding through the liquid-solid interface with no mass and kinetic hindrance favors axial rather than radial growth, leading to twin-free nanowires with a constant cylinder shape over unusual length. The achievement of GaAs NWs several tens of micrometers long showing a high surface to volume ratio may open the field of III-V wires, as already addressed with ultralong Si nanowires.

Quantitative and qualitative analysis techniques were employed to study the first stages of ultra-high vacuum plasma nitriding of the 42CrMo4 steel. At constant treatment temperature, maintained for all samples at about 360 °C, we have established the influence of treatment time on the chemical composition, thickness and electrical properties of the nitrided layer. In this purpose it was used a stacking atomic layer model describing the sample surface, which takes into account the attenuation depth of photoelectrons by the atomic monolayers. So, we have found that after 2 h of nitriding in laboratory conditions, 70% of the nitrided layer was composed of iron oxide. Also, I – V measurements indicate an influence of the nitride overlayer with increasing treatment time.

International audience; The nitridation of InP(1 0 0) surfaces has been studied using synchrotron radiation photoemission. The samples were chemically cleaned and then ion bombarded, which cleaned the surface and also induced the formation of metallic indium droplets. The nitridation with a Glow Discharge Cell (GDS) produced indium nitride by reaction with these indium clusters. We used the In 4d and P 2p core levels to monitor the chemical state of the surface and the coverage of the species present. We observed the creation of In-N and P-N bonds while the In-In metallic bonds decrease which confirm the reaction between indium clusters and nitrogen species. A theoretical model based on stacked layers allows us to assert that almost two monolayers of indium nitride are produced. The effect of annealing on the nitridated layers at 450 °C has also been analysed. It appears that this system is stable up to this temperature, well above the congruent evaporation temperature (370 °C) of clean InP(1 0 0): no increase of metallic indium bonds due to decomposition of the substrate is detected as shown in previous works [L. Bideux, Y. Ould-Metidji, B. Gruzza, V. Matolin, Surf. Interface Anal. 34 (2002) 712] studying the InP(1 0 0) surfaces.

International audience; The nitrides of group III metals: AlN, GaN and InN are very important materials due to their applications for short wavelength opto-electronics (light-emitting diodes and laser diodes). It is essential for the realization of such novel devices to grow high-quality nitride single crystals. In this paper, we report the first stages of the InP(1 0 0) surfaces nitridation in order to grow high-quality nitride films. Indeed, the nitridation process is an important step in the growth of nitrides [J. Vac. Sci. Technol. A 17 (1999) 2194; Phys. Status Solidi A 176 (1999) 595]. Previous works [Synth. Met. 90 (1997) 2233; Appl. Phys. Lett. 63 (1993) 1957] have shown that in situ Ar+ ions bombardment is useful on the one hand to clean the surface, and on the other hand to create droplets of metallic indium in well-controlled quantity. Then the indium metallic enrichment of the surface, monitoring by elastic peak electron spectroscopy (EPES) and Auger electron spectroscopy (AES) allows to prepare the III-V semiconductors surfaces to the nitridation step. The nitridated process has been performed with a high voltage plasma discharge cell and has been studied using quantitative Auger electron spectroscopy, elastic peak electron spectroscopy and electron energy loss spectroscopy (EELS), in order to optimize the conditions of InN layers formation.

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As2O3, As2O5 and Ga2O3. Large amount of oxygen is present at the surface before the surface cleaning. Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650–680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650–680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.

EPES (elastic peak electron spectroscopy) and AREPES (angle resolved elastic peak electron spectroscopy) are non destructive methods and very sensitive to the surface region. These techniques allow to measure the percentage η e of elastically backscattered electrons from the surface excited by an electron beam. Both methods are combined with Monte-Carlo (MC) simulations to interpret experimental results. In this work, we underline the importance of a micrometric scale roughness at the surface. The use of an original Monte-Carlo method was fruitful for the simulation, moreover 3D representations have been developed for visualization and qualitative interpretation of the results. For a more precise quantitative study, a 2D representation was necessary. The calculated results have been compared with published experimental ones got for different incidence angles and primary energies, on a silicon surface having triangular saw-tooth aspect (crenels) obtained by photolithography. We have observed that the effect due to the roughness increases with the incidence angle.

Quantitative X-ray photoelectron spectroscopy was used to characterize the native oxide film formed on 42CrMo4 steel surface by air exposure in normal conditions. In order to determine the thickness and composition of the oxide layer we have used a stacking layer model together with experimental XPS sputtering depth profiling. At a nanoscale study, to obtain quantitative results one must take into account fundamental parameters like the attenuation depth of photoelectrons. We have found that both lepidocrocit (γ-FeOOH) and magnetite (Fe3O4) were present and the total thickness of the oxide layer was 16 monolayers.

In this study, InP(1 0 0) surfaces were bombarded by argon ions in ultra high vacuum. Indium metallic droplets were created in well controlled quantities and played the role of precursors for the nitridation process. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. X-ray photoelectron spectroscopy (XPS) studies indicated that the nitrogen combined with indium surface atoms to create InN thin films (two monolayers) on an In rich-InP(1 0 0) surface. This process occurred at low temperature: 250 °C. Synchrotron radiation photoemission (SR-XPS) studies of the valence band spectra, LEED and EELS measurements show an evolution of surface species and the effect of a 450 °C annealing of the InN/InP structures. The results reveal that annealing allows the crystallization of the thin InN layers, while the LEED pattern shows a (4 × 1) reconstruction. As a consequence, InN related structures in EELS and valence bands spectra are different before and after the annealing. According to SR-XPS measurements, the Fermi level is found to be pinned at 1.6 eV above the valence band maximum (VBM).

The gas sensing, electrical and chemical properties of a novel toxic gas sensor structure based on n-InP epitaxial layers have been investigated. The gas sensing properties were investigated under extremely low concentration of toxic gas flow, nitrogen dioxide, in dry pure air. Responses, resistance changes ∃R, towards NO2 of the structures with different active InP layer thickness were compared. The influence of the surface states at InP surface on the initial resistance of the sensor structure was shown from numerical simulations. Furthermore, the changes of InP surface chemical properties due to gas exposition were stud-ied using the X-ray photoelectron spectroscopy (XPS).

This article investigates the nitridation effect of InP(100) semiconductor surfaces performed using a glow discharge cell. Auger electron spectroscopy and XPS were used to understand the different steps of this process. An important point is the initial quantity of metallic indium on the InP(100) surfaces. Indeed the indium droplets, created in well-known quantity, play the role of precursor. At a relatively low temperature of 523 K the system undergoes surface restructuring, which includes removal of the In droplets and the formation of two InN monolayers. Phosphorus-nitrogen bonds have been detected by the analysis of P LMM Auger peaks, and In-N bonds by analysis of the In 4d XPS peak. However, the presence or not of metallic indium inside this InN overlayer is crucial for passivation of the substrate. Ex situ photoluminescence measurements correlated to the electron spectroscopies results have shown the good passivation effect of the InP(100) surfaces by InN overlayers for 40 min of nitridation.

International audience; Auger electron spectroscopy (AES) was used to investigate the processes taking place during the initial stages of InP(100) surfaces nitridation. This AES study combined with electrical measurements (intensity-potential) shows that the processes greatly differ depending on the nitridation angles. Results show that with grazing angle for nitrogen flow, the nitridation process is more efficient. Results obtained with AES spectra are coherent with electrical measurements : Hg/InN/InP(100) Schottky diodes present better electrical characteristics in the case of a grazing flow. That means, the adsorption of nitrogen on the surface is more important for this configuration.

The cleaning and the nitridation of GaAs(1 0 0) substrates have been studied in this work by electron energy loss spectroscopy (EELS), elastic peak electron spectroscopy (EPES) and X-ray photoelectron spectroscopy (XPS). The nitridation of GaAs(1 0 0) was performed using a double differential pumping RF cell. We have studied in a first part, the surface modifications of the substrate during ionic cleaning. In a second part, we have determined the λ i ( E ) K ( E , E n ) curves from EELS spectra using Tougaard's procedure for cleaned-GaAs(1 0 0) and nitrited-GaAs(1 0 0) substrates. The thickness of the GaN overlayer was deduced from these results.

A double chamber vacuum system was constructed for the electron spectroscopy study of GaN layer growth. The samples are prepared by vacuum deposition from Knudsen-type Ga source in the afterglow atmosphere of active nitrogen produced by high-pressure RF-plasma source. The growth chamber is connected to the analysis chamber equipped with electron spectroscopy (XPS, AES) analysis, permitting to study the chemical composition of layers as a function of growing conditions. It is shown that the new RF-source is suitable for preparation of pure stoichiometric GaN layers.

Ion beam nitridation of indium phosphide (100) substrates was studied using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Argon ion bombardment of the InP(100) surface removes the native oxides and induces the formation of three-dimensional indium clusters. After cleaning, the samples were nitrided in an ultrahigh vacuum chamber using a home-made radiofrequency plasma source (13.56 MHz) that allows nitridation at low pressures (10−4 Pa). We have studied the influence of temperature on the nitridation mechanisms. For low temperature (T < 200°C), no variation of the metallic indium droplets was observed; for high temperature (T > 270°C), the first layers of the substrate were damaged. The optimal temperature for the nitridation of InP(100) was found to be 250°C.