Papers by Author: Alain Portavoce

Abstract: Nanostructures used to build current technology devices are generally based on the stack of several thin films (from few nanometer-thick to micrometer-thick layers) having different physical properties (conductors, semiconductors, dielectrics, etc.). In order to build such devices, thin film fabrication processes compatible with the entire device fabrication need to be developed (each subsequent process step should not deteriorate the previous construction). Solid-state reactive diffusion allows thin film exhibiting good interfacial properties (mechanical, electrical…) to be produced. In this case, the film of interest is grown from the reaction of an initial layer with the substrate on which it has been deposited, during controlled thermal annealing. In the case of the reaction of a nano-layer (thickness < 100 nm) with a semi-infinite substrate, nanoscale effects can be observed: i) the phases appear sequentially, ii) not all the thermodynamic stable phases appear in the sequence (some phases are missing), and iii) some phases are transient (they disappear as fast as they appear). The understanding of the driving forces controlling such nanoscale effects is highly desired in order to control the phase formation sequence, and to stabilize the phase of interest (for the targeted application) among all the phases appearing in the sequence.This chapter presents recent investigations concerning the influence of atomic transport on the nanoscale phenomena observed during nano-film reactive diffusion. The results suggest that nano-film solid-state reaction could be controlled by modifying atomic transport kinetics, allowing current processes based on thin-film reactive diffusion to be improved.

Abstract: Atomic redistribution of W and Fe in Si were studied using secondary ion mass spectrometry and transmission electron microscopy. W diffusion experiments performed during isothermal annealing and during Si oxidation show that W atoms should use at least two different diffusion mechanisms. Experimental diffusion profiles can be well simulated by considering the simultaneous use of three different W diffusion mechanisms: the dissociative and the kick-out mechanisms, as well as an original mechanism based on the formation of a W-Si self-interstitial pair located on the interstitial Si sub-lattice. Fe redistribution was studied during the oxidation of a Fe-contaminated Si wafer. Fe is shown to be first pushed-out in Si by the mobile SiO₂/Si interface, and thus to form Fe silicides precipitates at this interface. The silicide precipitates, which can exhibit a core-shell structure, appear to move with the SiO₂/Si interface thanks to an oxidation/dissolution mechanism in the SiO₂ and a nucleation/growth mechanism in the Si matrix. Furthermore, the rate difference between Si and Fe silicide precipitate oxidation leads to the formation of Si pyramidal defects at the SiO₂/Si interface.

Abstract: Ge and B diffusion was studied in nanocrystalline Si, and Pd and Si self-diffusion was studied in nanocrystalline Pd₂Si during and after Pd/Si reactive diffusion. These experiments showed that grain boundary (GB) diffusion kinetic is the same in micro-and nanoGBs, whereas triple junction (TJ) diffusion is several orders of magnitude faster than GB diffusion. In addition, GB segregation and GB migration can significantly modify atomic diffusion profiles in nanocrystalline materials, and atomic transport kinetics can be largely increased in nanograins compared to micro-grains, as well as during reactive diffusion, probably due to an increase of point defect concentration. These observations show that atomic transport in nanometric layers during reactive diffusion is complex, since GBs and TJs are moving and the proportion of GBs and TJs is changing during the layer growth.

Abstract: In this paper, we report investigations concerning the fabrication of a diluted Ge (Mn) solution using solid state Mn diffusion, and Mn/Ge reactive diffusion for spintronic applications. The study of Mn diffusion shows that the quasi-totality of the incorporated Mn atoms occupies Ge substitutional sites and probably exhibits two negative elementary charges. The solubility limit of Mn in Ge is comprised between 0.7 and 0.9 % (T  600 °C). We show that substitutional Mn atoms are not ferromagnetic in Ge and consequently that Ge (Mn) diluted magnetic semiconductor can not be produced. Beside the ferromagnetic signal from Mn₅Ge₃, ferromagnetic signals detected in the samples could be always attributed to surface or bulk Mn-Ge clusters. Furthermore, we show that the CMOS Salicide process is potentially applicable to Mn₅Ge₃ nanolayer fabrication on Ge for spintronic applications. During Mn (thin-film)/Ge reaction, Mn₅Ge₃ is the first phase to form, being thermally stable up to 310 °C and exhibiting ferromagnetic properties up to T_C ~ 300 K.

Abstract: Silicide growth via reaction between a metallic film and a Si substrate has been well documented. In general, atomic transport kinetic during the growth of silicides is considered to be the same as during equilibrium diffusion, despite the reaction and its possible injection of point-defects in the two phases on each side of the interface. To date, the main studies aiming to investigate atomic transport during silicide growth used immobile markers in order to determine which element diffuses the fastest during growth and in which proportion. The quantitative measurements of effective diffusion coefficients during growth was also performed using Deal-and-Groove-type of models, however, these effective coefficients are in general not in agreement with the interdiffusion coefficients calculated using the equilibrium diffusion coefficients measured during diffusion experiments. In general, atomic transport kinetic measurements during growth and without growth are performed using different types of samples for experimental reasons. In this paper, we discuss the possible use of ultrahigh vacuum in situ Auger electron spectroscopy in order to measure the effective diffusion coefficient during growth, as well as the equilibrium self-diffusion coefficients, in the same samples, in the same experimental conditions. The first results on the Pd-Si system show that atomic transport during Pd₂Si growth is several orders of magnitude faster than at equilibrium without interfacial reaction.

Abstract: An alternative solution for producing logic devices in microelectronics is spintronics (SPIN TRansport electrONICS). It relies on the fact that in a magnetic layer, the electrical current can be spin polarized. To fabricate such components, a material whose electronic properties depend on its magnetic state is needed. The Mn-Ge system presents a lot of phases with different magnetic properties, which can be used for spintronics. The most interesting phase among the Mn-Ge system is Mn₅Ge₃ because of its stability at high temperatures, its Curie temperature which is close to room temperature and its ability of injecting spin-polarized electrons into semiconductors. In this paper, we have combined Reflection High-Energy Electron Diffraction (RHEED) and X-ray Diffraction (XRD), to study the sequence of formation of Mn_xGe_y phases during reactive diffusion of both a 50 nm and a 210 nm thick Mn films deposited by Molecular-Beam Epitaxy (MBE) on Ge (111).

Abstract: The use of nanometric size materials as embedded clusters, nanometric films, nanocrystalline layers and nanostructures is steadily increasing in industrial processes aiming to produce materials and devices. This is especially true in today Si-based microelectronics with transistors made of a multitude of different thin film materials (B-, As-, and P-doped Si, NiSi (Pt), poly-Si, W, TiO_x, LaO, SiO₂, Al, H_fO₂), and exhibiting a characteristic lateral size of 32-22 nm. Size reduction leads to an increasing role of surfaces and interfaces, as well as stress and nanoscale effects upon important phenomena driving fabrication processes, such as atomic diffusion, phase nucleation, phase growth, and coarsening. Consequently, nanotechnology related to Material Science requires an investigation at the nanometric (or atomic) scale of elementary physical phenomena that are well-known at the microscopic scale. This paper is focused on nanosize effects upon diffusion in Si and Si reactive diffusion. We present recent results showing that the kinetic of lattice diffusion is enhanced in semiconductor nanometric (nano) grains, while grain boundary (GB) diffusion is not changed in nanoGBs. It is also shown that diffusion in triple-junction (TJ) is several orders of magnitude faster than GB diffusion, and that its effect cannot be neglected in nanocrystalline (nc) layers made of 40 nm-wide grains. Experimental results concerning Si sub-nanometric film reaction on Ni (111) substrate are also presented and compared to theoretical results giving new prospects concerning nanosize effects on reactive diffusion at the atomic scale.

Abstract: Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM) were used to study the reactive diffusion of one monolayer of silicon deposited at room temperature onto a Ni (111) substrate. We have done isochronal and isothermal kinetics by AES, and we observed in both cases a kinetics blockage on a plateau corresponding to around one third of a silicon monolayer. STM images and LEED patterns both recorded at room temperature just after annealing, reveal formation of an ordered hexagonal superstructure corresponding probably to a two-dimensional surface silicide.

Abstract: The Diffusion and Solubility of B Implanted in δ-Ni₂Si and Nisi Layers Is Studied by SIMS. it Is Observed that both Diffusion and Solubility Are Higher in δ-Ni₂Si than Nisi. the Redistribution of B during Ni Silicidation Is Also Studied. the SIMS Profiles Show the Presence of Concentration Step in the Middle of the Final Nisi Layer. this Profile Shape Is Explained in Light of the Results Obtained in Preformed Silicides. the Proposed Model Is Supported by Redistribution Simulations that Can Reproduce the Main Features of the Profile.

Abstract: With the development of nanotechnologies, the number of industrial processes dealing with the production of nanostructures or nanoobjects is in constant progress (microelectronics, metallurgy). Thus, knowledge of atom mobility and the understanding of atom redistribution in nanoobjects and during their fabrication have become subjects of increasing importance, since they are key parameters to control nanofabrication. Especially, todays materials can be both composed of nanoobjects as clusters or decorated defects, and contain a large number of interfaces as in nanometer-thick film stacking and buried nanowires or nanoislands. Atom redistribution in this type of materials is quite complex due to the combination of different effects, such as composition and stress, and is still not very well known due to experimental issues. For example, it has been shown that atomic transport in nanocrystalline layers can be several orders of magnitude faster than in microcrystalline layers, though the reason for this mobility increase is still under debate. Effective diffusion in nanocrystalline layers is expected to be highly dependent on interface and grain boundary (GB) diffusion, as well as triple junction diffusion. However, experimental measurements of diffusion coefficients in nanograins, nanograin boundaries, triple junctions, and interfaces, as well as investigations concerning diffusion mechanisms, and defect formation and mobility in these different diffusion paths are today still needed, in order to give a complete picture of nanodiffusion and nanosize effects upon atom transport. In this paper, we present recent studies dealing with diffusion in nanocrystalline materials using original simulations combined with usual 1D composition profile measurements, or using the particular abilities of atom probe tomography (APT) to experimentally characterize interfaces. We present techniques allowing for the simultaneous measurement of grain and GB diffusion coefficients in polycrystals, as well as the measurement of nanograin lattice diffusion and triple junction diffusion. We also show that laser-assisted APT microscopy is the ideal tool to study interface diffusion and nanodiffusion in nanostructures, since it allows the determination of 1D, 2D and 3D atomic distributions that can be analyzed using diffusion analytical solutions or numerical simulation.