Philippe Buffat | EPFL Graph Search

Born in Lausanne (Switzerland) in 1942. EPUL physics engineer diploma in 1967 and EPFL PhD in physics in 1976. From 1966 he studies at the Experimental Physics Laboratory (Prof. J.P. Borel) the physical properties peculiar to the very small size metal crystals and gets a PhD degree for his thesis "Abaissement de la température de fusion de petits cristaux d'or par effet de taille thermodynamique" (Lowering of the melting temperature of small gold crystals by thermodynamic size effect). In 1971, he starts to develop an electron microscopy facility available to all EPFL students and researchers (nowadays Centre Interdisciplinaire de Microscopie Electronique CIME) that he manages till 2007. In parallel he teaches the principles of electron microscopy and the Experimental methods of physics at the Physics/Basic Sciences School (SB). In addition, he trains a large part of the facility users. Honorary professor BS/EPFL he carries-out a free-lance research at CIME and in collaboration with the Institute of Crystallography of the Russian Academy of Sciences (ICRAS, Moscow) and the International Centre of Electron Microscopy for Material Science (IC-EM AGH Krakow) This activity covers all the techniques related to transmission and scanning electron microscopy applied to materials science and solid-state physics. His interest is directed towards the structure of nanocrystals, their size effects and behavior under strong electron irradiation, the phase transitions in perovskites, the characterization of nanophases, multilayer and interface structures of crystalline materials and bioceramics. More recently a large research part has moved to understanding/pointing-out the adequacy between the limits of the instruments and their interpretation means in regard of their use in a multiusers facility with a large turnover and a wide range of materials/structures. He is past-president (2006-2007) of the Société Française des Microscopies (Sfµ), honorary member of the Sfµ and of the Swiss Society of Optics and Microscopy (SSOM).

Structure, Oxygen Content and Electric Properties of Titanium Nitride Electrodes in TiNx/La:HfO2/TiNx Stacks Grown by PEALD on SiO2/Si

Philippe Buffat, Elena Suvorova Buffat

This work reports experimental results of the quantitative determination of oxygen and band gap measurement in the TiNx electrodes in planar TiNx top/La:HfO2/TiNx bottom MIM stacks obtained by plasma enhanced atomic layer deposition on SiO2. Methodological aspects of extracting structural and chemical information from (scanning) transmission electron microscopy imaging (bright field and high angular annular dark field), energy dispersive X-ray spectrometry and electron energy loss spectroscopy are thoroughly considered. The study shows that the oxygen concentration is higher in the TiNxOy bottom electrode (about 14.2 ± 0.1 at. %) compared to the TiNxOy top electrode (about 11.4 ± 0.5 at. %). The following average stoichiometric formulas are TiN0.52O0.20 top and TiN0.54O0.26 bottom for top and bottom electrodes, respectively. The amount of oxygen incorporated into TiNx during PEALD because of oxygen impurities in the plasma is minor compared to that because of diffusion from SiO2 and HfO2. This asymmetry, together with results on a sample grown on a Si substrate, shows that incorporating oxygen impurity from the plasma itself is a minor part compared to diffusion from the SiO2 substrate and HfO2 dielectric during the PEALD growth. We observe the presence of TiO2 at the interface between the Hf oxide layer and the Ti nitride electrodes as well as at the SiO2 interface. EELS analysis led to a band gap ranging from 2.2 to 2.5 eV for the bottom TiNxOy and 1.7–2.2 eV for the top TiNxOy, which is in fair agreement with results obtained on the top TiNx electrode (1.6 ± 01 eV) using optical absorption spectra. Measurement of sheet resistance, resistivity and temperature coefficient of resistance by a four-point probe on the top TiNxOy electrode from 20 to 100 °C corresponds to the typical values for semiconductors.

2022

Structure evolution, bandgap, and dielectric function in La-doped hafnium oxide thin layer subjected to swift Xe ion irradiation

Philippe Buffat, Elena Suvorova Buffat

High-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy provide information on the structural evolution, dielectric function, and bandgap values of nanocrystalline 10nm thick lanthanum doped hafnia (La:HfO2) layers in TiN/La:HfO2/TiN/SiO2/Si irradiated with 24, 46, 72, and 160MeV (0.2-1.2MeV/u) Xe ions. Swift heavy Xe ions were expected to create significant atomic rearrangements when passed through a solid losing energy mainly through electronic excitation and ionization of the target atoms. Local heating and subsequent rapid cooling in the region around the ion track can lead to re-solidification with the formation of a new stable crystalline phase, and direct crystal-to-crystal transformations are possible. The structure evolution of hafnia nanocrystals from the orthorhombic Pbcm in the pristine layer to the tetragonal P4(2)/nmc phase in the 160MeV Xe ion irradiated layer was observed. The mixture of tetragonal and orthorhombic phases was found in samples irradiated with ions of intermediate energies. Textured hafnia layers were formed as a result of ion irradiation. The changes in plasmon line shape and the blueshift of the plasmon energy-loss peak from 14.9eV in the pristine layer to 15.4eV in 46MeV and 15.9eV in 160MeV Xe ion irradiated hafnia reflect structural transformations, the increase in the Hf coordination number, and crystal orientations. Valence-electron energy-loss spectroscopy measurements showed a slight increase in the bandgap value from 6.1eV in the pristine sample to 6.2eV and 6.3eV in irradiated samples with 46 and 160MeV Xe ions, respectively, and dielectric functions changed insignificantly in irradiated hafnia layers.

AMER INST PHYSICS2020

Energy dependent structure of Xe ion tracks in YBCO and the effect on the superconductive properties in magnetic fields

Philippe Buffat, Elena Suvorova Buffat

The morphology and structure of damaged regions (tracks) produced by swift heavy 167, 77, and 46 MeV 132Xe23+ ions in YBCO-based second generation industrial high temperature superconductors wires (2G HTS) were studied using transmission electron microscopy. It was shown that ions produce tracks aligned along the ion trajectory but of morphology depending on their energy: continuous, nearly continuous, or discontinuous tracks like prolate ellipsoids of 10–15 nm in length and spheroids of 5 nm in diameter. The damaged regions of about 5 nm in diameter contain an amorphous material with a lower density compared to the pristine YBCO. The material density drops from 6300 kgm−3 for YBCO matrix to 3600 kgm−3 inside the tracks. Barium enrichment was found in the vicinity of the track at a distance of about 10 nm from the center of the defect. Ion damage tracks with different morphologies showed different effectivenesses as pinning centers. Tracks composed of discontinuous pinning centers—spheroids of about 5 nm in diameter—bring the best enhancement of the critical current density. It occurs for the lowest ion energy (46 MeV) in the range of energy loss of 8.9 keV/nm–4.7 keV/nm for Xe ions. The samples showed highest critical current densities of 56 MA/cm2 (4.2 K) and 3 MA/cm2 (77 K) in self-field, while in magnetic fields of 8 T, the values of 17 MA/cm2 (4.2 K) and 1.6 MA/cm2 (77 K) were achieved. The reduction of the superconducting volume corresponding to the amorphous radiation defects did not exceed 4% from the total sample volume.

2019