E. Le Bourhis

Structural, electrical, optical, and mechanical characterizations of decorative ZrOxNy thin films

The main objective of this work is the preparation of decorative zirconium oxynitride, ZrOxNy, thin films by dc reactive magnetron sputtering. Film properties were analyzed as a function of the reactive gas flow and were correlated with the observed structural changes. Measurements showed a systematic decrease in the deposition rate with the increase of the reactive gas flow and revealed three distinct modes: (i) a metallic mode, (ii) a transition mode (subdivided into three zones), and (iii) an oxide mode. The measurements of target potential were also consistent with these changes, revealing a systematic increase from 314to337V. Structural characterization uncovered different behaviors within each of the different zones, with a strong dependence of film texture on the oxygen content. These structural changes were also confirmed by resistivity measurements, whose values ranged from 250to400μΩcm for low gas flows and up to 106μΩcm for the highest flow rates. Color measurements in the films revealed a chan...

Measurement of the elastic constants of textured anisotropic thin films from x-ray diffraction data

The elastic constants (compliances sij) of a textured anisotropic thin film deposited on a substrate have been determined. Using x-ray diffraction to measure the intragranular strain and a tensile machine to deform in situ the samples, an analytical method is described and has been developed for fiber textured thin films. The determination of thin film compliances only requires the knowledge of the substrate elastic constants. In the case of a 260-nm-thin gold film, the compliances were found to be slightly different from the corresponding bulk material ones.

In situ diffraction strain analysis of elastically deformed polycrystalline thin films, and micromechanical interpretation

In situ tensile tests have been carried out under synchrotron radiation on supported gold (Au) thin films exhibiting a pronounced crystallographic texture. The 2θ shift of X-ray diffraction lines has been recorded for different specimen orientations and several loading levels in the elastic domain. The data obtained demonstrate the large strain heterogeneities generated within the specimen because of the intergranular interactions associated with the large elastic anisotropy of Au grains. To interpret these results, the use of a multi-scale micromechanical approach is unavoidable. The theoretical background of such methods is described, and the points where exact results can be obtained and where approximations have to be introduced are highlighted. It is shown that the Vook–Witt model, for which a general formulation is provided, is the exact solution for polycrystals exhibiting a laminate microstructure, which is a significant departure from the standard thin-film microstructures. Among several standard models used in the field, the self-consistent model is the only one that reproduces the experimental data correctly. This is achieved by accounting for the actual crystallographic texture of the specimen, and assuming pancake-shaped two-point statistics for the morphological texture. A discussion of the limitations of this approach, originally developed for bulk materials, is given for the specific case of thin films.

Development of a synchrotron biaxial tensile device for in situ characterization of thin films mechanical response

We have developed on the DIFFABS-SOLEIL beamline a biaxial tensile machine working in the synchrotron environment for in situ diffraction characterization of thin polycrystalline films mechanical response. The machine has been designed to test compliant substrates coated by the studied films under controlled, applied strain field. Technological challenges comprise the sample design including fixation of the substrate ends, the related generation of a uniform strain field in the studied (central) volume, and the operations from the beamline pilot. Preliminary tests on 150 nm thick W films deposited onto polyimide cruciform substrates are presented. The obtained results for applied strains using x-ray diffraction and digital image correlation methods clearly show the full potentialities of this new setup.

Indentation-induced crystallization and phase transformation of amorphous germanium

It has been known for about 15 years that when a Vickers indenter is loaded on to a crystalline semiconductor, such as silicon, a semiconductor to metallic phase transition occurs during indenter loading and on removal of the indenter the material within the residual indentation becomes amorphous. Here we report a completely opposite effect: when a Berkovich or Vickers diamond indenter is loaded onto a submicrometer thick film of amorphous germanium, it crystallizes and undergoes structural phase transitions. These observations are based on our transmission electron microscopy and Raman scattering investigations, which have been described. It has also been shown that the indentation-induced crystallization and phase transitions occur close to the indenter tip, where the plastic strains are the highest.

Elastic-strain distribution in metallic film-polymer substrate composites

Synchrotron x-ray radiation was used for in situstrain measurements during uniaxial tests on polymer substrates coated by a metallic goldfilm 400 nm thick deposited without interlayer or surface treatment. X-ray diffraction allowed capturing both components elastic strains and determining how these were partitioned between the metallic film and the polymeric substrate. For strains below 0.8%, deformation is continuous through the metal-polymer interface while above, the onset of plasticity in the metallic film induces a shift between film and substrate elastic strains.

Indentation of glass as a function of temperature

Abstract We have investigated the mechanical response of soda–lime–silica glass to a Vickers indentor applied at temperatures between room temperature and 600°C. The permanent deformation as well as the brittle deformation were analysed. We could determine the hardness as well as the fracture toughness of the glass as a function of temperature.

Nanoindentation of GaAs compliant substrates

Abstract We have investigated the indentation plastic flow into compliant GaAs wafers. The compliant substrates were fabricated by bonding two GaAs(001) wafers with an adequate twist angle. A dense network of pure screw dislocations was obtained at the interface. A Berkovitch indenter was loaded on the compliant substrates to maximum forces ranging between 600 and 4500 μN. The loading and unloading curves as well as the plastic zone size were measured and compared with that measured on bulk GaAs in the same conditions. Finally the fine structure of the dislocations generated by the indenter has been analysed.

Indentation response of glass with temperature

The mechanical response of different glasses to a Vickers indentor has been investigated between room temperature and Tg þ 50 C. The permanent deformation, from which hardness is estimated, as well as the brittle fracture characteristics, allowing for an evaluation of the fracture toughness, were measured and analysed. Comparison between a standard float glass and advanced glasses such as chalcogenide (with mainly covalent bonding) and metallic glasses was made to get a more general insight into high temperature indentation behaviour. As temperature increases, the glass response becomes more and more time-dependent, and in the vicinity of Tg the permanent deformation was observed to increase rapidly for all glasses. Further, while the standard float glass showed an enhanced apparent toughness at elevated temperatures due to a brittle to ductile transition, almost no change in apparent toughness was revealed in the GeAsSe glass emphasizing the time-dependent response of glass at elevated temperature. 2003 Elsevier Science B.V. All rights reserved.

Subsurface deformations induced by a Vickers indenter in GaAs/AlGaAs superlattice

During the past century, indentation mechanics has been extensively explored. This effort led to the development of instrumented microand nanoindentation techniques that have proved to be of first importance in materials science research. As reviewed recently by Tabor [1], indentation mechanics straightforwardly used today was developed mainly in view of experimental results found on metals [2–4]. Therefore, the use of indentation theory for other materials (for instance ceramics) assumes that plastic-flow characteristics are not much affected. Furthermore, Chaudhri showed using pearlite grains as markers that the measured plastic strains in heavily work-hardened mild steel deformed by indenters of various geometries were much higher than previously thought [5]. In a more recent work, the same author used the hardness-strain relation to map the strain field in high-conductivity copper deformed by spherical indenters [6]. He showed then that the strain-contour geometry was more complex than what had been observed previously in other metals [2]. Determination of the strain field under a Vickers indenter in semiconductors was studied recently in a GaAs/AlAs superlattice where interfaces were used as markers [7]. However, severe delamination occurred at the interfaces. In this paper, we have deformed a GaAs/AlGaAs superlattice by a Vickers indenter and have used focused-ion beam (FIB) technique to prepare cross-sectional thin foils through the center of the indent site for transmission electron microscopy (TEM) observation. These techniques let us get information on the plastic and brittle deformations underneath the indent site and allow us to determine the strain-field map. Superlattices were grown on (001) surfaces of GaAs single crystals by metal-organic vapor-phase epitaxy at 650 ◦C. GaAs and AlGaAs layers (30 of each) were grown alternatively to form a structure about 7.6 μm thick (period 254 nm). The composition of the Alx Ga1−x As (x = 0.85) alloy was chosen so that the misfit with GaAs was only e = 0.13% and the structure was almost lattice matched. The samples were deformed by a Vickers diamond pyramid at room temperature under 500 mN for 30 s. They were set in the indenting machine in such a way that the diagonals of the indentations were along a 〈110〉 direction. The central zone of the indent site was protected from the gallium FIB by a tungsten thin film. Two trenches were milled with a 30 kV FIB in such a way that a thin wall was left containing the selected zone that was transparent to the electron beam as described elsewhere [8]. The periodical structure of the superlattice can be observed in Fig. 1. AlGaAs layers appear in clear contrast while GaAs layers appear in dark contrast. At the bottom of the superlattice, the structure was started by an AlGaAs layer and a GaAs layer about 510 and 235 nm thick respectively. It should be noted that during the milling two layers on top of the structure on the left-hand side of the indent were lost despite the tungsten protection. The indent site is situated on the top of the median crack (observed perpendicularly to the indented surface). Curvature of the surface at the indent site and of layers underneath is obvious and is the result of the plastic deformation generated by the indenter. The curvature of the layers was observed to decrease with depth. For instance, it is about 158◦ for the third GaAs layer and about 176◦ for the 41st layer below. Interestingly the value found in the vicinity of the indent site (158◦) differs from the Vickers pyramid angle (148◦) because of the elastic recovery of the sample while the indenter was unloaded [8]. Using a higher-magnification image just beneath the indent site (Fig. 2), converging slip bands could be observed. They are inclined to the indented surface by about 54◦ and correspond to the gliding of dislocations in the {111} slip planes. On a higher contrast image (Fig. 3) slip bands in {111} planes diverging from the indent site could also be observed. Such a plastic-flow geometry is now well established for bulk semiconductors and was shown to operate here [8–11]. The slip bands correspond to the gliding of dislocations with Burgers vectors inclined to the indented surface that generates deformation of the layers on their path [8–10]. This finally results in the curvature of the superlattice that was described above. At the intersection of the converging slip bands (underneath the indent site and along the load axis direction), we observed a median crack that crossed the layers. This crack was propagated straight into the GaAs substrate on 4 μm. A median-radial crack system is commonly observed in brittle material under sharp contact [12]. Here the crack was initiated at