Marie-Vanessa Coulet

Phase transition in stoichiometric GaSb thin films: Anomalous density change and phase segregation

The crystallization of stoichiometric GaSb thin films was studied by combined in situ synchrotron techniques and static laser testing. It is demonstrated that upon crystallization, GaSb thin films exhibit an unusual behaviour with increasing thickness and concomitant decreasing mass density while its electrical resistance drops as commonly observed in phase change materials. Furthermore, beyond GaSb amorphous-to-crystalline phase transition, an elemental segregation and a separate crystallization of a pure Sb phase is evidenced.

Ge-doped GaSb thin films with zero mass density change upon crystallization for applications in phase change memories

In order to optimize materials for phase change random access memories (PCRAM), the effect of Ge doping on Ga-Sb alloy crystallization was studied using combined in situ synchrotron x-ray techniques, electrical measurements, and static laser testing. The present data emphasize that the crystallization temperature can be increased up to 390 °C with subsequent higher thermal stability of the amorphous phase; phase segregation is evidenced with GaSb, Sb, and Ge phases that crystallize in a two-step crystallization process. The Ge-doped GaSb films exhibit a larger electrical contrast as compared to undoped GaSb alloy (up to ×100). The optical contrast measured by laser testing is shown to follow the mass density change variations upon crystallization, with a negative contrast (higher value in amorphous state) whatever Ge-doping levels. In situ x-ray reflectivity measurements show that zero mass density change can be achieved by low Ge-doping. Ge-doped GaSb alloys look promising since a phase change material w...

Unusual crystallization behavior in Ga-Sb phase change alloys

Combined in situ X-ray scattering techniques using synchrotron radiation were applied to investigate the crystallization behavior of Sb-rich Ga-Sb alloys. Measurements of the sheet resistance during heating indicated a reduced crystallization temperature with increased Sb content, which was confirmed by in situ X-ray diffraction. The electrical contrast increased with increasing Sb content and the resistivities in both the amorphous and crystalline phases decreased. It was found that by tuning the composition between Ga:Sb = 9:91 (in at.%) and Ga:Sb = 45:55, the change in mass density upon crystallization changes from an increase in mass density which is typical for most phase change materials to a decrease in mass density. At the composition of Ga:Sb = 30:70, no mass density change is observed which should be very beneficial for phase change random access memory (PCRAM) applications where a change in mass density during cycling is assumed to cause void formation and PCRAM device failure.

Evidence for correlated structural and electrical changes in a Ge2Sb2Te5 thin film from combined synchrotron X‐ray techniques and sheet resistance measurements during in situ thermal annealing

The temperature-induced phase transition in an as-deposited amorphous Ge2Sb2Te5 (GST) thin film was studied by a unique combination of in situ synchrotron techniques (diffraction and reflectivity of X-rays) and sheet resistance measurements. The combination of these characterization techniques allowed the simultaneous extraction of structural (density and film thickness) and electrical characteristics of the GST film during its thermal annealing. It is shown that, at 425 (3) K, the appearance of diffraction peaks associated with a metastable crystalline cubic phase is unambiguously correlated to a density increase in combination with a layer thickness reduction and a resistivity switch towards a lower-resistance state. Under the present annealing conditions, the Ge2Sb2Te5 film consists of a polycrystalline layer capped by an amorphous layer that strongly degrades the electrical conductivity.

Density change upon crystallization of Ga-Sb films

Besides crystallization time and temperature, the mass density change upon crystallization is a key parameter governing the reliability of phase change random access memory. Indeed, few percentages density change induces considerable mechanical stress in memory cells, leading to film delamination with subsequent electrical failures. This letter presents an extensive study of density change upon crystallization in a series of Ga-Sb thin films with various antimony contents. The mass density of the films is precisely determined by x-ray reflectivity in both their amorphous and crystalline states. The variations of the density in crystalline and amorphous films according to the Sb content found to cross with a zero-density change for 70 at. % Sb. The peculiar behavior of Ga-Sb thin films upon crystallization may be linked to their stress state and mechanical properties.

Using water adsorption measurements to access the chemistry of defects in the metal–organic framework UiO-66

Tailoring defects in metal–organic frameworks is important for enhancing sorption and reaction properties. Defects in UiO-66 have been characterized for the first time by using water adsorption measurements. We found that the defect-induced hydrophilicity, quantitatively expressed by the Henry constant and the saturation water uptake, correlates well with the catalytic performance in the cyanosilylation of benzaldehyde.

Metal-Organic Frameworks as Catalyst Supports: Influence of Lattice Disorder on Metal Nanoparticle Formation

Abstract Because of their high tunability and surface area, metal‐organic frameworks (MOFs) show great promise as supports for metal nanoparticles. Depending on the synthesis route, MOFs may contain defects. Here, we show that highly crystalline MIL‐100(Fe) and disordered Basolite® F300, with identical iron 1,3,5‐benzenetricarboxylate composition, exhibit very divergent properties when used as a support for Pd nanoparticle deposition. While MIL‐100(Fe) shows a regular MTN‐zeotype crystal structure with two types of cages, Basolite® F300 lacks long‐range order beyond 8 Å and has a single‐pore system. The medium‐range configurational linker‐node disorder in Basolite® F300 results in a reduced number of Lewis acid sites, yielding more hydrophobic surface properties compared to hydrophilic MIL‐100(Fe). The hydrophilic/hydrophobic nature of MIL‐100(Fe) and Basolite® F300 impacts the amount of Pd and particle size distribution of Pd nanoparticles deposited during colloidal synthesis and dry impregnation methods, respectively. It is suggested that polar (apolar) solvents/precursors attractively interact with hydrophilic (hydrophobic) MOF surfaces, allowing tools at hand to increase the level of control over, for example, the nanoparticle size distribution.