Magali Putero

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.

Dopant Effect On NiGe Texture During Nickel Germanide Growth

In this paper, we report on the solid state reactions of Ni thin films (6nm to 12nm) deposited on Ge (001). Especially, the influence of the substrate dopants (As and BF2) is investigated. TEM and SIMS analyses point out the presence of an additional phase at the NiGe/Ge interface. EELS and SIMS measurements reveal that this phase contains Ni and Ge. On the another hand, ex-situ XRD spectra allows to determine a change in NiGe texture with Ge(001) doped or undoped. In situ XRD confirms that the kinetics shifts from simultaneous to sequential growth of nickel germanides, as predicted previously. Moreover, a recrystallization of NiGe is observed only on the undoped substrate. This recrystallization can be attributed to a contribution of a stress term in addition of the surface energy. This stress term becomes minor when temperature increases and Ni-rich phase is completely consumed. The presence of dopants, probably at the NiGe/Ge interface could reduce this stress and thus the recrystallization.

Conductive-bridge memory cells based on a nano-porous electrodeposited GeSbTe alloy

We report on the fabrication of memory devices based on a nanoporous GeSbTe layer electrodeposited inbetween TiN and Ag electrodes. It is shown that devices can operate along two distinct electrical modes consisting of a volatile or a non-volatile resistance switching mode upon appropriate preconditioning procedures. Based on electrical measurements conducted in both switching modes and physical analysis performed on a device after electrical stress, resistance switching is attributed to the formation/dissolution of a conductive filament from the Ag electrode into the GST layer whereas the volatile/non-volatile resistance switching is attributed to the presence of an interface layer between the GST and the Ag top electrode. Due to their simple, low-cost and low-temperature fabrication procedure, these devices could be advantageously exploited in flexible electronic applications or embedded into the back-end of line CMOS technology.

MgO monolayer epitaxy on Ni (100)

The growth of two-dimensional oxide films with accurate control of their structural and electronic properties is considered challenging for engineering nanotechnological applications. We address here the particular case of MgO ultrathin films grown on Ni (100), a system for which neither crystallization nor extended surface ordering has been established previously in the monolayer range. Using Scanning Tunneling Microscopy and Auger Electron Spectroscopy, we report on experiments showing MgO monolayer (ML) epitaxy on a ferromagnetic nickel surface, down to the limit of atomic thickness. Alternate steps of Mg ML deposition, O2 gas exposure, and ultrahigh vacuum thermal treatment enable the production of a textured film of ordered MgO nano-domains. This study could open interesting prospects for controlled epitaxy of ultrathin oxide films with a high magneto-resistance ratio on ferromagnetic substrates, enabling improvement in high-efficiency spintronics and magnetic tunnel junction devices.

Stability of GeTe-based phase change material stack under thermal stress: Reaction with Ti studied by combined in-situ x-ray diffraction, sheet resistance and atom probe tomography

In situ sheet resistance and x-ray diffraction measurements were used simultaneously during heat treatment to study Ti electrodes in contact with Ge-Te phase change materials. Ti is found to react with GeTe forming TiTe2 and Ge. Atom probe tomography analyses confirm the presence of these two phases after a 400°C heat treatment.

Fabrication and characterization of ECM memories based on a Ge 2 Sb 2 Te 5 solid electrolyte

This work deals with the study of electrochemical metallization memory cells (ECM) also called CBRAM (Conductive Bridge RAM). Memory stacks were fabricated by sputtering onto SiO2/Si substrates and were characterized by atomic force microscopy and a mercury drop probe. These stacks employ a Ge2Sb2Te5 (GST) layer as a solid electrolyte which has been barely employed in CBRAM devices. Electrical measurements demonstrate resistance switching of the stacks due to the formation/dissolution of metallic filaments within the GST layer. However, the memory elements featuring a silver top electrode do not exhibit such switching behavior but show instead an ohmic behavior. This result is interpreted through physical analysis revealing the presence of silver in each layer of the memory devices. Finally, a physical model is presented. This model was used to interpret adequately the bipolar resistance switching phenomenon observed in the memory stacks.