Thermo-desorption measurements during N-doped Ge-rich Ge2Sb2Te5crystallization

Ge-rich Ge₂Sb₂Te₅(GGST) is considered as one of the best candidates for industrial phase change memory production. GGST memory cells are generally embedded with Si or Ti nitride layers to prevent oxidation, as it leads to an undesired decrease of the GGST crystallization temperature. Furthermore, GGST films are usually doped with elements such as N, C, O, or Bi, aiming to delay GGST crystallization during the fabrication process as well as during memory cell operation. In this work, ultrahigh vacuum thermal desorption spectroscopy (TDS) was performed during isochronal annealing of a N-doped GGST film covered by a 10 nm-thick TiN_xlayer. Desorption is observed before GGST crystallization, but the comparison between TDS andin situx-ray diffraction measurements shows that the main desorption peak, observed between 653 K and 703 K, occurs after GGST full crystallization. The most prominent desorbing species are Ar, N₂, H₂, and H. These results show that the TiN_xpolycrystalline layer cannot prevent N atoms from leaving the GGST layer during annealing, suggesting a progressive change of the N-doped GGST chemical composition during thermal annealing and crystallization.

Kinetic Monte Carlo simulations of Ge-Sb-Te thin film crystallization

Simulation of atomic redistribution in Ge-Sb-Te (GST)-based memory cells during SET/RESET cycling is needed in order to understand GST memory cell failure and to design improved non-volatile memories. However, this type of atomic scale simulations is extremely challenging. In this work, we propose to use a simplified GST system in order to catch the basics of atomic redistribution in Ge-rich GST (GrGST) films using atomistic kinetic Monte Carlo simulations. Comparison between experiments and simulations shows good agreements regarding the influence of Ge excess on GrGST crystallization, as well as concerning the GST growth kinetic in GrGST films, suggesting the crystallized GST ternary compound to be off-stoichiometric. According to the simulation of atomic redistribution in GrGST films during SET/RESET cycling, the film microstructure stabilized during cycling is significantly dependent of the GST ternary phase stoichiometry. The use of amorphous layers exhibiting the GST ternary phase stoichiometry placed at the bottom or at the top of the GrGST layer is shown to be a way of controlling the microstructure evolution of the film during cycling. The significant evolution of the local composition in the amorphous solution during cycling suggests a non-negligible variation of the crystallization temperature with operation time.

Role of dislocation elastic field on impurity segregation in Fe-based alloys

Dislocation engineering in crystalline materials is essential when designing materials for a large range of applications. Segregation of additional elements at dislocations is frequently used to modify the influence of dislocations on material properties. Thus, the influence of the dislocation elastic field on impurity segregation is of major interest, as its understanding should lead to engineering solutions that improve the material properties. We report the experimental study of the elastic field influence on atomic segregation in the core and in the area surrounding edge dislocations in Fe-based alloys. Each element is found either to segregate in the edge dislocation core or to form atmospheres. The elastic field has a strong effect on the segregation atmosphere, but no effect on the dislocation core segregation. The theory is in good agreement with experiments, and should support dislocation engineering.

Growth and dissolution kinetics of ultra thin silicon films on Cu(100)

On Cu(100) surface, Auger electron spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM) were used to study (i) at room temperature (RT) the first steps of silicon growth and, (ii) at higher temperature, the dissolution process of silicon in Cu(100). The growth kinetics of Silicon onto Cu(100) at RT monitored by AES shows a quasi perfect layer-by-layer behaviour. After deposition at RT of about 5 silicon monolayers (ML), isochronal dissolution kinetics (rate of annealing of 1.5 degrees C/min) is recorded in a temperatures range [50-400 degrees C]. The slowdown observed in the kinetics dissolution for temperatures between 150 and 340 degrees C, reveals formation of an intermetallic superficial phase thermally stable in this range of temperature. LEED pattern and STM images show large domains of a rectangular (5 x 3) superstructure.