NiSi2 nanocrystals were synthesized and used as the floating gate for nonvolatile memory application. Vapor-solid-solid mechanism was employed to grow the NiSi2 nanocrystals by introducing SiH4 onto the Ni catalysts-covered SiO2/Si substrate at 600°C. The average size and density of the NiSi2 nanocrystals are 7∼10nm and 3×1011 cm-2, respectively. Metal-oxide-semiconductor field-effect-transistor memory with NiSi2 nanocrystals was fabricated and characterized. Programming/erasing, retention and endurance measurements were carried out and good performances were demonstrated.

High magnetic fields up to 22 T have been applied to determine the local bonding asymmetries in the Ge2As2Se7 and Ge2As2Se5 glasses by 75As NMR. The results are analyzed using computer simulations of the line-shape. The asymmetry parameter η of the electric field gradient at arsenic sites in Ge2As2Se7 is found to be about 0.2, indicating that the dominant arsenic structural units in the composition are nearly axially symmetric pyramids. In the Ge2As2Se5 glass, however, the 75As NMR spectrum exhibits no well-resolved structure, revealing the existence of highly asymmetric sites. The experimental data are fitted using the previously obtained distribution of quadrupole coupling constants from pulsed 75As NQR experiments. These NMR simulations assume a broad distribution of the asymmetry parameter. The results are in agreement with the NQR studies in the same compositions.

The storage principal of the Electrochemical Metallization Memory Cell is based on change of cell resistance induced by electro-chemical driven growth and rupture of a cupric or silver filament in an insulating matrix. This kind of switching was found in several materials as AgGeSe, CuGeS, silicon oxide or tungsten oxide [1].

During write operation copper or silver is oxidized at the corresponding electrode and copper or silver ions are driven out of the copper or silver anode into the insulating matrix due to the applied field, whereas the insulating matrix serves as solid electrolyte. The silver or copper ions migrate towards the cathode. At the cathode electrochemical reduction occurs, and deposition of metallic copper or silver takes place. Fast diffusion paths in the solid electrolyte matrix or preferred nucleation sites (seeds) at the boundary lead to filamentary growth. This growing cupric or silver dendrite finally reaches the anode and switches the device to a low resistance state.

Based on this switching mechanism a FEM simulation model was set up. To simplify the model space charges due to silver or copper migration are neglected. It is further assumed, that the conductivity in the solid electrolyte is only ionic. Hence, it is sufficient to solve the well-known Laplace equation to address the electric properties as well as ion migration. A “Level Set” method is used to track the boundary of the growing filament. The velocity of this boundary is proportional to the ionic current density calculated by Laplace equation. Based on this model simulations are applied to cell structures with multiple fast diffusion paths and seeds. Simulation results show that just one filament reaches the anode.

In a second step, Butler-Vollmer boundary conditions are introduced. This nonlinearity leads to an exponential dependence between switching time and switching voltage. As switching voltage increases, switching time decreases.

A simulation model capable of simulating ECM memory cells is presented. The model enables to simulate the behaviour of different cell geometries or different materials as solid electrolyte. Furthermore it gives deeper insight into the switching mechanism.

This work was supported by the European project EMMA “Emerging Materials for Mass storage Architectures” (FP6-033751).

We have demonstrated conformal deposition of amorphous GeSbTe films in high aspect ratio structures by MOCVD. SEM analysis showed the as-deposited GeSbTe films had smooth morphologies and were well controlled for void free amorphous conformal deposition. GeSbTe films adhere well to SiO2, TiN, and TiAlN. The morphology and adhesion are stable in 420°C post process. By annealing at 365°C, amorphous GeSbTe films converted into crystalline GeSbTe with polycrystalline grain sizes of 5nm. Film resistivity in the crystalline phase ranged from 0.001 to 0.1 Ω-cm, suitable for device applications. Phase change devices fabricated with confined via structures filled with MOCVD GeSbTe showed cycle endurances up to 1×1010 with a dynamic set/rest resistance of two orders of magnitude.

Epitaxial Ge2Sb2Te5 has been successfully grown on GaSb(001) by molecular beam epitaxy. The films show a tendency for void formation and rough morphology, but at the same time a very strong epitaxial orientation, cubic structure and a sharp interface to the substrate. The layers can be reversibly switched between the crystalline and amorphous phases using short laser pulses.