Phase Change Compound Research Articles

Phase Change Memory: Device scaling and challenges for material engineering in the GeSbTe compound system

The Phase Change Memory (PCM) relies on the peculiar properties of a chalcogenide material, able to be reversibly switched between two stable states, namely an amorphous phase, characterized by high electrical resistivity, and a poly-crystalline one, featuring low resistivity. The former is associated to the RESET state of the memory and the latter represents the SET state, thus enabling the storage of digital data. On one hand PCM device scaling is an important step in order to enable denser and low power applications, thanks to the shrink of dimensions and to the reduction of the operating power. On the other hand material engineering may represent a breakthrough in terms of reliability and performance enhancement, owing to the exploitation of the physical and chemical features of different phase change compounds. A smart combination of scaling and material engineering may result into wider application spectrum for PCM.

Fast Crystallization of the Phase Change Compound GeTe by Large-Scale Molecular Dynamics Simulations.

Phase change materials are of great interest as active layers in rewritable optical disks and novel electronic nonvolatile memories. These applications rest on a fast and reversible transformation between the amorphous and crystalline phases upon heating, taking place on the nanosecond time scale. In this work, we investigate the microscopic origin of the fast crystallization process by means of large-scale molecular dynamics simulations of the phase change compound GeTe. To this end, we use an interatomic potential generated from a Neural Network fitting of a large database of ab initio energies. We demonstrate that in the temperature range of the programming protocols of the electronic memories (500-700 K), nucleation of the crystal in the supercooled liquid is not rate-limiting. In this temperature range, the growth of supercritical nuclei is very fast because of a large atomic mobility, which is, in turn, the consequence of the high fragility of the supercooled liquid and the associated breakdown of the Stokes-Einstein relation between viscosity and diffusivity.

First-principles study of the amorphous In3SbTe2phase change compound

Ab initio molecular dynamics simulations based on density functional theory were performed to generate amorphous models of the phase change compound In${}_{3}$SbTe${}_{2}$ by quenching from the melt. In-Sb and In-Te are the most abundant bonds with only a minor fraction of Sb-Te bonds. The bonding geometry in the amorphous phase is, however, strongly dependent on the density in the range 6.448--5.75 g/cm${}^{3}$ that we investigated. While at high density the bonding geometry of In atoms is mostly octahedral-like as in the cubic crystalline phase of the ternary compound In${}_{3}$SbTe${}_{2}$, at low density we observed a sizable fraction of tetrahedral-like geometries similar to those present in the crystalline phase of the two binary compounds InTe and InSb that the ternary system can be thought to be made of. We show that the different ratio between octahedral-like and tetrahedral-like bonding geometries has fingerprints in the optical and vibrational spectra.

Materials Issues in the Development of High Data-Transfer-Rate Phase-Change Compounds

ABSTRACTA model is presented to calculate glass-transition temperatures. This model in combination with experimental data is used to evaluate archival-life stability of common phase change materials Ge2Sb2Te5 and doped eutectic Sb2Te compositions.On the basis of this model, novel high-data-rate phase change compositions have been identified near and on the pseudo-binary line InSb-GaSb in the ternary system Ga-In-Sb.

Optical and recording properties of short wavelength optical storage materials

Research interests in short wavelength optical storage have been growing considerably in recent years. In this paper, we first present a brief introduction to the materials requirements for short wavelength optical storage and basic considerations on the relation between reflectivity and optical constants of the materials, and then report our experimental results on the optical and short wavelength recording properties of the azo dye and the phase change compounds, Ge 2Sb 2Te 5 and Ag 8In 14Sb 55Te 23. The reflectivity contrast due to optical recording at 514.5 nm in the dye-polymer film is as high as 25%. The change in n and k of the phase change materials due to heat treatment can cause reasonably high reflectivity change at 500 nm, writing and erasing at 514.5 nm result in moderate reflectivity contrast. These suggest that the materials can be used for short wavelength optical storage.

The influence of target temperature and dose on metallic strengthening for titanium implantation into steel

The formation of intermetallic compounds on titanium implantation into H13 steel at different target temperature (-196, 150 and 400 °C) is studied. The steel strengthening mechanism of titanium implantation with different doses and target temperatures was discussed. The titanium ions are implanted in the energy range from 100 to 300 keV and dose range from 1 × 10 17 to 5.5 × 10 17 cm -2. The observations of phase change, microstructure and intermetallic compounds were carried out by high voltage (1 MV) electron microscopy and X-ray diffraction analysis. The hardness of the implanted layer increases one to 2.5 times for low temperature (-196 and 150 °C) implantation, two to three times for moderate temperature (MT) (400 °C) implantation. The wear rate decreases two to 2.6 times for low temperature implantation and ten times for MT implantation. The influence of target temperature and dose on metallic strengthening is discussed.